The effect of tail geometry on the slipstream and unsteady wake structure of high-speed trains

The effect of varying the roof-angle of the tail of a generic high-speed train on the unsteady wake structure and corresponding slipstream is investigated in a 1/10th scale wind-tunnel experiment. Insight into the slipstream and unsteady flow features are gained from 4-hole dynamic pressure probe measurements, surface-flow visualisations, measurements from a two-dimensional array of total pressure probes, and frequency analysis. The results show that increasing tail roof angle leads to a transition from an unsteady wake with a pair of streamwise vortices that exhibit sinusoidal, antisymmetric motion to an unsteady wake dominated by large-scale separation with vortex shedding from the sides of the train. It is the interaction of the streamwise vortices with vortex shedding from the sides that results in the widest unsteady wake, and consequently, the largest slipstream velocities.