Experimental characterization of the unsteady natural wake of the full-scale square back Ahmed body: flow bi-stability and spectral analysis
Résumé
In recent years, the increasing interest in reducing the aerodynamic drag of vehicles, such as station wagons, minivans or buses, has led research to focus on the characterization of square back bluff geometries. In this paper, the results of an extensive experimental campaign on the full-scale well-known body of Ahmed et al. (1984) are presented, for two height-based Reynolds numbers, ReH = 5.1 × 105 and 7.7 × 105. Eighty-one measurement points were used to map the base pressure field, while the wake topology was investigated by means of a series of ten 2D Particle Image Velocimetry planes. These measurements clearly show that the wake presents a bi-stable behavior, characterized by a random succession of switches between two well-defined mutually symmetric configurations, confirming the results from Grandemange et al. (J Fluid Mech 722:51–84, 2013b. doi:10.1017/jfm.2013.83) for the same model. For the presented results, the timescale of this phenomenonis of the order of 800 V/H. The sensitivity of the bi-stability to the yaw angle was also investigated, and considerations on how to take such a behavior into account in
post-processing this kind of field are given. High-frequency measurements were also carried out with four piezoelectric transducers and a synchronized two-component hot-wire. The results show a low-frequency spectral activity: peaks at StH = 0.13 and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at StH = 0.08 was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal The results show a low-frequency spectral activity: peaks at StH = 0.13 and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at StH = 0.08 was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal decomposition confirm the interpretation of the latter mode as the pumping of the recirculation bubble.