Detection of Lambda- and Omega-vortices with the temperature-sensitive paint method in the late stage of controlled laminar–turbulent transition

An experiment investigating the laminar–turbulent transition of a Blasius boundary-layer-like flow was set up in the laminar water channel at the Institute of Aerodynamics and Gas Dynamics, University of Stuttgart. The late stage of controlled transition with K-type breakdown was investigated with the temperature-sensitive paint (TSP) method on the flat-plate surface. Additional velocity measurements in the boundary layer were performed with the hot-film anemometry for better interpretation of the TSP results. The test conditions enable the TSP method to resolve the complete transition process temporally and spatially. Therefore, it was possible to detect the coherent structures occurring in the late stage of laminar–turbulent transition from the visualizations on the flat-plate surface: namely, $\Lambda$- and $\Omega$-vortices. The transition location is derived from the TSP visualizations with a gradient-based method and with the Turbulence Energy Recognition Algorithm (TERA) from the velocity measurements. The derived average transition location shows good agreement between the two techniques, but the TSP method detected a later beginning and earlier end of transition. Graphical abstract:

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