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In this Letter, we observed and analyzed some spatial evolution (rolling up, growing up, pairing, merging and breaking up) of vortexes by self-developed flow facility. Some typical phenomenons are observed directly by using the flow visualization. We changed the inner flow rectangular exits into different shapes (square, small rectangular, big rectangular, small elliptical, big elliptical, and circular) or taken advantage of three parallel jets engender planar-symmetric shear flows. Furthermore, with dynamic surveying technology, we research the typical flow field which we studied by using of auto-power spectrum, cross-spectrum and multiple-spectrum. The following conclusions may be drawn from our work. We have found toroidal vortexes and spiral vortexes both in two experiments above. The pairing of toroidal vortexes has the Batchelor's couples or tri-poles phenomena. The spiral vortexes change to different structures during the change of Reynolds and velocity ratio. It is also revealed that the production of vortexes is determined by the speed at exits, nothing to do with the shape of exits. Besides, after it is engendered, the toroidal vortexes deflect during the change of the speed, and turn to spiral vortexes behind a certain critical value of Reynolds number which is determined by the shape of outlets. The spiral vortexes have the left-hand and right-hand phenomena, and the direction of rotation of it will reverse if attend to another certain velocity. The screw pitch of spiral vortexes structure is in direct proportion to velocity ratio.
Author(s):
Yun-xing JIANG
Donghua University
China
Xi-lin XIE
Fudan University
China
Yu-xuan YU
Fudan University
China
Wei-wei MA
Donghua University
China