Experimental investigation of vortex ring evolution in polymer solution

TL;DR

This study experimentally investigates how polymer solutions, specifically PAMH, affect vortex ring formation, propagation, and vorticity distribution, revealing deviations from Newtonian water behavior and demonstrating phenomena like ring reversal.

Contribution

It provides new insights into vortex dynamics in non-Newtonian fluids, highlighting the effects of shear-rate dependent viscosity and elasticity on vortex ring properties.

Findings

Vortex ring properties deviate from water with increasing PAMH concentration.

Circulation remains between two water experiments, but enstrophy and vorticity do not.

Polymer solutions cause vortex core vorticity modification and exhibit ring reversal.

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 ($\eta_0$) and the infinite-shear-rate viscosity ($\eta_{\infty}$) increase. The relaxation time also increases 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 $\eta_0$ and $\eta_{\infty}$. We find that while the circulation of PAMH solutions lies 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 behavior 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.