Bistable Flow Patterns in a Free Surface Water Channel

Abstract

A series of experiments is reported on a free surface water tunnel with a slot inlet centered at mid-depth in which the flow exhibited equally probable bistable flow patterns. The two flow fields are strongly asymmetric even when the geometry is symmetric and consists of a long stall on one side of the inlet and a short stall on the opposite side. Flow visualization and laser-Doppler velocimetry were performed to examine the flow structure of both stable states in detail. Results showed that the mean flow and turbulence structure of the two bistable states are largely mirror images of each other within the separation zone. The effect of the wall on the flow is minor, only very close to the wall/free surface did the difference in wall constraints cause the two flow patterns to diverge. After reattachment both flows relax to free-surface channel flow. It is shown that the bistable flow pattern results from the interaction of the free shear layers and that only strong disturbances in the free shear layers can cause the flow in one stable state to switch to the other stable state. Bistable flow exists for geometries with asymmetry up to 10 percent for the expansion ratio studied.