A PURELY ELASTIC TRANSITION IN TAYLOR-COUETTE FLOW.

A non-inertial, viscoelastic instability in the flow of a dilute polymer solution between concentric rotating cylinders (i.e. , Taylor-Couette flow) has been discovered. For a Newtonian fluid, Taylor-Couette flow is well-known to exhibit a transition from a purely azimuthal flow to steady vortex flow (and then to more complicated vortex flows) as inertia is increased. In this study, we present experimental evidence that in the absence of inertia, elastic effects alone may lead to a cellular instability. A linear stability analysis for the Oldroyd-B fluid, which is successful in describing many features of the experimental fluid, is also presented. The small-gap stability analysis quantitatively predicts the critical Deborah number, De sub c, at which the instability is observed. In addition, the predicted dependence of De sub c, on the value of the dimensionless gap between the cylinders is in approximate agreement with the experiments. The instability appears to be driven by a coupling of the first normal stress difference to the radial momentum balance through the curvature of the streamlines. Hence, it may be related to viscoelastic instabilities in a whole class of rotating shearing flows.