This study presents results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with a rear slant angle of 25°. This analysis focuses on the half elliptic separation bubble that developps on the rear slanted surface and brings new information, improving the understanding of the flow unsteadiness. Flow investigations are carried out using hot wire probe measurements for velocity fluctuations in the plane of symmetry above the rear slanted surface and five unsteady flush mounted pressure taps (Kulite transducers) simultaneously acquiring static pressure fluctuations along the middle line of the slanted surface. Spectral analysis and Proper Orthogonal Decomposition of the output signal show the emergence of a low frequency unsteadiness and high frequency activities which, in accordance with bibliography about separated and reattaching flow configurations, is related to a global flapping of the separated shear layer and a large scale vortices shedding. Characteristic frequencies of both instabilities is given and physical effects of the low frequency unsteadiness is related with the flapping motion of the separated shear layer.

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