Reattachment characteristics of a backward-facing step flow in a duct

Spatial distributions of flow reattachment position on the bottom wall downstream of a backward-facing step in a duct have been measured by 2D PIV method. Investigations have been done for changing the flow Reynolds number, ranging from steady (Re=200) to unsteady (Re=1000) flow regimes for the stepped duct having an aspect ratio of 16 and expansion ratio of 2. It was found that non-uniform spanwise distribution of the reattachment length was observed in each case. Within the steady state, the reattachment length shows a unique distribution in the spanwise direction being protruded like a tongue over a channel, that is, having the maximum at the center of the duct but the minimum near the side wall. Its short length near the side wall reaches only 60% of that of the center of the duct. Generally, the reattachment length increases rapidly as the Reynolds number is increased and has the maximum at the center of the duct at Re=400. It should be noted that in this paper, as the Reynolds number is further increased Re>400, the reattachment length conversely decreases due to the flow unsteadiness occurring near the center of the duct. This flow unsteadiness expands to the side wall with the further increase of the Reynolds number, therefore, it makes the spanwise distribution of the flow reattachment position more complicated. ©2013 The Japan Society of Mechanical Engineers.