research report 157research report 157 hse health & safety executive the effect of adjacent...

HSE Health & Safety

Executive

The effect of adjacent buildings and topographical features on the wind pressure field around buildings

Prepared by BRE for the Health and Safety Executive 2003

RESEARCH REPORT 157

HSE Health & Safety

Executive

The effect of adjacent buildings and topographicalfeatures on the wind pressure field around buildings

Dr Philippa Westbury, Mr Liam Roche, Dr Bridget Pierce & Mr James Smith

BRE Environment Division

Bucknalls Lane Garston Watford

WD25 9XX

This work has been carried out to help develop guidance on siting open flue terminals for natural draught domestic gas boilers. It aims to investigate the effects of adjacent structures and topographical features on wind pressure fields around dwellings, and to identify scenarios where the likelihood of flue flow reversal is high. A combination of wind tunnel testing and computational modelling has been used. Ridge vent pressures on isolated dwellings have also been investigated. For the cases examined, the maximum pressure experienced by a flue terminal above a 45° pitch roof was found to exceed that for an isolated dwelling by up to 60% when very tall or wide structures (e.g. embankments) were located downwind of the dwelling. The percentage increase was higher for the 30° pitch roof, although absolute pressures for the 45° pitch roof were greater.

The greatest increased risk of flue flow reversal was found to occur with a background ventilation strategy rather than when a dominant opening was present, and when the adjacent structure was to one side of the dwelling, or offset or rotated relative to the dwelling. For these situations, large positive roof pressures occurred concurrently with negative wind-induced interior pressures.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2003

First published 2003

ISBN 0 7176 2750 0All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

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EXECUTIVE SUMMARY

This report describes a programme of wind tunnel testing and computer modelling which has been carried out to investigate the impact of adjacent structures and topographical features on the wind pressure field around dwellings, in order to determine building configurations that could increase the susceptibility of flues to flow reversal. The work is part of a wider programme to investigate the flue performance of open flued, natural draught domestic gas boilers, and to identify any safety implications arising from their increased efficiency which tends to cause greater sensitivity to adverse wind effects. The main objectives of the work are to investigate the effects of height and separation distance of adjacent structures and topographical features on wind pressure fields around dwellings, and to determine the sensitivity of the wind pressure field to additional factors such as dwelling geometry and orientation/geometry of the adjacent building. Furthermore, the work aims to identify scenarios where the likelihood of flue flow reversal is high due to positive pressures acting on the flue terminals and/or negative windinduced interior pressures. For this part of the work, a combination of wind tunnel testing and computational modelling has been used. The results from this work will help to develop guidance in BS5440 on siting open flue terminals.

The results show that, for a ‘standard’ domestic dwelling with a 45° pitch roof, the maximum positive pressure on the dwelling roof occurs when an adjacent structure is directly downwind of the dwelling, and the dwelling is immersed in the positive pressure field produced by the downwind structure. The magnitude of the pressure depends on the height and width of the adjacent structure, and its distance from the dwelling. The maximum pressure experienced by a flue terminal located above a 45° pitch roof may exceed that for the isolated dwelling by up to 60% when very tall or wide structures (e.g. embankments) are located close to the dwelling. Similarly to Upton et al. (1999b), increases in flue height up to about 2.5m (full scale) have not been found to significantly reduce the risk of flue flow reversal.

The percentage increase in maximum roof pressure with an adjacent structure present, relative to the isolated dwelling, is considerably greater for the 30° pitch roof than the 45° pitch roof. In addition, the eaves pressure for an isolated 30° pitch roof dwelling is negative, whereas it is positive with an adjacent structure close by. However, magnitudes of the maximum pressures remain lower for the 30° pitch roof than those for the 45° pitch roof. This indicates that the increased risk of flue flow reversal due to the presence of an adjacent structure is greater for the 30° pitch roof relative to the isolated dwelling case. However, the absolute risk for the 45° pitch roof is still greater.

For the range of additional factors investigated for the 45° pitch roof, maximum roof pressures have not been found to increase significantly when different factors are introduced. Factors that have been investigated include adjacent structure geometry, offset or rotation of the adjacent structure relative to the dwelling, location of the dwelling within a terrace, location of a dwelling adjacent to similar neighbours, geometry of topographical feature (hill or embankment) and terrain type.

The zone of influence over which a downwind adjacent structure of height H causes an increase in positive roof pressure relative to the isolated case has been found to extend to a distance of ~1.5H when the adjacent structure width, W, is relatively small in comparison to the dwelling eaves height, h (W ~ 2h). For wider adjacent structures (W ≥ 4h), the zone of influence extends over a distance of at least 2H. For all cases, the maximum roof pressure tends to decrease with increasing separation distance.

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The impact of the wind pressure field on the interior pressure has been investigated by considering two ventilation scenarios. The first is a worst-case ventilation scenario which occurs when an open window or ‘dominant opening’ is situated on the façade where the minimum external façade pressure acts. This results in the maximum possible difference between the roof and interior pressures. However, the maximum difference between the roof pressure and the interior pressure has been found to occur for the isolated dwelling rather than obstructed dwellings. This is because the adjacent structure tends to smooth out the variation in pressure over the dwelling, resulting in a smaller difference between maximum roof and minimum façade pressures.

The second ventilation scenario is for a dwelling where background ventilation only is provided. When the dwelling is fully enclosed by the positive pressure field, the interior pressure is positive and the net pressure (difference between the maximum roof pressure and the interior pressure) is smaller with the obstruction present than in the isolated case. This indicates that in a steady-state situation, the risk of flue flow reversal could be lower with the obstruction present.

When the adjacent structure is to one side of the dwelling, or is rotated or offset relative to the dwelling, the dwelling is only partially enclosed by the positive pressure field created by the adjacent structure. In this case, large positive roof pressures can occur concurrently with relatively large negative façade pressures and net pressures tend to be higher than for the isolated case. Thus the risk of flue flow reversal increases relative to the isolated case when background ventilation only is provided.

For the moderate wind speeds at which Upton et al. (1999a) identified high flue flow reversal risk, the influence of stack pressure on the interior pressure has been found to be significant when internal/external temperature differences are large. Stack-induced pressures have a larger relative effect when background ventilation only is provided than when a dominant opening is present. Negative stack-induced interior pressures occur when the external temperature is lower than the internal temperature, which will occur during winter months. These will tend to increase the risk of flue flow reversal, in addition to any adverse wind effects.

Thus the greatest increase in the risk of flue flow reversal with an adjacent structure present relative to the isolated case has been found to occur when the ventilation strategy is background ventilation, with no dominant opening provided by a window. The former ventilation strategy is more likely to be used when internal/external temperature differences are large (i.e. in winter), when the risk is further increased by the influence of the stack effect on interior pressure.

It is recommended that fanned draught systems should be considered for dwellings with roof pitches greater than or equal to 45°, both with and without an adjacent structure present. For dwellings with less steep roof pitches (30° or lower), it is recommended that alternatives to natural draught systems should be considered for the 30° roof pitch when an adjacent structure is at a sufficiently close distance to the dwelling.

Additional wind tunnel experiments could be considered for dwellings with lower roof pitches (for example, 30° pitch roof or flat roof), in order to provide a more detailed assessment of the impact of adjacent structures. In particular, the effect of size and separation distance of an adjacent structure on the roof pressure in the eaves region (which is negative for the isolated dwellings) could be investigated further.

Ridge vent pressures have been investigated for both 45° and 30° roof pitch dwellings located in 3- and 9-house terraces. For both roof pitches, ridge vent pressures are found to be negative for most wind directions. Furthermore, for the 45° case, when positive pressures occur on the upwind side of the ridge (0° and 30° wind directions), much greater negative pressures occur on

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the downwind side, and thus the net wind effect will tend to enhance ventilation from the terminal. For both roof pitches, the most problematic wind direction occurs when the wind is along the ridge. In this case, although the magnitude of the pressure is found to be small, sustained positive pressure 'events' can occur for a significant proportion of the time. The susceptibility of the terminal to adverse wind effects is found to increase slightly with increasing terrace length, when the wind is parallel to the ridge.

It is suggested that the use of ridge vents is preferable to flue terminals located close to the eaves, particularly in the case of the 45° pitch roof.

This report and the work it describes were funded by the Health and Safety Executive. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

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CONTENTS

EXECUTIVE SUMMARY

1. INTRODUCTION 1

2. SELECTION OF TEST SCENARIOS 32.1 Overall scope of the investigation 32.2 Selection of dwellings 32.3 Selection of adjacent buildings 52.4 Arrangements of dwellings relative to adjacent buildings 62.5 Selection of topographical features 92.6 Selection of wind directions 102.7 Additional sensitivity analyses 102.8 Selection of ridge vent cases 112.9 Inventory of tests carried out 12

3. EXPERIMENTAL METHODOLOGY 173.1 Wind tunnel modelling 173.2 Model construction 173.3 Measurement of pressures 203.4 Data analysis 21

4. COMPUTATIONAL MODELLING 23

5. RESULTS 275.1 Impact of adjacent structures and topographical features on the external pressure field over dwelling roofs 275.2 Effect of flue height on the external pressure at the flue terminal 435.3 Impact of adjacent structures and topographical features on maximum internal/external pressure differences in the case of a dominant opening 455.4 BREEZE modelling 505.5 Ridge vent investigation 60

6. DISCUSSION 676.1 Impact of adjacent structures and topographical features 676.2 Ridge vents 69

7. CONCLUSIONS 717.1 Impact of adjacent buildings and topographical features 717.2 Ridge vents 72

8. RECOMMENDATIONS 738.1 Impact of adjacent buildings and topographical features 738.2 Ridge vents 73

9. ACKNOWLEDGEMENTS 75

10. REFERENCES 77

11. Appendix A 79

12. Appendix B 87

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1. INTRODUCTION

Since April 1997, BRE has carried out a number of studies on behalf of HSE to investigate the flue performance of domestic gas boilers, and to identify any safety implications arising from the increased efficiency of open flued, natural draught appliances. These studies have investigated the susceptibility of flues to flow reversal, using a combination of full scale tests in BRE’s test houses, wind tunnel testing and computational modelling. From the full scale tests, climate and other factors which tend to increase the risk of flue flow reversals were identified for external flues discharging near to the building eaves (Upton et al. (1999(a)), and for internally run flues discharging to near the building eaves or to ridge vents (Upton et al. 2000). Later full-scale studies in combination with computational modelling investigated the effects of ventilation strategy on flue performance (Upton et al. 2001, Plathner & Stephen 2001).

The studies found that factors contributing to the risk of flue flow reversal included the wind speed and direction, the interior dwelling pressure resulting from the ventilation strategy, the type of flue terminal, and its location on the roof. The risk was found to be greater for high efficiency boilers than lower efficiency boilers, due to the reduction in flue gas temperatures causing a reduction in the available buoyancy forces for exhausting flue gases. Flues running predominantly externally were also more prone to flow reversal than those running predominantly internally, and the risk of flue flow reversal was greater when the boiler was starting up (cold flue) than when it had been firing for a period of time. High risk conditions tended to be initiated by external wind conditions producing positive pressures around the flue terminal; however, once flue flow reversals were initiated, it was found that maintenance of the reversed flow depended not only on externally induced wind pressures, but also on temperature conditions prevailing both within the flue and between the house and ambient environment.

A programme of wind tunnel testing was carried out in order to identify building shapes and configurations that would tend to cause flue flow reversal to occur due to the formation of positive wind pressure fields around the dwelling (Upton et al. (1999(b)). This work identified a number of building shapes for which flues might be subjected to positive pressures, including roofs with pitch angles of 45°, single storey flat roof extensions and 'L' shaped buildings. The tests also investigated wind pressure fields for a limited number of groups of buildings, and found that positive pressure fields were caused by taller adjacent buildings. It was recommended that further work was required to investigate the effect of adjacent buildings and topography on the wind pressure field around dwellings.

Previous investigations by other authors into the effects of adjacent objects (buildings or topography) on the pressure field over a dwelling include those by De Gids et al. (1974) and Lilly et al. (1996). Lilly et al. investigated the effects of adjacent embankments (of lower, equal height and higher than the building) and adjacent houses of heights h and 2h, where h is the dwelling height. De Gids et al. investigated the effects of taller buildings, ranging from 2h to 8h, and found that the effect of the taller buildings was strongly dependent on the ratio of the heights of the respective buildings and the separation between them.

This report describes a programme of wind tunnel testing and computational modelling which has been carried out to investigate further the effect of adjacent buildings and topographical features on the wind pressure field around buildings, in order to determine building configurations that could increase the susceptibility of flues to flow reversal. This work is required in order to extend the guidance in BS5440 on siting open flue terminals, particularly given the increased sensitivity of high efficiency open flued, natural draught boilers to adverse wind pressure effects. BS5440 currently recommends that 'Additional precautions may be needed in siting a terminal in certain circumstances, such as on a sloping site or near to a very

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large structure' (Section 5.1.6.1, Commentary and Recommendations). More specific guidelines are not currently given. Thus there is a need to investigate a wider range of configurations of dwellings with adjacent buildings and topographical features than has previously been investigated. This work follows on from the wind tunnel testing carried out previously by Upton et al. (1999(b)).

The main objectives of this work are as follows:

· To determine the impact of adjacent buildings or topography on the wind pressure field over the roof of a building, as a function of the type of object, its height, its distance from the building and the wind direction;

· To determine the implications of these findings for safe siting of open flue terminals, to minimise the risk of flue flow reversal occurring due to the presence of positive pressure fields or strong negative interior pressures;

· To extend the design guidance given in BS5440 on locating open flue terminals next to adjacent objects, in the light of these results.

The work uses a combination of wind tunnel testing and computational modelling to determine the impact of adjacent buildings and topographical features on the wind pressure field around a dwelling, and the interior dwelling pressures resulting from the wind pressure field.

In addition to the work described above, a study has been carried out to determine the wind pressures experienced by ridge vents located on isolated buildings, in order to determine the optimum siting for ridge vents.

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2. SELECTION OF TEST SCENARIOS

2.1 OVERALL SCOPE OF THE INVESTIGATION

2.1.1 Effect of adjacent buildings and topographical features

The arrangements of dwellings, adjacent buildings and topographical features were selected in order to achieve the following aims:

· To investigate the effects of height, separation distance and wind direction on the pressure field around the dwelling for different adjacent buildings and topographical features for one ‘standard’ domestic dwelling;

· To investigate the sensitivity of the pressure field to additional factors such as the dwelling geometry, the geometry of the adjacent building or topographical feature and the orientation of the adjacent building relative to the dwelling;

· To identify scenarios where flues could be particularly sensitive to flow reversals due to:

o Positive pressures acting on the flue terminals; o Negative interior pressures resulting from the wind pressure field around the

building.

