Turbulent Drag Reduction in a d-Type Rough Wall Boundary Layer with Longitudinal Thin Ribs Placed within Traverse Grooves : Higher-Order Moments and Conditional Sampling Analysis

Abstract

Large-scale coherent motions were investigated experimentally in a d-type rough wall boundary layer modified with longitudinal thin ribs. Higher-order moments and conditional averaged statistics were obtained at R_θ=1 300 and 5 000, where the local skin friction coefficients of modified flow are the same as and about 17% larger than those of smooth wall flow. The magnitude of triple velocity correlations is reduced in most of the layer by drag reduction control using longitudinal thin ribs. The higher-order moments and conditional averaged signals obtained by the VITA analysis show that the turbulence production model with a pair of streamwise vortices interacting with outflow from the roughness grooves is reasonable.