A Quasi-Unsteady Description of Windscreen Wiper Induced Flow Structures. A P Gaylard†, A C Wilson† and G S J Bambrook‡ Jaguar Land Rover, TASE & Weight† and Jaguar Land Rover Research‡ SYNOPSIS This paper draws on full scale wind tunnel flow visualisation and CFD simulation, carried out on a full-sized SUV geometry, to provide a description of windscreen wiper induced flow structures. The focus of this work is the effect of wipers on the local flow, rather than the more usual consideration of the aerodynamic forces exerted on the wipers. The flow structure was analysed for a series of fixed wiper arm orientations, as well as a bare windscreen (no wipers) reference case. This enabled the identification of a number of coherent vortex structures. Evidence is also presented which indicates that these structures are present in other vehicle types. Further, the vortex structures associated with the wiper blade and arm are seen to convect downstream, maintaining their coherence well onto the vehicle roof. It is suggested that during a dynamic wiper cycle these vortex structures are swept over the screen and roof. This raises the possibility of aeroacoustic sources remote from the wiper system location as well as interactions with open sunroofs.

1. INTRODUCTION

The presence of windscreen wipers is, of course, essential for the maintenance of forward vision during adverse weather conditions. It is also the case that they are aerodynamically active components, both subject to aerodynamic loading and a source of aeroacoustic noise. Initial consideration of wiper aerodynamics was mainly concerned with the forces experienced by the wiper system and ensuring that they did not degrade wiper function to an unacceptable degree [1,2]. Latterly they have been considered as an aeroacoustic source [3]; the precise mechanism of noise generation subjected to scrutiny by fundamental numerical and experimental investigations [4]. The aim of this paper is to provide an overview of the characteristic vortex structures generally associated with current wiper systems, by reference to previously published work, surface flow visualisation and numerical simulation.

2. PREVIOUS STUDIES

2.1 Review

Previous studies have looked at the important issue of maintaining an acceptable wiper performance in the presence of aerodynamic lifting forces acting on the arm and blade [5,6].

In an early study Clarke and Lumley [1] provided a comprehensive survey of the problems associated with windscreen wiper operation. Reduced scale water tank experiments (1/20th) were used to survey the flow structure over the screen of four different vehicles. Further, wiper arm pressure was measured both at full scale and on a wiper mounted in a rectangular duct. Finally, tests were carried out on a half scale test rig, representing the bonnet and windscreen of a car. The work considered the influence of vehicle geometry, along with the size and shape of the screen. Their motivation was increasing vehicle speeds and the advent of "wrap-around" windscreens. They were able to demonstrate both the typical radial flow pattern seen on windscreens and flow acceleration due to screen curvature. The use of an aerofoil to produce "anti-lift [sic] properties" was suggested, along with some other, less practical, ideas. Dawley [2], in a review of aerodynamic effects on automotive components, elucidated the flow pattern and pressure distribution over a 3/8th scale model of a two door hard top vehicle. This work again demonstrated the typical radial flow over an unobstructed windscreen. The author made the, now typical, observation that the passenger's side wiper arm tended to be parallel to the onset flow, whereas the driver's side wiper tends to be perpendicular to the onset flow, at least through a significant (perhaps considerable) portion of its travel. The use of a spoiler formed from a continuous arcuate aerofoil section was investigated. The relationship between its angle to a perpendicular onset flow and wiper lift force was investigated for a fixed speed. A "wiper blade wind tunnel" was used to generate this data; effectively a test wiper mounted in the collector of a wind tunnel. This demonstrated that a practical wiper-mounted aerofoil could be used to generate negative lift (referred to as "antilift" and now more commonly as "downforce".) Some fundamental work has been undertaken to determine the basic relationships between wiper geometry and aerodynamic forces. Barth [7], for example, provided insight into the relationship between the lift and drag forces acting on a wiper blade and the tilt angle of the blade with respect to the screen. Latterly, CFD models have been used to provide both quantitative and qualitative evaluations of wiper performance. Strumolo et al [8], for instance, constructed a CFD model of a simplified 3-box saloon that included a wiper system in the parked position and a "leaf screen cavity" (cowl). The main focus of this work was to understand the aerodynamic mechanisms underlying "water blow back" [sic]. Jallet et al [5] also provided a numerical simulation of wiper system aerodynamic behaviour. This work focused on validating the simulation technique for the prediction of both wiper drag and lift forces. A single wiper blade and arm were modeled mounted on a flat plane and perpendicular to a uniform onset flow. Two different wiper blade designs were evaluated, both with and without a spoiler. The