2.1.2 Ridge vent investigation

Arrangements of dwellings were selected in order to achieve the following aims:

· To investigate the effect of roof geometry on the pressure field at dwelling ridges;

· To examine the sensitivity of the pressure field to the location of a dwelling in a terrace, and the length of the terrace.

2.2 SELECTION OF DWELLINGS

The ‘standard’ domestic dwelling was chosen as a detached house with a 45° pitch roof. 45°

represents a commonly occurring roof pitch and a ‘worst case’ scenario, since a steeper roof pitch tends to cause higher positive pressures on the upwind side of the roof. The choice of a detached house, rather than a terraced house, allows a wider range of relative widths between the dwelling and adjacent buildings to be tested. The dimensions of the detached house were based on the dimensions for standard dwellings defined previously in Iles (1998). The roof shape was chosen to be a dual-pitch, rather than a hip roof. Most of the investigations were carried out using the ‘standard’ dwelling described above.

In order to determine the sensitivity of the wind pressure field to the geometry of the dwelling, the following dwelling geometries were additionally tested:

· Detached dwelling with 30° roof;

· Detached dwelling with 45° roof and flat roof extension;

· Dwelling within a three-house terrace.

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Dwellings with a roof pitch of 30° were selected as they are likely to be built more commonly in the future, although will tend to be less susceptible than 45° roof pitches to adverse pressure effects. The flat roof extension was identified as being particularly vulnerable to adverse pressure effects in Upton et al. (1999(b)). In the case of the terraced dwelling, the dwelling under investigation was tested both in the centre of the terrace and at one end.

Photographs of the dwellings are shown in Figures 1-4.

Figure 1 Pressure tapped model of 45° pitch roof dwelling

Figure 2 Pressure tapped model of 30° pitch roof dwelling

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Figure 3 Pressure tapped model of dwelling with flat roof extension

Figure 4 Pressure tapped model of 45° pitch roof dwelling in mid terrace location

2.3 SELECTION OF ADJACENT BUILDINGS

To investigate the effect of adjacent buildings, a number of geometries were selected, with dimensions defined as multiples of the eaves height, h, of the dwelling. The width is the crosswind dimension, while the breadth is the along-wind dimension when the wind direction is 0° or 180°. Parameters defining the height, width and breadth are shown in Figure 5.

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Figure 5 Definition of dimensions and wind directions

The following geometries were tested in order to determine the impact of adjacent building height and width on the wind pressure field over the dwelling:

· Flat-roof adjacent buildings of width 2h, with heights 2h, 4h and 8h;

· Flat-roof adjacent buildings of height 2h, with widths 2h, 4h and 8h;

· Flat-roof adjacent buildings of height 4h, with widths 2h and 4h;

All adjacent building breadths were h.

The following geometries were tested in order to determine the impact of adjacent buildinggeometry on the wind pressure field:

· 30° pitch roof adjacent building of height 4h and width 2h;

· 45° pitch roof adjacent building of height 4h and width 2h.

2.4 ARRANGEMENTS OF DWELLINGS RELATIVE TO ADJACENT BUILDINGS

The adjacent buildings and topographical features were arranged in different positions relative to the dwelling in order to test the following:

· The effect of separation distance between the dwelling and the adjacent building or topographical feature;

· The effect of offset of the adjacent building relative to the dwelling;

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· The effect of angle of rotation of the adjacent building relative to the dwelling.

The parameters defining the arrangements (separation distance, offset, rotation) are shown in Figure 6, along with the definition of wind direction in relation to the arrangement. Examples of the arrangements are shown in photographs (Figures 7 and 8).

Figure 6 Plan view offset and rotated building configurations

Wind pressures were also investigated for isolated dwellings in the absence of adjacent buildings or topographical features.

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Figure 7 4h x 2h adjacent structure at 2h distance

Figure 8 2h x 8h adjacent structure at 4h

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2.5 SELECTION OF TOPOGRAPHICAL FEATURES

Embankments and hills were selected to represent topographical features. The following geometries were tested in order to determine the effect of the topographical feature on the wind pressure field (the width is the long dimension of the feature and the breadth is the shorter dimension of the flat top as in figure 5):

· Embankment of height 2h, width 16h and breadth h, slope 90°;

· Hills of height h, 2h and 3h, width 16h and breadth 4h.

The following additional embankment geometries were tested in order to determine the effect of embankment breadth and slope on the wind pressure field:

· Embankment of height 2h, width 16h and breadth 4h;

· Embankment of height 2h, width 16h, breadth h, with slopes 30° and 60°.

Plans of embankments and hills, and definitions of separation distances, are shown in Figure 9a. A photograph of a large hill is shown in figure 9b. Remote slope was omitted from large hills.

Figure 9a Configurations of hills and embankments.

(Tops are horizontal and slopes reach ground level)

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Figure 9b Upwind hill of height 3h with base 2h from dwelling

2.6 SELECTION OF WIND DIRECTIONS

Preliminary studies identified that the worst case wind direction was with the adjacent building or topographical feature directly downwind of the dwelling (wind direction 180°). Thus the majority of cases were tested with this wind direction. In addition, a number of cases were tested with the adjacent building or topographical feature directly upwind of the dwelling (wind direction 0°). Where other wind directions could have a significant impact (for example, for wider adjacent buildings or topographical features, or where the adjacent building was offset or rotated relative to the dwelling), additional wind directions were tested.

2.7 ADDITIONAL SENSITIVITY ANALYSES

Additional sensitivity analyses were carried out to determine the following:

· The effect of surrounding terrain (suburban or urban) on the wind pressure field measured over the dwelling;

· The effect of similar dwellings neighbouring the dwelling under investigation (Figure 10).

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Figure 10 Detached dwelling with two identical neighbours

2.8 SELECTION OF RIDGE VENT CASES

Wind pressures were measured at positions on the roof coinciding with the locations of ridge vents. Tests were carried out for the following scenarios, in the absence of adjacent structures or topographical features:

· 3 house terrace with 30° dual pitch roof

· 3 house terrace with 45° dual pitch roof

In addition, a 9 house terrace with 45° dual pitch roof was tested in order to determine the effect of terrace length on the results. Each terrace was tested with the terrace perpendicular and parallel to the adjacent building (see Figure 11).

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Figure 11 Parallel and perpendicular terrace configurations with an adjacent building

2.9 INVENTORY OF TESTS CARRIED OUT

2.9.1 Adjacent buildings

The wind tunnel tests carried out have been grouped according to the following objectives, and are summarised in Tables 1(a) to 1(g). Note that some wind tunnel tests are common to more than one objective.

· Objective 1(a) - Effect of height and separation distance

· Objective 1(b) - Effect of width of adjacent building

· Objective 1(c) - Effect of dwelling geometry

· Objective 1(d) - Effect of adjacent building geometry

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· Objective 1(e) - Effect of adjacent building offset

· Objective 1(f) - Effect of rotation of adjacent building relative to dwelling

· Objective 1(g) - Effect of neighbouring dwellings

· Objective 1(h) - Effect of upwind terrain

2.9.2 Topographical features

· Objective 2(a) - Effect of embankment slope and breadth

· Objective 2(b) - Effect of hill

Table 1(a) Effect of adjacent building height and separation

Dwelling roof pitch

H W B Separation distance

45° 2h 2h h h 45° 2h 2h h 2h 45° 2h 2h h 3h

45° 4h 2h h 2h 45° 4h 2h h 4h 45° 4h 2h h 6h

45° 8h 2h h 4h 45° 8h 2h h 8h 45° 8h 2h h 12h

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Table 1(b)Effect of aspect ratio

Dwelling roof pitch


45° 2h 2h h h 45° 2h 2h h 2h 45° 2h 2h h 3h

45° 2h 4h h 2h 45° 2h 4h h 4h 45° 2h 4h h 6h

45° 2h 8h h 2h 45° 2h 8h h 4h 45° 2h 8h h 6h 45° 2h 8h h 8h

45° 4h 4h h 2h 45° 4h 4h h 4h 45° 4h 4h h 6h 45° 4h 4h h 8h

Table 1(c)Effect of dwelling geometry

Dwelling roof pitch / geometry

Location within terrace


45° - 4h 2h h 2h 45° - 4h 2h h 4h

30° - 4h 2h h 2h 30° - 4h 2h h 4h

45° + flat roof - 4h 2h h 2h 45° + flat roof - 4h 2h h 4h

45° terrace centre 4h 2h h 2h 45° terrace centre 4h 2h h 4h

45° terrace centre 4h 4h h 2h 45° terrace centre 4h 4h h 4h

45° terrace end 4h 2h h 2h 45° terrace end 4h 2h h 4h

45° terrace end 4h 4h h 2h 45° terrace end 4h 4h h 4h

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Table 1(d)Effect of adjacent building geometry

Dwelling roof pitch

Adjacent building roof

pitch


45° 30° 4h 2h h 2h 45° 30° 4h 2h h 4h

45° 45° 4h 2h h 2h 45° 45° 4h 2h h 4h

Table 1(e)Effect of adjacent building offset

Dwelling roof pitch

Offset of adjacent building


45° h 2h 4h h 2h 45° 2h 2h 4h h 4h

Table 1(f) Effect of rotation of adjacent building relative to dwelling

Dwelling roof pitch

Adjacent buildingrotation


45° 30° 2h 4h h 2h 45° 60° 2h 4h h 2h

Table 1(g) Effect of neighbouring dwellings

Dwelling roof pitch

Neighbouring dwellings present


45° Yes 2h 4h h 2h 45° Yes 2h 4h h 4h

45° Yes 4h 2h h 2h 45° Yes 4h 2h h 4h

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Table 1(h) Effect of upwind terrain

Dwelling roof pitch

Upwind terrain

Neighbouring dwellings present


45° Suburban Y/N 4h 2h h 2h 45° Suburban Y/N 4h 2h h 4h

45° Urban Y/N 4h 2h h 2h 45° Urban Y/N 4h 2h h 4h

Table 2(a)Effect of embankment slope and breadth

Dwelling roof pitch

Embankment slope

Embankment breadth

H W Separation distance

45° 90° h 2h 16h 2h 45° 90° h 2h 16h 4h 45° 90° h 2h 16h 8h

45° 90° 4h 2h 16h 2h 45° 90° 4h 2h 16h 4h

45° 60° h 2h 16h 2h 45° 60° h 2h 16h 4h

45° 30° h 2h 16h 2h 45° 30° h 2h 16h 4h

Table 2(b)Effect of hill

Dwelling roof pitch


45° h 16h 4h 2h 45° 2h 16h 4h 2h 45° 3h 16h 4h 2h

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3. EXPERIMENTAL METHODOLOGY

3.1 WIND TUNNEL MODELLING


External wind pressure fields were investigated using 1:80 scale models of dwellings and adjacent buildings or topographical features. The tests were carried out in BRE’s Environmental Wind Tunnel, which is used primarily for studies into wind flow around buildings. It is an openjet wind tunnel with a working section of approximately 2m wide by 1.5m high.

The model scale was chosen to be small enough to allow a sufficient range of adjacent object heights and separation distances to be represented, but large enough to allow sufficiently detailed pressure measurements to be made. The models were mounted on a turntable in order to test at any required wind direction. Appropriate mean velocity profiles and turbulence intensity profiles for the surrounding suburban or urban terrain were modelled in the upwind fetch of the wind tunnel at a scale to match the dwellings and adjacent objects. The aerodynamic roughness height parameter, z0, which is a measure of the terrain roughness, was 0.35m (full scale) for the suburban terrain simulation, and 0.70m (full scale) for the urban terrain simulation, which are within the range of z0 given by ESDU (1993) for these terrain types.

Measurements of surface pressures on the roof and façades of the dwelling, and pressures at two additional heights above the roof of the dwelling were made. These heights corresponded to flue heights of 0.6m and 1.5m at full scale.

3.1.2 Ridge vent investigation

Pressures at locations coinciding with ridge vents were carried out at 1:60 scale, on existing wind tunnel models that were used previously in Upton et al. (1999(b)). Measurements of surface pressure were made only. Suburban terrain was modelled in the upwind fetch of the wind tunnel using the same roughness simulation as in the previous investigation. Thus the roughness height parameter, zo, for this investigation was 0.26m (full scale).

3.2 MODEL CONSTRUCTION

Dwelling models were constructed in a similar way to the models used in Upton et al. (1999(b)). The models were constructed using 6mm perspex sheet and were instrumented in different ways, according to whether surface pressures, or pressures above the roof, or pressures at ridge vents were measured. These are described in the following sections. The adjacent buildings and topographical features were constructed as block models from wood, and were painted matt black. Non-instrumented block models of dwellings were also constructed from wood and painted matt black.

3.2.1 Surface tapped models

To measure surface pressures, dwelling models were instrumented using conventional surface pressure taps which consisted of 2mm diameter brass tubes inserted into the perspex model, with the ends flush with the external surface of the model. The taps were connected to the measurement system using small-bore pvc tubing. A restrictor was fitted in each line, in order to remove any distortion introduced into pressure traces by the tubing (Cook (1990)).

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Pressure-tapped models were constructed for dwellings with 45° and 30° roof pitches. An additional section which represented the flat roof extension was also constructed. When modelling terraces, non-instrumented block models were used alongside the instrumented model to represent additional houses in the terrace.

Pressure taps were densely distributed over the roofs of the instrumented models. In addition, pressure taps were introduced into the façades of the models, in order to measure pressures at locations coinciding with ventilation openings. The arrangements of pressure taps are shown in photographs in Figures 1 and 2.

3.2.2 Vertical tube models

To measure pressures at heights above the surface of the roof, dwelling models were instrumented with 2mm diameter brass tubes. Tubes were inserted in the roofs of the models and were fixed in a vertical position, independent of the roof angle. The height of the vertical tubes was adjustable, allowing pressures to be measured at any height above the roof surface. Surface taps were provided in the façades of the dwelling model, with an arrangement similar to that for the models with surface taps on the roof. Again, the vertical tubes and surface taps were connected to the pressure measurement system using small-bore pvc tubing and a restrictor was fitted in each line.

