矩形柱二维尾流的稳定性研究

An investigation of the stability of rectangular cylinder wakes

采用实验和数值模拟两种方法对长宽比为3∶1的矩形柱尾流进行了稳定性研究.实验发现当雷诺数接近临界值时,未加扰动的矩形柱尾流呈二维定常状态.当在柱体上游一定位置和下游靠近柱体的位置上沿垂直于来流方向施加一个宽度很小的短时脉冲射流扰动以后,扰动随时间放大,并出现旋涡脱落现象,并且这种扰动激发的旋涡脱落可以持续很长时间,不会衰减.而在下游较远处施加同样的扰动时,扰动将会衰减,不能激发出持续的旋涡脱落.数值模拟再现了上述实验结果,并且表明,当扰动强度(脉冲射流出口速度)较小时,不会出现持续的规则旋涡脱落,只有当扰动强度达到一定阈值时,旋涡脱落才能被激发.

The stability of wakes behind a two-dimensional rectangular cylinder of length to width ratio L/B=3.0 was studied by experiment and numerical simulation at Reynolds numbers close to the onset of vortex shedding.The experiment was carried out in a propeller-driven water channel of test section 1.5 m long,0.25 m wide and 0.2 m high.The cylinder wake was steady at a Reynolds number Re(80),and no obvious fluctuation of the wake was found in the range of an observation time of 20 minutes.A vertical impulse jet disturbance was then applied to the wake at a stream-wise station.The wake fluctuation was visualized by color liquid and the long-period evolution of the disturbed wake was recorded by a CCD camera.Results showed that persistent vortex shedding could be induced if the disturbance was applied at stations upstream of or in the near wake of the cylinder.At stations far down the stream of the cylinder,however,the impulse disturbance could not induce persistent regular vortex shedding.Numerical simulation at the same conditions recurred the results of experiment and showed that the strength of disturbance,defined by jet speed(the jet width and blow time is fixed at b/B=0.04 and t=1 s respectively),had a strong influence on the results.At a low jet speed u/U=15.6,the disturbance could not induce persistent vortex shedding.At u/U=31.2 or even higher,persistent vortex shedding could be excited if the disturbance was applied in the range-12<X/B<10.The above results show that there is an absolute instability region in which the impulse disturbance can rapidly induce vortex shedding,that strong nonlinear effect exists in the wake,and that the induction of vortex shedding is greatly influenced by the strength of the disturbance.