Measurements of the mean and the turbulent velocities have performed in the vicinity of a roughness element for the turbulent boundary layer developed over a k-type rough wall, which consists of two-dimensional square ribs (10 mm x 10 mm x 400 mm) arrayed with pitch ratio of 4. The Reynolds-number R_θ and k~+ based on momentum thickness 0 and roughness height k are about 500 and 50 respectively, and relative roughness height k /δ =0.156. LDV (Laser Doppler Velocitimerry) that it is possible to measure reverse flow was applied to investigate the momentum-exchange process in the open-face (y_T = 0(mm)) over cavities. The variation of the mean and turbulent quantities profiles at y_T = 0(mm) depend on the evolution of a free-shear layer formed behind roughness elements and the behavior of eddies in the cavity. Pressure drag coefficient acting on the roughness element in the local skin friction coefficient is 88% in the present k-type rough wall. The momentum exchange for the mean' flow and turbulence contribute 6% and 56% respectively to the local skin friction coefficient.
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