Keywords: underfloor aerodynamics, ballast flight, ballast projection, high-speed trains, vehicle aerodynamics, particle image velocimetry.

The increasing speeds of today's high-speed trains and the desire to travel even faster in the future have increased the interest in the aerodynamics of high-speed trains. The train induced aerodynamic loads on the trackbed are strong enough to dislodge the gravel stones (ballast) and set them into motion or even propel them airborne. This flying stone phenomenon underneath trains is referred to as ballast flight or ballast projection. To be able to asses the risk of ballast flight the flow field underneath high-speed trains has to be investigated.

Therefore measurements of the underfloor flow field for three different 1:50 generic high-speed train models hauled in a water towing tank over a smooth, a rough and a ground with sleepers were conducted with two-component high-speed particle image velocimetry (PIV). The PIV setup was arranged in a way that the vertical plane between the ground and the train at the centre line of the train could be measured. The train model speed was set to 4 m/s corresponding to a Reynolds number of 0.25 Mio calculated with the train model speed, reference length of 0.06 m and the kinemtic viscosity of water. The reference train model consisting of four cars, features inter-car gaps and two bogies per car. These features are removed and all gaps are closed for the smooth train configuration, while the rough train configuration reflects all gaps but no bogies.

The results obtained revealed that the underfloor flow is characterized by four regions, the head region, the Couette-like underfloor flow region, the near wake region and the far wake region. In the head region the flow is strongly accelerated followed by a deceleration before developing a Couette-like flow. At the end of the train in the near wake region, the highest velocities are reached after a strong acceleration that decays in the far wake region. Comparing the results for the three considered train models proved that bogies, bogie cavities, inter-car gaps or any other protruding objects increase the local and overall velocities and turbulence level in the underfloor flow and consequently increasing the risk of ballast flight. The rough ground and the sleepers had a positive effect with respect to the risk of ballast flight since they decelerate the underfloor flow and thus reduce the chance for ballast flight.

The results for the reference train model were also compared with full scale measurements on the Italian high-speed train ETR 500. The comparison disclosed that the main flow characteristics are captured by the downscaled train model measurements in the water towing tank.

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