wind tunnel experiments modelling the thermal effects within the vicinity of a single block building...

ARTICLE IN PRESS

Journal of Wind Engineering

and Industrial Aerodynamics 94 (2006) 621ndash636

0167-6105$ -

doi101016j

CorrespoE-mail ad

wwwelseviercomlocatejweia

Wind tunnel experiments modelling the thermaleffects within the vicinity of a single block building

with leeward wall heating

K Richards M Schatzmann B Leitl

Meteorological Institute University of Hamburg Bundesstrasse 55 20146 Hamburg Germany

Received 12 July 2005 received in revised form 8 February 2006 accepted 13 February 2006

Available online 17 April 2006

Abstract

A wind tunnel model aiming to simulate the thermal effects within the vicinity of a building with

leeward wall heating was set up The work was conducted within the scope of the European

ATREUS project (httpaixmengauthgratreus) in which micro-scale numerical models were used

to obtain data concerning the microclimatic conditions within the vicinity of buildings [AM

Papadopoulos N Moussiopoulos Towards an holistic approach for the urban environment and its

impact on energy utilization in buildings the ATREUS project J Environ Monit 6 (2004)

841ndash848] This data was then used as input data for generating typical weather data required as input

for building and heating ventilation and air-conditioning (HVAC) system models in order to study

the energy budgets of buildings and assess the performance of air-conditioning (AC) systems

However it was first necessary to validate these microscale numerical models for a simplified case

under different thermal conditions A series of wind tunnel experiments were conducted in which the

mean velocity and temperature field within the vicinity of a single block building (a cube) with

leeward wall heating were measured The ratio of Grashof number to the square of Reynolds

number GrRe2 was used to model thermal effects within the vicinity of the model but some

compromises were needed in order to obtain a practical model while at the same time fulfilling the

objectives of the task set Conditions representative of mixed and forced convection were modelled

Results showed some degree of flow modification within the recirculation region of the model for

both GrRe209 and 16 the recirculation length in both cases being shortened when compared to

the non-heated case The velocity field influenced the temperature distribution within the

recirculation region There was a rapid temperature drop away from the surface with the

temperature distribution reaching near ambient conditions within one model height downstream of

the heated face for GrRe216 In spite of the restrictions applied to the physical model the

see front matter r 2006 Elsevier Ltd All rights reserved

jweia200602003

nding author Tel 0049 40428385091 fax 0049 40428385452

dress krichardshondaracingf1com (K Richards)

ARTICLE IN PRESSK Richards et al J Wind Eng Ind Aerodyn 94 (2006) 621ndash636622

technique applied showed very good stability and repeatability during the entire measurement

campaign producing a reliable data set for the validation of the microscale numerical models

r 2006 Elsevier Ltd All rights reserved

Keywords Thermal effects Building Wind tunnel Grashof Similarity

1 Introduction

The heating and cooling requirements of buildings are strongly associated with themicro-climatic conditions that develop within their vicinity in particular influences due todirect solar radiation [1] Increasing urbanisation and the appearance of the urban heatisland effect have also become a factor in assessing the need and performance of air-onditioning (AC) systems [23] For example since 1987 more than 12 million AC unitshave been installed in Greece alone due to increased demand for building cooling as adirect result of changing urban climatic conditions [2] However the installation of theseunits which are often fac-ade mounted are exacerbating the problem as they themselves areheat sources rejecting heat directly in to the street canyon and in effect further influencingthe local air temperature and also the flow field within the vicinity of a building Inessence therefore the efficiency of AC systems are being severely compromised by anincrease in local air temperature which results from not only heating due to solar radiationbut also heat rejection from the AC unit itself As a consequence it is important tounderstand the local thermal conditions around a building and its influence on local airpatterns in order that the efficiency of AC systems maybe comprehensively assessedHowever knowledge of this kind and in particular thermal effects due to direct solarradiation within the vicinity of buildings is limited with only a handful of experimentalfield and numerical studies which show varying degrees of influence of thermal effects[4ndash17] In general the numerical-based studies [4ndash11] tend to predict strong influences onthe flow field within a street canyon as a direct result of wall or ground heating withthermally induced motion either acting with (to strengthen) or against (to alter) theexisting mechanically induced flow field These strong effects were not however reportedin the field or wind tunnel studies [12ndash17] A typical example of these differences is reportedin the combined numericalfield study of Louka et al [6] where the numerical model over-estimates the thermal effects for windward wall heating in a street canyon predicting twocounter-rotating vortices when only one recirculation vortex was observed from the fielddata Observations made during model-scale experiments [14ndash17] also show little influenceon the velocity field due to wall-heating except a strengthening due to the thermallyinduced upwards motion close to the heated surface No firm conclusions can be madefrom the data presented in the literature as to the degree of influence of thermal effectswithin the vicinity of buildingsWithin the scope of the European ATREUS project (httpaixmengauthgratreus)

micro-scale numerical models were used to obtain data concerning the microclimaticconditions within the vicinity of buildings (to include the influences of solar radiation) andthen used as input data for generating typical weather data required as input for buildingand HVAC system models to study the energy budgets of buildings and the performance ofAC systems In light of the previous experiences observed in the literature usingmicroscale numerical models to simulate thermal effects it was important that the models

