Effect of free surface perturbation on fully developed smooth open channel flow

Abstract

Light floater boards are commonly used in open channel flow studies to reduce free surface disturbances. However, the shear layer generated from the surface of the floaters, which has not been considered in previous research, may have unintended effects, and can alter the downstream flow characteristics. This aspect is investigated using particle image velocimetry at Reynolds number of 35,100 by varying the perturbation caused by the floaters. With the perturbation, the mean velocity profiles show a dip and the Reynolds shear stress becomes negative near the free surface. It is shown that, due to perturbations caused by ejections and sweeps of the inverted shear layer at the floater bottom surface, the joint probability density function of velocity fluctuations near the free surface shifts from ejections and sweeps to outward and inward interactions. The data indicate that the shear penetrates deeper into the flow towards the bed with increase in perturbation.