DISCUSSION ON THE WAKE VORTEX STRUCTURE OF A HIGH SPEED TRAIN BY VORTEX IDENTIFICATION METHODS

A flow field around a 1/30th-scale and simplified model of high speed train (HST) has been numerically calculated by the improved delayed detached eddy simulation, and the vortex structure in the near wake detailedly discussed as a focus. According to different vortex identification methods, it can be observed for the wake vortex structure that powerful vortices with high vorticity magnitude mostly appear in the vicinity of the tail; however, there are stable vortices with lower vorticity widespread in the near wake region. Based mainly on the findings and the newly-proposed definition of a vortex and physical meaning, there are conclusions given as follows. Shear deformation and high vorticity diffused play significant roles in forming those energetic vortices, due to boundary layers separated from the streamlined tail. And turbulent eddies have to be rotated and strained by the strong shears, thus resulting in prominent turbulent characteristics of the local complex flow. On the other hand, though strength of the vortices evidently drops, vortical vorticity is dominant inside the streamwise vortex cores when the strong shear strains rapidly decay. Under the circumstances, fluid particles rotate round the cores that get closer to the ground, and thus the interaction between the vortices and the ground becomes a dominant mechanism. The vortices have to be diminished at relatively low rate, but considering turbulence production, the flow mechanism can play an important role in self sustainment of turbulent eddies. As a result, the vortex structure is able to stably be in the wake flow.