Time-resolved PIV measurement of coherent structures downstream of step blockage in a narrow rectangular channel

Abstract

Experimental study of coherent structures in a narrow rectangular channel with 50 percent single side entrance blockage was conducted by time-resolved particle image velocimetry (TR-PIV) under Reynolds numbers from 2,180 to 21,800. With error and uncertainty analysis, the maximum standard uncertainty is less than 2% of bulk velocity (Wb) for time-averaged velocity and less than 2% Wb2 for Reynolds stress.The blocked channel accelerates fluid and forms strong streamwise velocity gradient, which dominates the flow field downstream the blockage. The shear-flow generates a large reverse flow vortex downstream blockage, which forms a second reverse flow in the corner of narrow channel near blockage. The streamwise flow reattaches the side channel wall downstream the reverse flow vortex. The vortices shedding from blockage interact with the reverse flow, forming a long and narrow region of high Reynolds stress near the inlet. The time-averaged velocity and Reynolds stress are strong in shear-layer and weak in the reverse flow and second reverse flow. The streamwise velocity gradient and Reynolds stress decay in streamwise direction. With Reynolds number increasing, normalized spanwise velocity and reattachment length increase, while normalized streamwise velocity decreases.The coherent structures downstream blockage has different PSD peak frequencies in shear-layer, reverse flow and second reverse flow. The Strouhal number decreases with Reynolds number increasing. The integral spatial–temporal length scales in the coherent structures are determined by the velocity gradient, which forms large turbulent fluctuations in the shear layer and reverse flow. With Reynolds number increasing, the spatial–temporal length scales increase because stronger velocity gradient generates larger coherent structures downstream blockage. The strong anisotropic turbulent flow in narrow rectangular channel with 50% single side entrance blockage exists in terms of Reynolds stress and spatial–temporal length scales.The experimental findings improve understandings of coherent structures in narrow channel with 50% single side entrance blockage. The experimental data can be used for mechanism study of entrance blockage accident and validations of turbulent models.