Effect of Surface Roughness on Inner-outer Interaction in a Turbulent Plane Couette-Poiseuille Flow

Abstract

When a rod surface roughness is introduced in a turbulent Couette-Poiseuille flow (CP-flow), it is known that the Reynolds stresses near the centerline decrease due to weakened very-large-scale motions (VLSMs) and roll-cell motions (Lee, Kim & Lee, Physics of Fluids, vol. 30, 2018, 105101). In the present study, we examine the origin of the weakened turbulent structures near the centerline in a CP-flow with the roughness (CPR-flow) using dataset from direct numerical simulation (DNS). The top-down and bottom-up interactions to organize a CP-flow is very similar to those found in earlier studies in turbulent channel/pipe and boundary layer flows. The circulations of roll-cells in the outer region induce congregation of negative streamwise velocity fluctuating structures (u) near the wall, leading to the generation of a large-scale ejection into the outer region. This large-scale ejection contributes to the formation of a negative VLSM when two adjacent negative large-scale motions (LSMs) merge, and the VLSM induces the circulation of a roll-cell motion by the pure kinematics. Similar process for the inner-outer interactions is found for the CPR-flow. However, because the impact of the surface roughness suppresses congregative motion of negative u-structures near the surface roughness, strong congregation by the roll-cells is observed to occur far from the wall, indicating that relatively small number of negative u-structures with weak strength contribute to the formation of a large-scale ejection for the CPR-flow. The weakened large-scale ejection decreases the strength of a VLSM, thus resulting in a weakened roll-cell motion.