横向振动方柱锁定区近尾迹结构

THE NEAR WAKE STRUCTURE FROM OSCILLATING SQUARE CYLINDER IN THE LOCK-IN RANGE

对雷诺数为150≤Re≤3600,振幅与柱宽比 A/h 为0.24和0.7条件下横向强迫振动单方柱的锁定现象、涡脱落过程和近尾迹特性,用流动显示和热线技术进行了研究,结果表明锁定区的范围随振幅比和雷诺数的增高而扩大,在大振幅比情况下,锁定区上、下临界折合速度值比小振幅比情况均有增高。流动显示结果清晰而系统地揭示了柱振动频率越过共振锁定点时,方柱上下两侧涡脱落规律发生的变化以及振幅的改变对近尾迹结构的影响。在低振幅比情况下,锁定区近尾迹结构基本为2S 型,在高振幅比情况下,由2S 型变为 P+S 型,这是非对称的尾迹结构,每一振动周期,从物体上脱落三个涡。在高频振动情况下,近尾迹由大尺度涡结构转化为小尺度涡结构。

The near wake structure and the vortex-shedding processfrom the square cylinder in the lock-in range have been investigatedexperimentally by means of a hot-wire and flow visualization tech-nique.The cylinder was forced to oscillate transverse to the freestream at amplitudes of 0.24h and 0.7h.Reynolds numbers of ex-periments are in the range of 150 to 3600.Present study indicates thatthe range of the synchronization was extended with increasing theoscillation amplitude and Reynolds numbers.The reduced velocity ofthe upper and lower end of the lock-in range increases when theoscillation amplitude of the cylinder is 0.7h.The flow visualizationshows that there are drastic changes in the near wake structure andthe vortex-shedding from the cylinder in passing from flow condi-tions below the range f_c/f_(vs)=l.O to above it.Especially,at highamplitude the flow structure of the near wake is different from theflow structure at small amplitude in the lock-in range.At A/h=0.24,the major vortex patterns of the near wakeare mode'2S'in the lock-in range.At A/h=0.7,the vortex patternsof the near wake change into mode'P-S'.The'P-S'mode is anasymmetric mode where the cylinder sheds a pair and a single vortexeach cycle.At higher value of f_c/f_(vs)(f_c——oscillating frequency ofthe cylinder,f_(vs)——the natural shedding frequency),the large-scalevortex pattern in thenear wake change into the small-scale vortexpattern.