ARTICLE IN PRESSK Richards et al J Wind Eng Ind Aerodyn 94 (2006) 621ndash636 623

chosen for this study were first validated A series of wind tunnel experiments weretherefore conducted measuring mean velocity and temperature field within the vicinity of asingle block building with leeward wall heating The single building case was selected asthe model case because of its relative simplicity which is vital when producing data for thevalidation of a numerical model Note that the physical effect of the AC unit on themodification of flow within the street was not assessed in this study

During the preparation and set-up of the wind tunnel model it became clear thatmodelling such thermal effects within the vicinity of buildings at scale was not easy Thereare very few model scale studies in this area to the authorsrsquo knowledge [141617] withonly one [16] considering specifically a building with surface heating The experimentscarried by Huizhi and his colleagues [16] were conducted in a water tank where as [1315]as with the current study were conducted using wind tunnels The primary aim of thefollowing paper is to address the issues and difficulties encountered during the currentstudy detailing the considerations and compromises made during the development of thewind tunnel model Results from the experiments are also discussed so as to put theseconsiderations in to context and to demonstrate the effectiveness and repeatability of theexperimental technique and conditions applied

2 Similarity criteria and model constraints

The influence of thermal effects with respect to the mechanical flow around the buildingmodel was modelled using the ratio of Grashof number to the square of Reynolds number[151619]

Gr

Re2frac14

bgH Tw T ref

U2ref

(1)

where b is the coefficient of thermal expansion g acceleration due to gravity H is theobstacle (in this case the model building) Tw is mean wall temperature and T ref ambienttemperature within the wind tunnel test section and U ref is a reference wind velocitymeasured upstream of the model The ratio GrRe2 is essentially the bulk Richardsonnumber or inverse Froude number 1Fr When GrRe2E1 then motion is induced by boththermal and mechanical effects However if GrRe2b1 then thermal effects are moresignificant

It is important within this ratio that the similarity criteria concerning Re and Gr arerespected as far as is practically possible At model-scale it is impossible to replicate full-scale Re Therefore it is commonplace to assume that provided Re is sufficiently high thegross structure of turbulence should no longer be affected by a change in Re thus achievingdynamic similarity between the two flows fields Even for the low wind speed conditionsrequired by the current model set-up this criterion could easily be satisfied anddemonstrated through measurement The difficulty came when trying to satisfy thermalsimilarity With reference to Eq (1) the Grashof Gr number is concerned with theconvective flow within the thermal boundary layer that forms close to the wall resultingfrom an applied temperature difference

Gr frac14bgH3 Tw T ref

n2 (2)


and n is kinematic viscosity of the fluid In contrast to the Reynolds number this quantityneeds more careful consideration The magnitude of this number is used to indicatewhether the free convective flow within the thermal boundary layer close to the heatedsurface is laminar or turbulent which in turn can significantly affect quantities such as heattransfer rate According to Bejan and Lage [20] GrE109 marks the transition from laminarto turbulent free convection for a vertical flat plate in a wide range of Prandtl numberPr frac14 0001ndash1000 Based on values calculated from field data [6] Gr1013 can be expectedfor DT frac14 10 1C (ie Tw T ref is Eq (2)) close to a wall subject to solar radiation whereH frac14 21m clearly implying turbulent convection within the thermal boundary layer 3pGrRe2p300 However in order to achieve such similarity at model scale lets say 1200 onewould need a model of over 1m high and DT frac14b200 1C to achieve Gr1010 But inreality the model size is restricted by the test facility and governed by blockage ratio F theratio of model frontal area to test section area (o5 for closed section wind tunnels)Therefore this means much smaller models that would require even higher DT to achievean equivalent Gr But excessive DT would lead to density effects where the air within thevicinity of the model becomes much less dense than it would under ambient or marginalheating conditions eg at 21 1C the density of air is r frac14 12 kg=m3 at 150 1Cr frac14 084 kg=m3 31 less dense than at 21 1C and at 300 1C r frac14 062 kg=m3 50 lessdense than at 21 1C There is the potential therefore to over exaggerate the phenomena oneis trying to model when applying elevated temperatures Ruck [16] applied rooftemperatures up to 400 1C and thus the observations made might have been overexaggerated in this case for this reason Consequently developing the physical modelbecame a question of compromise while still maintaining an element of realism The windtunnel models set-up in [1417] had some success in achieving GrRe23 but onlyconsidered 2D street canyons The depth of the canyon in the study of Kovar-Panskus etal [14] was twice the height of the model used here for similar model conditions andconsequently made the difference in the GrRe2 achievable Huizhi and his colleagues [17]used a water tow-tank that utilises static salty water with varying salinity making it mucheasier to achieve high GrRe2 Note that Ruck [16] was only able to achieve GrRe204despite applying surface temperatures up to 400 1C with a building height of 01m Themodel conditions applied in the current study will now be described