calculated drag and lift forces were seen to compare reasonably well with experiment (Differences of 1%-7% for drag and 2%-9% for lift with an onset flow of 40m/s). Billot et al [6] took this validated computational methodology and applied it to a wiper system at a mid-wipe position (30°), installed on a realistic car geometry. The mechanical distortion of the blade/arm under the spring load was calculated by an FE program prior to the aerodynamic simulation. The results of the aerodynamic simulation were then fed back into the FE model. Thus the authors were able to determine the effect of aerodynamic lift on the net pressure applied by the wiper blade, along its length, to the windscreen. Moving away from purely aerodynamic wiper performance considerations, Sanon and Jallet [3] examined windscreen wipers as a local aeroacoustic noise source. This was done by wind tunnel and road based test work, supported by CFD simulation. Again, realistic car geometry was used with the wipers fixed at a mid-wipe (30°) position.

2.2 Flow Topology

The work reviewed here also provides insights into the local flow structure around the wiper systems. For instance, Dawley [2] proposed a two-dimensional view of the flow structure over a wiper blade and spoiler. This comprised a recirculation in front of the blade with a region of separated flow behind it. The simulations of Stromolo et al [8] showed a time-averaged flow pattern with three recirculation regions (trapped vortices) around the wiper arm and blades.

(i) Leaf screen cavity (cowl). (ii) Upstream of the blade/arm. (iii) Downstream of the blade/arm. Taking slices through the trapped vortices (ii) and (iii) reveals a flow structure with some similarities to that suggested by Dawley [2], at least upstream of the wiper blade. It should be noted though that Dawley's two-dimensional reconstruction shows a turbulent separated zone downstream of the blade, rather than the coherent trapped vortex evident in later work. The simulations of Jallet et al [5] showed both of the trapped vortices upstream and downstream of the wiper blade/arm along with a wake. This work showed substantial lateral variation of the downstream vortex, due to interactions with flow through and over various small geometric features. The paper by Sanon and Jallet [3] contains experimental and computational flow visualisation for a mid-wipe (30°) configuration, with and without a spoiler. The downstream arm/blade vortex is clearly present along with the wake. The lateral (spanwise) variation in these structures caused by the stacked blade/arm elements is also evident.

This paper seeks to build on these observations, adding in additional data, to provide a comprehensive qualitative description of the flow structures induced by typical wiper systems. This is done by reference to surface flow visualisation obtained in a full scale wind tunnel, along with some CFD simulations.

3. WIPER LOADS AND DYNAMICS

In common with the published literature, this paper neglects the dynamic aspect of wiper performance. However, it is important to briefly review the complexities of both wiper dynamics and the time-varying aerodynamic forces that they are subjected to. The aerodynamics and mechanics of wiper systems is complex. As the wipers sweep across the windscreen the blade and arm orientation to the flow changes dynamically. Typically, on the driver's side, the wiper is almost perpendicular to the main flow through most of the sweep, as the flow over the windscreen is largely radial. Additionally, the angle of the blade to the screen surface is different on the up-sweep and down-sweep. During the upsweep, on the driver's side, the wiper moves with the prevailing flow; on the down-sweep it moves against it. Thus both the aerodynamic drag and lift forces on the wiper system (predominantly the blade and arm) vary substantially during the wiper sweep. The aerodynamic loads also vary with onset flow velocity. To sweep the blade across the screen the motor must overcome both surface friction and aerodynamic drag. Aerodynamic lift forces can tend to pull the blade away from the screen, degrading wiper performance. This force is opposed by a spring mounted in the wiper arm which pulls the arm towards the screen applying a load onto the blade. Historically, many wiper designs have included cantilevers to transfer the load from the arm out laterally along the blade. Recently, "Beam Blade" designs have become popular. These do not include cantilever elements, so the spring load is applied at the centre of the blade only. Finally, the blade is designed to conform to the contours of the windscreen as it sweeps over the surface. Thus the curvature of the wiper blade/arm system changes dynamically.

