Spectral proper orthogonal decomposition of Ahmed-body wake using time-resolved three-dimensional flow measurements*

The turbulent wake behind a flat-back Ahmed body was investigated using a methodology that combines time-resolved tomographic particle image velocimetry (tomo-PIV) measurements and spectral proper orthogonal decomposition (SPOD). The SPOD analysis shows four types of flow motions in the wake of the Ahmed body, each occurred at a specific frequency range. The bi-stability mode was asymmetric and consisted of a tilted toroidal vortex and two counter-rotating streamwise vortices. This motion resulted in significant variations in the separation bubble volume. A newly identified category of motions, referred to as swirling motions, was observed. The swirling motions consisted of rotational movements of the wake barycenter in the crossflow plane in addition to streamwise motions of the barycenter. The swiriling motions resulted in expansion and contraction of the separation volume similar to those associated with bubble pumping motions, as well as oscillations between asymmetric and symmetric states previously described as switching attempts. The vortex-shedding motions consisted of quasi-streamwise vortices that advected in the downstream direction and resulted in oblique movements of the barycenter. Furthermore, SPOD showed shear layer instabilities that generate small vertical and spanwise velocity fluctuations along the shear layers.