不同控制角下附加圆柱对圆柱涡激振动影响

在弹性支撑的圆柱周围布置直径更小圆柱会影响剪切层发展以及旋涡脱落,进而改变其涡激振动状态.通过不同的布置形式和附加小圆柱个数可以实现对圆柱涡激振动的促进或抑制.激励更大幅值的振动可以更好地将水流动能转化为可利用的机械能或电能,抑制其振动则可以实现对海洋平台等结构物的保护.采用基于迭代的嵌入式浸入边界法对前侧对称布置两个小圆柱的圆柱涡激振动进行数值模拟研究,系统仅做横向振动,其中基于主圆柱直径的雷诺数为100,质量比为2.0,折合流速为3~11.小圆柱与主圆柱的直径比为0.125,间隙比为0.125.结果表明,在研究的控制角范围内(30°~90°),附加小圆柱可以很大程度上改变圆柱涡激振动的状态.当控制角较小(30°)时,附加小圆柱对主圆柱的振动起抑制作用;当控制角为45°~60°时,圆柱的振动分为涡振和弛振两个阶段,在弛振阶段,圆柱振幅随折合流速增加而持续增加;当控制角较大(75°~90°)时,附加小圆柱的促进作用随着控制角增加而减小.进一步地,结合一个周期内不同时刻旋涡脱落以及圆周压强分布,解释了附加小圆柱对主圆柱涡激振动的作用机制.应用能量系数对圆柱系统的进一步分析发现,弛振阶段由流体传递到主圆柱的能量系数随折合流速的增加逐渐下降,旋涡结构的改变是产生这种变化的直接原因. 

EFFECTS OF THE ADDED CYLINDERS WITH DIFFERENT CONTROL ANGLES ON THE VORTEX-INDUCED VIBRATIONS OF A CIRCULAR CYLINDER

Vortex-induced vibrations of an elastically mounted circular cylinder will be altered through influencing the development of the boundary layer of the surface and the vortex shedding by the added smaller cylinders. The excitation or suppression of vortex-induced vibrations can be obtained by changing the arrangement and number of the small cylinders. In the former, more fluid energy can be transformed into mechanical energy or electricity while the latter can be applied to protect the structures. Numerical simulations of a transversely vibrating cylinder with two small cylinders behind were conducted, where the Reynolds number is 100, based on the main cylinder, the mass ratio is 2.0 and the reduced velocity is 3~11. The diameter ratio between the small and the main cylinder is 0.125 and the gap ratio is 0.125. Results indicate that the small cylinders can change the vibration of the main cylinder significantly in the simulated control angle range of 30°~90°. When the control angle is small (30°), the small cylinder suppresses the vibration of the main cylinder. The response can be divided into two branches, i.e. VIV-and galloping-branch, at the control angle of 45°~60°. The vibration amplitude increases monotonically with the increasing reduced velocity in the galloping branch. When the control angle is large (75°~90°), the promotion from the small cylinder decreases with the increase of the control angle. Furtherly, mechanisms of the small cylinders are explained by combining vortex shedding and pressure distribution around the cylinder of different instants in one period. Analysis of the energy coefficient indicates that the energy transferred from the fluid to the main cylinder decreases with the reduced velocity, which is caused by the variation of vortex structures.