Quantum Turbulence - Introduction

Introduction

The turbulence of classical fluids is an everyday phenomenon, which can be readily observed in the flow of a stream or river. When turning on a water tap, one notices that at first the water flows out in a regular fashion (called laminar flow), but if the tap is turned up to higher flow rates, the flow becomes decorated with irregular bulges, unpredictably splitting into multiple strands as it spatters out in an ever-changing torrent, known as turbulent flow. Turbulent flow comprises randomly sized regions of circulating fluid called eddies and vortices, which can be ordered, giving rise to large-scale motions such as tornados or whirlpools, but in general are completely irregular.

Under normally experienced conditions, all fluids have a resistance to flow, called viscosity, which governs the switch from laminar to turbulent flow, and causes the turbulence to decay (for example, after a cup of coffee is stirred it will eventually return to rest). A superfluid is a fluid which has no viscosity, or resistance to flow, meaning that flow around a closed loop will last forever. These strange fluids exist only at temperatures close to absolute zero, being in effect a more ordered and separate fluid state, arising due to the macroscopic influence of quantum mechanics brought about by the low temperatures involved.

Despite having no viscosity, turbulence is possible in a superfluid. This was first suggested theoretically by Richard Feynman in 1955, and was soon found experimentally. Since the flow of a superfluid is an inherently quantum phenomenon (see macroscopic quantum phenomena and superfluid helium-4), turbulence in superfluids is often given the name quantum turbulence to reflect the key role played by quantum mechanics. A recent overview of quantum turbulence is given by Skrbek.

In these so-called "superfluids", the vortices have a fixed size, and are identical. This is another startling property of superfluids, being very different from the random vortices in a classical fluid, and arises out of the quantum physics whose effects become observable on a larger scale at low temperatures. Quantum turbulence, then, is a tangle of these quantized vortices, making it a pure form of turbulence which is much simpler to model than classical turbulence, in which the myriad of possible interactions of the eddies quickly make the problem too complex to be able to predict what will happen.

Turbulence in classical fluid is often modelled simply using virtual vortex filaments, around which there is a certain circulation of the fluid, to get a grasp on what is happening in the fluid. In quantum turbulence, these vortex lines are real – they can be observed, and have a very definite circulation – and moreover they provide the whole of the physics of the situation.