Effects of Blockage Ratio on the Spatiotemporal Dynamics of Turbulent Flow Separation Around a Square Cylinder at Moderate Reynolds Numbers

The effects of blockage ratio (BR) on turbulent flows around square cylinders at moderate Reynolds numbers are investigated using a time-resolved particle image velocimetry (TR-PIV). The blockage ratios range from 2.5% to 15%, and the Reynolds numbers based on the freestream velocity and cylinder thickness are 3000, 7500, and 15,000. The flow dynamics are examined in terms of the mean flow, Reynolds stresses, frequency spectra, reverse flow area, and proper orthogonal decomposition (POD). The results show that the wake characteristics are nearly independent of the Reynolds number and blockage ratio. Spectral analyses of the velocity fluctuations demonstrate that the von Karman (VK) shedding frequency is independent of the Reynolds number and blockage ratio, however, the Kelvin-Helmholtz (KH) frequencies increase with increasing Reynolds number and blockage ratio. The probability density function of the reverse flow area shows unimodal and bimodal distributions for the lower (BR <= 5%) and higher (BR >= 10%) blockage ratios, respectively, and the mean reverse flow area and its standard deviation decrease with increasing blockage ratio. The results also show that the contributions from the first POD mode pair to the total energy increase with blockage ratio but independent of the Reynolds number. The POD mode coefficients show significant cycle-to-cycle variation at lower blockage ratios, suggesting that the energetic structures are comparatively less organized at lower blockage ratios. The spectra of the velocity fluctuations, reverse flow area, and POD mode coefficients all show dominant peaks at the fundamental shedding frequency.