Effect of Surface Roughness on Vortex Shedding in The Wake of a Circular Cylinder in a Subcritical Regime

An experimental study of vortex shedding in the wake of a circular cylinder at a subcritical Reynolds number of approximately 5 000 is presented, with emphasis on the influence of surface roughness. Planar particle image velocimetry (PIV) measurements were performed for a smooth cylinder and for a cylinder with uniformly distributed sand-grain roughness. Mean flow statistics show that surface roughness significantly reduces the size of the recirculation region and increases turbulence intensity in the near wake. The unsteady wake dynamics were analyzed using Oscillation Pattern Decomposition (OPD), which allows ident ification of the dominant vortex shedding mode and its higher harmonics. The results indicate a systematic decrease in Strouhal number for the rough cylinder compared with the smooth reference case. Phase aligned OPD mode topologies further reveal increase d streamwise spacing of vortical structures in the rough surface wake. The study demonstrates that surface roughness has a measurable effect on vortex shedding characteristics even in the subcritical flow regime.