Upstream flow separation is not a triggering factor in wake bi-stability of the square-back Ahmed body

Wake bi-stability is a common flow instability phenomenon observed in the square-back Ahmed body, which has a significant impact on aerodynamic performance and vehicle drag. Recent studies have indicated that upstream boundary layer separation is a key factor triggering wake bi-stability, and suppressing leading-edge separation using suction control methods can eliminate wake bi-stability [D. Ahmed and A. S. Morgans, “Wake bi-modality: The effect of upstream boundary layer dynamics” J. Fluid Mech. 975, A7 (2023)]. However, we found that the simulation time in that study was relatively short, which may not have fully captured the switching process of the bi-modal characteristics. Therefore, in this study, we conducted a large eddy simulation for the square-back Ahmed body over a longer timescale and employed an improved data analysis method to further explore the generation mechanism of wake bi-stability. The results show that even when there is no significant boundary layer separation at the leading edge of the body, the bi-modal flow characteristics in the wake region still persist, suggesting that the triggering mechanism may not solely depend on leading-edge separation. The findings of this study provide new insights into the optimization of wake control strategies and offer important guidance for vehicle aerodynamic design.