Turbulence Models and Model Closure Coefficients Sensitivity of NASCAR Racecar RANS CFD Aerodynamic Predictions 2017-01-1547

Cost benefit and teraflop restrictions imposed by racing sanctioning bodies make steady-state RANS CFD simulation a widely accepted first approximation tool for aerodynamics evaluations in motorsports, in spite of its limitations. Research involving generic and simplified vehicle bodies has shown that the veracity of aerodynamic CFD predictions strongly depends on the turbulence model being used. Also, the ability of a turbulence model to accurately predict aerodynamic characteristics can be vehicle shape dependent as well. Modifications to the turbulence model coefficients in some of the models have the potential to improve the predictive capability for a particular vehicle shape. This paper presents a systematic study of turbulence modeling effects on the prediction of aerodynamic characteristics of a NASCAR Gen-6 Cup racecar. Steady-state RANS simulations are completed using a commercial CFD package, STAR-CCM+, from CD-Adapco. Three turbulence models are included in this study: the realizable and AKN variants of the k − ε model, and the SST k − ω model. The a₁ coefficient for the SST k − ω model is slightly modified in order to investigate the sensitivity of this modeling constant on the veracity of predictions. The aerodynamic characteristics investigated include the force and moment coefficients, and front-to-rear downforce balance. In addition, the turbulence model dependence of the flow around the vehicle, particularly at some key locations in the wake region, will be analyzed. Although the flow features in the wake of this racecar show a discernable dependence on the choice of the turbulence model, this difference did not translate into a significant difference in the prediction of force coefficients.