Turbulent Structures in the Vicinity of a Roughness Element for the Boundary Layer Over a K-Type Rough Wall

Abstract

Turbulent structures in the vicinity of a roughness element have been investigated in the boundary layer developed over a k-type rough wall, which consists of two-dimensional bar roughness array with pitch ratio of 4. Measurements were made at a roughness Reynolds number k⁺≑120 in the fully rough regime. In order to discuss role of a separated free shear layer evolving behind trailing edge of a roughness element and vortices formed inside a groove, velocity and length scales were defined with mean and turbulence quantites and wall static pressure measurements.The length scales obtained from the wall static pressure distribution around a roughness element and oil film patterns show that "K-type"and "d-type"are classified with the criterion value of b/k(=groove width/roughness height)=2.5 In the K-type rough wall flow, the length scale is equal to roughness element height. Pressure drag acting on the roughness element in the total wall shear stress is 93% in the k-type rough wall flow, much higher than that of the d-type rough wall flow. Nevertheless, the turbulence velocity and length scales of the separated free shear layer can scale the fraction of pressure drag.