Spectral proper orthogonal decomposition of time-resolved three-dimensional flow measurements in the turbulent wake of the Ahmed body

This study investigated the turbulent wake flow behind a flat-back Ahmed body using a combination of time-resolved tomographic particle image velocimetry measurements and spectral proper orthogonal decomposition (SPOD). The experiments were conducted at a Reynolds number of Re_H = 10 000, which is defined as U_∞H/ν, where U∞ represents the free-stream velocity, H is the height of the body and ν represents the viscosity. The SPOD analysis revealed four distinct flow motions present in the wake of the Ahmed body, each occupying a specific range of Strouhal number, St_H . Here, St_H is defined as f × H/U∞, where f is the frequency of the motion. At the lowest resolved St_H of 0.007, the system exhibited a bi-stability mode, in which the wake switched between asymmetric states consisting of a tilted toroidal vortex and a streamwise vortex. In the next St_H range of 0.014 to 0.123, the flow demonstrated swinging/flapping motions, characterized by small spanwise and vertical movements of the wake barycentre. These movements were attributed to the tilting of the toroidal vortex. The third category included the vortex-shedding motions and consisted of quasi-streamwise vortices. These vortices advected in the downstream direction, causing oblique displacements of the barycentre in the streamwise-vertical plane. The peak energy of the vortex shedding was observed at St_H = 0.164. Finally, shear layer instabilities induced small vertical and spanwise velocity fluctuations along the shear layers at a high St_H of 1.147.