Keywords: aerodynamics, freight train, computational fluid dynamics, large eddy simulation.

The purpose of the research, presented in this paper, is to study the flow physics of the air flowing around a single-stacked container freight wagon model. This is desirable since the aerodynamic drag is responsible for a large part of the total energy consumption for a freight train, even though this part is smaller compared with e.g. for high speed trains. The amount of drag imposed on a wagon depends on how the air flows around the wagon, which can be understood by studying the flow physics of the air around it. Thus, if we would like to decrease energy consumption by decreasing the aerodynamic drag of a train, a good start is to first study how the flow behaves around it and where high contributions to the drag arises in the geometry. As a result of the non-intuitive and semi-chaotic nature of fluid dynamics, unexpected answers may sometimes be obtained.

The model in this work consists of a 11.8 long container placed on an eight-wheel freight-liner (four in the front and four in the rear). The geometry of the model is simplified in relation to a real freight wagon, but the main geometrical features of a container freight wagon is present in the model. Only one single alone-standing wagon is considered in this work.

To meet the purpose of the study the flow around the freight wagon model was simulated numerically. The numerical technique that was used was large eddy simulation (LES). The Reynolds number in the simulation was 100,000 based on the width of the container. The work was purely numerical and no direct comparisons to any wind tunnel experiments or other numerical studies could be done. However, the value of the drag coefficient (0.90) in the simulation correlated well with values from wind tunnel studies where similar types of container freight wagons were studied. The side force signal was found to contain larger oscillations and a broader spectrum of frequencies compared to that of the drag force signal. 74% of the drag force was found to come from the container. The rest comes from the undercarriage. The calculated spatial resolution in the simulation showed that the resolution was very fine even compared to standard requirements for well-resolved LES simulations. Visualization of the flow field with the help of streamlines showed that the flow around the front of the wagon is governed by a large separation originating from the leading edges. The flow under the wagon is more complex.

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