3 Physical model set-up and conditions applied

A 1100 scale model of a single block building (cube) was designed built and set-upin the multi-layer wind tunnel at the Meteorological Institute University of HamburgFig 1a [21] The wind tunnel is a closed section return-circuit type but unlike conventionalboundary layer wind tunnels the multi-layer wind tunnel has 9 fans not 1 This allows eachlayer of air to be heated and thus different stratification effects to be modelled Howeverfor this study we were only interested in neutral stratification conditions The model itselfwas unique in that only one of its vertical faces was heated (the leeward face) in order tosimulate the influence of solar radiation on one wall of an isolated building This made thedesign and specification of the model a real challenge as it was essential that the unheatedfaces remain cool relative to the heated face After numerous design and heating trails thefinal model (H frac14 019m) was made from Plaster of Paris with a 2mm aluminium metalplate fixed to form the leeward face (Fig 1b) The leeward face was heated from the insideusing a ceramic radiant heater powered via an autotransformer

ARTICLE IN PRESS

Fig 1 Physical model set-up (a) Set-up in the multi-layer wind tunnel (b) close-up of model and (c) co-ordinate

system (same as in numerical models)

K Richards et al J Wind Eng Ind Aerodyn 94 (2006) 621ndash636 625

The chosen model scale of 1100 was a compromise between achieving Re independenceobserving the allowable maximum blockage ratio F and the order of magnitude ofGrashof number achievable With reference to Eq (2) Gr is also partly dependant on themodel height for its magnitude and the larger the model the more potential there is for thethermal boundary layer to develop and for transition to turbulence to occur However wecould only practically achieve Gr108 for H frac14 019m implying laminar convection closeto the heated surface Re based on model height was 6291 and F frac14 16 One couldachieve larger values of Gr by applying higher surface temperatures Tw in Eq (2) but atthe risk of causing density effects at elevated temperature Consequently the meantemperature of the heated face of the model was restricted to Two200 C where air is 38less dense than it would be at 21 1C The fluid properties in the calculation of Gr wereevaluated at film temperature Tf where T f frac14 05 Tw thorn T ref

to account for changes in

these properties with increased temperature The very high values of GrRe2 observed inthe field could therefore not be achieved at model scale However because our primaryinterest was to measure the mean temperature and velocity field for validation purposesand not to necessarily replicate full scale conditions our compromise with respect to theGr was to neglect the character of the flow close to the surface and model ratios of GrRe2


representative of mixed and forced convection [19] We assumed that the heat energyintroduced into the wake flow field behind the model through the thermal boundary layerclose to the surface would be the same regardless of whether this layer was laminar orturbulent and thus have the same effect on the flow field With this conditionmeasurements were restricted to outside the thermal boundary layer The thermalboundary close to the surface was observed and estimated to be 8mm (004H) throughflow visualisationThe actual thermal conditions modelled are given in Table 1 GrRe201 represents

forced convection conditions and was applied in order to have a baseline case forcomparison to assess any thermal influences resulting from surface heating GrRe216according to Ref [19] is representative of a mixed convection condition and was thehighest ratio obtainable taking in to account the conditions and restrictions describedabove and in Section 2 The temperatures given in Table 1 are the overall averagesdetermined for all data of the entire measurement campaign (repeatability of conditions isdiscussed in the next section) The reader will observe in addition to Tw and T ref values ofT floor T roof TLS and TRS These are the mean temperature of the wind tunnel floor behindthe model the mean roof temperature of the model mean surface temperature on the leftside of the model and mean surface temperature on the right side of the modelrespectively These were monitored because it was impossible to completely isolate theheated face from the rest of the model and because the floor tended to heat due to radiationeffects from the heated face All surface temperatures were measured using E-type surfacemounted thermocouples (SMTC) located at a point representative of the mean for thatsurface under the applied heating condition For Tw 2 SMTC were place such that they arecombined value was equal to the mean heated surface temperature Tw (these can be seen inFig 1b) For practical reasons DT ie Tw T ref as opposed to U ref was used to govern theparameter GrRe2 and the low wind speed condition U ref05m=s measured at (15H 0125H) (lower than would normally be employed for the flow around buildings) wasapplied to maximise thermal effects See Fig 1c for co-ordinate system Re independenceof the flow field around the model was therefore assured through measurement for thisspecific low wind speed conditionThe ambient temperature T ref was monitored and recorded using a column of semi-

conductor temperature transducers that recorded the mean temperature within the windtunnel test section (seen upwind of the model in Fig 1a) The reference wind speed U ref

could not be measured directly during measurements due to low wind speedhighturbulence difficulties with the conventional Pitot-static tube We therefore relied on data

