Dr Christoph Heine and Gerd Matschke report on the use of computational fluid dynamics to plot wake flow behind qqqhigh speed trains.

Modern trains are lighter than those of past years. This is due in part to the replacement of a power car at the rear of the train with an unpowered driving trailer. This change has meant lower axle loads, reduced wear on ballast, and increased passenger capacity, since the end car can now be filled with seats. For a light-bodied driving trailer, the unsteady aerodynamic loads may become significant for the running behaviour, and this effect has become a concern for a number of railway operators in Europe.
In the BriteEuram-funded research project Railway Aerodynamics of Passing and Interaction with Dynamic Effects (RAPIDE), the partners have joined forces to investigate the boundary layer development along a modern high speed train and the wake flow characteristics behind the end car using computational fluid dynamics (CFD).
The Fluent CFD investigation was divided into three parts, corresponding to three sections of a moving train: the front car, the six mid-cars, and the trailing car. The boundary layer grows in thickness from the front to the trailing car, and when this thick boundary layer separates behind the trailing car, the points of separation on the train surface can periodically shift. This gives rise to aerodynamic oscillations about the longitudinal axis, which can cause discomfort to the passengers riding in the trailing car. The European organisations MIRA and SNCF performed boundary layer development calculations on the front and mid-car sections. Their results were then used by Deutsche Bahn to simulate the unsteady flow around and behind the German ICE 2 end car.
The end section modelled was 40m in length and positioned in a domain of length 60m, width 20m, and height 15m. A volumetric mesh of tetrahedral and prismatic cells was used. The profiles along the sides and on top of the train generated by the other partners in the project were used as inlet boundary conditions.
A steady-state simulation using the k-e turbulence model was initially performed on multiple processors. The symmetric solution showed low pressure on the shoulder areas of the end car and a high pressure region on the back face that results from the onset of separation. A transient calculation using the steady solution as a starting point. Using time steps of up to 0.01s, unsteady flow developed with a period of oscillation on the order of 1Hz. This frequency was found to be in good agreement with measurements reported by a Japanese railway company1.
Further runs were done using smaller time steps and a higher order turbulence model (RSM), yielding identical oscillations in the flow. Based on the CFD results, the aerodynamic coefficients were calculated. These forces and moments served as an input for multi body systems (MBS) calculations performed by Bombardier Transportation, and the running comfort was evaluated. Luckily, the oscillations were found to be far too weak to cause vehicle movements, so they would not cause any passenger discomfort.
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Dr Christoph Heine and Gerd Matschke are with Deutsche Bahn AG, Germany. For more information, visit www.fluent.com