Unsteady flow structures in the wake of a high-speed train

This paper reports an experimental investigation on the wake of a 1/50th-scale high-speed train (HST) with a slenderness ratio, L/W, of 15.7 (L and W are the length and width of the train model, respectively). Hot-wire anemometry, flow visualization and particle image velocimetry (PIV) measurements are conducted in a close-loop low-speed wind tunnel at a Reynolds number of 1.3 × 105, based on the free stream oncoming flow velocity U∞ and W. The results of both the frequency spectrum analysis and flow visualization suggest that the instantaneous near wake of a slender HST is dominated by a pair of large-scale counter-rotating streamwise vortices, which are shed alternatingly. Utilizing the proper orthogonal decomposition (POD) analysis for the PIV measurement results, the dynamic characteristics of the near wake are clarified. The first six POD modes, corresponding to the dominant coherent structures, are described in more detail. Each of the large-scale streamwise vortices presents increase/decrease alternatingly in size along with oscillating behaviour in both lateral and vertical direction, which is ascribed to the tilted vortex shedding from the bogie section of the trailing carriage. Moreover, the interaction between the streamwise vortices and the ground is also discussed.