Table 1

Thermal modelling conditions (U ref05m=s)

GrRe216 (1C) GrRe209 (1C) GrRe201 (1C)

Tw 176 79 30

T ref 24 24 24

T floor 38 28 25

T roof 50 34 25

TLS 39 29 25

TRS 40 30 25


obtained during preliminary tests (under the same conditions) using a 2D fibre-optic LaserDoppler Anemometer (BSAF80-LDA Dantecs) We were able to do this confidently dueto the excellent repeatability of the set-up model conditions and of measurements usingthis device (70011ms) plus the good stability in the fan speeds of the wind tunnel ie themaximum standard deviation of the actual velocity value recorded for each of the 9 fans bysonic-anemometers during all measurements was p701ms Whenever practical thevalue of U ref was checked using the BSAF80-LDA As a consequence of this assumptionthree values for GrRe2 were calculated using Eq (1) for U ref 0011ethm=sTHORN and then anaverage taken This approach took account of the sensitivity of GrRe2 due to naturalfluctuations in U ref With regard to Tw and T ref GrRe2 was less sensitive or Tw frac14 10

Cand T ref frac14 5

C All throughout the tests careful attention was paid to the conditions ofthe model and within the wind tunnel so as to ensure stable and repeatable experimentalconditions

An arrangement of sharp edged roughness elements and upstream vortex generatorswere used to simulate a turbulent atmospheric boundary layer approach flow (neutrallystratified) to the model (Fig 1) The aerodynamic properties of this approach flow weremeasured using a 2D fibre-optic Laser Doppler Anemometer (BSAF80-LDA Dantecs)In accordance with the Fluid modellingPhysical modelling guideline VDI 378312established by the German Engineering Society VDI [22] the modelled boundary layer flowdemonstrates the behaviour and characteristics of an urbaninner city like roughness (to ascale of 1100) with a power law exponent a frac14 052 roughness length z0 frac14 29m andconstant shear layer to 50m Fig 2

A series of measurement campaigns were conducted measuring the mean velocity andtemperature field within the vicinity of the model (to be referred to as the cube) underisothermal and thermal conditions 3-min averages of the mean longitudinal lateral andvertical components u v and w respectively the root mean square (RMS) values of eachvelocity component as well as the Reynolds shear stresses u0w0 or u0v0 were recorded using a

0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)

Theoretical profile = 052 [23]

Fig 2 Inflow profile characteristics (z frac14 100mm would represent total wind tunnel height at 1100) (a) Time-

averaged velocity profile (b) time-averaged velocity profile semi-log scale and (c) shear stress profile


2D fibre-optic Laser Doppler Anemometer (BSAF80-LDA Dantecs) with a systemaccuracy of 005ms With respect to the mean temperature field T 3-min averages werealso recorded using E-type thermocouples mounted in a streamlined holder Fig 1b Dataacquisition was performed using an IOtech DaqBook200 16-bit PC-based data acquisitioncard with the DBK83mdash14-channel thermocouple acquisition card According to IOtechspecifications E-type thermocouple accuracy with respect to the complete system set-up is705 1C between 0 and 300 1C For technical reasons the velocity and temperature fieldswere measured separately

4 Results

Figs 3a and b compares contour plots of T=T ref for y frac14 0 with the uw-vector fieldssuperimposed for GrRe216 and 09 respectively Note that the temperature field for GrRe2 was only measured to x=H frac14 15 as T=T ref frac14 1 at this point The temperature fieldmeasured for GrRe201 is not shown here as it was T=T ref frac14 1 at all measured pointsThe thermal plume that forms as a result of the heating of the air close to the surfaceis clearly visible for the condition GrRe216 (Fig 3a) and still visible for GrRe209(Fig 3b) As this warmer less dense air meets the cooler mechanically driven flow fromover the top of the cube it is washed downstream hence the elongated temperaturedistribution at the upper trailing edge of the cube The maximum temperatures recordedwere at this point The observed temperature drop away from the surface was large Forexample with reference to Fig 3a just 21 of Tw was recorded within 10mm (005H) ofthe heated face at y frac14 0 for z=H frac14 103 this equates to a temperature difference of 136 1CThe maximum recorded temperature for the heating condition GrRe216 was just 28of Tw at y frac14 04H at z frac14 103H This behaviour was not unexpected with the temperaturefield in the wake of the cube much reduced with respect to the wall conditions The resultsimply that the majority of heat is transported away vertically by the thermal plume and not

Fig 3 Contour of T=T ref superimposed with uw-velocity vector fields at y frac14 0 for GrRe216 and 09 (a) Gr

