正三角形排列刚性耦合三圆柱涡激振动特性及尾涡模式

Vortex-induced vibration features and wake modes of three rigidly coupled circular cylinders in equilateral triangular arrangements

采用基于嵌入式迭代的浸入边界法对等边三角形排列的刚性耦合三圆柱涡激振动进行了数值模拟研究。其中一个圆柱在上游放置,另外两个圆柱并排放置于下游,圆柱间刚性连接,系统仅在横向自由振动。圆柱间距比L^(*)分别为1.0、1.6、2.5和4.0,雷诺数为Re=100,质量比为m^(*)=2.0,折合流速为U_(r)=3.0-30.0。分析了不同间距比下圆柱振幅、流体力、振动频率和脱涡模式等。研究发现,随U_(r)的增大,各间距比下的振动响应均可划分为初始分支(initial branch,IB)、下端分支(lower branch,LB)和非锁定区域(desynchronized region,DS)。其中,非锁定区域又可进一步分为前非锁定区域(DS1)和后非锁定区域(DS2)。随折合流速的增大,圆柱振幅整体上先增后减,而随间距比的增大,圆柱振幅则先减后增。圆柱的最大振幅(A^(*)=1.11)出现在L^(*)=1.0、U_(r)=8.0处。当L^(*)=1.0、1.6和2.5时,圆柱振动存在锁定区间,振动频率锁定在固有频率附近,而L^(*)=4.0时,圆柱的振动频率随折合流速增大线性增大,不存在锁定区间。当L^(*)=2.5时,在DS2分支上,圆柱振动出现了两个强度相当、频率不同的分量,分别为低频驰振分量与高频涡振分量,而且由于复杂的柱间流体结构使得三圆柱升力频率存在较大差异。当L^(*)=1.6时,在DS分支上,圆柱下游出现宽-窄尾流,导致了下游圆柱所受升阻力均值和升力均方根不相等。

Vortex-induced vibrations(VIV)of three rigidly coupled circular cylinders in equilateral triangular arrangements were numerically investigated by using the iterative immersed boundary method.The three cylinders,with one placed upstream and the other two downstream side-by-side,are free to oscillate only in the cross-flow direction.The normalized spacings between the cylinders are L^(*)=1.0,1.6,2.5 and 4.0.The Reynolds number is Re=100,the mass ratio is m^(*)=2.0,and the reduced velocity is U_(r)=3.0-30.0.Characteristics of the vibration amplitudes,fluid forces,vibration frequencies and the vortex-shedding modes were investigated.It was found that with the increment of the reduced velocity,the vibration responses at different L^(*)can be categorized as the initial branch(IB),the lower branch(LB)and the desynchronized region(DS)which is further divided into the front desynchronized region(DS1)and the rear desynchronized region(DS2).With the increasing reduced velocity,the vibration amplitudes show a first-increase-then-decrease pattern as a whole,while with the increasing spacing ratio,the vibration amplitudes display a first-decrease-then-increase pattern.The maximum amplitude(A^(*)=1.11)is achieved at L^(*)=1.0 and U_(r)=8.0.At L^(*)=1.0,1.6 and 2.5,the vibration responses show the lock-in region where the vibration frequency locks onto the natural frequency of the cylinders,while at L^(*)=4.0,the vibration frequency of the cylinder increases linearly with the increasing reduced velocity,and no obvious lock-in occurs.Two comparable vibration components with different frequencies exist in the DS2 branch at L^(*)=2.5,and they are the low-frequency galloping component and the high-frequency VIV component,and the lift frequencies of three cylinders are obviously different because of complex gap flow between cylinders.In the DS branch of L^(*)=1.6,the wake behind two downstream cylinders shows a wide-narrow pattern,which leads to the mean drag and mean and r.m.s.lift of two downstream cylinders are not identical.