Experimental studies on wake structures of a circular cylinder with different surface roughness heights

In the present study, the effects of surface roughness height (k/D) on wake characteristics of a circular cylinder were examined experimentally in a water flume at a Reynolds number of 2520 using particle image velocimetry (PIV). The surface roughness was simulated by using 3D printed hemispheres uniformly distributed over the cylinder surface. Four relatively large roughness heights with k/D = 5%, 10%, 15% and 20%, and a coverage ratio of 79%, which were not explored systematically in the literature, were considered, and the results were compared with that of a smooth cylinder (k/D = 0%). It is found that the surface roughness disturbs the shear layers of the cylinder, affecting the vortex shedding frequency and the wake formation region. The Strouhal number decreases with the increase of k/D. This can be attributed to the forward movement of the shear layer separation points. This phenomenon is also confirmed by smoke-wire flow visualisation. Besides, the regularity of the vortex shedding and the magnitudes of the Reynolds stresses are enhanced with the existence of roughness. The proper orthogonal decomposition (POD) analysis shows that with the existence of surface roughness, the large-scale Kármán vortex structures is intensified. This study extends the understanding of the influence of k/D on flow characteristics of a circular cylinder.