4. SIMPLIFYING ASSUMPTIONS

Having reviewed the complexities of wiper aerodynamics it is understandable that the work published to date uses a range of simplifying assumptions. These are summarised in Table 1(below). The work reported in this paper uses similar simplifying assumptions. In the experimental work, the wiper sweep is represented by considering fixed positions on the windscreen. Thus the relative movement between the wiper and onset flow is not captured. The CFD model is similarly static and does not include the change in blade angle with respect to the screen seen between the up-sweep and down-sweep. Further, the wiper geometry has been adjusted and morphed to match the screen profile, but cannot deform as a result of the aerodynamic load.

However, this quasi-unsteady investigation is based on detailed vehicle and wiper system geometry.

Reference Authors

Clarke and Lumley

Dawley Sanon and Jallet

Fischer and Zuccini

Jallet et al

Billot et al

Barth Strumolo et al

Reference Number in Text

Simplifying Assumption

[1] [2] [3] [4] [5] [6] [7] [8] Experiment/CFD Y/N Y/N Y/Y Y/Y Y/Y Y/Y Y/N N/Y Two-Dimensional N N* N N** N N N N

Fixed Wiper Position Y Y Y Y Y Y Y Y

Flat Inclined Screen (No Vehicle)

N N N Y N N N N

Flat Horizontal Screen (No Vehicle)

N Y Y N Y N Y N

Normal Onset Flow Y Y Y N Y N Y N

Part vehicle Y N N N N N N N Simplified Vehicle N N N N N N N Y

Realistic Vehicle Y N N Y/N N N Deformable Wiper Y Y Y/N Y/N Y/N Y n/a N

*Three-dimensional experiment with two-dimensional flow structure proposed **Geometry invariant laterally with finite width Test rig comprised bonnet and screen. Full scale and model scale (water tunnel) tests carried out also.

Table 1 Simplifying Assumptions Made In Previous Work.

The following sections use the results of both experiment and CFD simulation to illustrate the wiper induced flow structures seen on a full sized SUV. First, the experiments and CFD simulations are described.

5. SIMULATION

5.1 Wind Tunnel Experiments

The experiments were performed in the MIRA Full Scale Wind Tunnel. This is a closed test section, open return facility with a 4.4m (H) by 7.9m (W) test section. Its four fans are able to develop a maximum flow velocity of 130 km/h. The empty tunnel has a background turbulence intensity of 1.8% and a 200mm characteristic length [9].

The flow visualisation data presented in this paper were obtained by testing either production level vehicles or aerodynamics simulators (buck) at zero yaw with a nominal test speed of 100 km/h. Time-averaged surface flow visualisation patterns were obtained using a proprietary UV fluorescent dye suspended in white spirit, running the wind tunnel for a period of 5-10 minutes.

5.2 CFD Simulation

Further insight into the flow structures of interest is provided by CFD simulation. It is presented here as indicative flow visualisation. No local measurements are provided to indicate its reliability. However, the CFD tool that has been used (Exa PowerFLOW™) has been previously validated for both aerodynamics [10,11] and aeroacoustics simulation [12,13]. The CFD model represents a fully-detailed full sized SUV with closed-cooling intakes. Onset-flow conditions have been matched to MIRA FSWT. Around the windscreen and wipers the smallest volume element (voxel) has a characteristic dimension of 1.1mm.

Simulation Parameter Bare Screen Parked Mid-Wipe No. Volume Elements /106 42.03 108

No. Surface Elements /106 6.05 14.39 Time Step Length /106 .s 6.187 6.187 6.187 Averaging Period /s 0.528 0.387 0.187 Reynolds Number /106 4.03 4.03 4.03

Table 2 Basic Details for the CFD (PowerFLOW™) Simulations

The CFD simulations have been interrogated to provide the following insights into the flow structure.

• Surface streamlines, to facilitate direct surface flow structure comparison with the wind tunnel experiments.

• Flow streamlines, to capture the off-surface flow structures. • Contour plots of vorticity magnitude ( ×u) on a vertical plane through the roof

header, enabling the identification of regions of high vorticity. • Vortex core detection using closed iso-surfaces of the λ2 parameter. (This is

the second largest eigenvalue of the symmetric tensor S2+ΩΩΩΩ2; where S and ΩΩΩΩ are the symmetric and antisymmetric parts of the velocity gradient tensor ∇∇∇∇u. Jeong and Hussain [14] proposed this approach and demonstrated that λ2 is negative in vortex cores. A physical interpretation of λ2 is that it identifies vortex cores as pressure minima due purely to vortical motion. Therefore, in theory, λ2 < 0 implies a vortex core. However, in practice a larger negative value is required to provide meaningful detection, thus a value of -200 was selected for this work†,‡. In addition, vortex core detection was improved by using the final data frame of the calculation (averaged over 2000 time steps), rather than an average over the complete simulation period.)

