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.
This paper reviews the dynamics of vortex rings and the control of flow by the manipulation of vortex rings. Vortex rings play key roles in many flows; hence, the understanding of the dynamics of vortex rings is crucial for scientists and engineers dealing with flow phenomena. We describe the structures and motions of vortex rings in circular and noncircular jets, which are typical examples of flows evolving into vortex rings. For circular jets the mechanism of evolving, merging and breakdown of vortex rings is described, and for noncircular jets the dynamics of three-dimensional deformation and interaction of noncircular vortex rings under the effect of self-and mutual induction is discussed. The application of vortex-ring manipulation to the control of various flows is reviewed with successful examples, based on the relationship between the vortex ring dynamics and the flow properties.
The interaction of streamwise and axisymmetric vortices in an axisymmetric jet is studied by flow visualization and velocity measurement. The jet is excited by azimuthal and axial perturbations to enhance streamwise and axisymmetric vortices. The three-dimensional views of the jet-boundary surface and streamwise vortices are constructed by applying the Taylor hypothesis to the jet cross-sectional images, and the interaction model of streamwise and axisymmetric vortices is proposed. The interaction of spanwise and streamwise vortices in a plane jet is also studied by velocity measurement. The experiment is carried out under conditions similar to those of the axisymmetric jet. The vortical structure is discussed on the basis of the three-dimensional views of phase-averaged vorticities. It is confirmed that the interacting vortical structure is similar to that in an axisymmetric jet. The entrainment mechanism is also discussed in relation to the vortical structure.
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