Axisymmetric and non-axisymmetric elastic and inertio-elastic instabilities in Taylor-Couette flow

Flow visualization is performed on two highly elastic, non-shear-thinning d ilute polymer solutions (polyisobutylene/ polybutene) in a wide gap Couette cell (R-1/R-2=0.827) over a range of shear rates and choices of relative c ylinder rotations. Axisymmetric structures of the type found in a narrow ga p cell (R-1/R-2=0.912) [B.M Baumert, S.J. Muller, Rheol. Acta 34 (1995) 147 -159; B.M. Baumert, S.J. Muller, Phys. Fluids 9 (1997) 566-586] were reprod uced. Additionally, helical and nonaxisymmetric standing wave patterns were visualized. In the more viscous, more elastic fluid (epsilon=De/Re=15.0) t ransitions were found to be purely elastic. At the lowest shear rates at wh ich transitions were observed, extremely slow-growing stationary axisymmetr ic counter-rotating vortices replace the purely azimuthal base flow. At rat es more than twice the critical, axisymmetric oscillatory flow precedes the onset of steady vortices. Non-axisymmetric structures an first observed at rates more than 5 times the critical. The pattern is initially highly regu lar: two m=1 helices superpose to generate a nonaxisymmetric standing wave pattern ('ribbons'). The ribbons quickly give way to intense merging struct ures with nearly axisymmetric cores. In the less viscous, less elastic flui d (epsilon=0.0562) the lowest rate transitions are to weak axisymmetric cou nter-rotating vortices. At progressively higher rates in the presence of ce ntrifugal destabilization several types of axially translating axisymmetric vortices are generated. Go-rotation of the cylinders gives rise to transla ting vortices at fewer shear rates than rotation of the inner cylinder alon e. At somewhat more than twice the critical rate, robust and hysteretic m=1 ribbons are produced. Brief periods of unpaired m=1 helices are also detec table very near the critical rate for the ribbons. The spectrum of spatial and temporal frequencies of the ribbons is found to broaden with further in creases of shear rate. At the highest shear rates examined for this fluid, the flow is non-axisymmetric and highly disordered. (C) 1999 Elsevier Scien ce B.V. All rights reserved.