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Show Conc:enlr.t.d neg.tJve vorUdtyhm vortltx ehMt Fig. 17: lliustration of the vorticity structure after roll up of the vonex sheet The vortex lines in the sheet are drawn together via the Kelvin-Helmholtz instability to fonn the large scale structures seen in the images of Figs. 6-9. Since the vortex lines follow a left-handed helix, so too does the roller, accounting for the clockwise tail observed in Fig. 10. Dispersed positive helical vorticity Concentrated negative helical vorticity Fig. 18: Illustration of the clustering of the negative helical vorticity after roll up of the vonex sheet The roll up causes vorticity to be pulled from the adjacent regions of the sheet and concentrate along the roller. As a result, the roller fonns a strong center of vorticity which draws the bulk fluid (with vorticity of opposite sign) around it, and perhaps into it, as the flow proceeds downstream. Fig. 19: Illustration of the vorticity structure of the flow as it approaches the walls. The clustering of the sheet vorticity after roll up causes the bulk fluid to be drawn out towards the walls and intertwine with the rolled up vorticity. When this happens, strong three-dimensional effects are expected to result, as illustrated in Fig. 13. After contacting the wall, the two streams of fluid (with opposite signs of vorticity) may split, with the outer fluid being preferentially drawn into the external recirculation zone and the inner fluid proceeding downstream in the flow, or they may be sufficiently convolved that they are convected together until dissipated by diffusion. - 16 - |