Experimental Investigation of Topological Changes in the Flow Field around High-Speed Trains with respect to Reynolds Number Scaling Effects

Abstract

Temperature-sensitive paint and oil-film flow visualization techniques are used to measure and visualize the near-wall flow field of high-speed train models subject to cross wind flow in a wind tunnel. An ICE3 and a NGT2 train model (next generation train) are investigated with respect to laminar-turbulent boundary layer transition and flow separation. Reynolds numbers in the range 0.20 <= Re <= 0.50x106 are realized, and the scaling of flow structures for different Reynolds numbers and yaw angles of the model is studied. The specific flow topology is correlated with force and moment coefficients, which are measured with a strain gauge balance. It is shown, that in the described Re-range laminar flow exists to an extent, which is not to be expected on a real train. Laminar-turbulent transition and laminar separation bubbles are present and their location and size depend on model geometry, Reynolds number and yaw angle. The Reynolds number scaling of the separation bubble is analysed in detail and an optimized transition tripping method is proposed to control the flow separation, leading to considerably less Re-dependency of the force and moment coefficients in the range 0.30 <= Re <= 0.50x106 and a better agreement with benchmark data given for higher Reynolds numbers.