Space-time diagramsof inertio-elastic Couette flow

Space time diagrams of Couette flow of a polymer solution in the regime of weak (a-b) to moderate (c) elasticity (Groisman and Steinberg, PRL 96). The primary instability is driven by inertial forces, but flow patterns above the instability threshold are modified by elastic effects. The flow was visualized with light reflecting Kalliroscope flakes under ambient illumination. Brightness profiles along a line parallel to the axis of the Couette-Taylor column were taken in consecutive moments of time (12.5 times per second) and plotted from top to the bottom. The diagram at the top corresponds to the well ordered Rotating standing waves (standing wave along the axis and traveling wave around the circumference of the Taylor column), which occur slightly above the inertial instability threshold. The diagram in the middle corresponds to higher Reynolds number, where the waves become less ordered with distinct patches of traveling and standing waves in the space-time domain. The diagram at the bottom is for the case of higher elasticity, when the vortex oscillations become essentially disordered and their velocity spectra become stochastic.