Poiseuille流通過往復旋轉圓柱的渦流抑制與其物理現象

Vortex Suppression and Other Physical Phenomena for Poiseuille Flow Past a Cylinder with Rotary Oscillation

在本論文中，我們藉由計算流體力學，研究二維剪流通過週期性往復旋轉之圓柱所造成的渦流現象，我們特別感興趣的是渦流的抑制（vortex suppression）效應。為此，我們將圓柱置於Poiseuille流的中線上，此流場係透過壓力梯度驅動，速度梯度呈線性分布。本論文涵括一個幾何參數和兩個運動參數，前者為阻塞係數B，定義為圓柱直徑與渠道寬度之比值，我們研究了B = 0.2與0.5之狀況；後者則為無因次化的圓柱往復旋轉周期T/T_s（其中T係表圓柱往復旋轉之周期，T_s則表圓柱靜止下的Kármán渦漩週期）與最高旋轉速度V_m/U_avg（其中V_m表圓柱往復轉動的速率峰值，U_avg則表入流的平均速度），我們對此二參數作了一系列的變化方式。我們發現隨著參數組合的不同，流體通過圓柱後之流場有許多有趣的現象，包括圓柱本身的阻力與升力係數之變化、渦流特性的變動等，同時我們也發現在適當的參數條件下，渦街可得到不錯的抑制。 In this paper, we computationally studied the physical phenomena of a two-dimensional shear flow past a circular cylinder with rotary oscillation. Of particular interest is the vortex suppression due to the rotary oscillation. In this particular study, the cylinder is placed in a Poiseuille flow which is driven by a constant pressure gradient in a channel of infinite length and whose velocity gradient on far-upstream cross sections varies linearly. There are a geometric parameter and two kinematic parameters in the present problem. The former one is the blockage ratio, B, defined as the ratio of the cylinder diameter to the channel width. In the present study, we choose B = 0.2 and 0.5 for study. The latter ones include the rotation ratio, ξ, defined as the peak rotation speed of the cylinder to the average incoming velocity, and the period ratio, P, defined as the rotary oscillation period of the cylinder to the vortex shedding period when the cylinder is fixed. We vary the two parameters systematically in computations. Many interesting flow phenomena are revealed, including the drag reduction, lift reduction, and various vortex flow patterns. In addition, we find that under a suitable combination of these parameters, the vortex street can be effectively suppressed.