Vertical tube models were constructed for dwellings with 30° and 45° pitch roofs, and for the flat roof extension. The 5 by 4 array of vertical tubes on each slope of the 45° pitch roof is shown in the photograph in Figure 12a. The 5 by 3 array of tubes on each slope of the 30° pitch roof model is shown in 12b.

Figure 12a Vertical tube model of dwelling with 45° pitch roof

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Figure 12b Vertical tube model of dwelling with 30° pitch roof

3.2.3 Models used for ridge vent tests

To measure ridge pressures, the models were fitted with surface taps located along the ridges of the 3 house terrace models, and were connected to the pressure measurement system in the same way as described previously. Terraces with roof pitches of 30° and 45° were modelled. Dummy block models of 3 house terraces were used to extend the 3 house terrace to a 9 house terrace. The instrumented terrace model and 2 dummy block models were arranged in different ways in order to allow pressures to be measured along half the length of the 9 house terrace, taking advantage of a plane of symmetry. The arrangement of pressure taps is shown in Figure 12c.

Figure 12c Ridge taps on 30° pitch model (45° model similar)

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3.3 MEASUREMENT OF PRESSURES

Pressures were measured using a Scanivalve pressure measurement system comprising ZOC electronic pressure scanners and a Digital Service Module (DSM3200), which allows the electronic pressure scanners to be interfaced to an Ethernet network. The system was run from a separate computer using DSMLINK software, allowing experimental setups for the measurement hardware to be configured. The following pressure scanners were used: a 64 channel scanner (ZOC33/64Px), 2 ´ 32 channel scanners (ZOC22B) and 1 ´ 32 channel scanner comprising 4 individual banks of 8 channel sensors (ZOC23B). Thus pressures could be measured at up to 160 points simultaneously, at a maximum sampling rate of 100kHz (depending on the number of scanners connected to the DSM at any one time). Each scanner module has its own unique factory set calibration coefficients, provided by the module supplier, and is installed with an RTD, allowing temperature compensation to be carried out.

For a particular dwelling model, pressure data were acquired at all vertical tubes and surface taps simultaneously, at a sampling rate which allowed high pressure 'events' that have been found to tend to cause flue flow reversal to be resolved. For example, Upton et al. (2000) found that typically a flue flow reversal may occur if a low to moderate wind speed (say, 2-4m/s) occurred for a period of 5-10 seconds. The sampling rate has been set to resolve this event using ~5 sampling points. The timescales (T), length scales (L) and velocity scales (V) are related between full scale and model scale in the following way:

Lm Lf =

TmVm V T ff

where the subscripts 'm' and 'f' denote model scale and full scale respectively.

Taking Lf = 80 (model scale 1:80), Vf = 3m/s and Tf = 10 seconds, if the wind tunnel speed Vm = 6m/s and Lm = 1, therefore the corresponding wind tunnel time period Tm = 0.06s. Thus a sampling frequency of ~100Hz is required to obtain ~5 sampling points in order to resolve the event. Pressures have been sampled at 100Hz over a period of approximately 10 seconds at model scale (corresponding to a sampling period of about 15 minutes at full scale).

A pitot-static tube was placed at a height of 300mm above the wind tunnel floor in the undisturbed incoming flow (slightly upwind and to one side of the model). The dynamic and static pressures from the pitot-static tube were measured using each of the transducer modules being used (1 to 3), and the average pressure difference, corrected to eaves height, was used to normalise the measured pressures to coefficient form using the standard definition:

CP p x - ps

= 21

2 r Vref

where: Cp is the pressure coefficient px is the pressure measured at the surface tap or vertical tube ps is the static pressure measured by the pitot-static tube Vref is the reference velocity (wind speed at eaves height in undisturbed flow) r is the air density 1 rVref

2 is the total pressure at reference height 2

20

To minimise the effect of system drift, data acquisition was carried out in blocks, with zero calibrations (to determine zero offsets) performed immediately before and after the wind tunnel was run. If the drift was large, the data set was repeated; otherwise, the average offset was removed from the data during analysis.

3.4 DATA ANALYSIS


Pressure records containing instantaneous sampled pressures have been analysed in the following ways:

· Time-averaged pressure coefficients have been evaluated over the roof (measured using both surface taps and vertical tubes), and on façades (measured using surface taps only);

· The minimum time-averaged façade pressure coefficients for each scenario have been identified in order to determine the maximum difference between the pressure field over the roof and the interior pressure (i.e. ‘worst-case’ internal/external pressure difference). The maximum pressure difference will tend to occur when an open window is located on the façade at a point coinciding with the minimum pressure. In this case, the interior pressure will be close to the measured façade pressure, since the window acts as a 'dominant opening' (Cook (1990));

· Time-averaged roof and façade pressures have been used as boundary conditions to a computational model (discussed further in Section 4) in order to predict the impact of the external wind pressure field on the interior pressure, and to determine the sensitivity of the interior pressure to factors such as the ventilation strategy, internal/external temperature difference and orientation of the dwelling relative to the adjacent structure.

Results are presented in Sections 5.1 to 5.3.

3.4.2 Ridge vents

Pressure records containing instantaneous pressures sampled over a period of approximately 10 seconds at model scale have been analysed in the following ways:

· Time averages have been evaluated for pressures measured at the ridge;

· Intermittency characteristics of the pressure fields have been plotted by determining the percentage of time that pressure coefficients are positive for time periods greater than 10 seconds (at full scale) at a nominal wind speed of 3m/s.

Results are presented in Section 5.5.

21

4. COMPUTATIONAL MODELLING

Computational modelling has been used to investigate further the effect of the wind pressure field caused by an adjacent building or topographical feature on the interior pressure. From this, the difference between the interior pressure and the pressure at a point over the roof coinciding with the flue terminal may be assessed. An indication of the maximum possible pressure difference resulting from an open window in a region of low pressure has been obtained from the simplified analysis described in Section 5.3. The main purpose of the computational modelling is to investigate a greater range of factors influencing the interior pressure, including ventilation opening areas, orientation of the building relative to the adjacent structure, wind speed and internal/external temperature difference.

Computational modelling has been carried out using the BRE's flow model, BREEZE. This is a flow/pressure analysis program which applies methods of network flow computation to calculate mass flows and pressure differences throughout a building. The building model used in this analysis is of BRE test house B54.2, and is similar to that used in Plathner & Stephen (2001). This model has been selected in order to provide consistency between previous BREEZE modelling work carried out for HSE and the current program of BREEZE modelling. Also, the model has previously been validated against test data, and its dimensions are similar to the scale model of the dwellings tested in the wind tunnel.

In all cases, the internal temperature was set to 20°C and the 50th percentile house envelope leakage was selected, corresponding to a leakage coefficient of 140.934m3.hr-1.Pa-1 (Plathner & Stephen (2001)). The trickle vent sizes and locations were chosen in accordance with Approved Document F1 (Department of the Environment and The Welsh Office (1995)). The kitchen window dimensions from the BRE test house were used, and the effective area of the window open 90° was calculated using CIBSE Guide A (1999).

The flue was modelled to represent closely the existing flue in building 54.2, and was situated within the kitchen. The main lineaments of flue construction are outlined as follows:

· Flue cross-sectional area of 0.012m2;

· Duct length of 6.24m which includes 4.74m to match base of duct to roof height and 1.5m for the flue above the roof;

· A wire mesh inlet;

· Standard cowling terminal type.

BREEZE is able to accommodate pressure losses both due to inlet/outlet terminal losses and friction losses along the length of the duct. To model the inlet/outlet pressure losses, the user needs to enter the velocity pressure loss factors. Based on values presented in CIBSE Guide C (1996), a loss factor of 0.5 was chosen for the inlet and 1.0 for the outlet. To model the friction losses, the user needs to enter a coefficient of friction. Based on values presented in CIBSE Guide C (1996), a coefficient of 0.03 was chosen. A sensitivity study was undertaken to determine the impact of reasonable variations of both the pressure loss factors and the coefficient of friction. This study showed that the results were fairly insensitive to variations in these parameters. Indoor temperatures were used as the flue temperature for the part of the flue up to the height of the roof, and external temperature were used for the part of the flue above the

23

height of the roof. This is a worst-case scenario, corresponding to the flue temperature likely to be encountered on start-up from cold, when buoyancy forces which would normally enhance the exhausting of flue gases are small.

The following scenarios, representing different pressure field regimes, have been modelled:

· Base case - isolated dwelling (no adjacent structures).

· Five additional scenarios:

(1) A dwelling within a positive pressure field caused by a downwind structure;

(2) A dwelling within a negative pressure field caused by an upwind structure;

(3) A dwelling with a large structure situated to one side of it;

(4) A dwelling with the adjacent structure offset relative to the dwelling;

(5) A dwelling with adjacent structure rotated relative to the dwelling.

For the base case and cases (1) and (2), sensitivity analyses have been carried out to determine the effects of the following factors on the interior pressure:

· Internal/external temperature difference;

· Ventilation openings;

· Open/closed doors;

· Orientation of kitchen relative to adjacent structure.

Four internal/external temperature differences (0°C, 5°C, 10°C and 15°C have been considered, corresponding to an internal temperature of 20°C and external temperatures of 20°C, 15°C, 10°C and 5°C, respectively. When the external temperature is below the internal temperature, the interior pressure is negative, and therefore will tend to have an adverse impact on flue performance. Ventilation openings have been varied by considering a scenario with background ventilation (i.e. trickle vents only) and a scenario with additionally an open window. Different kitchen orientations have been considered by placing the kitchen and open window on the upwind and downwind sides of the dwelling (as shown in Figure 13).

For the non-symmetrical cases ((3), (4) and (5)), the impact of the ventilation openings only has been investigated. The kitchen has been oriented so that the open window is located on the façade which experiences the minimum negative pressure (a 'worst-case' scenario).

24

Figure 13 Kitchen orientations S1 and S2 (C4 is kitchen with W marking kitchen window; arrow

denotes wind direction)

A description of the cases modelled, along with external pressure coefficients at locations coinciding with the kitchen window are given in Table 3. The external pressure coefficients have been obtained directly from façade pressure measurements carried out in the wind tunnel. For the base case and cases 1 and 2, the pressure coefficients are those measured at a location coinciding with the kitchen window, which is situated on either the upwind (S2) or downwind (S1) side of the dwelling. For cases 3, 4 and 5 the pressure coefficients correspond to the minimum negative façade pressures measured in each case, on any of the four facades.

The output obtained from the BREEZE model includes the kitchen pressure relative to atmospheric pressure (referred to as ‘interior pressure’). Results are presented in Section 5.4.

25

Table 3 Cases modelled using BREEZE

Case Description External pressure coefficient at window (S1)

External pressure coefficient at window (S2)

Base case Isolated dwelling -0.217 0.487

Case 1 8h x 2h adjacent structure

downwind at a separation 4h 0.050 0.586

Case 2 8h x 2h adjacent structure upwind at a separation 4h -0.135 0.060

Case 3 4h x 4h adjacent structure to one side at a separation 2h -0.625 -

Case 4 2h x 4h adjacent structure offset by 2h at a separation

2h, wind direction 15° -0.257 -

Case 5 2h x 4h adjacent structure rotated 60° at a separation

2h, wind direction 0° -0.232 -

26

5. RESULTS

5.1 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES ON THE EXTERNAL PRESSURE FIELD OVER DWELLING ROOFS

5.1.1 Introduction

Surface pressure tap measurements have been used in the first instance to determine the impact of adjacent structures and topographical features on the external pressure field over dwelling roofs. Vertical tube measurements are discussed later in Section 5.2. In the present section, the variation in the pressure field with wind direction has been investigated first, in order to determine wind directions with the most significant impact. The results of this investigation are discussed in Section 5.1.2. In subsequent sections, only results for the wind directions with the most significant impact are presented, except where asymmetries in the arrangements of adjacent structures and dwellings require testing of additional wind directions (for example, where the adjacent structure is rotated or offset relative to the dwelling). Section 5.1.3 presents results on the magnitude and extent of the adjacent structure’s zone of influence as a function of its dimensions. The results of sensitivity analyses are presented in Sections 5.1.4 - 5.1.8. Results for topographical features are discussed in Section 5.1.9.

5.1.2 Effect of wind direction

In order to determine the wind direction for which the maximum mean positive pressure over the roof occurs, the maximum mean Cp has been plotted as a function of wind direction for an isolated dwelling, a dwelling with adjacent structure of dimensions 4h x 2h (height of 4h, width of 2h) at a separation of 4h and a dwelling with adjacent structure of dimensions 2h x 16h at a separation of 4h (Figures 14, 15 and 16, respectively). The maximum positive surface pressure has been plotted along with pressures measured by the 12.5mm and 31.5mm tubes. For the isolated dwelling, the variation in maximum pressure with wind direction is roughly symmetrical about a 90° wind direction. For both the surface pressure and the vertical tube measurements, the maximum positive pressure occurs at near 0° and 180°. The minimum pressure occurs in the range 60° to 120°.

When an adjacent structure is present, the variation in maximum pressure with wind direction is no longer symmetrical about the 90° wind direction. Maximum pressures are equal to or greater than the isolated dwelling case, and occur for 180°, when the obstruction is downwind of the dwelling. When the obstruction is upwind of the dwelling, the maximum pressure is lower than the isolated case, and in the case of the 2h x 16h adjacent structure, is negative at wind directions close to 0°. Comparing surface pressure measurements with and without the adjacent structure present, these indicate that the Cp difference for a particular wind direction is greatest when the adjacent structure is upwind of the dwelling (0°), and is smallest when it is downwind (180°). Thus the wake downwind of an obstruction has a greater influence than the flow pattern produced by an obstruction on its upwind side. For both adjacent structures, the maximum surface pressure for the 90° wind direction is lower with the structure present than for the isolated dwelling, due to funnelling of wind between the dwelling and structure.

The results illustrate that the maximum positive pressures occur for the 180° wind direction. Since positive pressures will tend to contribute towards flue flow reversal, this wind direction has been considered primarily for situations where the arrangement of adjacent structure and dwelling has a plane of symmetry. The 0° wind direction has also briefly been considered.

