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 (10mm × 10mm × 400mm) arrayed with pitch ratio of 4. The Reynolds-number Rθ and k+ based on momentum thickness θ and roughness height k are about 500 and 50 respectively, and relative roughness height k / δ = 0.156. LDV (Laser Doppler Velocitimetry) that it is possible to measure reverse flow was applied to investigate the momentum-exchange process in the open-face (yT = 0(mm)) over cavities. The variation of the mean and turbulent quantities profiles at yT = 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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