Keywords: cross wind stability, high-speed train aerodynamics, wind tunnel experiment, vortex detection, particle image velocimetry, force measurements.

Cross wind stability is one of the most relevant safety topics for the development of modern high-speed trains. New materials and better construction principles reduce the weight of train cars. Combining double deck configurations with an increased cross-sectional area which are developed to increase passenger capacities with higher operational speeds, the effect of cross winds on the driving dynamic behaviour of the train increases dramatically.

Wind tunnel experiments were performed in the Side Wind Test Facility Göttingen (SWG) and the Cryogenic Wind Tunnel Cologne (KKK). The measurements in SWG were performed to study the leeward velocity field of the NGT2 double deck high-speed train under cross wind conditions with particle image velocimetry (PIV) and its influence on the induced aerodynamic loads with force measurements at Reynolds numbers of up to 4.00E+05. For detailed investigations of the Reynolds number effects on the aerodynamic loads the configurations used in the SWG were also used in the KKK with the same test setup up to Reynolds numbers of 7.50E+05. The KKK has the option to increase the Reynolds number by a factor of up to 5.5 by reducing the gas temperature using liquid nitrogen and to vary both Reynolds number and Mach number independently.

The influence of the averaged leeward velocity field of the NGT2 double deck high-speed train on the mean aerodynamic coefficients is demonstrated for different cross wind conditions. It can be shown that the characteristic of the upper quasi stationary leeward vortex depends on the inflow conditions and directly influence the force coefficient dominating train stability. A vortex detection algorithm was applied to detect the position and the core width of the vortices in the train wake field. A wavelet analysis using the 'lambda two' criterion was conducted which offers the advantage to be much less sensitive to the shear layer than the vorticity.

The significance of research studies in the field of unsteady aerodynamics will become more important for the definition of safety requirements for high-speed trains in the future. In addition to detailed investigations of the instantaneous flow at stationary inflow conditions, the interest in more realistic studies will increase. This can be realised by the application of moving model tests.

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