SPECTRAL PROPER ORTHOGONAL DECOMPOSITION OF THE JUNCTION FLOW BEHIND A CANTILEVERED SQUARE CYLINDER

The wake of a cantilevered square cylinder of height-to-width ratio h/d = 4 protruding a thin laminar boundary layer is investigated using time-resolved stereoscopic particle image velocimetry at a Reynolds number (Re) of 10600. The PIV are processed using proper orthogonal decomposition (POD) and the time-domain spectral proper orthogonal decomposition (SPOD). Spectral analysis in the wake region in the vicinity of the ground plane (z < d) show fluctuation energy concentration centered on the vortex shedding frequency (fs), as well as weaker contributions at 0.1, 0.4, 0.9, 1.1, and 2 fs. It is shown that SPOD is significantly more successful than POD in separating coherent contributions within narrow spectral bandwidths and resolving spatial modes. Each of the identified frequencies is represented in a separate mode pair, which permits a better interpretation of the related dynamics. Notably, the improved modal separation assisted in associating the 0.4 fs spectral contribution to the interactions between the horseshoe and Kármán vortices in the wall-body junction region.