Exp Fluids (2008) 45:537–546 DOI 10.1007/s00348-008-0499-z
RESEARCH ARTICLE
Experimental investigation into wing span and angle-of-attack effects on sub-scale race car wing/wheel interaction aerodynamics S. Diasinos Æ A. Gatto
Received: 29 October 2007 / Revised: 9 March 2008 / Accepted: 22 March 2008 / Published online: 13 April 2008 Ó Springer-Verlag 2008
Abstract This paper details a quantitative 3D investigation using LDA into the interaction aerodynamics on a subscale open wheel race car inverted front wing and wheel. Of primary importance to this study was the influence of changing wing angle of attack and span on the resulting near-field and far-field flow characteristics. Results obtained showed that both variables do have a significant influence on the resultant flow-field, particularly on wing vortex and wheel wake development and propagation.
1 Introduction Current open wheel race car performance depends heavily on the effectiveness and efficiency of its aerodynamics. Appendages such as inverted wings, diffusers, barge boards and splitter plates are currently used to enhance performance, but the physics and mechanisms through which this is achieved, particularly via various component interactions, remains poorly understood. From a design perspective, the ultimate goal of the designer is to contribute to the downforce produced by a race car while minimizing drag. This enhances the mechanical grip of the tires on the track, allowing the vehicle to corner at speeds which would otherwise be impossible. On modern open S. Diasinos Toyota F1, Koln, Germany e-mail:
[email protected] A. Gatto (&) Department of Mechanical Engineering, School of Engineering and Design, Brunel University, UB8 3PH Uxbridge, England e-mail:
[email protected]
wheeled race cars, approximately 30% of the total downforce produced originates from the front wing (Metz 1987; Dominy 1992). However, the close proximity of the front wheels to the front wing can have a significant influence on this performance metric (Katz 2006; Agathangelou and Gascoyne 1998). Surprisingly, very little information relating to open wheel/inverted wing interaction exists in the open source literature. This is most probably due to the sensitive nature of this information in the extremely competitive and lucrative businesses at the pinnacle of motor racing, particularly Formula 1. There are however, an ever increasing number of investigations appearing in the literature concerned specifically with investigating an inverted wing or wheel in isolation (Zerihan and Zhang 2000; Katz 1986; McManus and Zhang 2006). Two such investigations for the isolated wing case were conducted recently by Zerihan and Zhang (2000) and Ranzenbach and Barlow (1996). Independently, they both found that a ‘‘downforce loss phenomenon’’ can occur with a small, subsequent reduction beyond a critical ride height. This was later found to be fundamentally caused by the development of severe adverse pressure gradients on the wing undersurface as the ride height was reduced. Zerihan and Zhang (2003) followed up this work with an additional 3D experimental analysis, using LDA, to determine the effect of varying ride height and front wing angle of attack (AOA) on the overall wake structure, together with the formation and displacement of tip vortices. Results from this analysis showed that as the wing approached the ground, the region of separated flow and the size of the wake increased, moving towards the ground plane as it travelled downstream. With subsequent further reductions in ride height, the vortex strength was found to increase up until a critical ride height at which a loss of downforce, and consequently wing tip vortex strength, occurred.
