Experimental Investigation on Roughness-Induced Transition Under the Influence of Freestream Turbulence

The impact of freestream turbulence (FST) on a roughness-perturbed, laminar boundary layer is investigated in a laminar water channel. Both wire-mesh grid and rising air bubbles are used to induce random disturbances to the flow. The transition Reynolds number depends significantly on the level of random fluctuations. This confirms that the lower transition limit, which is relevant to most engineering applications, depends on external noise and is thus difficult to predict without precise knowledge of related flow parameters. Moreover, visualizations and harmonic forcing with a vibrating wire suggest that the near and far wake may respond differently to freestream turbulence. The near wake can effectively be forced in a region at the height of the roughness, whereas the far wake is less sensitive to this type of forcing.