IMPINGEMENT OF CIRCULAR TURBULENT JETS ON ROUGH BOUNDARIES

This paper presents the results of a laboratory study of the perpendicular impingement of circular turbulent jets on rough walls placed at large impingement distances. The jet of air, with velocity U0 of 45-90 m/s and diameter d at the nozzle of either 6.4 or 12.7 mm, impinged on a wall with a roughness ks of 1.73, 8.23, or 15.18 mm. Measurements were taken of the boundary shear stresses created by the jet, as well as velocity profiles in the wall jet region. The maximum shear stress on the wall, expressed in the dimensionless parameter [(tau)0m(H/d)2/(rho)U2 0 ], depends on the relative roughness ks/H and is about 2.5-5 times as large as the value for smooth walls. The boundary shear stress increases to a maximum (tau)0m at a radial distance from the jet centerline r of about 0.12H, where H is the height of the jet above the boundary, and decreases as r increases further. This is shifted slightly toward the jet centerline as compared to the jet impinging on a smooth wall. In the wall jet region, the boundary shear stress varies as 1/r2. The velocity distribution in the wall jet region was found to be similar.