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As one of the first investigations to consider the realistic flow conditions over an isolated rotating wheel, Fackell and Harvey (1975) published both qualitative and quantitative experimental results using a moving ground at a Reynolds Number (based on wheel diameter) of 5.3 9 105. In this work, it was demonstrated that spinning the wheel and having it in contact with the ground is of vital importance to obtain appropriate lift and drag values. In previous studies this was not, necessarily, considered critical (Stapleford and Carr 1970). Perhaps, the most striking of the results presented by Fackrell (1974) was the detection of surface pressure coefficients greater than two in the upstream region of the contact patch (i.e the cross-sectional area of the wheel in contact with the ground) due primarily to the effects of viscous jetting (Fackrell 1974). This jetting effect was found to be a bi-product of the interaction between the oncoming airstream and the wheel/road juncture, producing an effective and significant ‘‘pumping’’ of the air out into the freestream from either side of the wheel. Considering the specific case of a front wing and wheel operating in close proximity, Thisse (2004) conducted a small number of computational investigations using FluentÓ on differing endplate designs with the front wheel included. The analysis was compiled at fixed wing span, chord, ride height, and AOA with results obtained indicating as much as a 36% reduction of wing downforce with the wheel present over the isolated wing case. This was thought to be due to lower wing surface exposure to the high pressure region formed forward of the wheel contact patch. Curiously, results also showed that the flow separation position on the wheel occurred aft of the position of maximum wheel height with the wing installed. Mortel (2003) and Cumming (2002) also presented results for the wing/wheel interaction case with the former investigating improved endplate designs and the later, the effect of wing sweep. Unfortunately, neither offered any great insight into the aerodynamic interaction of a wing and wheel, but they did indicate that the inclusion of the wheel made the flow over the wing significantly more three-dimensional.
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position of the wing surface was also used as a reference for ride height which was defined as the distance from this position to the ground plane. For this investigation, ride height was fixed at h = 0.13 c. The overall wheel diameter and width were 1.17 and 0.631 c, respectively, with both wheel edges incorporating shoulder radii of 0.067 c to more accurately depict a real tire cross-section. The minimum distance between the endplate and the front of the wheel was maintained at 0.087 c for all cases investigated, with the wing span adjustable from O = 0–100% overlap of the wheel width. The range of AOA investigated was from 0° to 12° with increments of 4°. A picture of the setup used is shown in Fig. 1. 2.2 Experimental setup All experimental results were obtained using a 225 mm by 340 mm open circuit, closed test section, wind tunnel. A moving ground was also built and installed in the wind tunnel with a belt span of 200 mm (93% of the tunnel width) and an overall length of 990 mm. A purpose built duct, located upstream of the belt (Fig. 2a), was used to remove the boundary layer formed over the tunnel floor to accurately simulate moving model conditions. An example of the velocity profile at a location -2 c upstream of the model station, under operating conditions without the model installed, is shown in Fig. 3. The nominal freestream turbulence level in the wind tunnel was measured at 0.15%, with all experiments conducted at a freestream velocity (Vo) of 10 ms-1 ± 0.05 ms-1. This gave a test Reynolds number, based on wing chord, of 5.11 9 104. The wing, wheel, and endplate used for all experimental testing was manufactured from polished acrylic to ensure a smooth, transparent surface (Fig. 1). The wheel was mounted in position via a faired sting connected to the test section wall and incorporated deep groove ball bearings to allow free rotation from contact with the moving ground
2 Experimental setup and apparatus 2.1 Inverted wing and wheel geometry For all experimental investigations, the wing comprised a single element, constant chord (c = 0.075 m), NACA 4412 aerofoil profile. A rectangular endplate with a rounded leading edge and a tapered trailing edge was fitted at the wing tip. For all AOA conditions considered, the base of the endplate was fixed at a position 0.04c below the lowest vertical position of the wing surface. The lowest vertical
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Fig. 1 Inverted wing and wheel setup
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Fig. 2 a Overall layout of test rig, b LDA measurement planes
belt. The scale of the experimental wind tunnel model was approximately 1:7.5. The support system for the wing/endplate combination allowed an AOA adjustment resolution of ±0.05°, with the span of the wing, wheel track, ride height of the endplate, and distance between the endplate and the wheel all positioned to better than ±0.1 mm. Under experimental test conditions, the belt of the moving ground, at no stage, was observed to lift up and was operated at 10 ms-1± 0.2 ms-1. The maximum test section blockage for the wing/wheel combination was calculated at 9.5%. At this level, it is envisaged that blockage effects will cause a certain degree of flow constrainment (i.e reduced vortex expansion and
