International Journal of Automotive Technology, Vol. 17, No. 2, pp. 281−288 (2016) Copyright © 2016 KSAE/ 089−10 DOI 10.1007/s12239−016−0028−7 pISSN 1229−9138/ eISSN 1976−3832 EXPERIMENTAL STUDY COMPARING RACECAR AERODYNAMIC DOWNFORCE-GENERATING DEVICES USING SCALE MODEL NASCAR COT S. HELLMAN1), M. UDDIN2, 3)*, P. T. TKACIK2, 3) and S. D. KELLY3) 1)Dantec Dynamics Inc., Holtsville, NY 11742, USA 2)North Carolina Motorsports and Automotive Research Center, UNC Charlotte, Charlotte, NC 28223, USA 3)Department of Mechanical Enginnering and Engineering Science, UNC Charlotte, Charlotte, NC 28223, USA (Received 4 December 2014; Revised 19 May 2015; Accepted 18 September 2015) ABSTRACT−The performance and safety of the rear wing and spoiler employed on the National Association of Stock Car Auto Racing (NASCAR) COT (car of tomorrow) racecar are experimentally studied using 10 % scale models in a water channel. Particle image velocimetry is used to qualitatively examine the differences in flow structures between the two downforce-generating devices under 0 and 180-degree yaw cases. The latter is important due to an issue with the COT flipping into the air when at extreme yaw (i.e. during a crash). At zero yaw, it is observed that smaller length scales of the flow structures in the wake of the wing compared to those in the wake of the spoiler, provide more predictable handling for racecars in close proximity and may allow more safe and competitive racing. At 180-degree yaw, it is observed that wake-structure interactions may not allow proper operation of anti-flipping devices (roof flaps) on the winged car. In the extreme yaw case, local flow scales are examined and show much stronger Reynolds number (Re) dependence for the wing than the spoiler. KEY WORDS : NASCAR, Wing, Spoiler, Wake, Particle image velocimetry (PIV), Experiment, Aerodynamics, Water tunnel NOMENCLATURE car used in the series and improve upon safety, performance, competition and cost efficiency for the teams D : length scale (NASCAR, 2006). The COT must drive at high speeds h : height (m) (over 300 km/h) and in close proximity to other vehicles Re : reynolds number which made aerodynamics play a large part in its design U : freestream longitudinal velocity (m/s) and development. Along with an entirely new chassis and w : wheel base body, the COT implemented two new aerodynamic υ : kinematic viscosity (m2/s) features which included a front, bumper-mounted splitter and a rear, deck-mounted wing, whereas the previous SUBSCRIPTS vehicle (popularly known as the Car of Yesterday (COY)) employed a rear deck-mounted spoiler only. Both of these L : local new features were intended to generate downforce. While ∞ : freestream condition the COT successfully achieved most of its design goals, it was found to have a serious safety issue under extreme yaw 1. INTRODUCTION (near 180º). When turned around backwards as it might during a crash, the COT was prone to generating lift which The The National Association of Stock Car Auto Racing could cause the car to rise off the ground and flip. This (NASCAR) Sprint Cup series is a widely viewed form of problem had existed in the COY, and a robust device automobile racing in the US with a fan base of known as a roof flap was already put in place to counteract approximately 75 million. After a number of dangerous it. Roof flaps, shown in Figure 1, are passive aerodynamic and even fatal accidents, NASCAR decided to design a devices that consist of flat plates located at the rear of the new racing vehicle which culminated in the COT (car of vehicle’s roof which are hinged at the front edge. Under tomorrow) in 2007. It was designed by NASCAR’s normal driving conditions, the flow over the roof keeps the Research and Development Center to replace the previous roof flaps down; however, when turned at extreme yaw, low pressure is intended to pull them up into the oncoming *Corresponding author. e-mail: [email protected] air. When extended into the air, the roof-flaps increase drag 281.