Experimental investigations of the combined influence of free-stream turbulence and roughness on the laminar-turbulent transition

• Applicant: Privatdozent Dr.-Ing. Christoph Wenzel
• Subject Area: Fluid Mechanics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567603033

Over the past years a worldwide unique experimental setup has been established at the laminar water channel at IAG that allows to systematically analyze the influence of free-stream turbulence (FST) on the flow around roughness elements. Based on this setup it is the primary goal of this proposal to develop an in-depth understanding and description about the influence of FST on the roughness-induced transition. Among others, this includes the determination of a valid empirical correlation between the transition Reynolds number, cylinder size and aspect ratio, and FST. To this end, a clear characterization of the grid dependence of the quasi-stationary streaks (Klebanoff modes) initiated by turbulence grids in the boundary layer has been carried out in the last year for the first time. Based on this preliminary work, an empirical relation for the time-averaged transition Reynolds number as a function of the FST and the roughness-based Reynolds number $Re_{kk} = u_k k /\nu$ is to be determined as part of this project. Subsequently, the influence of the Klebanoff modes, which are naturally excited by the FST, on the flow past the cylinder is investigated. In addition, preliminary investigations have shown that the modes emerging in the cylinder wake are clearly influenced by FST, a finding that has not yet been found in the literature. However, since the character of these modes is directly related to the critical Reynolds number, their understanding is highly relevant for the fundamental description of roughness-induced transition and could provide clues to the causes of numerous previously unexplained results in the literature on roughness-induced transition.

• DFG Programme: Research Grants