Re216 and (b) GrRe209


re-entrained into the wake via recirculation region This could be due to the modified flowpattern observed as a consequence of the thermal effects Fig 4 The flow direction withinthe wake was modified to predominantly upward motion towards the heated face relativeto the flow field measured under isothermal conditions (cold cube case) Such a change inthe velocity field was not expected and it was initially thought that these effects were aconsequence of the floor which was unavoidably heated through radiation duringmeasurements (refer to Table 1) However a further test showed this not to be the case andthat these were actually thermal effects due to wall heating and not influences from thefloor The thermally induced vertical upward motion acts against the low velocity airwithin recirculation region altering its direction by drawing the air towards the heated faceand into the thermal plume As a consequence the relative magnitude of vertical motion

Fig 4 Comparing the mean velocity vector fields at y frac14 0 for isothermal and thermal conditions GrRe216

(note the magnitude of the vector lines have been enhanced for clarity) (a) Mean velocity field isothermal

conditions and (b) mean velocity field thermal conditions GrRe216


within this region was increased on average by 9 for the case GrRe216 (+100at a few points) when compared with the isothermal case These effects were observed forGrRe209 albeit weakened As a consequence of this flow modification the length ofthe recirculation region measured at z=H frac14 0125 was shortened for by 14 and 7 forGrRe216 and GrRe209 respectively The increase in vertical movement is reflected inthe turbulent kinetic energy field Fig 5 The plots show contours of turbulent kineticenergy superimposed with the uw-velocity vector field for GrRe216 09 and for the coldcube caseFor GrRe216 at y frac14 025H modification to the bulk vertical velocity field was still

observed as seen for y frac14 0 but the effects were no longer felt at y frac14 05H in spite of higherrecorded T=T ref at the upper trailing edge The mechanical flow field around the side ofthe cube becomes dominant at this point negating any influence due to thermal effects ForGrRe209 there was little change to the velocity flow field at y frac14 025H and again noeffect was felt at y frac14 05H There will no doubt be a threshold GrRe2 at which these effectsare no longer seen and the mechanically driven flow of the wake field becomes dominantbut this was not investigated hereFig 6 shows the lateral distribution of T=T ref at different zH The lateral influence of

the temperature field tended to increase with height to z=H frac14 103H due to the rising and

Fig 5 Non-dimensioned turbulent kinetic energy field and uw-velocity vector fields at y frac14 0 for GrRe216 09

and Cold Cube (a) GrRe216 (b) GrRe209 and (c) Cold Cube

ARTICLE IN PRESS

Fig 6 Contour plots of T=T ref for GrRe216 at different vertical levels zH (a) z=H frac14 07 (b) z=H frac14 103 (c)z=H frac14 113 and (d) flow field schematic modified from Ref [21]


expanding thermal plume hence the higher recorded temperatures at the upper trailingedge Two high temperature points or hot spots developed with height and their locationmight suggest that they coincide with the development of 2 lateral contra-rotating vorticeswhich commonly form at the vertical trailing edges of a cuboid shape [2425] and thatthere is an accumulation of temperature within the low velocity core of these structuresFig 6d The uv-velocity vector field has been superimposed onto Fig 6a and traces of avortex can be observed Unfortunately there is insufficient velocity data to show thesefeatures in detail as LDA measurements were very time consuming but the generalincrease in the magnitude of the vertical velocity component Fig 4 might suggest astrengthening of these features Fig 7 compares contour plots of non-dimensionedtemperature T=T ref for GrRe209 and 16 for x=H frac14 057H and x=H frac14 125H The hot-spots are again clearly visible and the vertical temperature distribution seen at the edges ofthe cube suggest further influence from the flow field structure on temperature field assuggested in Fig 6 [2425] The figure also clearly demonstrates how quickly the heatwithin the flow field is dissipated and the temperature distribution is T=T ref 1

ARTICLE IN PRESS

Fig 7 Non-dimensioned temperature field for GrRe216 and 09 at different xH downstream of the cube (a)

GrRe209 x=H frac14 057 (b) GrRe216 x=H frac14 057 (c) GrRe209 x=H frac14 125 and (d) GrRe216

x=H frac14 125

K Richards et al J Wind Eng Ind Aerodyn 94 (2006) 621ndash636632

The repeatability and stability of the methods applied as described in the previoussection were closely monitored and assessed during the 4-month measurement campaignBecause T is directly related to T ref and T ref tended to rise slowly during measurementdue to the wind tunnel being closed section meant that defining the repeatability of T

during measurement would be senseless The repeatability on the non-dimensionedquantity T=T ref and of course GrRe2 were therefore determined through repeatedmeasurements at different intervals during the campaign The repeatability on T=T ref for


each heating condition was 701 and is demonstrated in Fig 8 for GrRe216 and 09The repeatability on GrRe2 was 702 This satisfactory overall repeatability comes fromthe fact that the mean wall temperature Tw was not only repeatable but also stable towithin a few degrees during each day of measurements and day after day Fig 9 Inaddition because the test section temperature ie T ref had the tendency to rise during windtunnel operation measurements were performed in batches for a specified temperaturewindow so as to ensure consistent average T ref and thus consistent thermal modellingconditions This also helped towards the good repeatability and confidence in the resultsthus providing a reliable data set for the validation of the microscale numerical modelsused The reader should be cautious if comparing these results for the validation of his orher own simulations as the experiments described here were set up and tailored specificallyfor the purposes of the numerical models used and the resources available to the modellers

