Transition to Turbulence due to Microresonators Oscillating in Superfluid Helium

Orateur : David SCHMORANZER (Ph.D., Charles University, Prague)

I would like to present and discuss the latest experiments performed by the Prague superfluidity group on the transition to turbulence in os cillatory flows of superfluid He-4. Special attention will be devoted to the results obtained with double-paddles, miniature tuning forks and microwires, both in the two-fluid regime (T > 1 K) and at mK temperatures, focusing on the similarities and differences between classical and quantum turbulence. Specifically, by analysing the drag forces acting on the various oscillators in the two-fluid regime, we have established a classical-like scaling law describing the laminar flow of the normal component, and later used it to provide a more detailed description of the initial instabilities triggering the transition to turbulence. At very low temperatures, three different fluid-dynamical critical velocities along the path to turbulence have been observed, linked to previous experiments, and associated with i) multiplying quantized vortex loops attached to the surface of the oscillator, ii) quantized vortices propagating into the bulk superfluid, and iii) the predominantly quantum flow starting to become polarized and mimicking classical turbulent eddies.

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