Vortex ring formation for low Re numbers

The dynamics of formation and evolution of vortex rings with low Reynolds numbers created in a piston-cylinder arrangement are studied. The ratio of the piston displacement L (m) to the nozzle diameter D (0) determines the vortex size and evolution. Experiments with different conditions are presented: translation velocity of the piston and stroke ratio L (m) /D (0) for 150 < Re < 260. Measurements of the 2D velocity field were obtained with a PIV technique. The vortex circulation was computed considering a vortex identification scheme (Q criterion). The results show that there is a critical value of L (m) /D (0) above which the circulation inside the vortex cannot increase and remains constant. For the Reynolds numbers studied, we found that the limit stroke ratio is 4 a parts per thousand currency sign L (m) /D (0) a parts per thousand currency sign 6. As Re decreases, the vortices become "thicker"; therefore, they are able to accumulate more vorticity and increase their circulation.