27

Maximum mean Cp over the roof of isolated 45 degree pitch dwelling for different wind directions

0 0 30 60 90 120

150

180

Cp

-0.6

-0.4

-0.2

0.2

0.4

0.6

Surface 12.5 tubes 31.5 tubes

Wind direction

Figure 14 Effect of wind direction on the maximum roof pressure

Isolated dwelling

Maximum mean roof Cp on dwelling with 4h x 2h obstruction at 4h

0

0.2

0.4

0.6

0 30 60 90 120

150

180

Cp

)

-0.6

-0.4

-0.2

Surface 12.5 tubes 31.5 tubes Isolated (surface

Wind direction


4h x 2h adjacent structure at 4h separation

28

0 0

Wi

Cp

f

I ( f )

M aximum mean roof Cp on dwelling with 2h x 16h obstruction at 4h

-0.6

-0.4

-0.2

0.2

0.4

0.6

30 60 90 120 150 180

nd direction

Sur ace

12.5 tubes

31.5 tubes

solated sur ace


2h x 16h adjacent structure at 4h separation

5.1.3 Effect of adjacent structure dimensions and separation distance

The effect of adjacent structure dimensions has been investigated by varying the heights and widths of adjacent structures, and the separation distances between dwellings and adjacent structures. When the wind direction is 0° (obstruction upwind) or 180° (obstruction downwind), the maximum roof pressure occurs on the roof centre line. Thus the mean roof pressure coefficient Cp has been plotted as a function of its location (1 to 12) along the roof centre line. Locations 1 and 12 are adjacent to the eaves, while locations 6 and 7 are either side of the ridge. For 180°, locations 1 to 6 are on the upwind side of the roof furthest away from the adjacent structure, and locations 7 to 12 are on the downwind side. For 0°, locations 1 to 6 are on the downwind side (again furthest away from the adjacent structure) and locations 7 to 12 are on the upwind side.

Figures 17 and 18 illustrate the effect that an adjacent structure (in this case with dimensions 8h x 2h) has on the mean external roof pressure field when wind directions are 180° and 0°, respectively. When the adjacent structure is downwind of the dwelling (180°), the pressure field over the whole roof is more positive relative to the isolated dwelling. When the adjacent structure is upwind (0°), the pressure field is more negative on the upwind side of the roof, but less negative on the downwind side, relative to the isolated dwelling. For the 180° case, pressures become monotonically less positive with increasing separation distance, converging towards the isolated case at large separation distances, when the dwelling is outside the influence zone of the adjacent structure. On the upwind side of the roof of the 0° case, pressures monotonically increase with increasing separation distance, again converging to the isolated case at large separation distances.

29

Roof centre line Cp 8h x 2h obstruction at 4h, 8h and 12h downwind

0 1 2 3 4 5 6 7 8 9

Cp

4h 8h

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12

12h Isolated

Tap position on roof centre line (upwind eaves to downwind eaves)

Figure 17 Effect of separation distance on the roof centre line Cp

8h x 2h obstruction downwind (180° wind direction)

Note: on this graph and all similar ones, “upwind eaves to downwind eaves” indicates that upwind is to the left of the graph and downwind to the right

0

0.2

0.4

0.6

0.8

1 2 3 4 5 6 7 8 9

i i(

Cp

4h 8h 12h

8h x 2h obstruction at 4h, 8h and 12h upwind roof centre line Cp

-0.6

-0.4

-0.2

10 11 12

Tap posit on on roof centre l ne downwind eaves to upwind eaves)

isolated

Figure 18 Effect of separation distance on the roof centre line Cp

8h x 2h obstruction upwind (0° wind direction)

30

In both cases, the variation in Cp with separation distance is greatest on the side of the roof which is closest to the adjacent structure. For example, for the 180° case, the variation in Cp with separation distance is greater for locations 7 to 12 (closest to the adjacent structure) than for locations 1 to 6. Similarly, for the 0° case, the variation in Cp with separation distance is greater for locations 7 to 12 (again, closest to the adjacent structure) than for locations 1 to 6. In both cases, the adjacent structure also tends to reduce the variation in pressure between the upwind and downwind sides of the roof.

Figures A1 to A3 in Appendix A illustrate the variation in the impact of directly downwind structures of width 2h and height 2h, 4h and 8h respectively. The separation distances plotted have been chosen as a function of the adjacent structure height, (multiplied by 0.5, 1 and 1.5). The figures illustrate the increase in the magnitude and extent of the influence zone with height of structure; for a given separation distance, the magnitude of the positive pressure field increases with increasing structure height.

In Figures A4 to A7 in Appendix A, graphs have been plotted to illustrate the impact of adjacent structures of the same height 2h, but with varying widths 2h, 4h, 8h and 16h. The magnitude and extent of the influence zone increases with increasing width. These results are summarised in Figure A8 for a single separation distance 2h.

Figure A9 shows the effect of distance for an adjacent building of height 2h and width 4h downwind. It is clear that the case with the building at 6h distance is closer to the isolated case than the cases at distances of 2h and 4h. With a larger adjacent building of 4h high and 4h wide downwind, figure A10 shows the greater increases in the roof centre line Cp with a monotone increase in the effect at every point on the centre line as the distance of the building decreases.

Figures A11-A14 of Appendix A illustrates the variation in the impact of adjacent structures with the adjacent structure upwind of the dwelling. The significant effect, common between the cases, is that pressures on the upwind slope are reduced by comparison with the isolated case, with the effect increasing with proximity.

Figure A11 shows that for adjacent buildings of height 2h at a distance of 2h upwind the pressures on the upwind roof of the dwelling are reduced. The effect increases with the width of the adjacent building, with all mean roof pressures being brought below zero for cases where it is above 4h wide. Pressures on the downwind slope are increased compared to the isolated case, but not to above zero, except in the case of a 16h wide building, where all centre line pressures are reduced.

Figure A12 shows the effect of the distance of an adjacent upwind building of height 2h and width 2h. The pressures on the upwind roof slope are reduced, with the effect increasing as the adjacent building becomes closer. As in figure A11, pressures on the downwind roof slope are increased, but not to above zero.

Figure A13 shows the effect of a building of height 4h and width 2h. Pressures are decreased across the whole roof of the dwelling, particularly on the upwind slope, with a much greater effect when the adjacent building is nearer.

Figure A14 shows the effect of a building of height 4h and width 4h at different distances upwind. On the upwind roof slope, pressures are strongly reduced, with all pressures being below zero if the adjacent building is at 6h or nearer. For buildings at 4h or further, downwind roof pressures are increased, but not above zero, but when the neighbouring building is at 2h, all roof pressures are reduced.

31

The influence of the downwind adjacent structure is illustrated more clearly by examining the variation in maximum Cp difference with normalised separation distance (Figure 19). The maximum Cp difference is defined as the difference between the maximum roof and minimum façade pressure coefficients. Separation distance, x, has been normalised by the adjacent structure height, H. The zone of influence of the adjacent structure is the value of x/H at which the maximum Cp difference tends to the isolated case.

Variation of max Cp difference with separation distance

0

1

i

=4 =4 =8

0.2

0.4

0.6

0.8

1.2

Max

Cp

diffe

renc

e

2h x 2h 8h x 2h solated 4h x 4h 2h x 4h 2h x 8h

W/h=2

W/h = 2

W/h:

0 0.5 1 1.5 2 2.5 3

x/H

Figure 19 Variation of maximum Cp difference with separation distance

for adjacent buildings (labelled height x width)

When the adjacent structure is relatively narrow in comparison to the dwelling (W/h = 2, where W is the width of the adjacent structure and h is the dwelling eaves height), the zone of influence is approximately 1.5 times the height of the adjacent structure. When the adjacent structure is wider than the dwelling (W/h ≥ 4), the zone of influence extends to at least 2 times the height of the adjacent structure.

The impact of these results on the likelihood of flue flow reversal is discussed in Section 6.

5.1.4 Effect of dwelling roof pitch

Decreasing the pitch of a roof decreases the range of pressures on the roof. Figure 20a illustrates that for an isolated dwelling, roof centre line Cp is less positive on the upwind slope and less negative on the downwind slope in the 30° pitch case compared to the 45° pitch one.

32

The effect of roof pitch on roof centre line Cp for an isolated dwelling

0 0 1C

p

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

30 pitch 45 pitch

Position on roof centre line (downwind eaves to upwind eaves)

Figure 20a Effect of dwelling roof pitch on the roof centre line Cp

Figures 20b and 20c show the effect of a downwind obstruction on a dwelling with a 30° pitch roof and a 45° pitch roof respectively. In both cases the pressure is increased at all points. Figure 20d shows that on the upwind slope the effect on the pressures is greater in the case of the 30° pitch roof, with the peak pressures for the 30° and the 45° roofs being significantly closer in the obstructed case than when the dwellings are isolated.

The effect of a 2h x 4h obstruction at 4h downwind on roof centre line Cp for a 30 degree pitched roof

0 0 1

Cp itch

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

Position on roof centre line

30 p30 pitch +obs

(downwind eaves to upwind eaves)

Figure 20b Effect of a 2h x 4h downwind obstruction on 30° pitch roof centre line Cp

33

The effect of a 2h x 4h obstruction at 4h downwind on roof centre line Cp for a 45 degree pitched roof

0 0 1C

p itch

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

45 p45 pitch +obs


Figure 20c Effect of a 2h x 4h downwind obstruction on 45 pitch roof centre line Cp

The effect of roof pitch on roof centre line Cp with a 2h x 4h obstruction at 4h downwind

0 0 1C

p

30 pi45 pi

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

tch tch

30 pitch +obs 45 pitch +obs


Figure 20d Comparison of effects of a 2h x 4h obstruction at 4h upwind

on 30 and 45 pitch roof centre line Cp

5.1.5 Effect of location of dwelling in a terrace

When a dwelling is located in a terrace of similar dwellings, the effect of a nearby building on the roof pressure field is dependent on both the location of the adjacent building and the position of the dwelling in the terrace.

34

Figures 21a and 21b examine the case where a dwelling has an adjacent structure, which is in the same position downwind of it in each case, but the dwelling is either in the middle or at the end of a terrace of three similar houses. The long axis (i.e. ridge) of the terrace is perpendicular to the wind direction. Figure 21c looks at the case where the ridge of the terrace is parallel to the wind. Here again the dwelling may be either at the end of the terrace near the adjacent building or in the middle of the terrace.

With the terrace perpendicular to the wind direction, and an obstruction downwind, mid-terrace dwellings have much higher roof pressures than end-terrace cases. In both of the cases in Figures 21a and 21b the end-terrace case has even lower roof pressures than in the case where there is no downwind obstruction, whereas the mid-terrace case is comparable to the case where the dwelling is detached.

In Figure 21c it is clear that the mid-terrace house again has higher roof pressures, despite being further from the downwind obstruction, contrasting with results for a detached dwelling where there was a monotone decrease in roof pressures as the dwelling was moved further upwind of a moderate sized obstruction. It seems that the increase in the roof pressures due to being in the middle of a terrace is greater than the decrease due to the greater distance from the nearby building.

Roof centre line Cp for end-terrace, mid-terrace and detatched dwellings with a 4hx2h building at 2h

downwind

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

1 2 3 4 5 6 7 8 9Cp

mi

i

10 11 12

d-terrace end-terrace detached solated

Tap position on roof centre line (downwind eaves to upwind eaves)

Figure 21a Effect of location in terrace on roof centre line Cp for the terrace ridge perpendicular to

the wind direction; downwind building of height 4h and width 2h at 2h distance

35

Roof centre line Cp for end-terrace, mid-terrace and detached dwellings with a 4h x 4h building at 4h

downwind

0.6

0.4

0.2

0

12 -0.2

-0.4

-0.6

mi

i lated

d-terrace end-terrace detached so

1 2 3 4 5 6 7 8 9 10 11


Figure 21b Effect of location in terrace on roof centre line Cp for a terrace ridge perpendicular to the

wind direction; downwind building of height 4h and width 4h at 4h distance

Effect of a 4h x 2h building at 2h downwind along ridge of a terrace

Cp

0.18 0.16 0.14 0.12 0.1

0.08 0.06 0.04 0.02

0 1 2 3 4 5 6 7 8 9 10 11 12

end-terrace centre-terrace


Figure 21c Effect of location in terrace on roof centre line Cp for the terrace ridge parallel to the

wind direction; downwind building of height 4h and width 2h at 2h distance

36

5.1.6 Effect of adjacent structure roof pitch

Figure 22 examines the effect of the roof pitch of a neighbouring building of height 4h and width 2h. The measured roof centre line Cp values indicate that the effect of the roof pitch of the adjacent building in this case were quite small, with little difference in the range of roof pressures measured. There is an indication that the highest pressure on the roof of the dwelling occurs in a different place for the cases with a pitch roof building downwind, but this effect is a small one. These results show that in this scenario the pitch of an adjacent structure has no significant effect on the risk factors for flue flow reversal.

0 1 2 3 4 5 6 7 8 9

(

Cp

PiPi

Effect of adjacent structure roof pitch 4h x 2h neighbour at 2h downwind

roof centre line Cp

-0.6

-0.4

-0.2

0.2

0.4

0.6

10 11 12

Tap position on roof centre line downwind eaves to upwind eaves)

Flat roof neighbour tch 30 neighbour tch 45 neighbour

Isolated

Figure 22 The effect on roof centre line Cp of the pitch of the roof of a downwind structure of

height 4h and width 2h

5.1.7 Effect of offset or rotation of adjacent building relative to dwelling

Figure 23 shows the effect on maximum roof Cp of a 2h x 4h obstruction at 2h downwind, offset by 0h (symmetric), h or 2h, for each wind angle from 0 to 180. The maximum Cp occurs for an adjacent structure offset by 2h, when the wind angle is 15°; however, the increase in the maximum Cp relative to the isolated case is reasonably small.

37

0 0

ic

2h x 4h obstruction at 2h, offset by 0, h or 2h maximum roof Cp

-0.2

-0.1

0.1

0.2

0.3

0.4

0.5

0.6

15 30 45 60 75 90 105 120 135 150 165 180

Wind angle

Max

roof

Cp Offset 2h

Offset h SymmetrIsolated

Figure 23 Maximum roof Cp for a range of wind angles for a dwelling with a 2h x 4h neighbour at

2h distance, offset by 0, h or 2h

Figure 24 shows the maximum roof pressure for a range of wind angles for dwellings with a 2h x 4h neighbour at 2h, rotated by 0, 30° or 60°, For the range of wind angles modelled , there is little evidence that there is greater risk in the angled cases than the symmetric case. The only angle where the maximum roof pressure is greater in the angled cases is when the neighbour is directly upwind of the dwelling, when the amount of shelter provided by the neighbour decreases dramatically as it is rotated. However for almost all other wind angles, the rotated neighbours cause lower maximum roof pressures than in the symmetric case, indicating probable lower risks of flue flow reversals.