08

09

10

11

12

13

14

15

0 2 5 6 7

Individual repeated measurement

TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43

Fig 8 Repeatability of T=T ref for different days (note only a few data are shown here but for data points shown

multiply points were measured for each date)

170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw

Consecutive data recorded 3-minute averages

93 5 7

Fig 9 Stability of Tw during measurements for different days


[2627] However the trends observed in the data with respect to the thermal effects on theflow field are informative and the physical model conditions and the methods applied givesome guidance for future experiments It was the authors wish when compiling this paperto share in the experiences of modelling thermal effects at model scale and offer

5 Conclusions

A series of wind tunnel measurements were carried out to record the mean wind andtemperature field within the vicinity of a single block building with leeward wall heatingwith the purpose of providing data for the validation of microscale numerical models Theinfluence of thermal effects with respect to the mechanical flow around the model buildingwere modelled using GrRe2 but the task of realising the physical model set-up andsimulating sufficient thermal effects at scale were difficult The model had be small enoughso as not to create lsquolsquoblockagersquorsquo in the test facility but large enough to satisfy Reynoldsnumber independence for the low wind speed conditions required to maximise thermaleffects while at the same time achieving a suitable order of magnitude of Grashof numberIn the end the compromise was made not to replicate typical full-scale conditions but tomodel conditions with respect to GrRe2 representative of mixed and forced convectionResults from the experiments showed the flow direction within the wake recirculation

zone to be modified and strengthened to predominantly upward motion towards theheated face in the centre plane y frac14 0 as a consequence of thermal effects due to wallheating Initially it was thought that these effects were a consequence of the floor whichwould heat up through radiation from the heated surface However a further test showedthis not to be the case and that these were actually thermal effects due to wall heating Thethermally induced vertical upward motion was seen to act against the lower velocity airwithin recirculation region altering its direction by drawing the air towards the heated faceand resulting in a shortening of the recirculation region by up to 14 compared with theisothermal caseThe thermal plume that forms as a result of the heating of the air close to the surface was

clearly visible and the observed temperature drop away from the surface was large ieDT frac14 136 1C=10mm from the surface This behaviour was not unexpected with thetemperature field in the wake of the cube much reduced with respect to the wall conditionsThe influence of expected bulk flow structures were seen on the temperature field Thetemperature field resulting from wall heating was really only felt within the wake region upto 075H downstream of the heated faceWhile the physical model set-up was not comparable to conditions measured in the field

[6] (GrRe216 modelled here equated at to only DT frac14 15 1C at full-scale for H frac14 21m)it was successful for the objectives set within the ATREUS project [12627] Throughcareful set-up of the physical model and constant monitoring and recording of conditionsduring measurements good stability and repeatability of the conditions was ensuredproviding a comprehensive data set of mean temperature and velocity measurements forthe numerical modellersValuable lessons have been learnt from this study and it is the authors hope that this text

will aid any future modeller in such a task However if one wants to replicate large scaleconditions with regard to thermal effects around buildings then this method using windtunnels is not appropriate and the author might suggest perhaps the use of water tunnels as


used by Huizhi et al [17] Nevertheless the method applied is however ideal for use insuch validation cases as the conditions were controllable and the results repeatable

Acknowledgements

The authors wish to acknowledge the European Commission and in particular DGResearch for funding the ATREUS project within the framework of Research TrainingNetworks under Contract No HPRN-CT-2002-00207 (2002ndash2005) The author wouldalso like to thank Rainer Knut Thomas Glanert and Gopal Krishan for their technicalcontributions during the project

References

[1] AM Papadopoulos N Moussiopoulos Towards an holistic approach for the urban environment and its

impact on energy utilization in buildings the ATREUS project J Environ Monit 6 (2004) 841ndash848

[2] AM Papadopoulos The influence of the street canyons on the cooling loads of buildings and the

performance of air conditioning systems Energy Build 33 (2001) 6001ndash6607

[3] M Santamouris N Papanikolaou I Livada I Koronakis C Georgakis A Argiriou DN

Assimakopoulos On the impact of urban climate on the energy consumption of buildings Sol Energy 70

(2001) 201ndash216

[4] VT Ca T Asaeda M Ito S Armfield Characteristics of wind field in a street canyon J Wind Eng Ind

Aerodyn 57 (1995) 63ndash80

[5] PG Mestayer J-F Sini M Jobert Simulation of wall temperature influence on flow and dispersion within

street canyons Third International Conference on Air Pollution Proto Carras Greece Turbulence and

