Crossflow-Dominated Laminar-Turbulent Transition Downstream of an Isolated Roughness Element of Small Height

Abstract

The effect of an isolated roughness element on laminar-turbulent transition has been thoroughly investigated in the range of roughness Reynolds numbers Re_k for which the laminar-turbulent transition front advances to the immediate vicinity of the isolated roughness, a behaviour often referred to as direct tripping. Experimental studies for this range have been conducted in both two-dimensional and three-dimensional boundary layers. Radeztsky et al. showed that roughness elements with Re_k values orders of magnitude smaller can already have a significant impact on the location of laminar-turbulent transition. In this investigation, the effect of an isolated roughness element with a particularly low roughness Reynolds number of Re_k = 0.55 (compared to typical critical values for direct tripping of Re_k >> 100) is studied experimentally in a three-dimensional boundary layer. Detailed spatially scanned hot-wire anemometry measurements are performed in the SPECTRA-B configuration. The development of steady and unsteady instabilities is characterized in comparison to a reference case without artificial roughness and a case with spanwise-periodic forcing.