38

2h x 4h obstruction at 2h, angled at 0, 30 and 60 maximum roof Cp

0.6 M

ax ro

of C

p0.5

0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

0 105 120 135 150 165 18015 30 45 60 75 90

Isolated Angle 30 Angle 60 Angle 0

Wind angle

Figure 24 Maximum roof Cp for a range of wind angles for a dwelling with a 2h x 4h neighbour at

2h, rotated by 0, 30° or 60°

5.1.8 Effect of neighbouring dwellings

Figures 25 and 26 compare the roof centre line Cp for a dwelling with a 2h x 4h building at 2h downwind in the cases where it is on its own and where there are two identical dwellings located at the width of the dwelling to each side. It is apparent that roof pressures are somewhat higher in the case with the two neighbours, but that the peak roof Cp is very similar. The effects on risk of flue flow reversal might be expected to be small, as sizeable increases are only visible at points where the roof pressure is negative.

39

Effect of neighbours 2h x 4h obstruction at 2h downwind

roof centre line Cp

0 1 2 3 4 5 6 7 8 9C

p

i

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

10 11 12

solated no neighbours neighbours


Figure 25 Roof centre line Cp for a dwelling with a 2h x 4h building at 2h downwind, with

and without two nearby neighbours

Effect of neighbours4h x 2h obstruction at 2h downwind

roof centre line Cp

0 1 2 3 4 5 6 7 8 9C

p

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

10 11 12

Isolated no neighbours neighbours


Figure 26 Roof centre line Cp for a dwelling with a 4h x 2h building at 2h downwind,

with and without two nearby neighbours

40

5.1.9 Effect of adjacent topographical features

Topographical features such as embankments and hills can have a large effect on the pressure field around a building. Relevant factors are the distance to the feature, its height, breadth and the angles of its slopes.

Figure 27a illustrates the effect of the height of a small downwind hill with the base always at 2h separation. It is clear that the higher the hill, the larger the pressure on the roof of the dwelling, with peak coefficients being higher than in the isolated case and positive over the whole centre line, rather than just one half of it as in the isolated case. The effect is much greater on the downwind side of the roof (closest to the hill), although risks are clearly higher on the upwind slope.

Figure 27b shows the effect with a hill of different heights with base at 2h upwind of the dwelling. The effect here is strikingly different to cases (for example figure A14) where there is a large building upwind. In the case of a hill of height h, the roof pressures are decreased slightly where they are highest, but when the hill is of height 2h, maximum roof pressures are increased. In both cases roof pressures on the downwind side of the roof are substantially increased, but not above zero.

Figure 28a illustrates the effect of the downwind dimension (breadth) of a downwind embankment (both examples have height 2h and crosswind dimension 16h). The effects of two vertical embankments of breadth h and 4h are shown. It is clear that the smaller embankment increases the pressures on the upwind slope of the roof much more than the larger embankment.

Figure 28b compares the effects of downwind embankments with different slopes. In each case the distance of the dwelling to the top of the embankment is the same, as are the height, width and breadth of the top of flat part of the embankment. In this case, all of the embankments have a similar effect on the upwind roof slope (a small increase in pressure coefficients compared to the isolated case). On the downwind slope, pressure coefficients are increased, with the greatest increase being caused by the 60° embankment and the least by the vertical embankment.

41

0 1 2 3 4 5 6 7 8 9

Cp

Hill h Hill 2h Hill 3h i l

Roof centre line Cp with hills of different heights at 2h downwind

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12


so ated

Figure 27a Roof centre line Cp with hills of different heights at 2h downwind

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

1 2 3 4 5 6 7 8 9

(

Cp

Hill upwind of dwelling roof centre line Cp

10 11 12

Tap position on roof centre line downwind eaves to upwind eaves)

Hill h Hill 2h isolated

Figure 27b Roof centre line Cp with hills of different heights at 2h upwind

42

0 1 2 3 4 5 6 7 8 9

)

Cp

Roof centre line Cp with an vertical embankment of different breadths at 2h downwind

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12

Tap position on roof centre line (upwind eaves to downwind eaves

Breadth h Breadth 4h Isolated

Figure 28a Roof centre line Cp with embankments of height 2h

and different breadths at 2h downwind

0 1 2 3 4 5 6 7 8 9

(

Cp

Roof centre line Cp with embankments of different slopes at 4h downwind

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

10 11 12

Tap position on roof centre lineupwind eaves to downwind eaves)

Slope 90 Slope 60 Slope 30 Isolated

Figure 28b Roof centre line Cp with embankments of different slopes at 4h downwind

5.2 EFFECT OF FLUE HEIGHT ON THE EXTERNAL PRESSURE AT THE FLUE TERMINAL

Pressures have been measured at 2 flue terminal heights above the dwelling roof, using vertical tubes of length 12.5mm and 31.5mm (model scale). The results of the vertical tube measurements are compared with surface pressure measurements with the adjacent structure present, and additionally with surface pressure measurements for the isolated dwelling.

43

Figure 29 shows vertical tube measurements for an adjacent structure of 8h x 2h placed at a separation distance of 4h downwind of the dwelling. This figure illustrates the decrease in maximum positive pressure with an increase in height above the roof. Additional graphs have been plotted for adjacent structures with the same width and different heights (2h x 2h, 4h x 2h) in Appendix B, along with adjacent structures with the same height and varying widths (2h x 4h, 2h x 16h). The results show that the difference in maximum positive Cp between the surface measurements and the 12.5mm tubes is generally about 0.1, and between the surface measurements and the 31.5mm tubes is about 0.2. The variation in Cp with height above the surface is generally smaller for the downwind side of the dwelling (where roof pressures tend to be negative) than the upwind side (where roof pressures are positive).

8h x 2h obstruction at 4h downwind roof centre line Cp

0.8

0.6

0.4

0.2 Cp

0

-0.2 i

12.5 tubes

31.5 tubes

surface

solated

-0.4

-0.6

0

Position on roof centre line (dow nw ind eaves to upw ind eaves)

Figure 29 Variation in Cp with height above the roof, 8h x 2h obstruction at 4h downwind

Figure 30 shows vertical tube measurements for an 8h x 2h adjacent structure at a separation distance of 4h upwind of the dwelling. Again, additional cases have been presented in Appendix B. The variation in Cp with height above the roof again is greatest on the upwind side of the roof, and in the case of the 8h x 2h obstruction, the pressure becomes more negative with height above the roof. However, for some of the cases plotted in Appendix B where the height of the adjacent structure is closer to the dwelling height, the pressure becomes slightly less negative with increasing height, although remains negative.

44

8h x 2h obstruction at 4h upwind roof centre line Cp

Cp

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

0

i

31.5 tubes 12.5 tubes surface solated

Position on roof centre line (upwind eaves to downwind eaves)

Figure 30 Variation in Cp with height above the roof

8h x 2h obstruction at 4h upwind

5.3 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES ON MAXIMUM INTERNAL/EXTERNAL PRESSURE DIFFERENCES IN THE CASE OF A DOMINANT OPENING

The roof centre line Cp difference has been calculated as the roof centre Cp values minus the minimum façade Cp. This represents a 'worst-case' scenario, when the flue terminates in a region where maximum positive pressure is encountered on the roof centre line, and the boiler is situated in a room with an open window. In this case, the interior pressure will be close in value to the external pressure at a location coinciding with the open window. Therefore, the Cp difference profiles plotted are a measure of the maximum internal/external pressure difference that may occur with an open window.

Results for three scenarios have been plotted in Figures 31 to 33, where the adjacent structure is located downwind of the dwelling. The scenarios include a 8h x 2h downwind obstruction (Figure 31), a 4h x 4h downwind obstruction (Figure 32) and an 2h x 16h downwind obstruction (Figure 33), and represent a range of different adjacent structure geometries. The maximum Cp difference is approximately equal to 1.0, and generally occurs for the isolated case or for a situation where the adjacent structure is far enough from the dwelling that it does not have a large influence on the dwelling pressure field. In these cases, the profiles are shifted upwards since the minimum façade pressure is negative (Cp ~ -0.6). In cases where the dwelling is very close to the downwind adjacent structure, the minimum façade pressure becomes positive (for example, at downwind separation distances of 2h, for the 2h x 16h and 4h x 4h adjacent structures, minimum façade pressures are equal to 0.07 and 0.09 respectively). Thus the profiles are shifted downwards. For all other separation distances where the adjacent structure has a significant effect, the minimum façade pressures are negative, although they are smaller in magnitude than the isolated case.

45

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 5 6 7 8 9

(

Cp

diffe

renc

e 4h 8h 12h isolated

8h x 2h obstruction downwind roof centre line Cp difference

10 11 12

Tap position on roof centre line upwind eaves to downwind eaves)

Figure 31 Effect of separation distance on the roof centre line Cp difference


4h x 4h obstruction downwind roof centre line Cp differe nce

1.2

1

0.8

0.6

0.4

0.2

0

-0.2

1

i

8h

6h

4h

2h

solated




46

Cp

diff

eren

ce

0

1

1

iti(

2h

4h

i

2h x 16h obstruction downwind roof centre line Cp diffe rence

-0.4

-0.2

0.2

0.4

0.6

0.8

1.2

Pos on on roof centre line upwind eaves to downwind eaves)

Cp

diff

eren

ce

solated



Results for one example of an upwind adjacent structure, 8h x 2h, are shown in Figure 34. Again the maximum Cp difference occurs for the isolated case. With the adjacent structure present, the minimum façade pressures are -0.18, -0.22 and -0.30 for separation distances of 4h, 8h and 12h respectively. Thus the minimum façade pressure becomes less negative when the dwelling is close to the adjacent structure, and profiles are shifted upwards by an amount which increases with increasing separation distance.

47

8h x 2h obstruction upwind roof centre line Cp difference

Cp

diffe

renc

e1.2

1

0.8

0.6

0.4

0.2

4h 8h 12h isolated

0

-0.2

-0.4

1



2h x 16h obstruction upwind (0° wind direction)

Results for non-symmetrical cases, where the adjacent structure is offset or rotated relative to the dwelling, are shown in Figures 35 and 36. The variation in maximum Cp difference with wind angle is plotted. The graphs show that the maximum Cp difference again occurs for the isolated case.

These results show how an adjacent structure tends to moderate the pressure difference encountered over the dwelling, both for the upwind and downwind cases. The results also demonstrate that, for the situation where a dominant opening is located close to where the minimum façade pressure occurs, the internal/external pressure difference will tend to be larger for the isolated dwelling than dwellings with adjacent structures.

48

0

0

2h x 4h obstruction at 2h, angled by 0, 30 or 60 maximum Cp difference

0

1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Cp

diffe

renc

e Isolated Angle 0 Angle 30 Angle 60

15 30 45 60 75 90 105 120 135 150 165 180

Wind angle

Figure 35 Effect of wind angle on the maximum Cp difference2h x 4h obstruction at 2h, angled by 0°, 30° and 60°

2h x 4h obstruction at 2h, offset by 0,h and 2h maximum Cp difference

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

ic

I

Cp

diff

eren

ce

Offset 2h

Offset h

Symmetr

solated

15 30 45 60 75 90 105 120 135 150 165 180

Wind angle

Figure 36 Effect of wind angle on the maximum Cp difference

2h x 4h obstruction at 2h, offset by 0, h and 2h

49

5.4 BREEZE MODELLING

5.4.1 Sensitivity of interior pressure to internal/external temperature difference

Results for the sensitivity analyses for the base case, case 1 and case 2 are presented first. Figures 37 to 39 show the impact of internal/external temperature difference on the kitchen interior pressure for a standard reference meteorological wind speed of 4m/s. The reference wind speed has been corrected to the local wind speed at eaves height, taking into account the effect of the surrounding suburban terrain. In all cases, the minimum interior pressure occurs when the internal/external temperature difference is at a maximum of 15° C (i.e. the internal temperature is 20° C and the external temperature is 5° C), which will occur during winter months. At this temperature difference, the interior pressure is negative. When background ventilation is provided, the minimum interior pressures for the base case, upwind case and downwind case are -2.0Pa, -2.4Pa and -1.6Pa respectively (i.e. the wind pressure field has a small influence on the interior pressure). With an open window, the sensitivity of the interior pressure to the internal/external temperature difference decreases, and the interior pressure becomes more dominated by the external wind pressure at the open window, although at this wind speed, pressure differences associated with buoyancy forces still have a significant effect. At larger wind speeds, it is likely that the internal/external temperature difference will be less significant in the case of an open window. In subsequent models, the internal/external temperature difference has been taken as 15° C, as this has the largest adverse effect on the kitchen interior pressure.

-3

-2

-1

0 0 5 10 15 20 25

; /s ; window; /s

Isolated dwelling

-2.5

-1.5

-0.5

0.5

External temperature

Inte

rior p

ress

ure

(Pa)

S1 background; doors open, met wspeed 4mS1 doors open, met wspeed 4m

Figure 37 Effect of internal/external temperature difference on kitchen interior pressure

Isolated case – orientation S1 (kitchen downwind side of house)

50

Adjacent structure downwind 0 5 10 15 20 25

-3

-2

-1

0

-2.5

-1.5

-0.5

0.5

Inte

rior p

ress

ure

(Pa)


; /s ; window; /s

S1 background; doors open, met wspeed 4mS1 doors open, met wspeed 4m

Figure 38 Effect of external/internal temperature difference on kitchen interior pressure

Adjacent structure downwind

-3

-2

-1

0 0 5 10 15 20 25

Adjacent structure upwind

-2.5

-1.5

-0.5

0.5


Inte

rior p

ress

ure

(Pa)

S1; background; doors open, met wspeed 4m/s S1; window; doors open, met wspeed 4m/s

Figure 39 Effect of external/internal temperature difference on kitchen interior pressure


51

5.4.2 Sensitivity of interior pressure to wind speed, as a function of dwelling ventilation strategy and orientation

Figures 40 to 45 illustrate the sensitivity of the interior pressure to the wind speed, as a function of the ventilation strategy for the dwelling and the orientation of the dwelling relative to the wind direction. Figures 40 and 41 show these results for the base case (isolated dwelling). For both orientations, the interior pressure is much more sensitive to the wind speed for the open window case than the background ventilation case, and as would be expected rapidly becomes more positive with increasing wind speed when the kitchen is on the upwind side of the house (orientation S2), and more negative with increasing wind speed when the kitchen is on the downwind side of the house (orientation S1). The variation of interior pressure with wind speed over the range 0m/s to 4m/s (at eaves height) is less than 0.5Pa for the background ventilation case, and the pressure remains negative. For orientation S2 (open window), the variation in pressure is ~2Pa (more than 4 times greater) over the same range of wind speeds. Note that the wind pressure coefficient at a location coinciding with the open window is -0.217, and at higher wind speeds, the interior pressure is close to the external wind pressure at the window. The small variation of interior pressure with wind speed in the background ventilation case arises due to the averaging out of positive and negative façade and roof pressures over the dwelling, since the ventilation openings are relatively well distributed. This will tend to result in a net zero interior pressure in the absence of the stack effect.

