11MSEC-0015 Experimental Analysis of Flow Past NASCAR COT Rear Wing Versus Spoiler Traveling Forward and Backwards using PIV Measurements and Flow Visualization Author, co-author list (Do NOT enter this information. It will be pulled from participant tab in MyTechZone) Affiliation (Do NOT enter this information. It will be pulled from participant tab in MyTechZone) Copyright © 2011 SAE International ABSTRACT There have been claims that the rear wing on the NASCAR Car of Tomorrow (COT) race car causes lift in the condition where the car spins during a crash and is traveling backwards down the track at a high rate of speed. When enough lift is generated, the race car can lose control and even become airborne. To address this concern, a new rear spoiler was designed by NASCAR to replace the wing and prevent this dangerous condition. This paper looks at the flow characteristics of both the rear wing and the new spoiler using particle image velocimetry (PIV) to provide both quantitative and qualitative analysis as well as hydrogen bubble flow visualization. These experiments are done in a continuous flow water tunnel having a cross section of 1.0 m2 using a simplified 12% scale model COT body with either a wing or spoiler attached. Flow structures are identified and compared for both the wing and spoiler under Reynolds number conditions between 1x105 and 3x105. We also review the same conditions when the car is traveling backwards as it might during a crash. This paper highlights the differences and similarities between the two devices, providing insights into the advantages and disadvantages of the new design. INTRODUCTION The NASCAR Car of Tomorrow (COT) is a racing vehicle designed to operate at high speeds (>300 kph) in close proximity to other racecars. It was introduced in 2007 as a completely new chassis and body design from the previous generations of cars in the racing series. Aerodynamic research played a crucial role in the design and evolution of the vehicle for both performance and safety. The wing (Figure #) was designed to provide downforce (negative lift) on the rear axle while producing a “smoother” or less turbulent wake than would the same car fitted with a spoiler. The intent of this was to allow trailing vehicles to approach in the car’s wake without losing control due to the stochastic airflow [1]. (“Stochastic” reads strange. How about “highly turbulent”?) (I think it might also be appropriate to mention the highly effective ‘Splitter’ that provides downforce at the front of the car). While the winged car met these design criteria, a safety issue arose in situations where the car was turned around backwards during high-speed travel—not an uncommon situation in this race series. When this happened, it was found that the car would generate rear lift. This lift led to loss of handling control and could even cause the vehicle to rise off of the road surface completely and fly into the air. This problem existed for previously designed racecars in the series and a solution was found and already in use. The solution consisted of devices commonly referred to as “roof flaps.” The “roof flaps”, shown in Figure #, are passive aerodynamic panels located at the rear of the roof that are hinged at the leading edge. When the car is traveling forward and travelling smoothly through somewhat ambient air, the roof flaps remain flat and do not disturb the flow. When the car is turned around backwards, the wake is a high speed flow of air being dragged along and results in a low-pressure bubble at the rear of the roof that pulls the flaps up and into the air stream. When the panels are extended, they provide a large amount of drag and spoil the lift over the car and prevent the car from lifting [2]. These roof flaps were carried over to the COT design; however, they did not function consistently on the winged vehicle. Page 1 of 11 Figure # - Parametric model of COT with rear wing attached. http://www.creativecrash.com/rhino3d/marketplace/3d- models/vehicle/cars/c/nascar-cot-stock-cars-joe-gibbs-racing-pack Figure # - Illustration of roof flap function. Nelson, G. et al [2] This roof flap image shows the air going right through the spoiler and might not be what we want. There have been many suggestions made as to why the winged COT had this safety issue; however, there is no research on the topic in the open literature. There have been two popularly made suggestions as to why this safety issue exists with the winged car, and it is unclear which, if either, is responsible. The first suggestion is that the wing is designed to create downforce when traveling forward but when facing backwards, creates lift instead. The second suggestion is that the dominant effect is the ineffectiveness of the roof flaps in deploying. NASCAR has chosen to