On the influence of free stream turbulence on roughness-induced boundary-layer transition

This study experimentally examines how free stream turbulence (FST) alters roughness-induced transition by exposing cylindrical roughness elements of diameter-to-height ratios $\eta =1,2,3$ to three FST levels ( $ \textit{Tu}\approx 0.05\,\%$ , $1.15\,\%$ , $1.55\,\%$ ). Although the roughness Reynolds number $ \textit{Re}_{\textit{kk}}$ is varied solely through the free stream velocity, a detailed characterisation of the incoming FST and resulting Klebanoff modes shows that their variation remains moderate, enabling a controlled assessment of FST effects. The FST consistently promotes earlier transition for all $\eta$ at subcritical global-instability Reynolds numbers, but its influence on the underlying instability mechanisms depends strongly on $\eta$ . For the ‘thick’ cylinders ( $\eta =2,3$ ), the global instability remains remarkably robust: the dominant varicose mode and its frequency scaling show virtually no sensitivity to FST. In contrast, the ‘thin’ cylinder ( $\eta =1$ ) exhibits an extreme susceptibility: already moderate FST suppresses the sinuous mode predicted by global stability theory at supercritical Reynolds numbers. These findings provide a clear, experimentally grounded explanation for why global stability predictions often fail to materialise the structures in real, turbulence-contaminated environments and establish FST as a decisive factor governing whether, when and how roughness-induced instabilities manifest.