The sensitivity of the interior pressure to ventilation strategy and orientation is shown in Figures 42 and 43 for case 1 (downwind obstruction) and in Figures 44 and 45 for case 2 (upwind obstruction). In both cases, the variation in interior pressure for the background ventilation case is greater relative to the base case, and the pressure tends to increase with wind speed for case 1 and decrease with wind speed for case 2. This is because in case 1, the averaging of façade and roof pressures over the dwelling results in net positive wind-induced interior pressures, and in case 2 it results in net negative wind-induced interior pressures. The sensitivity of interior pressure to wind speed for the open window case relative to the background ventilation depends on the magnitude of the external pressure coefficient at a location coinciding with the window. For example, for case 1, the external pressure coefficient at the window for orientation S1 is close to zero, and therefore the variation is relatively small. Conversely, the external pressure coefficient at the window is large and positive (0.586) for orientation S2 and therefore the interior pressure rises rapidly with wind speed relative to the background ventilation case. For case 2 (adjacent structure upwind of dwelling), the largest variation occurs when the open window is on the downwind façade of the dwelling (orientation S1), when the pressure coefficient is negative. The pressure coefficient on the upwind façade of the dwelling is close to zero, and therefore when the window is on this façade (orientation S2) the variation is small.

52

-3

-2

-1

0 0 1 2 3 4 5

Wi /s)

Isolated dwelling

-3.5

-2.5

-1.5

-0.5

nd speed (m

Inte

rior p

ress

ure

(Pa)

S1; background; doors open, T diff 15 S1; 90 deg window; doors open, T diff 15

Figure 40 Effect of wind speed on kitchen interior pressure


-3

-2

-1

0

1

2

3

4

5

0 1 2 3 4 5

)

; i

; i ;

Isolated dwelling

Wind speed (m/s)

Inte

rior p

ress

ure

(Pa

S2 background; doors open, T d ff 15

S2 90 deg w ndow doors open, T diff 15

Figure 41 Effect of wind speed on kitchen interior pressure

Isolated case – orientation S2 (kitchen upwind side of house)

53


-3

-2

-1

0

1

2

3

4

5

6

0 1 2 3 4

(

0.5 1.5 2.5 3.5 4.5 Inte

rior p

ress

ure

Pa)

Wind speed (m/s)

S1; 90 deg wi

S1; background; doors open, T diff 15

ndow; doors open, T diff 15

Figure 42 Effect of wind speed on interior pressure

Adjacent structure downwind – orientation S1 (kitchen downwind side of house)


-3

-2

-1

0

1

2

3

4

5

6

0 1 2 3 4

(

0.5 1.5 2.5 3.5 4.5 Inte

rior p

ress

ure

Pa)

Wind speed (m/s)

S2; 90 deg wi




Adjacent structure downwind – orientation S2 (kitchen upwind side of house)

54

-3

-2

-1

0 0 1 2 3 4 5

Wi /s)

S2; 90 deg wi


-2.5

-1.5

-0.5

nd speed (m

Inte

rior p

ress

ure

(Pa)




Adjacent structure upwind – orientation S2 (kitchen downwind side of house)

-3

-2

-1

0 0 1 2 3 4 5

/s)

S1; 90 deg wi iff 15


-2.5

-1.5

-0.5

Wind speed (m

Inte

rior p

ress

ure

(Pa)


ndow; doors open, T d


Adjacent structure upwind – orientation S1 (kitchen upwind side of house)

55

5.4.3 Sensitivity of interior pressure to wind speed, as a function of internal door setting

Figures 46 to 48 illustrate the sensitivity of the interior pressure to the internal door setting (open or closed) for the base case and cases 1 and 2. With the window open, the internal door setting does not have a significant influence on the interior pressures, which are again dominated by the external pressure at the window. In the case of background ventilation, the influence of internal door setting is greatest for the isolated case. This is likely to be due to the greater variation of pressure over different façades of the dwelling, which will tend to cause variations in interior pressure between different rooms in the dwelling when doors are closed, but will be equalised over the inside of the dwelling when doors are open. In the isolated case, the kitchen is on the downwind side of the dwelling, and therefore when doors are closed the negative external pressures acting on leakage and ventilation paths to the kitchen from outside will tend to dominate the kitchen interior pressure.

Isolated case

Inte

rior p

ress

ure

(Pa)

0 0 0.5 1 1.5 2 2.5 3 3.5

-0.5

-1

-1.5

-2

-2.5

-3

-3.5

4 4.5

Wind speed (m/s)

il

i l

S1; background; doors open, T diff 15 S1; 90 deg w ndow; doors open, T diff 15 S1; background; doors c osed, T diff 15 S1; 90 deg w ndow; doors c osed, T diff 15

Figure 46 Effect of door opening and ventilation strategy on interior pressure


56


-3

-2

-1

0 0 1 2 3 4 5

)

-3.5

-2.5

-1.5

-0.5 In

terio

r pre

ssur

e (P

a

Wind speed (m/s)

; ;; indow;; ; l i; indow; l

S1 background doors open, T diff 15 S1 90 deg w doors open, T diff 15 S1 background doors c osed, T d ff 15 S1 90 deg w doors c osed, T diff 15


Case 1 – orientation S1 (kitchen downwind side of house)

j

-3

-2

-1

0 0 1 2 3 4 5

Wi ( )

()

; ; i ;; l; i ; l i

Ad acent structure upwind

-3.5

-2.5

-1.5

-0.5

nd speed m/s

Inte

rior p

ress

ure

Pa

S1 background; doors open, T diff 15 S1 90 deg w ndow doors open, T diff 15 S1 background; doors c osed, T diff 15 S1 90 deg w ndow doors c osed, T d ff 15


Case 2 – orientation S1 (kitchen downwind side of house)

57

5.4.4 Results for Cases 3, 4 and 5

Results for cases 3, 4 and 5 are shown in Figures 49 to 51. For cases 3, 4 and 5, the dwelling has been orientated so that the open window is located where the minimum negative external pressure occurs. For case 3, the proximity of a large adjacent structure to one side of the dwelling causes extremely large external pressures on the façade closest to the adjacent structure. Therefore, the sensitivity of the interior pressure to wind speed is very large, and the interior pressure becomes large and negative at high wind speeds. For the other cases, the minimum interior pressures are similar to those found in the isolated case.

5.4.5 Further comments on interior pressures

Interior pressures expressed in coefficient form for the base case and 5 additional cases are shown in Table 4 for scenarios with no internal/external temperature difference (i.e. interior pressure is generated solely by wind). For open window cases, the interior pressure is approximately the same as the external pressure at a location coinciding with the window (see Table 3 in Section 4), since an open window acts as a ‘dominant opening’. For background ventilation cases, the interior pressure is closer to the external pressure field averaged over the whole dwelling, since openings are small and reasonably well distributed. For this reason, the interior pressure does not vary greatly with orientation, and is considerably less negative than the most negative pressure on the façade. The latter point is most clearly illustrated for cases 3, 4 and 5 where the open window has been chosen to coincide with the most negative pressure. In these cases, interior pressures are considerably more negative when there is a dominant opening than when the ventilation is well distributed.

The table also illustrates the magnitude of the positive and negative interior pressures experienced for the background ventilation cases, relative to the external pressure fields presented in Section 5.1. Considering the upwind case, Figure 18 shows that the maximum roof pressure for an 8h x 2h obstruction at a distance 4h upwind of a dwelling is -0.17, which is more negative than the interior pressure (-0.09). Thus, when background ventilation is provided, the net pressure (roof pressure minus interior pressure) remains negative, which will tend to enhance flue performance. Considering the downwind case, Figure 17 shows that the maximum roof pressure for an 8h x 2h obstruction at a distance 4h downwind of a dwelling is 0.55, which is more positive than the interior pressure (0.16). Therefore, the net pressure remains positive, although the positive interior pressure helps to reduce the magnitude of the net pressure relative to the roof pressure. Note that the net pressure for this case is similar to the isolated case (~0.4). The greatest net pressure occurs for Case 3, where there is a large structure to one side of the dwelling. In this case, the maximum roof pressure is 0.35, and the interior pressure is -0.22, giving a net pressure of 0.57.

Table 4 Interior pressures expressed in coefficient form (Cp)

Case Orientation S1 Orientation S2

background open window background open window

Base case (isolated dwelling) -0.01 -0.21 0.00 0.48 Case 1 (downwind adjacent structure) 0.16 0.05 0.18 0.58 Case 2 (upwind adjacent structure) -0.07 -0.13 -0.09 0.06 Case 3 (side adjacent structure) -0.22 -0.62 - -Case 4 (offset adjacent structure) -0.01 -0.25 - -Case 5 (rotated adjacent structure) -0.11 -0.23 - -

58

-8

-7

-6

-5

-4

-3

-2

-1

0 0 1 2 3 4

Wi /s)

S1; 90 deg window;

Side adjacent structure

0.5 1.5 2.5 3.5 4.5

nd speed (m

Inte

rior p

ress

ure

(Pa)


doors open, T diff 15


4h x 4h adjacent structure to one side – orientation S1 (kitchen downwind side of house)

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0 0 1 2 3 4 5

Wind speed (m/s)

S1; 90 deg wi

Offset adjacent structure

Inte

rior p

ress

ure

(Pa)




Adjacent structure at 2h upwind – orientation S1 (kitchen downwind side of house)

59

Rotated adjacent structure

-4

-3

-2

-1

0 0 1 2 3 4 5

/s)

(

S1; 90 deg wi

-3.5

-2.5

-1.5

-0.5

Wind speed (m

Inte

rior p

ress

ure

Pa)




Adjacent structure upwind – orientation S1 (kitchen downwind side of house)

5.5 RIDGE VENT INVESTIGATION

5.5.1 Influence of wind direction on ridge vent pressures

Ridge vent pressure coefficients are affected by five principal factors – wind direction, the location of the ridge vent on the roof, the pitch of the roof, the length of the building and the nearby buildings and terrain. In this study, the first four factors have been studied.

Figures 52a and 52b show the effect of changing wind directions on the ridge vent pressure coefficients, as measured on wind tunnel models, for a 45° pitch roof and a 30° pitch roof respectively. Note that the ridge vent locations are at a distance of 3mm (model scale) from the ridge, corresponding to a full scale distance of 0.24m. Thus for the 0° wind direction, the ridge vents are on the upwind side of the ridge, whereas for the 180° wind direction, the ridge vents are on the downwind side of the ridge.

There is a dramatic contrast in the behaviour between the 45° and 30° cases. In the 45° case positive pressures are only seen when the wind is from near to 0° (0° or 30° in the cases examined). By contrast, with a 30° pitch roof, positive pressures only occur in the case where the ridge of the dwelling is parallel to the wind direction.

A direct comparison of the worst cases for each are given in Figures 52c and 52d. Figure 52c illustrates the difference when the ridge is perpendicular to the wind: the 45° pitch roof dwelling has positive pressures over a large fraction of the ridge, whereas the 30° pitch roof has negative pressures along the whole ridge. In the case where the ridge is parallel to the wind direction (Figure 52d), the pattern is similar for each of the two buildings (lower pressure at the windward

60

end and higher pressure at the leeward end) but the 30° pitch roof dwelling has slightly higher pressure along the whole of the ridge. For the 45° case the pressure remains negative along the entire ridge (although is close to zero at the leeward end), whereas for the 30° case, the pressure becomes positive (although again is close to zero) at the leeward end of the ridge.

-1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

(

Cp

0 30 60 90

Ridge vent Cp for different wind directions on an isolated 45 degree pitched roof dwelling

-0.8

-0.6

-0.4

-0.2

0.2

17 18

Ridge vent position upwind to right in angled cases)

120 150 180

Figure 52a Effect of changing wind direction on ridge vent pressure coefficients for a 45° pitch roof

dwelling

61

0 1 3 5 7 9 11 13 15 17

Ri l i( )

Cp

0 30 60 90 120 150 180

Ridge vent mean pressure coefficients for different wind directions for a 3 house terrace with a 30 degree pitch roof

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.1

dge vent ocat on upwind to right for angled cases

Figure 52b Effect of changing wind direction on ridge vent pressure coefficients for a 30° pitch roof

dwelling

Comparison of roof vent Cp for 30 and 45 pitch roof 3 house terrace perpendicular to wind direction, ridge vents facing wind

0 1 2 3 4 5 6 7 8 9

Cp

-0.25

-0.2

-0.15

-0.1

-0.05

0.05

0.1

0.15

10 11 12 13 14 15 16 17 18 45 pitch 30 pitch

Ridge vent position (left ridge end to right)

Figure 52c Ridge vent Cp for 30° and 45° pitch roof dwellings with ridge perpendicular to the wind

direction

62

Comparison of roof vent Cp for 30 and 45 pitch roof 3 house terrace, with wind from 90 degrees

0.1 C

p0

-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

45 pitch 30 pitch

Roof vent position (from downwind end to upwind end)

Figure 52d Ridge vent Cp for 30° and 45° pitch roof dwellings with ridge parallel to the wind

direction

5.5.2 Intermittency in ridge pressures

The intermittency in ridge pressures has been analysed in order to determine the proportion of time that the pressure occurs for a continuous period greater than 10 seconds at a nominal wind speed of 3m/s. As an illustration of the intermittency characteristics of the pressure, the variation in pressure with sample number has been plotted in Figure 53. Individual measurement points have been chosen on the 30° and 45° pitch roofs. The graph shows that large fluctuations in pressure about the mean can occur. These may be sustained over a range of timescales. The variation in pressure over time is associated with fluctuations in the flow patterns caused both by atmospheric turbulence and the unsteady flows generated by the dwelling itself.