replace the wing with a deck-mounted spoiler (Figure #) to alleviate the problem. The effectiveness of the spoiler as well as the underlying physical cause of the wing’s safety issue have yet to be fully understood. Page 2 of 11 Figure # - Parametric model of COT with deck mounted spoiler. http://www.creativecrash.com/rhino3d/marketplace/3d- models/vehicle/cars/c/cot-nascar-2010-collection NASCAR as well as independent race teams have done extensive experimental and computational testing on these vehicles, but have generally not made their results publically available. Additionally, research on these vehicles has been primarily done in wind tunnels and using CFD. While both of these methods provide useful lift and drag measurements, they do not provide the same insight as experimental flow visualization in a water tunnel. Flow visualization on these vehicles has been done using smoke sheets in the airflow stream as well as through the use of complex wake imaging systems [3]. The use of smoke visualization is limited, as it does not generally provide detailed images of coherent structures in a turbulent flow. Wake imaging systems employing pressure probes are usefulgood tools for validatingto help validate results, but still do not provide detailed flow structure visualization. CFD analysis can provide detailed flow structure images; however, accurate predictions in regions of flow separation (as in the wake) can be difficult to make and the validity of these results should always be verified with experimental results [4]. The objective of this study is to qualitatively characterize the two aerodynamic devices (rear wing and spoiler) and evaluate the effectiveness of the new design (the spoiler) using an adaptive correlation particle image velocimetry (PIV) technique in a continuous-flow water tunnel with 10% scaled models. The wing and spoiler are compared under forward- and reverse-facing vehicle conditions. Because only the flow differences between the wing and the spoiler are under investigation, a simplified car body model with a smooth underbody and no ground plane is employed. Velocity field measurements are taken on the central plane of the model in the spanwise direction. The resulting velocity fields are time averaged and the data is analyzed and discussed. While the Reynolds numbers are much lower for this study than those in the full-scale case, experiments done in other water tunnels have proven to be qualitatively accurate when vortex flows are more dominant than the viscous flow effects [5]. Data obtained from NASA’s water tunnel visualization studies on models as diverse as combat aircraft and gurney flaps have shown excellent correlation with tests in wind tunnel and real-world experiments at Reynolds numbers that are many orders of magnitude higher [5-7]. EXPERIMENTAL EQUIPMENT AND SETUP This section describes the equipment and methodology used to perform the experiments in this paper. The PIV system consists of a double-head Nd-Yag Laser employing a submersed laser light periscope, timing controlled CCD camera, synchronizer, and computer for control and post processing. The test section of the continuous-flow recirculating water tunnel is1 m deep x 1 m wide x 3 m long and fully visible through tempered glass windows. The output energy of the Nd-Yag laser is #### mJ with a double wavelength of 512 nm and 1064 nm. The CCD camera has 8-bit and has #### x #### pixel resolution. The mean thickness (front to back depth), of the vertical laser light sheet is 2 mm which is approximately 0.4% of the characteristic length (% CL) of the model. The seeding particles are silver-coated hollow glass spheres with a mean diameter of #### mocrons. The model vehicles, shown in Figure #, are thermoformed clear polycarbonate shells mounted to stainless steel bases. These 10% scale models are 510 mm x 190 mm x 150 mm (length x width x height). The models are fixed inverted in the tunnel to allow future un-submerged load cell force measurements. They are attached on the underside of the cars by two thin stainless vertical steel plates, which are only 1.5 mm thick (<0.3% CL) in the spanwise direction and therefore do not have a significant effect on the underbody flow and trailing wake. Both models have the same geometric shapeproperties; however, one model has a wing affixed to the rear deck while the other has a spoiler. On the winged car model, the wing does not include endplates that are used in the actual COT (see Figure #) because only the center plane is being Page 3 of 11 analyzed and these should not have a significant effect. This exclusion of endplates allows a more complete profile view of the wing for the PIV camera. Additionally, the chord of the rear pivoting wing on the COT is adjustable from 0 to 16 degrees from horizontal.