Experimental characterization of flow unsteadiness in the centerline plane of an Ahmed body rear slant

This study presents the results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with rear slant angle of 25 degrees. This analysis focuses on the 3D separated and reattaching zone that develops on the rear slanted surface and provides new information, improving the understanding of the flow unsteadiness. Flow investigations were performed using particle image velocimetry, hot wire anemometry, and unsteady flush-mounted pressure transducers in the plane of symmetry above the rear slanted surface. Spectral analysis and proper orthogonal decomposition of the output signals show the emergence of low frequency unsteadiness and high frequencies activity. Characteristic timescales of both instabilities are provided and the physical effect of the low frequency unsteadiness is related to a flapping motion of the separated shear layer, while the high frequency activity is associated with a large-scale vortex emission. The results focus on the centerline plane of the rear slant and more specifically demonstrate relevant similarities with the unsteady mechanisms of 2D recirculated flow.