圆柱尾迹涡的三维演化及结构特征

NUMERICAL STUDY ON THE MECHANISM FOR THREE-DIMENSIONAL EVOLUTION OF VORTEX AND THE STRUCTURAL FEATURES IN THE WAKE BEHIND A CIRCULAR CYLINDER

应用无粘涡丝运动学理论和局部诱导近似(LIA)方法,以Lagrange观点跟踪涡丝在背景流场中运动,用数值方法研究了中等Re数(≈10~3)下圆柱分离尾迹中Kármán涡和涡辫区涡丝的三维演化的机制和动力学过程,及其结构特征。背景流场考虑为尾迹时间平均速度流场和Kármán涡街流场。初始展向小扰动为指数形式和谐波形式。结果指出:Kármán涡和涡辫区中的涡丝具有展向不稳定性,形成流向涡量。在尾迹的初期输运过程中,表现出有序的大、小尺度涡结构。并进一步分析了其产生的机理。

In this paper, the mechanism and dynamic process for three-dimensional evolution of the Karman vortices and the vortex filaments in the braid regions, and the structural features in the wake behind a ciccular cylinder at moderate Reynolds number (≈103) have been studied numerically. The vortex filament in the wake field are tracked by the inviscid vortex dynamics and the localized-induction approximation (LIA) method. The time-averaged velocity field of the wake and Karman vortex street flow field are considered as two kinds of the background flow field. The deformation of vortex filament is assumed to be periodic along the spanwise direction. The initial small disturbances used are exponential and harmonic disturbances in a period, respectively. The results show that Karman vortices and the vortex filaments in the braid regions have the spanwise instability, and the streamwise vorticity arises from the instability. The large and small scale organized vortical structures in the process for three-dimensional transition are expressed. The mechanism of their evolution is analyzed further in the paper.