Diffusion vol 1 1995 pp 109ndash116

[6] P Louka G Vachon J-F Sini PG Mestayer J-M Rosant Thermal effects on the airflow in a street

canyonmdashNantes lsquo99 experimental results and model simulations Water Air Soil Pollut Focus 2 (2002)

351ndash364

[7] J-F Sini S Anquetin PG Mestayer Pollutant dispersion and thermal effects in urban street canyons

Atmos Environ 30 (15) (1996) 2659ndash2677

[8] J-J Kim J-J Baik A numerical study of thermal effects on flow and pollutant dispersion in urban street

canyons J Appl Meteorol 38 (8) (1999) 1249ndash1260

[9] J-J Kim J-J Baik Urban street-canyon flows with bottom heating Atmos Environ 35 (20) (2001)

3395ndash3404

[10] S Bohnenstengel KH Schlunzen D Grawe Influence of thermal effects on street canyon circulations

Meteorol Z 13 (5) (2004) 381ndash386

[11] X Xie Z Huang J Wang Z Xie The impact of solar radiation and street layout on pollutant dispersion in

street canyon Build Environ 40 (2005) 201ndash212

[12] Y Nakamura TR Oke Wind temperature and stability conditions in an eastndashwest oriented urban canyon


[13] M Santamouris N Papanikolaou I Koronakis I Livada DN Assimakopoulos Thermal and airflow

characteristics in a deep pedestrian canyon under hot weather conditions Atmos Environ 33 (27) (1999)

4503ndash4521

[14] A Kovar-Panskus L Moulinneuf E Savory A Abdelquari J-F Sini J-M Rosant A Robins N Toy A

wind tunnel investigation of the influence of solar-induced wall heating on the flow regime within a simulated

urban street canyon J Water Air Soil Pollut Focus 2 (2002) 555ndash571

[15] K Uehara S Murakami S Oikawa S Wakamatsu Wind tunnel experiments on how thermal affects flow

in and above urban street canyons Atmos Environ 34 (10) (2002) 1553ndash1562

[16] B Ruck Wind-tunnel measurements of flow field characteristics around a heated model building J Wind

Eng Ind Aerodyn 50 (1ndash3) (1993) 139ndash152

[17] L Huizhi L Bin Z Fengrong Z Boyin S Jianguo A laboratory model for the flow in urban street

canyons induced by bottom heating Adv Atmos Sci 20 (4) (2003) 554ndash564


[19] J Gryzagoridis Combined free and forced convection from an isothermal plate Int J Heat Mass Transfer

18 (1975) 911ndash916

[20] A Bejan JL Lage The Prandtl number effect on the transition in natural convection along a vertical

surface J Heat Transfer 112 (1990) 787ndash790

[21] M Schatzmann J Donat S Hendel G Krishan Design of a low-cost stratified boundary-layer wind

tunnel J Wind Eng Ind Aerodyn 5455 (1995) 483ndash491

[22] VDI-guideline 3783Part 12 lsquoPhysical Modelling of Flow and Dispersion Processes in the Atmospheric

Boundary LayermdashApplication of wind tunnelsrsquo Beuth Verlag Berlin 2000

[24] WH Schofield E Logan Turbulent shear flow over surface mounted obstacles ASME J Fluids Eng 113

(1994) 405

[25] SR Hanna GA Brigg P Rayford RP Hosker Handbook on atmospheric diffusion Technical

Information Centre US Department of Energy ISBN0-87079-127-3 1982

[26] R Dimitrova J-F Sini K Richards M Schatzmann CFD investigation of airflow around a simple

obstacle with single heating wall Atmospheric Sciences and Air Quality Conference (ASAAQ2005) 27ndash29

April 2005 San Francisco California

[27] S Vardoulakis R Dimitrova K Richards D Hamlyn G Camilleri M Weeks J-F Sini R Britter C

Borrego M Schatzmann N Moussiopoulos Numerical model inter-comparison for a single block building

within ATREUS 10th International Conference on Harmonisation within Atmospheric Dispersion

Modelling for Regulatory Purposes Crete Greece 17ndash20 October 2005

Further reading

[18] FT DePaul C M Shieh Measurements of wind velocities Atmos Environ 33 (24ndash25) (1986) 4143ndash4150

[23] ESDU Characteristics of Atmospheric Turbulence Near to the Ground Part II single point data for strong

winds (neutral atmosphere) No 85020 1985

Wind tunnel experiments modelling the thermal effects within the vicinity of a single block building with leeward wall heating

