Measurements of Water Surface Profile and Velocity Field at a Circular Pier

Flow around a 60-mm-diameter pier on a smooth bed was measured in an open-channel flume. By varying the approach flow velocity and water depth, a wide range of subcritical flow conditions was produced. Water surface elevation was measured at 0, 90, and 180° to the approach flow direction near the surface of the pier; and three-dimensional velocity vector field around the pier was measured in two horizontal planes, one close to the bed and the other near the free surface. The velocity field measurements were obtained using a stereoscopic particle image velocimetry system. It was found that the change in water surface elevation around the pier was related to the Froude number and relative water depth. However, no direct relationship between the Froude number and the measured velocity fields was found. The approach flow conditions affected the pier flow field mainly behind the pier; the flow pattern was related to the pier Reynolds number. It was also found that the direction and magnitude of the ensemble-averaged velocity field was more dependent on the pier Reynolds number near the bed.