Vortex-induced vibrations of two cylinders in tandem arrangement in the proximity-wake interference region.
Vortex-induced vibrations of two cylinders in tandem arrangement in the proximity-wake interference region.
复制标题
DOI：
10.1017/s0022112008004850
复制
发表时间：
2009
影响因子：
3.7
通讯作者：
Sotiropoulos, Fotis
中科院分区：
文献类型：
--
作者：
Borazjani, Iman;Sotiropoulos, Fotis
在雷诺数Re = 200条件下，我们研究了两个相同的二维弹性圆柱体在接近尾迹干涉下串联的涡激振动（VIV），该系统具有一个（横向振动）和两个（横向和直线）自由度（分别为1-DOF和2-DOF）。对于高雷诺数下的1自由度系统，计算结果与现有实验结果定性一致。与这些实验相似，我们的模拟表明：(1)与孤立圆柱相比，串联布置的运动幅度更大，锁定区域更宽；(2)低减速时，前气缸的振动幅值大于后气缸；(3)在减小速度阈值以上，后缸激发出的涡激振动幅值明显大于前缸。通过分析模拟流型，我们确定了导致这种复杂响应的涡激激励机制，并阐明了每种情况下激发的近尾迹涡量动力学和涡脱落模式。我们表明，在低减速时，涡旋脱落提供了初始激励机制，这导致了两个圆柱体之间的垂直分离。然而，当这种垂直分离超过一个气缸直径时，很大一部分来流能够通过两个气缸之间的间隙，间隙流动机制开始主导涡动动力学。间隙流能够周期性地迫使前气缸的顶部或底部剪切层进入间隙区域，引发一系列非常复杂的涡-涡和涡-气缸相互作用，产生压力梯度，导致与涡脱落相一致的大振荡力，从而导致实验观察到的较大振动幅值。当涡旋脱落为主要机制时，前缸振动幅值大于后缸振动幅值。这种趋势的逆转超过阈值降低的速度与间隙流的开始有关。通过对二自由度系统的一系列仿真，进一步说明了间隙流的重要作用。我们发现，当间隙流动机制被触发时，2-DOF系统可以发展并维持与相应的（相同降低速度）1-DOF系统相当的大的涡激振动幅度。然而，对于足够高的减速速度，2-DOF系统中的两个圆柱体彼此接近，从而显着减小了间隙区域的大小。在这种情况下，间隙流被完全消除，两个圆柱体作为一个整体一起振动，振动幅值比间隙流活跃的相应1自由度的幅值低50%。三维模拟也进行了检验二维模拟的充分性描述串联系统的动态响应在Re = 200。结果表明，当间隙流活动时，即使尾迹过渡到弱三维状态，但三维模态太弱，无法影响系统的动态响应，与二维计算结果相同。
We investigate numerically vortex-induced vibrations (VIV) of two identical two-dimensional elastically mounted cylinders in tandem in the proximity–wake interference regime at Reynolds number Re = 200 for systems having both one (transverse vibrations) and two (transverse and in-line) degrees of freedom (1-DOF and 2-DOF, respectively). For the 1-DOF system the computed results are in good qualitative agreement with available experiments at higher Reynolds numbers. Similar to these experiments our simulations reveal: (1) larger amplitudes of motion and a wider lock-in region for the tandem arrangement when compared with an isolated cylinder; (2) that at low reduced velocities the vibration amplitude of the front cylinder exceeds that of the rear cylinder; and (3) that above a threshold reduced velocity, large-amplitude VIV are excited for the rear cylinder with amplitudes significantly larger than those of the front cylinder. By analysing the simulated flow patterns we identify the VIV excitation mechanisms that lead to such complex responses and elucidate the near-wake vorticity dynamics and vortex-shedding modes excited in each case. We show that at low reduced velocities vortex shedding provides the initial excitation mechanism, which gives rise to a vertical separation between the two cylinders. When this vertical separation exceeds one cylinder diameter, however, a significant portion of the incoming flow is able to pass through the gap between the two cylinders and the gap-flow mechanism starts to dominate the VIV dynamics. The gap flow is able to periodically force either the top or the bottom shear layer of the front cylinder into the gap region, setting off a series of very complex vortex-to-vortex and vortex-to-cylinder interactions, which induces pressure gradients that result in a large oscillatory force in phase with the vortex shedding and lead to the experimentally observed larger vibration amplitudes. When the vortex shedding is the dominant mechanism the front cylinder vibration amplitude is larger than that of the rear cylinder. The reversing of this trend above a threshold reduced velocity is associated with the onset of the gap flow. The important role of the gap flow is further illustrated via a series of simulations for the 2-DOF system. We show that when the gap-flow mechanism is triggered, the 2-DOF system can develop and sustain large VIV amplitudes comparable to those observed in the corresponding (same reduced velocity) 1-DOF system. For sufficiently high reduced velocities, however, the two cylinders in the 2-DOF system approach each other, thus significantly reducing the size of the gap region. In such cases the gap flow is entirely eliminated, and the two cylinders vibrate together as a single body with vibration amplitudes up to 50% lower than the amplitudes of the corresponding 1-DOF in which the gap flow is active. Three-dimensional simulations are also carried out to examine the adequacy of two-dimensional simulations for describing the dynamic response of the tandem system at Re = 200. It is shown that even though the wake transitions to a weakly three-dimensional state when the gap flow is active, the three-dimensional modes are too weak to affect the dynamic response of the system, which is found to be identical to that obtained from the two-dimensional computations.