Keywords: detached eddy simulation, aerodynamic train model, CRH1, proper orthogonal decomposition, slipstream, train aerodynamics.

Two different train geometries, the aerodynamic train model (ATM) and the CRH1, are studied in order to compare the flow fields around the trains. This paper focuses on the flow structures and flow topologies in the wake. The flow is simulated with detached eddy simulation and decomposed into modes with proper orthogonal decomposition and dynamic mode decomposition, respectively. The topology of the flow is found to be different for the two train geometries, where the flow behind the ATM separates with two counter-rotating vortices, while the flow behind the CRH1 separates with a separation bubble. The difference in flow topology is seen, for instance, in the mean pressure at the tail, the mean flow in the wake and streamlines of the flow. Despite the different flow topology, there are also similar flow structures in the wakes behind the ATM and the CRH1, such as vortex shedding.

In order to measure the slipstream effect of the two vehicles, the velocity in a ground fixed point has to be extracted from the train fixed flow field. The resulting velocity is averaged with an equivalent of 1s time average at full scale. The contribution of the DMD modes to slipstream has been analyzed and it is found that the same flow structure that is dominant in energy is also important for the slipstream.

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