Materials World Network: Collaborative Research on Simple Forms of Quantum Turbulence - Production, Decay and Visualization

Turbulence is universal in high velocity fluid flow. It is of near universal importance for meteorology, ships, aircraft, and the chemical industry. Despite much effort by scientists, mathematicians and engineers, the complexity of the phenomenon is such that it is still not well understood and presents major theoretical challenges. Quite generally, energy is fed into the fluid on a relatively large length scale, and is then successively transformed by non-linear processes to smaller and smaller scales, until it is ultimately converted to heat by dissipative processes. In classical turbulence, this dissipation is due to viscosity.Quantum turbulence (QT) arises in superfluids (4He below 2K or 3He below 2 mK). At very low temperatures T, where the liquid possesses no normal fluid component, it may be a simpler phenomenon because the fluid motion is constrained by quantum effects, yielding turbulence consisting of discrete vortex lines each with one quantum of circulation. The energy cascade cannot be terminated by viscosity because the liquid is inviscid; in 4He the energy is believed to flow through a second turbulent cascade, formed from Kelvin waves on the vortices, until dissipated on very small length scales by phonon radiation (direct generation of heat). Much of this theory is speculative and awaits experimental verification.Our proposed work is experimental, on 4He. Its two linked parts each depend on close collaboration between the partners. The first involves investigation of large-scale properties of the especially simple form of QT produced by a steadily moving grid at a very low T. It involves only the pure superfluid and can be expected to be approximately homogeneous and isotropic (cf. much recent work on the complex and ill-characterized QT produced by oscillating structures). The second is a pioneering project aimed at a flow visualization over the whole superfluid T-range (existing methods cannot be applied at very low T). The first part requires a superconducting grid-drawing motor that is already at an advanced stage of development (Florida, Lancaster and B'ham). It is a technically difficult venture: the component carrying the grid must be levitated to avoid frictional heating, and it must be moved in a controlled way. Feasibility studies and tests have already been completed. We are now moving to production of the first working device, in Florida. In Florida, the gradual decay of the QT will be monitored from the heat it produces, i.e. calorimetrically. In Lancaster the decay will be followed by repeated probing with a beam of small charged vortex rings (with crucial help from Manchester, who recently developed the technique). These complementary experiments will provide evidence relating both to the existence of a range of large length scales at which QT may behave classically, and to the nature of the turbulence on smaller (quantum) length scales.In the second part, the flow will be visualized by use of He2* excimer molecules as seed particles that can be imaged by laser-induced fluorescence. Above 1K in 4He they are expected to track the motion of the normal fluid; below 1K they are likely to be trapped by vortex lines and so can track their positions and motion. The development of this technique could revolutionize the study of QT. The present grant will cover feasibility studies of the motion of the normal fluid in 4He both in thermal counterflow and in grid turbulence above 1K (Florida, Yale, advice from B'ham), including direct observation of the energy spectrum providing, we hope, direct evidence for approximate Kolmogorov scaling and of possible deviations due to intermittency or formation of coherent structures. Visualization of vortices below about 0.4K will ultimately require knowledge of the molecule-vortex capture cross-section, which will be measured electrically for a vortex array produced by uniform rotation in the Manchester rotating dilution refrigerator.

The investigations of quantum turbulence (QT) that we propose represent cutting-edge basic research. We seek fundamental scientific understanding, and we do not anticipate immediate commercial applications. Nonetheless, given the universality and importance of turbulence, the work promises to have a powerful cultural and technological impact in the longer term. We envisage impact in the following areas - (a ) Academic/scientific/engineering, as discussed under Academic Beneficiaries. (b) Public. Everybody is familiar with turbulence. It arises for water in e.g. sinks, baths, rivers, and the sea. It also arises in the atmosphere, and most people are familiar with e.g. storms, hurricanes, and admonitions to fasten their seat-belts on encountering turbulent conditions. Hence an improved understanding of turbulence promises to make a significant cultural contribution to the life of ordinary people. One can readily imagine long-term benefits for museum displays, textbooks, and popular science writing. (c) Industry. Although immediate industrial applications are not envisaged, the research impinges on an area of enormous practical and financial importance. Much of the energy used in air or sea transport, for example, goes into the creation of turbulence. If improved understanding leads eventually to even a small reduction in turbulence production, the corresponding energy savings would pay for the cost of the present research programme many times over. We note that the cooling of large superconducting magnets (e.g. for the LHC at CERN) is achieved through the forced flow of liquid He-4 at about 1.9K. Here again, any advances in reducing the turbulent dissipation will increase the performance and reliability of the magnets. How will we ensure that the new knowledge reaches those likely to benefit from it? Well, for (a) we will mainly use specialist low temperature conferences and physics journals, but also ranging more widely to include classical turbulence conferences and journals. One of us (WFV) already has a track record of bridging this interdisciplinary divide, including direct communication and discussion with some of the leading researchers on classical turbulence and delivering invited papers at their conferences. For (b) we envisage the possibility of articles in popular science magazines (e.g. New Scientist) or editorial matter (e.g. Nature or Physics World) where one of us (PVEMcC) has taken the opportunity of EPSRC-supported PCTF training and has a track record. In relation to (c), it is difficult to plan, but we will remain alert to potential applications of results that emerge from the work and will immediately discuss any promising results with the offices for exploitation and commercial liaison in the three UK universities involved.