, Volume 224, Issue 2, pp 383-397
Date: 13 Nov 2012

Vortex ring formation for low Re numbers

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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 ≤ L m /D 0 ≤ 6. As Re decreases, the vortices become “thicker”; therefore, they are able to accumulate more vorticity and increase their circulation.