Experimental investigation of turbulent flow over a sharp-crested circular cylindrical segment

A circular cylindrical segment is a sharp-crested weir with a circular downstream curve profile. Overflow characteristics of circular cylindrical and circular-crested weirs have been studied extensively over the past century. Meanwhile, the available literature on the circular cylindrical segment is comparatively limited. Previous studies have often employed the potential flow theory to predict the mean velocities, pressure, and discharge coefficient, which may not be able to accurately predict the flow characteristics for a circular cylindrical segment. The aim of the present study is to experimentally investigate turbulent flows over circular cylindrical segments and to provide equations that can predict the mean velocities along the weir. Three upstream face angles & alpha; = 90 degrees, 45 degrees, and 30 degrees, and three water depths over the crest Dc/h = 0.7, 0.4, and 0.2, were tested to simultaneously characterize the effects of both the upstream face angle and water depth. The instantaneous velocities were measured using a planar particle image velocimetry system in an open recirculating water channel. The results showed the presence of a recirculation bubble at the crest of the weir for all test conditions. The size of this recirculation zone decreases as the water depth or the upstream face angle decreases. At a far enough distance from the recirculation zone, the mean angular momentum varied linearly in the circumferential direction. Meanwhile, a rigid-body rotation region was detected near the free surface, which grows thicker in the circumferential direction.