The deformation of a pseudo-elliptic vortex ring with circulation F was studied by a three-dimensional vortex blob method which included the viscous diffusion of vorticity. The circumference of this vortex ring consisted of two parallel line segments of the same length connected by two semicircles of the same radius R, the length of the major axis being denoted by L. The cross section of the vortex ring was represented by multiple vortex blobs with overlapping cores, while the circumference of the vortex ring was divided into a number of such sections. Numerical calculations were performed for the aspect ratios L /2R ~" 2.0-12,0, the initial core radius LT cs = 0.20R, and Reynolds number F /v c~1500, v being the kinematic viscosity. The vurtex rings yielded four fundamental patterns of deformation which included the axis switching and the split, depending on the aspect ratio, A flow-visualization experiment confirmed three of the four fundamental patterns, and was in sufficient agreement with the computation for the period of the axis switching and the time of the split.
The vortical structure of a circular water jet was investigated by a flow visualization technique. The jet was excited by axial and azimuthal perturbations to stabilize and enhance the largescale axisymmetric and streamwise vortices. A laser fluorescent dye and a laser light sheet were used to visualize the vortical structure, and the whole view of the structure was captured by applying the Taylor hypothesis to the cross-sectional images and by scanning a laser light sheet in the streamwise direction. The visualized images reveal the details of the complicated structure of axisymmetric and streamwise vortices, and it is confirmed that the streamwise vortices have fundamental effect on the entrainment of ambient fluid. From the images, the length of jet boundary was calculated to estimate the mixing effect. The result suggests that the jet mixing is significantly increased by the break-down of the vortices enhanced by axial and azimuthal perturbations. We also discussed the jet diffusion effect in consideration of the jet widths obtained by velocity measurement. The result indicates that the vortical structure including streamwise vortices plays an important role to enhance diffusion.
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