Analysis of the wake of an AR 4 square cylinder using time-domain spectral proper orthogonal decomposition*

The near-wake of a cantilevered square cylinder of height-to-width ratio h/d = 4 protruding a thin laminar boundary layer of thickness δ/d = 0.21 is investigated using time-resolved stereoscopic particle image velocimetry (PIV) at a Reynolds number of 10600. PIV data are processed using time-domain spectral proper orthogonal decomposition (SPOD), which offers greater flexibility through constraining the spectral bandwidth for coherent contributions. Simultaneously acquired wall-pressure measurements are used to synchronize planar PIV data for 3D reconstructions of the coherent velocity field using a pressure-based estimator. Unlike POD, SPOD modes tend to contain only one dominant frequency, allowing for a better modal separation of the coherent field dynamics and improved estimator performance. Here, near the ground plane, spectral analysis show fluctuating energy concentration at the vortex shedding frequency (f_s), as well as weaker contributions at 2f_s, a low-frequency instability at 0.1f_s, inter-harmonics of interactions at 0.9f_s and 1.1f_s. Of particular interest, for illustrating the benefits of the SPOD approach, is a broadband spectral concentration at 0.4f_s, which could be related to interactions between shed vortices and the horseshoe vortex system.