artifical delay of flow separation) on the flowfield. It should be noted, however, that the main focus of this work is considered comparative, thereby reducing somewhat the significance of blockage effects on the overall trends and correlated results. To obtain all experimental data, LDA surveys were performed over four planes located, between, and aft of the wing and wheel combination. As indicated in Fig. 2b, the location of these planes were referenced to the wheel center plane and staggered at x = -0.63 c, x = 0.75 c, x = 1.50 c, and x = 3.00 c, respectively. All LDA results were taken in an area of interest 1.63 c high by 2.1 c wide referenced 2 mm from the left test section wall (Fig. 1) and
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Fig. 3 Velocity profile -2 c upstream of wheel centreline; empty test section, mid-span
1 mm off the belt surface. LDA measurements within these planes, were taken at approximately 400 points within these boundaries, with measurement points selected to be more densely packed in close proximity to the wing and wheel. For the plane located at x = -0.63 c, access of the LDA beams was inhibited by the model, allowing therefore, only partial data collection at this measurement station. The 3D Dantec LDA system used to measure all velocities comprised a 5 W Ar–Ion laser and was configured to operate in coincident, backscatter mode. The LDA probes were mounted on a computer controlled traverse which was adjustable to within 0.01 mm over identical translation ranges in the X, Y, and Z directions of 1,010 mm. Procedures outlined by Benedict and Gould (1996) were used to estimate the 95% confidence interval for the LDA results with the accuracy of non-dimensional velocity magnitude found to be better than ±0.03. A single, Laskin type atomizer, using vegetable oil to generate seeding particles, was incorporated into the test setup to seed the flow and obtain all velocity data. At each measurement point analyzed, within each measurement plane, the final value for the three components of velocity obtained was acquired from averaging more than 2,000 instantaneous samples.
3 Results and discussion 3.1 Wheel without wing, comparison of stationary and rotating wheel Results obtained from the rotating and stationary isolated wheel cases are presented in Fig. 4. As with all following results, to aid in both qualitative and quantitative analyses, both constant non-dimensional streamwise velocity
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iso-contours (Vx/Vo) as well as the in-plane flow direction and relative magnitude (arrows indicate Vyz) are presented. On first inspection, the two main wheel vortex structures found in previous investigations are clearly evident in all but the foremost measurement plane (Fackrell 1974; Axon 1998). These vortices are setup in the recirculation zone behind the wheel from flow entrained from the freestream (McManus and Zhang 2006; Zhang et al. 2006). Clearly visible also at the measurement plane X = -0.63, is a stagnation region located at approximately mid-height for both wheel configurations. Flow physics dictate that flow stagnation also occur at the junction of the wheel/road interface (i.e the contact patch), however the proximity of the measurement plane to this region gives very little indication of its existence. The high pressure region generated at this location is known to accelerate and eject the flow cross-stream, where a complex interaction with the freestream occurs around the wheel edge (McManus and Zhang 2006). Considering more closely the development of the wheel vortices, from Fig. 4c–h, the general trend is for the vortex size and separation distance to increase with further propagation downstream. The wheel vortex separation distance is particularly evident when comparing Fig. 4c (0.9 \ Y \ 1.55) to Fig. 4g (0.75 \ Y \ 1.9) and Fig. 4d (0.85 \ Y \ 1.65) to Fig. 4h (0.5 \ Y \ 1.9). These results show that through rotating the wheel, decreases in wheel vortex separation distance by up to 20% (at X = 3) can be achieved over the stataionry wheel configuration. Also clear from comparing Fig. 4c, e to d, f is that the general wheel vortex shape appears significantly ‘‘flatter’’’ in the non-rotating case than that observed for the rotating configuration. This result has been inferred by past publications (Fackell and Harvey 1975; McManus and Zhang 2006) and is thought to result from the differing amounts of the viscous jetting flow and the different flow separation positions between the two configurations. Further comparisons between the rotating and stationary cases in Fig. 4 also show subtle differences in the complex near wake structure directly behind the wheel (Fig. 4c, d). First, the results indicate that the wheel wake for the rotating case is thinner and marginally higher (0.95 \ Y \ 1.6, Z = 1.5) than that found for the stationary configuration (0.8 \ Y \ 1.65, Z = 1.4), indicating delayed separation over the top of the wheel. This finding is further supported by differences found in the degree of flow entrainment down the centre of the rear of the wheel at this measurement location. From direct comparison of the in-plane velocity results between the two cases, velocity magnitudes of up to 10% greater where found for the stationary case. Supplimental to this result is that this increased degree of flow entraiment is seen to extend closer to the ground plane for this case (indicated in Fig. 4d). In previous work, (Fackell and Harvey
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Fig. 4 Comparison of stationary and rotating isolated wheel results
1975; McManus and Zhang 2006), this increased entrainment for the stationary case was found to be due to the decelerating flow on the rear surface of the wheel being energized by the entrained main streamwise flow around the sides. This promotes the central region of attached flow as it
is directed down the rear face of the wheel and toward the ground. Additionally, this characteristic of flowfield, and the subsequent bi-furcation of the flow as proximity to the ground increases, clearly implies the existence of a downstream stagnation line on the ground plane (Fig. 4f).
