Visualization of the vortical structure of a circular jet excited by axial and azimuthal perturbations

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 large-scale 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.