FLOW REGIMES IN MODEL VISCOELASTIC FLUIDS IN A CIRCULAR COUETTE SYSTEM WITH INDEPENDENTLY ROTATING CYLINDERS

Flow visualization of two highly elastic, nonshear-thinning polyisobut ylene/polybutene fluids in the gap between concentric cylinders was pe rformed over a range of shear rates and choices of relative cylinder r otations. The observed secondary flows are discussed in terms of desta bilizing elastic and centrifugal forces. In the more viscous, more ela stic fluid, instabilities are found to be independent of the choice of rotating cylinder and due entirely to elasticity. At the lowest shear rates examined, the first detectable secondary flows are steady count er-rotating vortices forming after a shearing time more than five orde rs of magnitude greater than the characteristic relaxation time of the fluid. At somewhat higher shear rates, a much more rapidly appearing oscillatory flow is observed to evolve into the steady vortex structur e. In the less elastic fluid, the structure first detectable at the lo west shear rates is again steady vortices regardless of the choice of driving cylinder, At all shear rates examined, only elastic stationary vortices are observed in the absence of centrifugal destabilization ( outer cylinder rotating). Secondary flows are significantly stronger i n the presence of the centrifugal destabilization due to a rotating in ner cylinder. Interaction of elasticity and centrifugal forces is roun d to generate a number of axially translating vortex structures, many of which are described here for the first lime. At a shear rate more t han five times the critical, another family of instability is observed which closely resembles a purely elastic instability observed by Baum ert and Muller (1995). These experimental results are expected to prov ide a challenging test of numerical simulations of these viscoelastic flows. (C) 1997 American Institute of Physics.