Effects of Reynolds number on the asymmetric wake characteristics of a notchback Ahmed body

The reduction of aerodynamic drag of green vehicles such as electric and hydrogen vehicles is crucial for increasing driving range and market acceptance. The notchback Ahmed body is a simplified model of a sedan that can be used to understand the generation mechanisms of drag on ground vehicles to develop effective flow control strategies. However, unlike the square-back Ahmed body, the wake characteristics of the notchback are not well-understood. In this study, the effects of Reynolds number on the wake characteristics of a notchback Ahmed body with an effective backlight angle β = 17.8° are investigated using Reynolds-Averaged Navier-Stokes (RANS). The height-based Reynolds number was varied from 5×10³ to 5×10⁴ to demonstrate the effects of low-speed urban and highway driving conditions. To select the best model for predicting the asymmetric wake structure of the 17.8° notchback Ahmed body, detailed assessments of 12 eddy-viscosity (EV) and four second-moment closure turbulence models were performed. The predictions based on Reynolds number of 5×10⁴ were compared with the experimental results of Sims-Williams et al. 2011 and large-eddy simulations (LES) of He et al. 2021. Interestingly, the Spalart-Allmaras one-equation EV model was the only model that accurately predicted the asymmetric wake structure, and the aerodynamic coefficients were similar to the experiment and LES. Accordingly, the Spallart-Allmaras model was chosen for the Reynolds number study. The results showed that, as the Reynolds number decreases, the wake structure transitions from the asymmetric to symmetric state and this was associated with a significant increase in drag. The critical Reynolds number for the transition was found to be 1×10⁴.