The formation of vortex rings in shear-thinning liquids

The dynamics of formation and evolution of vortex rings in non-Newtonian shear-thinning liquids generated in a piston-cylinder arrangement was studied. The ratio of the piston displacement L-m to the internal cylinder diameter D-o, as well as the mean piston velocity U-p determine the vortex properties and evolution. Experiments with different conditions are presented: translation velocity of the piston and stroke ratios L-m/D-o. Measurements of the 2D velocity field were obtained with a Ply technique. The vortex circulation F was computed considering a vortex identification scheme (Q criterion). The Reynolds number was in the range 138 < Re-o < 616. The Reynolds number for these vortices was computed in terms of the 'power-law' model parameters: the power index n and the consistency m. Considering different shear-thinning liquids and fixed Reynolds number, we observed that the vortex circulation decreases with the power index n. We show that the total circulation ejected from the cylinder is reduced when the thinning property of the liquid increases (decrease n); thus, the circulation confined inside the vortex ring, is reduced too. A value of the non-dimensional vortex circulation Gamma/DoUp approximate to 2 may indicate a saturation condition beyond which it is not possible to increase the vortex circulation for any Reynolds number. (C) 2012 Elsevier B.V. All rights reserved.