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Published on October 11, 2022
1. Introduction
The purpose of this study is to create an aerodynamic package to increase total downforce while minimizing total drag force generated for a Toyota GR Supra adhering to the Global Time Attack (GTA) Super Lap Battle (SLB) Limited class rules. The 2-dimensional (2D) model was created in SolidWorks and a computational fluid dynamic (CFD) simulation was performed using the native Flow Simulation tool. The tests were performed with different freestream velocities to understand the effects of the aerodynamic package throughout the different speeds the car experience – slow speeds before corners and fast straights – with an increase of downforce in slow speeds a priority. The optimal geometries and parameters of the aerodynamic devices were found parametrically. The results of the 2D simulation are a good starting point in designing the different aerodynamic components to create a coherent 3-dimensional (3D) aerodynamic package for motorsport applications.
2. Methods
The 2-D centerline side profile of the Toyota GR Supra was created in Solidworks by tracing the side profile from a reference photo of the car shown in Figure 1. The wheelbase is 97.2 in. and the reference photo was scaled to the appropriate size in Solidworks. Then, the side profile was traced using lines and splines. The front bumper is assumed to be in front of the front axle and the middle section of the bumper is also assumed to be in line with the front axle for simplicity. This is done to adhere to A. Chassis/Exterior/Aero, line number 11 regarding the allowable location of the front splitter. The ride height is assumed from the reference photo and the road was modeled accordingly.
Figure 1: Side profile used to generate the 2-D solid model
Table 1: Values used to calculate Reynold's Number. The velocity is converted from KPH to m/s. The calculation was done for 25, 50, 75, 100, 125, and 150 KPH.
To choose the correct flow characteristic for the CFD simulation model, the Reynold’s Number was calculated to understand the flow regime. The length is 4.38 (m), which is the total length of the car. The velocity ranged from 25, 50, 75, 100, 125, and 150 KPH to simulate the various low and high speeds. The Reynold’s Number was calculated using the values in Table 1 and the results range from 2 to 12 million, which is in the turbulent flow regime.
The CFD simulation domain was set up to provide adequate room for the solver to compute the fluid reaction while minimizing the mesh and solving time. The domain is 1x the length of the car from the front bumper to the front of the domain and 6x the length of the car from the front of the bumper to the rear of the domain. The domain is 2x the height of the car from the road to the top of the domain and the domain is 50 mm below the road to the bottom of the domain. This was set up to ensure that any significant effects downstream would be sufficiently captured in the simulation.
The global mesh was set to broad. This is done to keep the global mesh cells to keep mesh and solve times low. The only mesh refinement is near the surface of the car to better simulate the effects of the car on the airflow. A 5% mesh convergence test was performed.
3. Results
The results of the CFD simulation from 20 KPH to 150 KPH are shown in Figure 2. An increase of 7-10% in total downforce while an increase of 43-55% in total drag force was achieved.
During the design phase for the rear diffuser and front splitter, a 15% increase in total downforce was achieved with only a 21% increase in drag force compared to the stock configuration of the car at 25 KPH free stream velocity. Adding the rear wing, the improvement in downforce decreased to 7% and the drag force increased to 43% when compared to the stock configuration of the car at 25 KPH free stream velocity.
Figure 2: CFD results of the stock and aerodynamic package
4. Conclusion
The final aerodynamic package adhering to the rule book is shown in Figure 3. An increase in downforce was achieved although minimizing drag can be argued. From the results during the design phase, the addition of the rear wing is more sensitive compared to the addition of the rear diffuser and front splitter to the overall downforce and drag force produced. An extensive study of the optimal airfoil geometry and location is required to maximize the downforce and minimize the drag force produced. The 2D CFD simulation results are a great start to optimizing the 3D model of the car as it gives valuable insight into improving the aerodynamic performance of a car.
Figure 3: The final aerodynamic package adhering to the GTA SLB Limited class rules
Moving forward, all three of the aerodynamic devices: front splitter, rear diffuser, and rear wing, have the capability to be improved when designing in 3D space. Since the prescribed restriction for the front splitter and rear wing is a rectangular box, increasing the lift-to-drag ratio is plausible as there are geometries to be optimized to the sides of the car. The longitudinal expansion of the diffuser is also an area where performance can be extracted within the rules. Overall, the results from the 2D simulations are promising to improve the performance of the car.
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