Abstract
Multiple scenarios have been discovered by which laminar flow undergoes a transition to turbulence in Newtonian fluids. Here we show in non-Newtonian fluids a transition sequence to ‘elastic turbulence’ due to elasticity from polymers, with negligible inertia. Multiple dynamic states are found linking the base flow to ‘elastic turbulence’ in the flow between a rotating and stationary disk, including circular and spiral rolls. Also, a surprising progression from apparently ‘chaotic’ flow to periodic flow and then to ‘elastic turbulence’ is found. These transitions are found in experiments where either shear stress or shear rate is incrementally increased and then held at fixed values; the modes found following stable base flow are ‘stationary ring’, ‘competing spirals’, ‘multi-spiral chaotic’ and ‘spiral bursting’ modes, followed then by ‘elastic turbulence’. Each mode has a distinct rheological signature, and accompanying imaging of the secondary-flow field (simultaneous with rheological measurement) reveals kinematic structures including stationary and time-dependent rolls. The time-dependent changes in the secondary-flow structure can be related to the time-dependent viscosity in the case of several of the modes. Finally, the effect of polymer concentration on the transitional pathway modes is studied systematically.
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