SYMMETRIC AND ANTISYMMETIC CHARACTERIZATION OF TURBULENT FLOW PAST A SQUARE CYLINDER OF LOW ASPECT RATIO

In this paper, the turbulent wake velocity fields past a square cylinder of a small height-to-width aspect ratio and submerged in a turbulent boundary layer are measured with time-resolved particle image velocimetry (TR-PIV). Proper orthogonal decomposition (POD) is applied to characterize the spatial-temporal TR-PIV data of flow fields and extract the large-scale coherent motions in the cylinder wake. To reduce the effect of data error propagation and reinforce the existing symmetry property of the cylinder wake, a 'pre-filter' strategy of symmetrization is adopted. The low-order energetic modes are then used to reconstruct and represent the dominant fluctuating characteristics of near-wake flow. The antisymmetric and symmetric POD modes successfully reveal the occurrence and mechanism of two different types of vortex shedding process from the cylinder, namely the Karman- and arch-type vortices.