Introduction

Similarity criteria and model constraints

Physical model set-up and conditions applied

Results

Conclusions

Acknowledgements

References

bm_fur


technique applied showed very good stability and repeatability during the entire measurement

campaign producing a reliable data set for the validation of the microscale numerical models

r 2006 Elsevier Ltd All rights reserved

Keywords Thermal effects Building Wind tunnel Grashof Similarity

1 Introduction

The heating and cooling requirements of buildings are strongly associated with themicro-climatic conditions that develop within their vicinity in particular influences due todirect solar radiation [1] Increasing urbanisation and the appearance of the urban heatisland effect have also become a factor in assessing the need and performance of air-onditioning (AC) systems [23] For example since 1987 more than 12 million AC unitshave been installed in Greece alone due to increased demand for building cooling as adirect result of changing urban climatic conditions [2] However the installation of theseunits which are often fac-ade mounted are exacerbating the problem as they themselves areheat sources rejecting heat directly in to the street canyon and in effect further influencingthe local air temperature and also the flow field within the vicinity of a building Inessence therefore the efficiency of AC systems are being severely compromised by anincrease in local air temperature which results from not only heating due to solar radiationbut also heat rejection from the AC unit itself As a consequence it is important tounderstand the local thermal conditions around a building and its influence on local airpatterns in order that the efficiency of AC systems maybe comprehensively assessedHowever knowledge of this kind and in particular thermal effects due to direct solarradiation within the vicinity of buildings is limited with only a handful of experimentalfield and numerical studies which show varying degrees of influence of thermal effects[4ndash17] In general the numerical-based studies [4ndash11] tend to predict strong influences onthe flow field within a street canyon as a direct result of wall or ground heating withthermally induced motion either acting with (to strengthen) or against (to alter) theexisting mechanically induced flow field These strong effects were not however reportedin the field or wind tunnel studies [12ndash17] A typical example of these differences is reportedin the combined numericalfield study of Louka et al [6] where the numerical model over-estimates the thermal effects for windward wall heating in a street canyon predicting twocounter-rotating vortices when only one recirculation vortex was observed from the fielddata Observations made during model-scale experiments [14ndash17] also show little influenceon the velocity field due to wall-heating except a strengthening due to the thermallyinduced upwards motion close to the heated surface No firm conclusions can be madefrom the data presented in the literature as to the degree of influence of thermal effectswithin the vicinity of buildingsWithin the scope of the European ATREUS project (httpaixmengauthgratreus)

micro-scale numerical models were used to obtain data concerning the microclimaticconditions within the vicinity of buildings (to include the influences of solar radiation) andthen used as input data for generating typical weather data required as input for buildingand HVAC system models to study the energy budgets of buildings and the performance ofAC systems In light of the previous experiences observed in the literature usingmicroscale numerical models to simulate thermal effects it was important that the models






Gr

Re2frac14

bgH Tw T ref

U2ref

(1)




n2 (2)





ARTICLE IN PRESS












Table 1



Tw 176 79 30

T ref 24 24 24

T floor 38 28 25

T roof 50 34 25

TLS 39 29 25

TRS 40 30 25



Cand T ref frac14 5




0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)






4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur






Gr

Re2frac14

bgH Tw T ref

U2ref

(1)




n2 (2)





ARTICLE IN PRESS












Table 1



Tw 176 79 30

T ref 24 24 24

T floor 38 28 25

T roof 50 34 25

TLS 39 29 25

TRS 40 30 25



Cand T ref frac14 5




0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)






4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur





ARTICLE IN PRESS












Table 1



Tw 176 79 30

T ref 24 24 24

T floor 38 28 25

T roof 50 34 25

TLS 39 29 25

TRS 40 30 25



Cand T ref frac14 5




0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)






4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur






Table 1



Tw 176 79 30

T ref 24 24 24

T floor 38 28 25

T roof 50 34 25

TLS 39 29 25

TRS 40 30 25



Cand T ref frac14 5




0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)






4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



Cand T ref frac14 5




0

20

40

60

80

100

00 05 10 15 201

10

100

1000

000 025 050 075 100

u (ms) uwu 2 (m2s2)

0

40

80

120

160

200

000 050 100 150 200

z (m

)

Measured profile

z0 = 29m

u (ms)(a) (b) (c)






4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



4 Results















ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur












ARTICLE IN PRESS




ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur

ARTICLE IN PRESS



x=H frac14 125






08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



08

09

10

11

12

13

14

15

0 2 5 6 7


TT

ref

150405 270405

130405 310305

GrRe2 ~09 GrRe2 ~16

1 43



170

172

174

176

178

180

1 11 13 15

+010405120405

150405180505

240505310505

Tw


93 5 7




5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



5 Conclusions








Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



Acknowledgements


References







(2001) 201ndash216








351ndash364






3395ndash3404


Meteorol Z 13 (5) (2004) 381ndash386







4503ndash4521












18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur



18 (1975) 911ndash916








(1994) 405










Further reading





Introduction



Results

Conclusions

Acknowledgements

References

bm_fur

wind tunnel experiments modelling the thermal effects within the vicinity of a single block building...

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