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 Lm to the nozzle diameter D0 determines the vortex size and evolution. Experiments with different conditions are presented: translation velocity of the piston and stroke ratio Lm/D0 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 Lm/D0 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 ≤ Lm/D0 ≤ 6. As Re decreases, the vortices become “thicker”; therefore, they are able to accumulate more vorticity and increase their circulation.