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3.2 Wheel and wing, wing span 100% From Figs. 5 and 6, it is immediately apparent that the flow characteristics have become much more complex with the addition of the front wing at full span (silhouette indicated). This is particularly the case in the flowfield directly behind and around the wheel boundaries. In this region, the overall flowfield is clearly more asymmetric with an additional primary wing vortex clearly visible at the wingtip (i.e Fig. 5g at Y = 1.4, Z = 0.15). This vortex is generated via a pressure differential setup around the wing tip/endplate structure through wing undersurface suction (Zerihan and Zhang 2003). This vortex rotates in an anti-clockwise direction and when comparing Fig. 5e, g, as expected, clearly becomes larger with increasing angle of attack. Together with these characteristics, increasing wing angle of attack also produces more significant crossflow across the wheel face as can be most vividly shown when comparing Fig. 5a, g. For these two particular cases, a change in angle of attack of 12° has increased the local in-plane nondimensional velocity magnitude Vyz/Vo, from essentially zero at Y = 1.25, Z & 0 to Vyz/Vo = 0.4. As the angle of attack is reduced however, the ability of this vortex to promote crossflow velocity and overcome the high pressure region within the contact patch clearly diminishes with Vyz/Vo at Y = 1.25, Z & 0 for AOA = 4° and 8° being Vyz/Vo & 0 and Vyz/Vo = 0.1, respectively. The primary vortex, under these conditions, would posses an enhanced ability to move around the outside of the wheel rather than along the front face, and finally, on its inside. Further to the previous discussion, increasing the AOA of the wing up to 12° is seen to fundamentally shift the aerodynamics in the vicinity of the contact patch. At these extreme conditions, the enhanced crossflow generated creates conditions where the initially symmetric viscous jetting (Fig. 4a, c) becomes highly asymmetric (Fig. 5c–h). The primary effect of this resulting asymmetry is seen both at this measurement station (X = 0.75) as well as further downstream. From comparing Figs. 6 to 4, a significantly increased degree of wake asymmetry is also clearly evident. The resultant wake, with the wing installed, leads to a general bias toward the inside of the wheel, entraining the flow to this side, and ultimately setting up the predominant, final anti-clockwise flow rotation condition as shown in Fig. 6b, d, f, h. In Fig. 6b, d, f, h, the flowfield has become much less distinct to individual flow characteristics, effectively being now, only a mixture of the more specific flow features shown upstream. Another interesting aspect of note with the addition of the front wing is the movement of the stagnation position at mid-wheel height as angle of attack is increased. From an AOA = 0°–12°, the height of the stagnation position was found to move from Z = 0.5 to Z = 0.55. Adjacent to this
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region, at mid-wheel height, evidence also exists for a substantial degree of flow acceleration (Vyz/Vo = 0.7–0.9) over the top of the endplate and into the mainstream flow along the outside of the wheel (Fig. 5a, c, e, g). This flow characteristic, which intensifies with AOA increase, is also prominent when the AOA = 0o (Fig. 5a), which would be expected due to the proximity of the stagnation region at mid-wheel height. On further comparison of results from Figs. 4 and 5, the wake behind the wheel with the addition of the front wing was also found to be significantly more compact than that shown for the isolated wheel case (both stationary and rotating) shown in Fig. 4. Additionally, the wake height with the addition of the front wing seems to have reduced slightly to approximately the top of the wheel between 1.2 \ Z \ 1.3 (Fig. 5b, d, f, h) when compared to both wheel alone cases (1.4 \ Z \ 1.5). In this instance, the wing clearly assists the flow in overcoming the obstacle of the wheel thereby tending to reduce the likelihood of premature flow separation over the top of the wheel and the subsequent higher wheel wake. As can be seen in Fig. 5b, d, f, h, this decrease in wake height with the wing installed was also found to reduce the flow velocity entrained to the floor by upwards of 30% compared to the isolated wheel cases. 