HORIZONTAL VORTEX RING MOTION IN LINEARLY STRATIFIED MEDIA

Laminar vortex rings propagating horizontally in linearly stratified m edia were investigated in a water tank using the laser-induced fluores cence flow visualization technique. The vortex rings were observed to develop an asymmetry where the upper part of the ring became larger in the cross-sectional area than the lower part. The asymmetry was a res ult of the instability of the interface on the upper portion of the ri ng; the upper half of the vortex was heavier than its immediate surrou ndings, whereas the lower half was lighter. When the circulation-based internal Froude number was smaller than 1.3, the vortex rings retaine d their overall geometry and continually decreased in size by leaving significant amounts of dyed fluid behind in their wake. At Froude numb ers greater than 3.2, the asymmetry was more pronounced, both in the r elative position of the upper and lower cores as well as their sizes, and the overall vertical extent of the rings did not decrease until so me time after the vortex formation. These observations led to a two-le vel classification of the collapse process. The collapse time and dist ance increased with Froude number at any Reynolds number. For a given Froude number and discharge time, increasing the Reynolds number delay ed the vortex ring collapse. The normalized collapse time and distance for the lower Froude number vortex rings in the present study were fo und to scale with the composite parameter Re Fr-2/3. (C) 1997 American Institute of Physics.