† Freed, D. M. Letter to author. Dated 12th July 2006. ‡ Zaehring, E., Letter to author. Dated 12th July 2006.

6. FLOW STRUCTURES

6.1 Bare Windscreen – No Wipers.

In the absence of the disruption caused by the presence of the windscreen wipers the flow over a typical automotive windscreen is largely radial. This can be clearly seen in the surface flow visualisation presented in Figure 1. The lower part of the screen also shows the influence of a trapped lateral (“cowl”) vortex. The shear layer reattachment line is clearly visible, along with the reversed flow region associated with this trapped vortex.

Figure 1 Surface Flow Visualisation On A Bare Windscreen.

The CFD simulation shows the same surface flow features (Figure 2). Some of the additional field data available in the numerical simulation is presented in Figures 3 and 4. Both of these show the vorticity present in the plane of the roof header. Vorticity in both the boundary layer and a-pillar vortex is evident.

Figure 2 Surface Streamlines On A Bare Windscreen.

Shear layer reattachment line

Figure 4 attempts to provide detection of the vortex cores present in the flow via iso-surfaces of λ2. Multiple vortex cores are evident within the cowl region. A small trapped vortex is also seen at the front of the roof header, caused by the forward facing step formed by the windscreen glazing inset. This metric also seems to be sensitive to vorticity in the a-pillar and roof header boundary layers.

Figure 3 Flow Streamlines (Blue: Cowl Vortex; Red: Reattaching Shear Layer

And Attached Flow) On A Bare Windscreen.

Figure 4 Iso-Surfaces Of λλλλ2 On A Bare Windscreen.

6.2 Parked Wipers

Introducing windscreen wipers, in their parked position, clearly disrupts a region of the previously radial flow (Figure 5). The surface flow visualisation clearly shows a

region of separated flow consistent with the blade/arm wake. However, the reduced energy in the flow downstream of the arm/blade has reduced the effectiveness of the flow visualisation technique.

Figure 5 Surface Flow Visualisation On A Windscreen With Parked Wipers.

More detailed insights into the flow structure are provided by the CFD simulation. Figure 6 shows a near-wiper flow structure similar to that described by Stromolo et al [8], i.e. a cowl vortex along with vortices upstream and downstream of the wiper blade/arm, the latter forming the wiper/arm wake.

Figure 6 Surface Streamlines On A Windscreen With Parked Wipers.

The flow streamlines shown in Figure 7 show some fluid being drawn from behind the wiper arm by entrainment into the cowl vortex. Some disruption of the vortices upstream of the blade and arm by the lower arm on the driver's wiper system (B) is also evident. The vortex core plot (Figure 8) confirms this assessment.

Figure 7 Flow Streamlines Over A Windscreen With Parked Wipers. (Blue: Cowl And Windward Wiper Vortex; Green: Leeside Wiper Vortex; Red: Reattaching Shear Layer And Attached Flow).

Figure 8 Iso-Surfaces Of λλλλ2 Over A Windscreen With Parked Wipers.

6.3 Mid-Wipe

Moving the wipers to a mid-wipe position changes the orientation and position of some of the vortex structures previously identified. Figure 9 shows that the vortices on either side of the wiper blade/arm now convect downstream over the roof header (A). The vortex upstream of the driver's side wiper arm/blade is very clearly seen (ii).

B

B

Figure 9 Surface Flow Visualisation On A Windscreen With Wipers At The Mid-

Wipe Position.

The arm/blade vortices, (ii) and (iii) clearly convect downstream off the screen and over the roof header (A). This could give rise to an aeroacoustic noise source relatively remote from the wiper system.

Figure 10 Surface Streamlines On A Windscreen With Wipers At The Mid-Wipe

Position.