3.3 Wheel and wing, wing span 0% Results for the wing/wheel combination as the front wing angle of attack is increased for the minimum span condition are shown in Fig. 7. At first inspection, there exists a much larger vortex located at Y = 0.5, Z = 0.2 (Fig. 7c) which travels on the inside of the wheel and towards the wind tunnel wall (symmetry plane). This larger vortex, which is now augmented by the high pressure region of the contact patch, is also shown to translate vertically upwards from Z = 0.2 to Z = 0.3 (Fig. 7d) with change in wing AOA. This result was thought to be due mainly to the added contribution of flow upwash with increase in wing AOA. Additionally, as can be seen from Fig. 7e–h, this flow feature also remains one of the dominant flow features for both angles of attack, and together with the left wheel vortex, produces a general clock-wise rotation on the overall downstream wake field (Fig. 7g, h). This final result is in direct opposition to the result found at the same measurement locations for the full span condition shown in Figs. 5 and 6. Another interesting contrast with the results presented for the longest wing span tested, is that for the shortest wing span configuration, for the largest angle of attack, there is a clear absence of any dominant crossflow within the contact patch region (Figs. 5g, 7b). Additionally, the mid-wheel height stagnation position remains relatively unaffected with AOA change at this decreased wing span
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Fig. 5 Comparison of results for maximum wing span; X = -0.63 and X = 0.75
condition. However, the acceleration of the flow over the end plate clearly remains, albeit, now moving inboard instead of outboard as described in Sect. 2. Interestingly, for the short wing span, the maximum in-plane velocity magnitude within this region was found to be measurably smaller with Vyz/Vo = 0.8 at AOA = 12° compared to Vyz/Vo = 0.9 found in Fig. 5. Under these conditions, the high pressure regions at both the mid-height of the wheel
and on the upper wing surface now tend to react against each other, leading to lower flow velocities.
4 Conclusion An investigation into the effect of changing wing span and angle of attack on the interaction aerodynamics of a sub-
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Fig. 6 Comparison of results for maximum wing span; X = 1.5 and X = 3
scale inverted wing and wheel, in ground effect, is presented. Experimental results obtained using 3D LDA are presented and were used to show various component
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interaction aerodynamic phenomenon in an attempt to broaden understanding of this complex interaction. It was demonstrated that when the wing span is adjusted across
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Fig. 7 Comparison of results for minimum wing span
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the width of a wheel at different AOA, the primary wing vortex plays a significant role in the resulting flow physics. Primarily, at low wing spans the primary wing vortex travels on the inside of the wheel producing a complex asymmetric wake structure with general rotational characteristics in the clockwise direction when viewed from in front of the combination. At larger spans the reverse was found. The primary mechanism for changing wing AOA was to change the size, strength and degree of movement of the tip vortex generated from the main wing element. It is hoped that while the conditions specified in this paper are specific and sub-scale, there are clearly defined trends and characteristics in the results presented that may give the designer an initial, useful, and macroscopic insight into this highly complex flow phenomenon.
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