Experimental and numerical investigations on laminar-turbulent transition behind roughness with free-stream turbulence

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The influence of roughness on the stability of boundary layers is a known but not yet fully understood problem in fluid mechanics. New theoretical methods can now calculate a global critical Reynolds number, but at the same time it was recognized that this does not correspond to the transition Reynolds number in the experiment. Background disturbances in the experiment were suspected as an explanation. Consequently, this also means that the transition Reynolds number depends on the degree of turbulence.The aim of this research project is to investigate the extent to which flow instabilities behind roughness elements in the boundary layer actually depend on free-stream turbulence. For this purpose, the transition Reynolds number is first determined in the undisturbed boundary layer. The degree of turbulence is then slowly increased, causing the transition Reynolds number to drop according to our expectations. This relationship is determined and tested for universality by parameter studies for different roughness diameters.These experiments are supported by direct numerical simulations to analyze phenomena in the global flow field and to better understand the underlying instability mechanisms. In addition, calculations using linear stability theory are performed by us or our cooperating colleagues and compared with the experiment and direct numerical simulations.The experimental and numerical results are of high scientific and practical relevance. On the one hand, we provide urgently needed experimental data, which are necessary for the further development of theoretical methods. To this end, we adapt our setups to the theoretical investigations. In addition, our experiments provide precise and robust data that can be used in practical applications with known turbulence levels.