HOPF-BIFURCATION IN CREEPING CONE-AND-PLATE FLOW OF A VISCOELASTIC FLUID

This paper analyzes the bifurcations which occur in the creeping flow of a viscoelastic fluid subjected to a constant shearing motion in the gap between an inverted cone and a plate. We show that when the Debor ah number, a dimensionless relaxation time of the fluid, exceeds a cri tical value the base 'viscometric' flow loses stability and a Hopf bif urcation occurs. The nature of the bifurcation depends on the retardat ion parameter beta, defined as the ratio of polymer viscosity to the z ero shear rate viscosity of the fluid. Our analysis shows that for 0.9 8 less than or equal to beta less than or equal to 1, bifurcation is s upercritical and subcritical for beta less than or equal to 0.97. The analysis is facilitated by assuming that the gap between the cone and the plate is small. Center manifold theory is then used to derive appr opriate amplitude equations in a neighborhood of the critical Deborah number.