基于涡旋识别方法的高速列车尾涡结构的讨论

利用改进型延迟分离涡模拟方法对缩尺比例1:30的高速列车简化模型的绕流流场进行数值计算,主要针对近尾流区的涡旋结构展开具体讨论. 通过不同的涡旋识别方法,发现在尾涡结构中,高涡量的强涡旋主要聚集于尾车附近,而涡量较低但处于相对稳定状态的涡旋分布在大部分尾流空间中. 对此,主要基于最新提出的涡旋定义及其物理意义认为,由于边界层在尾部发生的流动分离,剪切变形以及高涡量的扩散对强涡旋的形成发挥着重要的作用,而涡旋会被较强的剪切旋转拉伸,使得局部复杂的流动表现出突出的湍流特性;另一方面,尽管涡强度明显下降,但是在强剪切应变迅速衰减的情况下,流向涡核中的涡旋涡量是主要的,此时,在较接近地面的情况下,流体微团以涡核为中心的旋转运动使得涡旋与地面之间的相互作用成为主导的流动机制. 虽然涡强度会相对缓慢地衰减,但是从湍流能量产生的角度,该机制对涡旋的自维持发挥重要的作用,从而使尾涡结构能够相对稳定地存在于尾流流动中. 

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.