Velocity Measurements in Asymmetric Turbulent Channel Flows

Abstract

Velocity measurements, using a two‐dimensional laser Doppler velocimeter (LDV), were made in turbulent channel flows of water, at a Reynolds number of 60,000, over a flat stationary sediment bed with covers of different roughness characteristics. The measurements confirmed the noncoincidence of the planes of maximum velocity and zero Reynolds stress. Significant diffusion of momentum and kinetic energy took place from the rough to the smooth surface. As the roughness of the cover was increased, the vertical transfer of vertical velocity fluctuations near the bed decreased, resulting in a decrease in the sediment‐suspension mechanism. The proper length scale in the outer region was the height of the plane of zero total stress from the corresponding surface. When the distance from each surface was normalized with this length scale, the measured mean streamwise velocity defects agreed with the log law, and the measured streamwise and vertical velocity fluctuations agreed with the exponential variations formulated in 1986 by Nezu and Rodi.