Experimental Study of Shallow Open Channel Turbulent Flows Over Rough Walls

An experimental study was undertaken to investigate the effects of bed roughness on the turbulence characteristics of shallow open channel flows. The measurements were performed in a recirculating open channel flow over a reference smooth bed and a three-dimensional rough bed (36-grit sandpaper). The velocity measurements were conducted using a high resolution particle image velocimetry (PIV) system. The Reynolds number based on the depth of flow (h) and freestream velocity (U_e) varied from 21000 to 30000 and the Froude number ranged from 0.46 to 0.65. Two smooth bed experiments were conducted to investigate the effect of Reynolds number on the open channel flow. The mean velocities and Reynolds stresses for the two smooth cases were observed to be weakly dependent on Reynolds number. The effect of bed roughness was observed to penetrate into the outer layer of the boundary layer. The results show that bed roughness significantly increased the skin friction coefficient, wake parameter, boundary layer parameters, as well as the mean velocity, Reynolds stresses and the energy budget terms. A two-point correlation analysis showed that the coherent structures were also significantly modified by bed roughness.