Figure 10 provides a very clear visualisation of the cowl vortex (i) and downstream wiper blade/arm vortices (iii). (However, the upstream vortex (ii) is not seen in the simulation, likely due to limitations imposed by the spatial resolution of the CFD model.) The contributions of the wiper knuckle (C) and blade/arm gap (D) to the driver's side wiper system wake are also evident. The downstream convection of the arm/blade vortices is shown very clearly in Figures 11 and 12. Vorticity peaks are seen over the roof header that correlate with the rotating fluid shown by the flow streamlines and the vortex cores highlighted by the λ2 plot. An additional (though less coherent) vortex structure is generated by the driver's side knuckle (iv).

(iii)

(i)

D

C

A A

(ii)

A

B

C

D

A

(iii)

Figure 11 Flow Streamlines Over A Windscreen With Wipers At The Mid-Wipe Position. (Blue: Cowl And Windward Wiper Vortex; Green: Leeside Wiper Vortex; Red: Reattaching Shear Layer And Attached Flow).

Figure 12 Iso-Surfaces Of λλλλ2 Over A Windscreen With Wipers At Mid-Wipe

(Zero Degrees Yaw).

6.4 Complete Sweep

Having explored the relevant flow structures in some detail experimental, surface flow visualisation is presented in for five fixed positions representing a complete wipe cycle is presented in Figure 13. (If wiper movement were considered this would only represent a half cycle.)

(iii) (ii)

(iii) (ii)

(i) (i)

(iv)

C

D

A A

The individual images follow the sequence below. (a) Parked. (b) 30 degrees. (c) Mid-wipe. (d) Passenger's side

wiper at 90 degrees. (e) End of wipe. As has been frequently observed, the angle of the passenger's side wiper system to the local flow results in relatively little disruption of the flow over the screen. However, vortices still form parallel to the blade and arm on the upstream and downstream sides. The lower arm (close to the spindle) also provides a limited amount of local flow distrubance. The driver's side arm and blade spend much of the sweep cycle nearly perpendicular to the local onset flow. Hence they perturb the flow more strongly. Coherent upstream and downstream vortices form along the blade/arm which convect downstream. A quasi-unsteady interpretation of these images suggests that the wiper blade/arm vortices, (ii) and (iii), persist over most of the wipe cycle and, convecting downstream, are repeatedly swept over the screen and roof. In addtion, for part of the wipe the driver's wiper arm knuckle generates a wake

(a)

(b)

(c)

(d)

(e)

Figure 13 Five Fixed Wiper Positions

vortex (iv) which is swept over the screen and roof. Finally, the cowl vortex (i) remains in a fixed postion, though when the wiper system approaches the parked position there is interaction between the cowl vortex and wiper arm/blade vortices.

7 OBSERVATIONS FOR OTHER VEHICLES

The flow structures described for this particular SUV geometry are commonly seen for other, very different, vehicles. Figure 14, for example, shows the wiper-induced flow structure for a luxury saloon (a) and sports car (b). These observations indicate that the description presented is reasonably general. In particular, the convection of the arm/blade vortices over the roof header onto the roof is very clear in both cases. In the case of the luxury saloon (a) they pass over the closed sunroof opening, raising the possibility of an interaction between these vortices and open sunroofs. Of course, the relative size and strength of the structures may well vary with screen rake and wiper system design.

Figure 14 Surface Flow Visualisation On The Windscreen Of A Luxury Saloon

(a) And Sports Car (b), With The Wipers At Mid-Wipe.

8 CONCLUSIONS

A description of the flow topology associated with windscreen wiper systems has been proposed. Although there are many small features associated with the geometric complexity of these systems, large coherent vortex structures can be identified. The main elements in the flow structure are:

(i) cowl vortex; (ii) upstream wiper arm/blade vortex; (iii) downstream wiper arm/blade vortex; (iv) driver's side knuckle wake vortex. It is asserted that (ii) and (iii) are swept laterally across the screen and roof during wipe cycles, with (iv) being present for a part thereof. However, the authors freely

(a) (b)

acknowledge the limitations of this work. In particular the use of static wiper positions in an attempt to understand a dynamic process entails a degree of risk. Finally, one of the more interesting features of this work has been the identification of the extent to which the wiper arm/blade vortices convect downstream. This raises the possibility of an aeroacoustic noise source relatively remote from the wiper system. Further, the possibility exists of an Interaction between these vortices and sunroof openings. ACKNOWLEDGEMENTS The authors would like to thank Jaguar and Land Rover for permission to publish this paper and their colleagues for many helpful suggestions. REFERENCES 1 Clarke, J. S. and Lumley, R. R. Problems Associated With Windscreen

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