Affiliations: Department of Mechanical Engineering, Hokkaido
Institute of Technology, Sapporo 006-8585, Japan
Abstract: 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.