Experimental investigation of vortex ring evolution in polymer solution

Abstract

We have conducted experiments on the effect of polymer solutions on the formation and propagation of vortex rings. We study this effect in aqueous solution of hydrolyzed polyacrylamide (PAMH) at different concentrations. Addition of PAMH imparts shear-rate dependent viscosity and elasticity to the solvent. With increasing concentration of PAMH both the zero-shear-rate viscosity (η 0 ) and the infinite-shear-rate viscosity (η ∞ ) increase. The relaxation time also increase with the increase in concentration. We generate vortex rings using a piston-cylinder mechanism in a glass tank and measure vortex ring properties such as ring position, ring circulation, enstrophy, kinetic energy and peak vorticity using particle image velocimetry (PIV). Experiments are conducted by (1) matching impulse, and (2) matching Reynolds number. We show that, at constant impulse, vortex ring properties in PAMH solution deviate from that in Newtonian water. As the concentration of the PAMH solution increases, so too does the deviation from water. We further perform experiments in water by matching the limiting Reynolds numbers of the PAMH solution corresponding to both η 0 and η ∞ . We find that while the circulation of PAMH solutions lie between the circulation curves of the two extreme Reynolds number matched water experiments for an extended period of time, the enstrophy and peak vorticity do not. We attribute this behaviour to the modification of vorticity distribution within the core of vortex rings in PAMH solutions. We also study the effect of polymer solutions on the formation number. We find that the formation number remains the same even in polymer solutions. We also demonstrate, using planar laser induced fluorescence (PLIF), the phenomenon of ring reversal. Once the vortex ring stops, it begins to unwind and retract by translating and rotating in the opposite direction. We attribute this behaviour of ring reversal to the elastic properties of polymer solutions.