CFD BASED ANALYSES OF THE EFFECTS OF 2017 NASCAR XFINITY RACING SERIES AERODYNAMIC RULE CHANGES by Nazarii Olkhovskyi A thesis submitted to the faculty of The University of North Carolina at Charlotte in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Charlotte 2019 Approved by: ____________________________ Dr. Mesbah Uddin ____________________________ Dr. Jerre Hill ____________________________ Dr. Peter Tkacik ii ©2019 Nazarii Olkhovskyi ALL RIGHTS RESERVED iii ABSTRACT NAZARII OLKHOVSKYI. CFD Based Analyses of the Effects of 2017 NASCAR Xfinity Racing Series Aerodynamic Rule Changes. (Under the direction of DR. MESBAH UDDIN) Xfinity racing series is an American stock car racing series organized by NASCAR. For the 2017 racing season, NASCAR introduced new regulations for this series in order to decrease aerodynamic influence on vehicle performance. Using only open-source Computational Fluid Dynamics (CFD) tools this work describes the differences in the aerodynamic performance between the 2016 and 2017 TOYOTA Camry Xfinity race-cars. During the CFD validation process, it was observed that none of the standard turbulence models, with default turbulence model closure coefficients, were able to provide the race-car aerodynamic characteristics prediction with an acceptable accuracy as compared to experiments, and that a slight modification to closure coefficient values was required. This work demonstrates that it is possible to generate CFD predictions which are highly correlated with the experiential measurements by modifying the turbulence model closure coefficients, and it is possible to tune the standard k-ω SST turbulence model. This work presents the differences between the flow fields around a pre- and post-2017 race-car models using the high fidelity CFD simulations was obtained. iv ACKNOWLEDGEMENTS I would like to thank my thesis advisor Dr. Mesbah Uddin, Director of the NC Motorsports and Automotive Research Center at the University of North Carolina Charlotte. He was always ready to help whenever I had questions about my research or writing. I felt his support and respect for my ideas and desire to point me to the right direction when it was needed. I would also like to thank the members of my thesis committee, Dr. Peter Tkacik and Dr. Jerre M. Hill, for their involvement and essential input in the evaluation of my research. My gratitude goes to Joe Gibbs Racing and Toyota Racing Development for providing research facility, data and costly resources that were necessary for the completion of this work. At last, I would like to express my gratitude to my family for supporting me along the way, encouraging throughout the years of my study, providing help and valuable feedback, which was very important to me. This work would not have been made without them. v TABLE OF CONTENTS ABSTRACT iii ACKNOWLEDGEMENTS iv TABLE OF CONTENTS v LIST OF TABLES vi LIST OF FIGURES vii LIST OF ABBREVIATIONS ix CHAPTER 1: INTRODUCTION 1 1.1 Objective 4 CHAPTER 2: METHODS 6 2.1 Theory 8 CHAPTER 3: GEOMETRY 10 3.1. Mesh 11 3.2. Boundary Conditions 16 3.3. Simulation 16 3.4. Settings 17 CHAPTER 4: RESULTS AND DISCUSSION 18 CHAPTER 5: CONCLUSION 33 REFERENCES 35 vi LIST OF TABLES TABLE 1: Standard k-ω -SST model CFD results vs wind tunnel results. 19 TABLE 2: Modified k-ω SST model CFD results vs wind tunnel results. 20 TABLE 3: Mesh sensitivity. 20 TABLE 4: Force coefficients at ride height RH2. 21 TABLE 5: Force coefficients at ride height RH2. 22 vii LIST OF FIGURES FIGURE 1: 2016 TOYOTA Camry NASCAR Xfinity race car. 2 FIGURE 2: 2017 TOYOTA Camry NASCAR Xfinity race car. 2 FIGURE 3: 2016 and 2017 Rear spoiler difference. 3 FIGURE 4: 2016 and 2017 Front splitter difference. 3 FIGURE 5: 2016 and 2017 Front splitter height difference. 4 FIGURE 6: NASCAR Xfinity racing series Toyota Camry. 10 FIGURE 7: Computational domain. 11 FIGURE 8: Volume refinement regions. 12 FIGURE 9: Surface mesh. 13 FIGURE 10: Volume mesh plot slice through y-z plane. 14 FIGURE 11: Volume mesh plot slice through x-y plane. 14 FIGURE 12: Volume mesh plot slice through x-z plane. 15 FIGURE 13: Splitter volume mesh plot slice through x-z plane. 16 FIGURE 14: Wall shear stress and oil flow lines (a1=0.31). 18 FIGURE 15: Wall shear stress and oil flow lines (a1=0.5). 19 FIGURE 16: Difference between 2016 and 2017 aerodynamic rule packages. 22 FIGURE 17: Drag coefficient difference contribution by parts. 23 FIGURE 18: Lift coefficient difference contribution by parts. 23 FIGURE 19: Pressure coefficient distribution over 2016 Toyota Camry. 24 FIGURE 20: Pressure coefficient distribution over 2017 Toyota Camry. 24 FIGURE 21: Pressure sensor array underneath the car. 25 FIGURE 22: Pressure sensor array on top of the deck lid. 25 viii FIGURE 23: Pressure sensor data. 26 FIGURE 24: Pressure coefficient distribution under 2016 Toyota Camry. 