Effect of freestream turbulence on flow structure around slanted-back Ahmed body

Abstract

This study explores the influence of freestream turbulence (FST) on the flow around Ahmed bodies with rounded (RL) and squared (SL) leading edges, both featuring a 25° slanted rear surface. Time-resolved particle image velocimetry was conducted at Re = 0.17 × 10⁵ under FST intensities of 1.5% (baseline), 5.0%, 10.0%, and 15.0% generated by regular and fractal grids. The SL geometry enlarged the roof recirculation bubble but did not alter wake length, while suppressing downwash and reducing vortex shedding frequency. For both geometries, increasing FST shortened the roof bubble through earlier shear-layer reattachment, whereas wake length remained unchanged. Over the RL slant, bubble size increased with FST up to 10% before stabilizing, linked to stronger interactions between the C-pillar vortex and the separated shear layer. Turbulence statistics showed geometry-dependent effects. In RL, FST suppressed Reynolds stresses over the roof and wake, while promoting transition via elongated streaks dominated by streamwise fluctuations. In SL, FST increased streamwise Reynolds stresses and turbulent kinetic energy (TKE) on the roof through energy redistribution to low-frequency motions (St < 0.23), though wall-normal and shear stresses were minimally affected. In both cases, wake TKE decreased due to weakened wall-normal stresses and disrupted vortex interactions. Frequency spectra and Spectral Proper Orthogonal Decomposition (SPOD) analysis revealed that in the RL geometry, FST suppressed dynamically important frequencies, including roof bubble pulsation, and disrupted the flapping motion and spectral coherence of wake recirculation bubbles. In the SL geometry, SPOD and quadrant-based analysis showed that FST amplified low-frequency flapping over the roof and promoted persistent C-pillar vortices that dominated the upper shear layer, disrupting classical vortex shedding in the wake. The results offer critical insights for refining aerodynamic models and developing flow control strategies tailored to real-world turbulent conditions.