Three-dimensional Instability of the Flow Induced around an Oscillating Cylinder in a Density Stratified Fluid at Rest

研究了静止的线性分层流体中垂直圆柱体的横向振动诱导的流动结构。密度梯度抑制了三维不稳定性的发生。这种不稳定性表现为沿圆柱轴有规则间隔的条纹流。每一条纹流都是沿着圆柱体表面振荡脱落的。当无因次振幅或频率增加到临界值时，沿圆柱轴线交替排列的规则间隔的条纹流动与相邻的条纹流动的振荡运动相差180ﾟ。当无因次振幅或频率增加到临界值时，沿圆柱轴线交替产生的条纹流动不会沿着圆柱表面振荡，而是以与均匀流体相同的方式沿圆柱振荡方向直线延伸。

The structures of the flow induced by a vertical circular cylinder performing transverse oscillations in a linearly stratified fluid at rest are investigated. The density gradient inhibits the onset of the three-dimensional instability. The instability appears as regularly spaced streaked flows along the cylinder axis. Each streaked flow is shed being oscillated along the cylinder surface. Regularly spaced streaked flows arrange themselves alternately along the cylinder axis as the oscillating motion of a streaked flow is 180ﾟ out of phase with adjacent streaked flows.When the non-dimensional amplitude or frequencies are increased beyond critical values, the streaked flows generated alternately along the cylinder axis do not oscillate along the cylinder surface but extend straight in the direction of cylinder oscillation in the same manner as in the homogeneous fluid.