Experimental investigations of critical roughness heights and roughness-induced laminar-turbulent transition in a laminar boundary layer

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The influence of wall roughness on laminar-turbulent transition has just attained increasing relevance in connection with efforts on drag reduction for boundary layers. To back up theoretical investigations which are pursued in several European countries, there is a clear need for an experimental investigation to validate the most recent developments in linear stability theory and their usefulness for predicting the critical roughness height in a laminar boundary layer. The present experiments are intended to yield insights into the acting mechanisms and to check and validate assumptions and results of the theory. A very unique low-speed water channel facility with carefully controlled low-turbulence level is available for the present research. Flow visualizations, hot-film measurements, investigations with linear stability theory and PIV-measurements will be performed at the University of Stuttgart. Base-flow data, results of 3D global instability, and DNS data will be contributed by international partners from KTH Stockholm, Sweden and ENSAM (ParisTech), Paris, France. In return, the experimental results will be shared by all three research groups. This offers a lot of synergies since the requested DFG-funding is restricted to the experimental work.