Diss. Stockholm : Kungliga tekniska högskolan. Institutionen för farkost och flyg, 2012

Train transportation is a vital part of the transportation system of today. As the speed of the trains increase, the aerodynamic effects become more important. One aerodynamic effect that is of vital importance for passengers’ and track workers’ safety is slipstream, i.e. the induced velocities by the train. Safety requirements for slipstream are regulated in the Technical Specifications for Interoperability (TSI). Earlier experimental studies have found that for high-speed passenger trains the largest slipstream velocities occur in the wake. Therefore, in order to study slipstream of high-speed trains, the work in this thesis is devoted to wake flows. First a test case, a surface-mounted cube, is simulated to test the analysis methodology that is later applied to two different train geometries, the Aerodynamic Train Model (ATM) and the CRH1.The flow is simulated with Delayed-Detached Eddy Simulation (DDES) and the computed flow field is decomposed into modes with Proper Orthogonal De-composition (POD) and Dynamic Mode Decomposition (DMD). The computed modes on the surface-mounted cube compare well with prior studies, which validates the use of DDES together with POD/DMD. To ensure that enough snapshots are used to compute the POD and DMD modes, a method to investigate the convergence is proposed for each decomposition method. It is found that there is a separation bubble behind the CRH1 and two counter-rotating vortices behind the ATM. Even though the two geometries have different flow topologies, the dominant flow structure in the wake in terms of energy is the same, namely vortex shedding. Vortex shedding is also found to be the most important flow structure for slipstream, at the TSI position. In addition, three configurations of the ATM with different number of cars are simulated, in order to investigate the effect of the size of the boundary layer on the flow structures. The most dominant structure is the same for all configurations, however, the Strouhal number decreases as the momentum thickness increases. The velocity in ground fixed probes are extracted from the flow, in order to investigate the slipstream velocity defined by the TSI. A large scatter in peak position and value for the different probes are found. Investigating the mean velocity at different distances from the train side wall, indicates that wider versions of the same train will create larger slipstream velocities.