Smoke-flow visualization of instabilities behind a cylindrical roughness element

The flow around a geometrically simple cylindrical roughness element submerged in a laminar boundary layer is a seemingly simple problem in fluid mechanics. However, a more in-depth analysis reveals intricate phenomena in specific parameter ranges, illustrating that more research is required. A smoke-flow visualization study in a large parameter space was performed. The experimental setup features a cylindrical roughness element connected to a linear traverse, enabling almost continuous variation of the roughness height. Three synchronized high-speed cameras were used to record the smoke streaks, illuminated by a continuous laser sheet. Results reveal that the instability in the roughness wake switches from convective to global instability at a critical Reynolds number. While the convective instability always exhibits a varicose shape, the shape of the global instability depends on the geometrical properties of the roughness element. Depending on the ratio of roughness diameter and height, the shape can be varicose or sinuous. Furthermore, frequency information is extracted from the smoke-flow visualization images and indicate that, in the sinuous instability regime, the dominating frequency in the wake can be well predicted using a Strouhal number.