Effects of freestream turbulence intensity on flow dynamics and vortical structures around a high-drag Ahmed body

Abstract

This study investigates the effects of freestream turbulence (FST) on the spatiotemporal flow dynamics around an Ahmed body with a slanted angle of β = 25° using time-resolved particle image velocimetry. Three FST intensities (Tu = 5.0%, 10.0%, and 15.0%) were generated using regular and fractal grids, and the results were compared with those obtained at a baseline turbulence level of Tu = 1.5% achieved in the absence of the grids. The size of the roof recirculation bubble decreased monotonically with increasing FST; however, the wake bubble size is independent of FST. Over the slanted surface, the bubble size increased with FST for Tu ≤ 10.0% and remained constant at Tu = 15.0%. The increase in the bubble size was attributed to a stronger spatial and temporal correlation between the C-pillar vortex and the separated shear layer from the slanted surface. FST also promotes turbulence transition by generating elongated streaks characterized by higher streamwise than wall-normal velocity fluctuations. The results also show that FST suppresses the magnitudes of Reynolds stresses over the roof and in the wake region and has no significant effects on the turbulence levels over the slanted surface. The spatiotemporal features were analyzed using velocity spectra and two-point spatial and space–time correlation coefficients, while spectral proper orthogonal decomposition was used to study vortical structures linked to dynamically important frequencies. FST suppressed the dynamically important frequencies of the vortical structure, including roof bubble pulsation, disrupted the flapping motion of recirculation bubbles in the wake, and reduced their spectral coherence.

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