Figures 54a and 54b show the variation with location on the ridge and wind direction of the percentage of time that sustained positive pressure ‘events’ occur for the two roof pitches. Generally, when the mean pressure is large and negative (e.g. for the downwind end of the terrace), sustained positive pressure events never occur (the percentage of time is zero). When the mean pressure is small and negative (e.g. for the 90° wind direction for both roof pitches), sustained positive pressure events can occur for a small proportion of the time (~10%).

63

Isolated dwelling ridge vent pressure

50

40Pr

essu

re (P

a)

30

20

10

0

-10

-20

-30

-40

1

i

i

11 21 31 41 51 61 71 81 91 101 111 121 131141 151 161 171181 191

45 p tch max tap

30 p tch max tap

Sample number

Figure 53 Variation in pressure with sample number

F r e q u e n c y o f s u s t a in e d p o s i t i v e p r e s s u r e o n 4 5 d e g r e e r o o f f o r d i f f e r e n t w i n d d i r e c t io n s

6 0

5 0

4 0

3 0

2 0

1 0

0 3 0 6 0 9 0 1 2 0 1 5 0 1 8 0

0

-1 0 1 2 3 4 5 6 7 8 9

R i d g e n u m b e r (e n d o f r id g e t o m i d d l e )

Figure 54a Percentage of time that sustained positive pressure events occur: 45° pitch roof

Perc

enta

ge o

f tim

e C

p po

sitiv

e fo

r m

ore

than

9 s

econ

ds re

al ti

me

64

-2

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9

)

0 30 60 90 120 180

F req u e n cy o f su stain e d p o sitiv e p res su re ev e n ts w ith 30 degre e p itc h ro o f

R id g e ve n t pos itio n (e n d o f ridg e to m id d le

Perc

enta

ge o

f tim

e Cp

pos

itive

for m

ore

than

9

seco

nds

real

tim

e

Figure 54b Percentage of time that sustained positive pressure events occur: 30° pitch roof

5.5.3 Effect of terrace length on ridge pressure

The variation in pressure has been investigated for ridge vents on 45° pitch roof dwellings in a terrace of nine. The results are shown in Figure 55 for the 'worst case' 0° wind direction (wind perpendicular to the ridge), and compared with the three-house terrace case. For these cases, the profiles along one half of the complete terrace are shown. Comparing the two profiles, for both terrace lengths, the maximum positive pressure occurs towards the midpoint of the terrace. However, in the case of the 9-house terrace, the pressure becomes positive between about half and two thirds of the way from the centre to the end.

Figure 56 shows the pressure coefficients at ridge vents on a terrace of 9 houses with a 45° pitch roof, for the cases where the ridge is parallel to the wind direction. Ridge vent pressures become positive for all houses after the upwind two houses on the terrace, and are greater in magnitude than in the case of the 3 house terrace by Cp ~ 0.1.

65

3-house and 9-house terraces

0 1 3 5 7 9

i

Cp

Variation in ridge vent mean Cp

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0.05

0.1

0.15

11 13 15 17 19 21 23 25 27

Tap posit on (from centre to end of terrace)

3-house terrace 9-house terrace

Figure 55 Variation in pressure along the ridge for winds perpendicular to the ridge

Three- and nine-dwelling terraces (Upwind half of the terraces only)

i

0 1 4 7

Cp

Mean ridge vent Cp for a 45 pitch roof terrace of 9 houses parallel to the w nd direction

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.1 0.2

10 13 16 19 22 25 28 31 34 37 40 43 46 49 52

Location of ridge vent (location 1: upwind end of terrace)

Figure 56 Variation in pressure along the ridge for a terrace of 9 houses with a 45° pitch roof

parallel to the wind direction

66

6. DISCUSSION

6.1 IMPACT OF ADJACENT STRUCTURES AND TOPOGRAPHICAL FEATURES

The wind tunnel results have shown how adjacent structures influence the pressure over the roof of a dwelling, and in particular how an adjacent structure downwind of the dwelling produces larger positive pressures over the roof of the dwelling than for the isolated case. Any increased likelihood of flue flow reversal due to the presence of the adjacent structure, relative to the isolated case, depends on a number of factors including:

· The increased magnitude of positive pressures on the roof at locations coinciding with the flue terminal, compared to the isolated case;

· The impact of the wind pressure field on the interior pressure, which is a function of the ventilation strategy (i.e. the size and distribution of ventilation openings);

· The relative timescales over which flue pressures and interior pressures respond following any changes in wind speed or direction.

The likelihood of flue flow reversal also depends on the location of the flue on the roof, although the following discussion assumes that the flue could be located at any point on the roof including the point at which the maximum positive pressure occurs.

Considering all the experimental results obtained for the 45° pitch roof dwelling, the largest maximum positive pressure coefficient has been found to occur when the dwelling is located next to a 2h x 16h obstruction. In this case, the maximum surface pressure coefficient is 0.64, which is approximately 50% greater than the isolated dwelling value of 0.43. Comparing vertical tube results, the maximum pressure coefficient measured by the 31.5mm tubes with the 2h x 16h obstruction present is 0.38, which is approximately 60% greater than the isolated case value of 0.24. Thus in the case of the 45° pitch roof, the pressure at the flue terminal could be up to 60% greater when a large obstruction is present than when the building is isolated.

Note that the difference in roof pressures between the isolated and obstructed cases is larger for the dwelling with the 30° pitch roof than the 45° pitch roof. For example, in the case of an obstruction with dimensions 2h x 4h placed at a distance of 2h downwind of the dwelling, the maximum positive surface pressure coefficient for the 30° pitch roof is 0.28. This is 75% greater than the maximum pressure coefficient for the isolated case (Cp=0.16). In contrast, for the 45° pitch roof adjacent to the same obstruction, the obstructed and isolated case values are 0.46 and 0.43, respectively, which is a difference of 7%. In addition, for the isolated building with a 30° pitch roof, the pressure along the eaves is negative, whereas with the adjacent structure present, the pressure becomes positive. In contrast, the pressure along the eaves for the 45° pitch roof dwelling is positive both with and without the adjacent structure present.

The highest positive pressures will tend to occur when the adjacent structure is very close to the dwelling (in the cases discussed above, the obstruction was at a distance of 2h from the dwelling). However, an increase in positive roof pressures relative to the isolated dwelling will tend to occur over a separation distance which is related both to the width of the adjacent structure and its height. For example, for adjacent structures which are relatively narrow in comparison to the dwelling dimensions (W = 2h), an increase in positive roof pressure will occur up to a distance of ~1.5h, where h is the height of the adjacent structure. However, for adjacent structures which are relatively wide in comparison to the dwelling dimensions (W ≥ 4h), an increase in positive roof pressure will tend to occur for a distance of at least 2h.

67

Sensitivity analyses carried out for the 45° pitch roof dwelling indicate that for the cases investigated, the maximum positive pressure on the roof of the dwelling is not significantly increased by the adjacent structure geometry, the offset or rotation of the adjacent structure relative to the dwelling, the location of the dwelling in a terrace or adjacent to neighbours, the type of terrain or the topography. However, given the results for the 30° pitch roof, it is possible that roof pressures for this dwelling are more sensitive to these factors.

The change in roof pressures due to the presence of an adjacent structure needs to be considered alongside the impact that the corresponding change in dwelling façade pressures has on the interior pressure. Investigation of the maximum Cp difference (i.e. the maximum roof pressure minus the minimum façade pressure) provides an indication of a possible ‘worst-case’ scenario. In this scenario, a dominant opening such as a window is located in the room in which the boiler is housed, at a position which coincides with the minimum external façade pressure. This analysis has shown that, although there is still a net positive pressure between the roof and façade which may be larger or smaller than the roof pressure, this net pressure is smaller than for the isolated dwelling case. This is because the adjacent structure tends to smooth out the variation in pressure over the dwelling, resulting in a smaller difference between roof and façade pressures. Thus the risk of flue flow reversal is greater for the isolated dwelling than the dwelling with a downwind obstruction, when a dominant opening is located where the minimum façade pressure occurs. This is also true for an obstruction located upwind of the dwelling, although net pressures are considerably less positive than in the case of a downwind obstruction.

Considering scenarios where background ventilation only is provided, and there is no dominant opening, BREEZE modelling has shown that for an isolated dwelling, the interior pressure coefficient is close to zero, and therefore the net pressure is approximately equal to the roof surface pressure (net Cp is 0.44). When a downwind obstruction is present, the interior pressure is positive, and therefore the net pressure is smaller in magnitude than the roof pressure. For example, for an 8h x 2h downwind obstruction at 4h separation, the maximum roof surface pressure coefficient is 0.55, and the interior pressure coefficient is 0.16 (c.f. Figure 17). Thus the net Cp is 0.39, which is 11% lower than the isolated case. For an 8h x 2h upwind obstruction, the maximum roof surface pressure coefficient is -0.17, and the interior pressure coefficient is -0.09. Thus the net Cp is -0.08, which is 118% lower than the isolated case (c.f. Figure 18). The highest positive net pressure has been found to occur when an extremely large obstruction (4h x 4h) is placed to one side of the dwelling, in which case the net pressure is 0.57 (30% higher than the isolated case). This is the result of a relatively high negative interior pressure (Cp = -0.22) induced by the acceleration of flow around the side of the adjacent structure.

For cases where the adjacent structure is offset or rotated relative to the dwelling, net pressures are 0.56 and 0.54 respectively (maximum roof pressure coefficients are 0.55 and 0.43 respectively, and interior pressures are negative). These values are 27% and 23% greater than the isolated case, respectively. Net pressure coefficients are therefore also greater than the isolated case when the adjacent structure is placed asymmetrically relative to the dwelling. In this case, the dwelling is only partially enclosed by the positive pressure field created by the adjacent structure, and large positive roof pressures can occur concurrently with relatively large negative façade pressures.

For cases where the obstruction is downwind of the dwelling and arranged symmetrically, the dwelling is more fully enclosed by the obstruction’s positive pressure field. In a steady-state situation the apparent increase in the risk of flue flow reversal due to increased positive pressures on the roof is moderated by the corresponding increase in interior pressures, as a result

68

of increased positive pressures acting on the façades. In fact, for the 8h x 2h adjacent structure, the net positive pressure has been found to be smaller than for the isolated case. However, for a reasonably airtight building with ventilation opening areas which are small in comparison to the building dimensions, the timescale over which the interior pressure changes as a result of a change in wind speed and/or direction is likely to be longer than the timescale for changes to flue pressure. Thus it is possible that a change in wind speed and/or direction could cause an immediate increase in positive pressure at the flue terminal, but that the influence of positive pressure on the façades might only be experienced by the dwelling interior after a certain period of time. The implications for flue flow reversal risk depend on the relative lengths of following timescales:

· The timescale for the interior pressure to respond to changes in the wind pressure field caused by a change in wind speed and/or direction;

· The timescale required for adverse wind pressures acting at the flue terminal to cause sustained flue flow reversal.

Lawson (1980) gives an order of magnitude estimate of the time taken for a volume within a building reach pressure equilibrium as:

= 1.21 x 10 4 - Vo D pti

AD

where ti is the time taken for pressure equilibrium to be reached, Vo is the volume of the space, AD is the volume of the openings and Dp is the average pressure across the openings. Taking a mean wind speed of 5ms-1 and an average pressure coefficient difference of 0.6 gives Dp = 15Pa. If the volume of a room Vo = 100m3 and the trickle vent area AD = 0.004m3, the timescale to reach equilibrium is approximately 12 seconds. Thus the timescale for the interior pressure to respond to changes in the wind pressure field is approximately the same as that required for adverse wind pressures acting at the flue terminal to cause sustained flue flow reversal (10-15 seconds). Therefore, the response of the interior pressure following changes in wind speed and/or direction is likely to be sufficiently fast so that dynamic effects are not an important consideration in the present analysis.

6.2 RIDGE VENTS

Pressures have been measured on either side of the ridge, at a small distance from the ridge (0.24m at full scale). These measurements are representative of the pressures which would be experienced on either side of a ridge vent terminal. The results indicate that for both the 45° and 30° roof pitches, the pressures are negative for most wind directions. Furthermore, for the 45° case, when positive pressures occur on the upwind side of the ridge (0° and 30° wind directions), much greater negative pressures occur on the downwind side, and thus the net wind effect will tend to enhance ventilation from the terminal. For both roof pitches, the most problematic wind direction occurs when the wind is along the ridge. In this case, although the magnitude of the pressure is found to be small, sustained positive pressure 'events' can occur for a significant proportion of the time. For example, in the case of the 45° roof, although timeaveraged pressures were found to be no greater than zero, positive pressure events greater than 9 seconds in length occurred for around 10% of the time, when the wind speed was moderate (~3m/s). For this wind direction, slightly higher positive mean pressures are found to occur for the 9 house terrace than the 3 house terrace, and thus the susceptibility of the terminal to adverse wind effects is found to increase with increasing terrace length.

69

7. CONCLUSIONS

7.1 IMPACT OF ADJACENT BUILDINGS AND TOPOGRAPHICAL FEATURES

· In the case of a ‘standard’ domestic dwelling with a 45° pitch roof, the maximum positive pressure on the dwelling roof occurs when an adjacent structure is directly downwind of the dwelling, and the dwelling is immersed in the positive pressure field produced by the downwind structure. The magnitude of the pressure depends on the height and width of the adjacent structure, and its distance from the dwelling. For the cases investigated, the maximum increase in positive pressure is found to occur when the dwelling (eaves height h) is located close to an embankment (height 2h). In this case, the maximum pressure experienced by a flue terminal located above the roof may exceed that for the isolated dwelling by up to 60%.

· The reduction in maximum positive pressure with increasing height above the roof, as measured by the vertical tubes, is similar for the isolated and obstructed cases. Typically, the maximum pressure coefficient reduces by 0.1 at a distance of 12.5mm above the roof and by 0.2 at a distance of 31.5mm above the roof, relative to the maximum surface pressure coefficient. Since maximum positive surface pressures have been measured over the range ~0.40 to ~0.65, pressure at height of up to 2.5m above the roof may remain positive, and thus increases in flue height will not significantly reduce the risk of flue flow reversal.

