A study of the aerodynamics of a generic container freight wagon using Large-Eddy Simulation

Artikel i vetenskaplig tidskrift, 2014

In this work simulations using the Large Eddy Simulation technique have been made of the flow around a generic container freight wagon model. The model consists of one 11.8 m standard length container placed on a wagon. Details of the undercarriage such as wheels are included, but the container is generic and smoothed in comparison to a real freight wagon. The Reynolds number of the flow is 105 based on the container width of 2.354 m. Two cases have been considered in the study, one case where the wagon is standing alone and one case where it is submerged into a train set with wagons ahead and behind the wagon. The latter case is simulated using periodic boundary condition. Both the time-averaged and the instantaneous flow around the wagon for the two cases are described. For the single wagon case, it is found that the separation bubble formed on the roof of the container oscillates back and forth in the streamwise direction and that this oscillation is in phase with oscillations found in the upper shear layer of the ring vortex in the wake. The mechanism that is causing the synchronization of the oscillations of the separation bubble at the front and the upper shear layers in the wake is found to be waves of vorticity being shed from the separation bubble. The time-averaged ring vortex in the near wake of the single wagon is found to be inclined due to the disturbance of the undercarriage details on flow in the lower shear layer. The lower center of the ring vortex is located closer to the base face than the upper center. The drag coefficient of the wagon in the periodic case was found to be only 10% of that of the single wagon case. This is due to two symmetrical counter-rotating vortices found in the gaps which make the train set appear as a single body to the oncoming flow and shielding the wagon from any direct impingement of the flow. The counter-rotating vortices in the gap are found to inhibit periodic oscillations in the lateral direction. These oscillations cause vortical structures to form by the air that is pushed out from the gap and these flow structures cause a dominating oscillation of non-dimensional frequency St=0.12 in the side force signal.