Effect of Roughness Pitch Ratio on the Structures in the Vicinity of a Roughness Element for the Turbulent Boundary Layer Developing over k-Type Rough Walls Arranged with Two-Dimensional Square Ribs

Abstract

The effect of roughness pitch ratio P to turbulent structure has been invistigated in the vicinity of a roughness element for turbulent boundary layers developing over rough walls, which consist of two dimensional square ribs arrayed with P=3, 4 and 8. The Reynolds numbers based on momentum thickness θ and roughness height k are over 500 and 60 respectively, which almost satisfy the conditions that the flows are confimed as turbulent boundary layer and fully rough regime. The measurements of velocity components were measured with LDV system. The distributions of the mean and turbulent quantities in the vicinity of the roughness element depend on the development of a separated free shear layer behind the roughness element and formation of eddies in the groove. For P=3 and 4 the flow separated from the downstream top corner of the roughness element reattaches to the upstream top corner of the down stream roughness element, and for P=8 to the bottom surface between roughness elements. The ratio of pressure drag coefficient acting on the roughness element to the local skin friction coefficient avaraged over a roughness pitch length approaches to almost 100% over 4 of P.