Numerical Study of the Unsteady Flow Structures Around Train-Shaped Body Subjected to Side Winds

Paper in proceeding, 2006

Large-eddy simulation (LES) is made to investigate the flow around a generic train under side wind conditions. Two different side wind yaw angles are used in the investigation: 35 and 90 degrees. The Reynolds numbers based on the height of the train and the freestream velocity are 300000 and 370000 for the 90 and 35 yaw angles, respectively. Two computations on two different meshes are made for each simulation in order to check the effect of the mesh resolution on the results. The fine and the coarse meshes give similar results for each simulation meaning that the results are mesh independence. The results are also verified against experimental data that have been collected on the numerical model at the same Reynolds number. Good agreement is obtained between the LES results and the experimental data. The LES results showed that the flow around the train at $90^o$ yaw angle is dominated by unsteady vortex shedding. Both the large-scale instability associated with the shedding of large scale vortices from the recirculation region to the far wake flow and the small-scale instability associated with the small-scale Kelvin Helmholtz instability are clearly exist yielding highly unsteady flow. The LES results for the $35^o$ side wind yaw angle show that two flow regimes exist in the wake. The first flow regime is the steady vortex lines in the upper part of the wake flow. It changes into unsteady shedding after some distance from their onset on the surface of the train. The second flow regime is the unsteady movement of the lower part of the wake vortices. They attach and detach from the surface of the train in a regular fashion. The influence of side wind yaw angle and the wake structures on the aerodynamic coefficients is discussed in the paper. The paper gives a picture of the unsteady flow and its instabilities around trains at large and low yaw angles.