Keywords: Star-CCM+, high-speed train, inter-car-gap, weak cross-wind, train car offset.

The paper presents RANS based three-dimensional simulations of a generic high-speed train comprising two and half cars, including the end car. The CFD code Star-CCM+ was used with the k-epsilon realizable turbulence model and a polyhedral mesh comprising 35 million points was used for the train configuration. The main objective of the work presented in this paper is to numerically investigate a high-speed train configuration, particularly, to assess the influence of coach offset and weak cross-wind in the ICG flow structure and drag force. The coach offset is a consequence of the misalignment between two consecutive cars. It was considered that the offset results from a pure translation of the car downstream the inter car gap (ICG) by 20 mm.

The cross-winds studied in this paper create very small yaw angles (0.59 degrees) resulting of a side horizontal velocity component of 2 m/s and a streamwise component of 100 m/s. Interest is related to their influence on drag as well as on other force components and moments. Particularly the influence of cross-wind in the ICG is quantitatively evaluated, as well as the combined action of offset and weak cross-wind.

The results show a very complex flow structure inside the inter car gap with a distorted swirling fluid motion and the presence of strong secondary flows forming cellular structures. The drag for each ICG accounts to 4 % of the two and half car total drag.

Small offset values have little influence in the overall coefficient of drag for the whole train but large differences, up to 30 % relative to the reference case, may occur in the ICG mainly in terms of pressure drag. Boundary layer influence occurs only downstream of the offset ICG, mainly on the side of the train and underneath, influencing the bogie coefficient of drag. The flow becomes asymmetric in an offset ICG geometry.

The 2 m/s cross-wind considered influenced only the coefficient of drag for the whole train by less than 1 %. There was no indication of flow separation although a flow twist occurs in the ballast bed leeward side. A slight asymmetry also occurs in the bogies as the flow twist increases. Regarding the offset geometry under cross-wind, previous effects add up resulting in a flow with similar asymmetries in the nose, ICG, bogies and boundary layer development. The ICG drag coefficient increased by 34 % and the constribution to the total train drag coefficient was 3 %.

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