Bm. Baumert et Sj. Muller, FLOW REGIMES IN MODEL VISCOELASTIC FLUIDS IN A CIRCULAR COUETTE SYSTEM WITH INDEPENDENTLY ROTATING CYLINDERS, Physics of fluids, 9(3), 1997, pp. 566-586
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.