27 FIGURE 25: Pressure coefficient distribution under 2017 Toyota Camry. 27 FIGURE 26: Splitter centerline pressure. 28 FIGURE 27: Pressure coefficient distribution under 2016 splitter. 29 FIGURE 28: Pressure coefficient distribution under 2017 splitter. 29 FIGURE 29: Center jet under 2016 Toyota Camry at z=h/2. 30 FIGURE 30: Center jet under 2017 Toyota Camry at z=h/2. 30 FIGURE 31: Jet centerline velocity magnitude at z=h/2. 31 FIGURE 32: Pole lap speed by track. 32 ix LIST OF ABBREVIATIONS CAD Computer-Aided Design CFD Computational Fluid Dynamics JGR Joe Gibbs Racing NASCAR National Association for Stock Car Auto Racing OpenFOAM Open source Field Operation And Manipulation RANS Reynolds-Averaged Navier-Stokes TRD Toyota Racing Development SST Shear Stress Transport 1 CHAPTER 1: INTRODUCTION Stock car racing organized by National Association for Stock Car Auto Racing (NASCAR) has three major national level series. The most prestigious is Monster Energy NASCAR Cup Series. The Xfinity Series is considered as the final step before entering the top level, and the Gander Outdoors Truck Series is the first step for young drivers in national series. There are only a few articles published about stock car aerodynamic performance and methods of simulation. Most recent articles were published by Chen Fu et al. (2017, 2018, 2019) where they investigate various turbulence models and effect of closure coefficients on gen 6 NASCAR Cup cars. Passive and active blowing effects were investigated and published by senior director of aerodynamics at NASCAR Eric Jacuzzi et al. (2018, 2019). Experiments were performed on NASCAR Xfinity cars in order to achieve a more competitive field and interesting racing. Catranis (2018), in his work, describes cost effectiveness of cloud based CFD simulations on example of 2016 and 2017 NASCAR Xfinity Chevrolet Camaro race cars. This work is the closest to the research and provides additional information for comparison. FIGURE 1 shows 2016 Toyota Camry NASCAR Xfinity race car on the race track. According to 2016 rules, all cars should have standard spoiler with optional ears attached at the ends. Additionally, all cars should have flat splitter attached at the bottom of front fascia. 2 Before the beginning of 2017 season NASCAR introduced major aerodynamic rule change for Monster Energy NASCAR Cup Series and Xfinity Series with the intention to reduce aerodynamic forces and improve racing. Due to this change, drivers could not rely as much on vehicles’ performance thus putting more importance on driving skills. Spoiler height was reduced, which was supposed to reduce read down force and drag of the vehicle. FIGURE 1: 2016 TOYOTA Camry NASCAR Xfinity race car. FIGURE 2: 2017 TOYOTA Camry NASCAR Xfinity race car. 3 FIGURE 2 shows 2017 TOYOTA Camry NASCAR Xfinity race car on the race track. Comparing to 2016 aerodynamic rules, 2017 features shorter spoiler with different shape. Spoiler width was also reduced to protect spoiler in the event of hitting a wall on the race track. No spoiler ears became available for 2017 race season. Splitter surface area was also reduced as can be seen on FIGURE 4. An overlay of 2016 and 2017 spoilers is shown on FIGURE 3 to see how both spoilers compare to each other. FIGURE 3: 2016 and 2017 Rear spoiler difference. In order to maintain aerodynamic balance of the car, front down force was reduced by making front splitter 6 in shorter on sides and 2 in shorter in the center, which is shown on FIGURE 4. In order to mimic Monster Energy NASCAR Cup Series rules, splitter height was changed from 5 to 4 inches, which allowed easier inspection process and is shown on FIGURE 5. FIGURE 4: 2016 and 2017 Front splitter difference. 4 FIGURE 5: 2016 and 2017 Front splitter height difference. These aerodynamic changes were applied to use at all race tracks, except Daytona International Speedway, Talladega Superspeedway, and Indianapolis Motor Speedway. 1.1 Objective The main goal of this work is to analyze 2017 NASCAR Xfinity aerodynamic rule changes using RANS approach in CFD simulation. For the simulation it was decided to use only open source software to investigate its capabilities and in case of successful results decrease expenses to perform simulations. To ensure accurate results an exact CAD model of both 2016 and 2017 race cars were created along with full kinematic ride height model. It is well known fact that k-ω SST model tends to predict early separation and as a result larger wake region, but it is able to predict flow structures around race car better than other turbulence models. That is why wind tunnel results were used to improve k-ω SST model. After robust simulation was developed, cars were tested at different ride heights to make sure CFD captures changes in force coefficients and correlated with wind tunnel results.