Impact of unsteady suction actuation on drag and wake dynamics of a vehicle model

This study employs wall-resolved large-eddy simulations to investigate the effectiveness of unsteady suction actuation applied near the upper edge of the slant surface of a 25° Ahmed body at a Reynolds number of 7.6 × 105. A key approach to this work is the validation of the baseline flow, which achieves a drag coefficient of 0.31, closely matching the benchmark experimental from Ahmed et al. [“Some salient features of the time-averaged ground vehicle wake,” SAE Trans. 93, 473–503 (1984)]. From this validated baseline case, the application of an unsteady suction jet with an optimal momentum coefficient Cμ=2.6×10−3 leads in a drag value of 0.274 and reduction of approximately 12%. This study establishes a clear connection between targeted momentum injection and drag reduction through detailed analysis of modifications to the separation bubble, shear layer development, and wake topology. It is found that the suction jet effectively delays flow separation while promoting earlier reattachment and suppressing large-scale coherent structures. Quantitative evaluations confirm a 28% shortening of the recirculation bubble length. The analysis of first- and second-order statistics reveals that turbulence intensifies locally near the actuation location. However, the controlled wake exhibits a shortened recirculation region in both instantaneous and mean resolved velocity fields, and the so-called C-pillar vortices display reduced structural coherence. Additional insight is provided through the energy spectra, and the resolved turbulent kinetic energy budget evaluated at downstream locations. We observed that the suction effect significantly alters the flow characteristics over the slant surface and in the near wake of the vehicle model, leading to a reduced drag coefficient and controlled lift, which are essential for improving overall energy efficiency.