The Temperature-Sensitive paint technique (TSP) has proven to be a valuable tool for visualizing near wall flow structures on high-speed train models as used in low-speed wind tunnels for crosswind investigations. It shows the laminar-turbulent transition on the model as well as the structure of separation bubbles and their Reynolds number scaling. The footprints of the leeside vortices at their point of origin on the train's nose can be detected and the changes in vortex dynamics for yaw angle variations become visible in the TSP images. A better understanding of the aerodynamics appearing at low Reynolds numbers in train testing (0.25 Mio < Re < 1 Mio) can help explaining the Reynolds number scaling of force and moment data, as well as for the results obtained in different wind tunnels when using different model scales, surface quality, level of detail on the models and the like, since all these quantities are influencing the flow structures occurring in the near wall flow field. Thus, it seems to be possible to implement model modifications like suited boundary layer tripping and suction to overcome the laminar separation on the windward roof edge, for example. This will enable us to assemble more reliable force and moment coefficients even in conventional, low speed wind tunnels. An accompanying TSP analysis then can help checking the functioning of these modifications. Furthermore, TSP wall-flow visualization can be used to validate numerical simulations of high speed train flow for given Reynolds numbers, since it is easy to plot the wall friction distribution on the model for CFD results using DNS, LES or RANS. The CFD should bear at least comparable flow structures like the separation bubble or the complex vortex topology on the nose. Furthermore, CFD needs the laminar or turbulent state of the boundary layer as an input. At the time of writing, numerical work using the DLR TAU code is in progress at the DLR Institute of Flow Technology simulating the complete wind tunnel situation including test section walls, different ground simulations, the representation of the bogies and even the struts where the force balance has to be mounted. Comparison to TSP results will be used to calibrate the numerical tools. In addition, the TSP results showing the wall-flow topology can help finding the right locations for the PIV and LDA experimenters, so they can concentrate on the interesting flow structures with their techniques.


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