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LDV (Laser Doppler Velocimetry) measurement has been made near a rough surface to clarify the effect of momentum transports to pressure distribution acting on a roughness element. The measurements were performed for an equilibrium boundary layer developing over the rough surface that the roughness height is proportional to a streamwise distance. The momentum thickness and roughness Reynolds numbers are 6000 and 150, respectively. The accuracy of the mean velocities and turbulence in LDV measurement was checked by a momentum balance method and the estimated value was compared with the local skin friction coefficient measured by a direct measurement. Near the rough surface except for a cavity region, it was confirmed analytically and experimentally that the averaged shear stress profile given as a sum of apparent shear stress due to waviness of mean streamlines, Reynolds shear stress and viscous stress per a roughness pitch length is approximately equal to the wall shear stress. In the cavity region, the averaged shear stress profile can be expressed as a function of an integral of the mean pressure difference on downstream and upstream sidewalls of the cavity, and takes positive value except for the bottom region. In the present rough surface, the gradient of spatially-averaged Reynolds shear stress with respect to height will mainly contribute to the mean pressure difference on sidewalls in the cavity.

The Flow Dynamics in Near-Wall Turbulence for the Boundary Layer over a Rough Surface with Two-Dimensional Square Ribs