Vortices and Turbulence at Very Low Temperatures
July 2, 2007 — July 6, 2007
Coordinators:
- Carlo F. Barenghi (University of Newcastle, Newcastle upon Tyne, UK, Great Britain)
- Yuri Sergeev (Univ. of Newcastle u. Tyne , Newcastle upon Tyne, UK, Great Britain)
Superfluidity (fluid motion without viscous dissipation) takes place at very low temperatures near absolute zero in quantum fluids, such as the two isotopes of liquid helium, He4 and He3, and ultra-cold Bose-Einstein-condensed atomic gases. The practical engineering applications of superfluidity include the cooling of superconducting magnets used in particle physics, hospitals and infra-red detectors.
A remarkable feature of superfluids is that quantum mechanics imposes a constraint on the rotational motion, which can only take the form of very thin vortex filaments of fixed (quantised) circulation. Therefore, unlike ordinary vortices which can be weak or strong, all superfluid vortices have the same strength and differ from each other only in the geometry (length and curvature).
Superfluid turbulence thus manifests itself as a disordered tangle of thin vortex filaments.
In the last few years experiments have shown that there are remarkable analogies and similarities between classical turbulence in ordinary fluids and superfluid turbulence in liquid helium. For example, the classical -5/3 Kolmogorov energy spectrum has been observed in superfluid helium continuously stirred by counter-rotating propellers; a second example is the classical -3/2 temporal decay law, which has been measured in turbulence created by towing a grid in a sample of liquid helium initially at rest. A third example is the analogy between the classical Richardson cascade and the newly-discovered Kelvin waves cascade in superfluid turbulence.
It has become apparent that progress in superfluidity requires more input from the fluid mechanics community in terms of ideas and methods. Viceversa, classical fluid mechanicists are becoming interested in superfluid turbulence, which, although conceptually simpler than classical turbulence, retains important fundamental aspects, such as the nonlinear interaction of classical inviscid Euler flows.
Additional interest in fluid dynamics near absolute zero arises from experiments on turbulent convection using cryogenic helium gas, which has achieved unprecedented turbulence intensity and open up the exploration of new scaling laws.
AIM OF THE COURSE
The object of this course is to introduce the basic concepts of superfluid hydrodynamics and classical vortex dynamics, then present the current problems and open questions of quantised vorticity and turbulence in its various aspects. The range covers theory and experiments, heat transfer, grid turbulence and rotating turbulence, He3 and He4, atomic Bose-Einstein condensates. The emphasis is on the basic ideas and the open problems.
The aim is to encourage the collaboration of fluid mechanicists and low temperature physicists and to produce mutual benefits.
ORGANIZATION OF THE COURSE
The course is organized allowing time for questions and discussions.The six sets of lectures will contain basic material as well descriptions of problems of current research interest.
AUDIENCE OF THE COURSE
The course is addressed to scientists, professionals and graduate students of physics, engineering and applied mathematics with general interest in fluids and vortices.
Finally, in his special lecture Professor Sreenivasan will report recent experimental results on the visualization of vortex lines.