FLOW VISUALIZATION OF THE ELASTIC TAYLOR-COUETTE INSTABILITY IN BOGERFLUIDS

Flow visualization is performed on an elastically-dominated instabilit y in several similar Boger fluids in Taylor-Couette flow The onset and evolution of secondary flow are observed over a range of shear rates using reflective mica platelet seeding. Sequences of ambiently and she et-illuminated images were digitally processed. Rotation of the inner cylinder was ramped from rest to its final value over a time on the or der of a polymer relaxation time. Dilute solutions of high molecular w eight polyisobutylene in oligomeric polybutene manifest a flow transit ion at a Deborah number, De(s) = lambda(s) gamma approximate to 1.5 wi th a Taylor number of 0.00022 in a cell with dimensionless gap ratio d elta = 0.0963. At this transition, simple azimuthal shearing is replac ed by steady, roughly square, axisymmetric counter-rotating vortices g rossly similar to the well-known Taylor vortex flow that is observed a t De(s) = 0, Ta = 3612. At De(s) = 3.75, Ta = 0.0014, an axisymmetric oscillatory secondary flow develops initially but is replaced by the s teady vortices. At De(s) = 7.5, Ta = 0.0054, the oscillatory and vorte x flow coexist and possess an irregular cellular cross-section. A wide span of growth rates is observed: the ratio of onset to polymer relax ation time ranges from 170 000 at De(s) = 1.5 to 0(10) at De(s) > 5. T he role of inertia was explored through changing the solvent viscosity . A transition similar to the one that occurs at De(s) = 3.75, Ta = 0. 0014, from the base azimuthal shearing flow to axisymmetric vortices, was also observed with a much lower viscosity fluid at De(s) = 3.3, Ta = 74.