The effects of wall roughness on the flow dynamics behind a near-wall square cylinder

Flow separations behind a square cylinder positioned at different vertical locations in smooth- and rough-wall turbulent boundary layers are experimentally studied using time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and cylinder height is 12 750, and the incoming turbulent boundary layer thicknesses over the smooth and rough walls are, respectively, 3.6 and 7.2 times the cylinder height. For both the smooth and rough wall conditions, measurements were performed for wall-mounted cylinders with no gap between the cylinder and the wall, and also for cases where the gap between the bottom surface of the cylinder and the wall was set to 0.3, 0.5 and 2.0 times the cylinder height. The results show that wall roughness reduces the mean reattachment length behind the wall-mounted cylinders by 29%, but does not significantly influence the mean recirculation length behind the offset cylinders. Wall roughness also suppresses the Reynolds stresses in the lower shear layer emanating from the undersurface of the offset cylinders. The dominant frequencies behind the wall-mounted cylinder are similar to the dominant frequencies of the incoming turbulent boundary layer regardless of the wall roughness conditions, but the fundamental frequencies in the wake behind the offset cylinders are dictated by the vortex shedding motions. The dominant shedding frequencies behind the offset cylinders are nearly independent of gap ratio when scaled by the free-stream velocity, even though the cylinder is deeply submerged in a thick incoming turbulent boundary layer. Reconstruction of the vorticity field based on proper orthogonal decomposition shows that with a gap of 0.3 times the cylinder height, wall roughness reduces the size of vortical structures downstream the gap exit, and suppresses regular vortex shedding motion behind the cylinder. Graphical abstract