Wall Roughness Effects on Turbulent Flow Past a Near-Wall Square Cylinder

Abstract

Separated turbulent flows induced by two-dimensional square cylinder with different gap heights from a solid wall submerged in a thick turbulent boundary layer are investigated using a time-resolved particle image velocimetry system. Two different upstream wall conditions are implemented to investigate the effects of upstream wall roughness on gap flow. The examined gap ratios between the cylinder and the wall include 0.0, 0.5, and 2.0. The incoming Reynolds number based on the cylinder height and freestream velocity is 12, 750 and the boundary layer thicknesses are 3.6 and 7.2 times the step height, respectively, for the smooth and rough upstream wall conditions. The results are presented in terms of mean flow, Reynolds stresses, and proper orthogonal decomposition. Notably, the wall-roughness effects are most pronounced on the vertical Reynolds stresses and act to reduce the vortex shedding frequency.