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

研究了垂直圆柱在静止线性分层流体中进行横向振荡所引起的流动结构。密度梯度抑制了三维不稳定性的发生。不稳定性表现为沿圆柱体轴有规则间隔的条纹流。每条条纹流都沿着圆柱体表面振荡。当条纹流的振荡运动与相邻的条纹流相位差为180‰时，条纹流沿圆柱体轴交替排列。当无量纲振幅或频率增加到临界值以上时，沿圆柱体轴方向交替产生的条纹流不再沿圆柱体表面振荡，而是像均质流体一样沿圆柱体振荡方向直线延伸。

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.