Bifurcation phenomena in Taylor-Couette flow of elastic polymer solutions

The results of a detailed study of the four-/six-cell primary flow exchange mechanism in Taylor-Couette flow of viscoelastic fluids are presented. The non-Newtonian fluids were aqueous solutions of low- and high-molecular-wei ght polyacrylamide. Results demonstrate that steady bifurcation phenomena i nvolving non-trivial nonlinear flows are qualitatively unchanged from the N ewtonian case for both fluids. Moreover, experimental results obtained with low-molecular-weight polyacrylamide fluid exhibit close agreement with num erical data for the Newtonian case. Interestingly, time-dependent flows of both polymer solutions are radically different to those found in a Newtonian fluid. The principal effect is an extremely long timescale phenomenon which gives rise to a shift in Hopf bif urcation points at which oscillatory flows arise. We also present evidence for the existence of multiple flow states which depend upon the shear histo ry of the polymer solution. The results provide evidence for a long-term ch ange in the conformation of individual polymer molecules and set a signific ant challenge for models.