Published online by Cambridge University Press: 26 April 2006
Experimental observations and linear stability analysis are used toquantitatively describe a purely elastic flow instability in theinertialess motion of a viscoelastic fluid confined between arotating cone and a stationary circular disk. Beyond a criticalvalue of the dimensionless rotation rate, or Deborah number, thespatially homogeneous azimuthal base flow that is stable in thelimit of small Reynolds numbers and small cone angles becomesunstable with respect to non-axisymmetric disturbances in the formof spiral vortices that extend throughout the fluid sample. Digitalvideo-imaging measurements of the spatial and temporal dynamics ofthe instability in a highly elastic, constant-viscosity fluid showthat the resulting secondary flow is composed of logarithmicallyspaced spiral roll cells that extend across the disk in theself-similar form of a Bernoulli Spiral.
Linear stability analyses are reported for the quasi-linear Oldroyd-Bconstitutive equation and the nonlinear dumbbell model proposed byChilcott & Rallison. Introduction of a radial coordinatetransformation yields an accurate description of the logarithmicspiral instabilities observed experimentally, and substitution intothe linearized disturbance equations leads to a separable eigenvalueproblem. Experiments and calculations for two different elasticfluids and for a range of cone angles and Deborah numbers arepresented to systematically explore the effects of geometric andrheological variations on the spiral instability. Excellentquantitative agreement is obtained between the predicted andmeasured wavenumber, wave speed and spiral mode of the elasticinstability. The Oldroyd-B model correctly predicts thenon-axisymmetric form of the spiral instability; however,incorporation of a shear-rate-dependent first normal stressdifference via the nonlinear Chilcott–Rallison model is shown to beessential in describing the variation of the stability boundarieswith increasing shear rate.