Patterns in elastic Couette flow
Snapshots of different patterns appearing in the Couette-Taylor flow in a highly elastic polymer solution (Groisman and Steinberg, PRL 97). The flow was visualized with light reflecting Kalliroscope flakes and laser sheet illumination. A laser beam expanded to a thin sheet of light was illuminating the working fluid in the gap between the cylinders in the plane of the axis of the Couette set-up. That produced a thin cross-section of the flow in the gap, which was viewed at the right angle. The upper and lower borders of the strips a-e correspond to the outer and the inner cylinders, respectively. The length of the photographed region is 16 times the thickness of the gap, d. The photograph a at the top corresponds to high velocity and large Weissenberg number, Wi, and shows a chaotic state of Disordered oscillations. The pattern a appears after a direct transition from the Couette flow to the Disordered oscillation, when Wi is raised. As Wi is decreased afterwards, some well defined persistent structures appear in the chaotic flow, which have a form of dark rings, spindle-shaped in the cross-section, b. At still lower Wi, as seen in c and d, the secondary vortex flow decays to a set of spatially confined oscillating rings with the spindle-shaped cores. The persistent cores correspond to regions of intensive radial flow from the outer to the inner cylinder. Finally, at even lower Wi, the oscillations stop and the perturbation of the basic shear Couette flow, as seen in e, is a set of randomly spaced solitary vortex pairs, which are stationary, axisymmetric and spatially confined. Flow lines in the solitary vortex pair, a diwhirl, are schematically shown at the bottom on the left.