Experimental study of flow over a cylinder with rectangular ridges in the transitional subcritical regime

The wake flow modifications induced by variations in macroscopic rectangular ridge height on a cylinder surface were investigated experimentally using time-resolved particle image velocimetry. Experiments were conducted in a low-turbulence wind tunnel at Re=5760 in the transitional subcritical regime, with additional measurements at Re=1660 and 3900 performed for validation against reference data. Ridge height, expressed as the non-dimensional parameter h/D, was systematically varied across six cases (0–0.033). A broad range of flow diagnostics was employed, including velocity components, streamline patterns, phase-averaged fields, Reynolds stress distributions, vorticity contours, and power spectral density (PSD) analysis. The results show that increasing ridge height causes the wake vortex to form closer to the cylinder, enhances turbulence intensity, and shortens the effective shear layer. PSD analysis further reveals that larger ridge heights lead to lower Strouhal numbers and higher spectral amplitudes. These findings identify the macroscopic surface modulation as a key geometric parameter for wake manipulation, demonstrating that it can reproduce wake characteristics typically associated with higher Reynolds numbers, even when Re is fixed. This highlights new opportunities for passive and active flow control strategies in the transitional regime.