Turbulent breakthrough in superfluid helium
Aug 27, 2003
Richard Feynman once described turbulence as the last great unsolved problem of classical physics. The flow of a classical fluid can be described by a dimensionless number known as the Reynolds number and the flow becomes turbulent when this number - which depends on the velocity of the flow, as well as other features - exceeds a certain value. Now Antti Finne of Helsinki University of Technology and colleagues in Japan, Finland, the Netherlands, Russia and the Czech Republic have discovered a new criterion for the onset of turbulence in superfluid helium-3 that does not depend on velocity (A Finne et al. 2003 Nature 424 1022).
A superfluid consists of two components: a viscous normal component and a frictionless superfluid that can flow without any viscosity. Helium-3 becomes a superfluid when it is cooled below 2.7 mK, and enters a so-called “B-phase” when it is cooled below a critical temperature of 2.2 mK.
Another characteristic of a superfluid is that its rotational motion is quantized into vortex filaments. Finne and co-workers used nuclear magnetic resonance techniques to investigate the behaviour of “loops” of vortex filament injected into a rotating superfluid as a function of temperature.
For temperatures above 60% of the critical temperature they found that the loops grew longer and aligned themselves with the axis of rotation. However, below 60% of the critical temperature, the loops grew into a turbulent tangle, although they eventually straightened themselves out. Numerical simulations suggest that the growth of Kelvin waves on the loops may be responsible for the turbulence. This breakthrough could help shed new light on turbulence in classical liquids.
This work is described in more detail in Waves and turbulence cause a stir in superfluids in the August issue of Physics World.