· The zone of influence of an adjacent structure can be determined approximately from results obtained here for the 45° pitch roof, and depends on the height and width of the adjacent structure. For adjacent structures which are relatively narrow in comparison to the dwelling dimensions (W = 2h), a significant increase in positive roof pressure will occur up to a distance of ~1.5h, where h is the height of the adjacent structure. However, for adjacent structures which are relatively wide in comparison to the dwelling dimensions (W ≥ 4h), an increase in positive roof pressure will tend to occur for a distance of at least 2h. In all cases, the maximum pressure tended to decrease with increasing separation distance.

· The impact of an adjacent structure on pressures over the roof of a 30° pitch roof dwelling is greater than for a 45° pitch roof. For the scenario investigated, the same adjacent structure increased maximum roof surface pressures relative to the isolated dwelling cases by 75% and 7% for the 30° pitch and 45° pitch respectively. In addition, pressures along the eaves which were negative for an isolated 30° pitch roof dwelling became positive with the adjacent structure present. However, maximum roof pressures were still found to be considerably lower for the 30° pitch roof than the 45° pitch roof.

· For the range of additional factors investigated, the maximum roof pressure has not been found to increase significantly with the introduction of these factors. Sensitivity to the following factors has been investigated: adjacent structure geometry; offset or rotation of the adjacent structure relative to the dwelling; location of the dwelling within a terrace; location of a dwelling adjacent to similar neighbours; geometry of topographical feature (hill or embankment); terrain type.

· For a worst case scenario where the interior pressure is dominated by the minimum external façade pressure due to the existence of an open window or ‘dominant opening’ where the minimum pressure occurs, the maximum difference between the roof pressure and the interior pressure is greatest for the isolated dwelling. This is because an adjacent structure

71

tends to smooth out the variation in pressure over the dwelling, resulting in a smaller difference between maximum roof and minimum façade pressures.

· When the dwelling is reasonably airtight and background ventilation only is provided, the interior pressure tends to be positive when the adjacent structure is downwind of the dwelling and the dwelling is fully enclosed by the positive pressure field. In the steady-state case, the net positive pressure (difference between the maximum roof pressure and the interior pressure) is smaller with the obstruction present than in the isolated case, for which the interior pressure is close to zero.

· With background ventilation, and when the adjacent structure is to one side of the dwelling, or is rotated or offset relative to the dwelling, the dwelling is only partially enclosed by the positive pressure field created by the adjacent structure. In these cases, large positive roof pressures can occur concurrently with relatively large negative façade pressures and net positive pressures tend to be higher than the isolated case.

· For the moderate wind speeds at which Upton et al. (1999a) identified high flue flow reversal risk, the influence of stack pressure on the interior pressure has been found to be significant when internal/external temperature differences are large. In this case, negative interior pressures occur when the external temperature is lower than the internal temperature.

· The greatest increase in the risk of flue flow reversal with an adjacent structure present relative to the isolated case has been found to occur when the ventilation strategy is background ventilation, with no dominant opening provided by a window. The former ventilation strategy is more likely to be used when internal/external temperature differences are large (i.e. in winter), when the risk is further increased by the influence of the stack effect on interior pressure.

7.2 RIDGE VENTS

· For both the 45° and 30° roof pitches, ridge vent pressures are found to be negative for most wind directions. Furthermore, for the 45° case, when positive pressures occur on the upwind side of the ridge (0° and 30° wind directions only), much greater negative pressures occur on the downwind side, and thus the net wind effect will tend to enhance ventilation from the terminal.

· For both roof pitches, the most problematic wind direction occurs when the wind is along the ridge. In this case, although the magnitude of the pressure is found to be small, sustained positive pressure 'events' can occur for a significant proportion of the time. For example, in the case of the 45° roof, although time-averaged pressures were found to be no greater than zero, positive pressure events greater than 9 seconds in length occurred for around 10% of the time, when the wind speed was moderate (~3m/s).

· When the wind is parallel to the ridge, slightly higher positive mean pressures are found to occur for the 9 house terrace than the 3 house terrace, and thus the susceptibility of the terminal to adverse wind effects is found to increase with increasing terrace length.

72

8. RECOMMENDATIONS

8.1 IMPACT OF ADJACENT BUILDINGS AND TOPOGRAPHICAL FEATURES

· The risk of flue flow reversal has previously been found to be of concern for isolated dwellings with roof pitches greater than 45° (Upton et al. 1999(b)). The present work has found that the risk could increase further with an adjacent structure present, due to the combination of higher wind pressures at the flue combined with larger negative pressures experienced on building façades affecting interior pressures. Again, the risk has not been found to reduce significantly by increasing the height of flues above the roof. Therefore, similar recommendations apply as previously, namely that fanned draught systems should be considered for dwellings with roof pitches greater than 45°, both with and without an adjacent structure present.

· For dwellings with less steep roof pitches (30° or lower), the risk of flue flow reversal for the isolated dwelling was previously found to be less of a concern. However, since the increase in the risk of flue flow reversal in the presence of an adjacent structure has been found to be more significant for the 30° pitch roof dwelling than the 45° pitch roof dwelling, it is recommended that alternatives to natural draught systems should be considered for the 30° roof pitch when an adjacent structure is present and at a sufficiently close distance to the dwelling.

· In the present work, mainly the 45° pitch roof case was addressed. While results from the 45° pitch roof are useful for guidance on 30° pitch roofs and flat roofs (for example, the zone of influence of an adjacent structure), additional wind tunnel experiments for the 30° pitch roof and flat roof cases could be considered, in order to provide more information on the impact of an adjacent structure on wind pressures over these roofs. In particular, the effect of size and separation distance of the adjacent structure on negative pressure in the eaves regions of the roofs could be investigated further.

8.2 RIDGE VENTS

· Comparing the results of the present investigation with results obtained previously for isolated buildings with similar roof pitches, the risk of positive pressure events occurring at ridge vents on 45° pitch roofs is significantly smaller than the risk of these events occurring at flue terminals located near the eaves. In the case of 30° pitch roofs, the risk is slightly reduced for ridge vents relative to flue terminals located near the eaves. Therefore, it is suggested that the use of ridge vents is preferable to flue terminals located close to the eaves for 45° pitch roofs.

73

9. ACKNOWLEDGEMENTS

This work was funded by the Health and Safety Executive under Contract RSU Ref: 4355/R41.124.

Thanks are due to Stuart Upton and Paul Blackmore for their help with this work.

75

10. REFERENCES

1. BS 5440-1 (2001). Installation and maintenance of flues and ventilation for gas appliances of rated input not exceeding 70kW (1st , 2nd and 3rd family gases) - Part 1 : Specification for installation and maintenance of flues. BSI, 389 Chiswick High Rd, London, W4 4AL. ISBN 0 580 33229 2.

2. CIBSE Guide A (1999) Environmental design, Chartered Institution of Building Services Engineers, 1999.

3. CIBSE Guide C (1986) Reference data, Chartered Institution of Building Services Engineers, 1986.

4. Cook, N.J. (1990) The designer’s guide to wind loading of building structures. Part 2 Static Structures. Building Research Establishment Report, Butterworths.

5. De Gids, W.F. & den Ouden, H.P.L. (1974) Three investigations of the behaviour of ducts for natural ventilation, in which an examination is made of the influence of location and ehight of the outlet, of the built-up nature of the surroundings and of the form of the outlet. IG-TNO, Delft, Report number 529, November 1974. TNO Bouw, POB 49, 2600 AA Delft, The Netherlands.

6. Department of the Environment and the Welsh Office, The Building Regulations 1991, Approved Document F, 1995 Edition.

7. ESDU Data Sheet 82026 Amendments A-D (1993) Strong winds in the atmospheric boundary layer. Part 1: Mean hourly wind speeds, April 1993.

8. Iles, P.J. (1998) Standard dwellings for energy modelling, BRE Client Report CR44/98, BRE, Garston, January 1998.

9. Lilly, J.P. & Williams, T.P. (1996) The Design, Installation and Performance of Open Flue Terminals. Paper No: T317. Proceedings of the Institute of Gas Engineers Conference, Harrogate, May 1996.

10. Plathner, P.S. & Stephen, R.K. (2001) The effect of ventilation strategies on flue performance. Part 2: BREEZE modelling of internal house pressures. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report number 205016.

11. Upton, S.L. Blackmore, P.A. & Tily, P.J. (1999(a)). Flue Performance of Domestic Gas Burning Appliances. Aim 1. Full-scale Test House Measurements of the Flue Performance of Open-Flued Gas Fired Appliances. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report Number CR 271/99.

12. Upton, S.L., Blackmore, P.A. & Tily, P.J. (1999(b)) Flue Performance of Domestic Gas Burning Appliances. Aim 2. Wind tunnel tests on various model building configurations for safe flue installation. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report number CR109/99.

77

13. Upton, S.L., Blackmore, P.A. and Tily, P.J. (2000). Additional full-scale test house measurements of the flue performance of open-flued gas fired appliances using internal flues. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report number 200175.

14. Upton, S.L. & Tily, P.J. (2001) The effect of ventilation strategies on flue performance. Part 1. Full-scale test house measurements. BRE, Garston, Watford, Herts, UK, WD25 9XX. Report number 205015.

15. Lawson, T.V., Wind Effects on Buildings, Volume 1: Design Applications, Applied Science Publishers, 1980.

78

11. Appendix A

2h x 2h obstruction downwind roof centre line Cp

Cp

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

h 2h 3h i10 11 12 solated


Figure A1 Effect of the distance of a 2h x 2h downwind

obstruction on the roof centre line Cp


Cp

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

2h 4h 6h

10 11 12 isolated




79

Roof centre line Cp 8h x 2h obstruction at different distances downwind

0.8 C

p 0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

4h

8h

10 11 12 12h

Isolated


Figure A3 Effect of the distance of an 8h x 2h downwind


2h x 2h obstruction downwind roof ce ntre line Cp

Cp

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

h

2h

3h

i10 11 12 solated




80


0.8 C

p

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

2h

4h

i10 11 12

solated





0 1 2 3 4 5 6 7 8 9

Cp

2h

4h

i

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12 solated

Position on roof centre line(upwind eaves to downwind eaves)



81


0 1 2 3 4 5 6 7 8 9

Cp

2h 4h i

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12

solated

Position on roof centre line(upwind eaves to downwind eaves)



0 1 2 3 4 5 6 7 8 9

( )

Cp

i

2h

4h

8h

2h high obstructions of different widths, 2h downwind roof centre line Cp

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.8

10 11 12

Position on roof centre line upwind eaves to downwind eaves

solated

16h

Figure A8 Effect of the width of an obstruction of

height 2h on the roof centre line Cp

82


0.8 C

p

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

2h

4h

6h

i10 11 12 solated


Figure A9 Effect of the distance of a 4h x 2h obstruction

on the roof centre line Cp


Cp

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1 2 3 4 5 6 7 8 9

i

8h

6h

4h

2h10 11 12

solated




83

2h high obstructions of different widths at 2h upwind roof centre line Cp

Cp

Cp

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

1 2 3 4 5 6 7 8 9

i

2h

4h

8h10 11 12

solated

16h


Figure A11 Effect of the width of a 2h high upwind obstruction on the roof centre line Cp

2h x 2h obstruction upwind roof centre line mean Cp

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

h

2h

3h

isolated 1 2 3 4 5 6 7 8 9 10 11 12

Position on roof centre line (downwind eavesto upwind eaves)

Figure A12 Effect of the separation distance of a 2h x 2h upwind


84

4h x 2h obstruction upwind roof centre line Cp

Cp

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

1 2 3 4 5 6 7 8 9

2h

6h

i10 11 12

solated


Figure A13 Effect of the distance of a 4h x 2h upwind


4h x 4h obstruction upwind roof ce ntre line Cp

-1

0

0.2

0.4

0.6

1 2 3 4 5 6 7 8 9

Cp

2h

4h

6h

8h

i

-0.8

-0.6

-0.4

-0.2 10 11 12

solated


Figure A14 Effect of the distance of a 4h x 4h obstruction

upwind on the roof centre line Cp

85

12. Appendix B

2h x 2h obstruction at h downwind roof centre line Cp

Cp

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

0 150


Figure B1 Roof centre line Cp for a 45 pitch roof dwelling

with a 2h x 2h building at h downwind


i

31.5 tubes 12.5 tubes Surface taps solated

0.6

0.4

0.2

Cp 0

-0.2

-0.4

-0.6

31.5 tubes 12.5 tubes Surface taps isolated 0 150



with a 4h x 2h building at 2h downwind

87


0.8

0.6

0.4

0.2 Cp

0 0 1 5 0

-0.2

-0.4

-0.6

i

31.5 tubes

12.5 tubes

surface

solated





0.6

0.4

0.2

00 1 5 0

Cp -0.2

-0.4

-0.6

-0.8




i

31.5 tubes

12.5 tubes

surface

solated

88

2h x 16h at 2h dow nw ind roof centre line Cp

0.8

0.6

0.4

0.2

Cp 0 0 1 5 0

-0.2

-0.4

-0.6

-0.8

i

31.5 tubes

12.5 tubes

surface

solated

Position on roof centre line (dow nw ind eaves to upw ind eaves)

Figure B5 Roof centre line Cp for a 45 pitch roof dwelling with a 2h x 16h

building at 2h downwind

2h x 2h obstruction at h upwind roof centre line Cp

Cp

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

0 150



with a 2h x 2h building at 2h upwind

i


89


Cp

0.6

0.4

0.2

0 0 1 5

f

i

31.5 tubes

12.5 tubes

Sur ace taps

solated

0

-0.2

-0.4

-0.6

-0.8


Figure B7 Roof centre line Cp for a 45 pitch roof dwelling with a

4h x 2h building at 2h upwind


0 0 150Cp

-0.6

-0.4

-0.2

0.2

0.4

0.6

31.5 tubes 12.5 tubes surface isolated




90


0.6

0.4

0.2

0

Cp -0.2

-0.4

-0.6

-0.8

0 150




2h x 16h at 2h upwind roof centre line Cp

i


0.6

0.4

0.2

00 1 5 0

Cp -0.2

-0.4

-0.6

-0.8

i

31.5 tubes

12.5 tubes

surface

solated




91

Printed and published by the Health and Safety ExecutiveC30 1/98

Printed and published by the Health and Safety Executive C1.10 09/03

ISBN 0-7176-2750-0

RR 157

9 78071 7 6275 09£25.00

research report 157research report 157 hse health & safety executive the effect of adjacent...

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