Turbulence in classical fluids is important and challenging both from a theory perspective and for many practical applications. This research, a collaboration between the University of Florida, Yale University, and the Universities of Lancaster, Manchester and Birmingham in the United Kingdom, aims to understand how classical turbulence is modified in a superfluid, such as liquid helium, in which flow is severely restricted by quantum conditions associated with the quantization of angular momentum and a complete lack of viscosity. Quantum turbulence has a character that depends on the temperature. Just below the superfluid transition temperature a superfluid behaves as a mixture of a normal (classical-like) fluid and a superfluid; either or both fluids can be turbulent, which gives rise to a rich variety of possible turbulent patterns At much lower temperatures only the superfluid component remains, exhibiting most clearly the fundamental aspects of quantum turbulence. The study of quantum turbulence would be greatly facilitated if the patterns of turbulent flow could be visualized. The use of micron-sized particles of hydrogen as tracers has been pioneered elsewhere, but the investigators in this project are implementing the use of much smaller particles in the form of metastable diatomic helium molecules, which will be observed by laser fluorescence. At temperatures somewhat below the superfluid transition such molecules are expected to follow the normal fluid, while at much lower temperatures they are expected to follow the quantized vortex motion in the superfluid component. In the earlier stages of the work the investigators will develop this technique for application in the more accessible temperature range where the molecules track the normal fluid, but where many important aspects of quantum turbulence have still to be studied and understood. The preliminary experiments will pave the way towards applications at lower temperatures. Also, since a very simple form of turbulence can be generated in the wake of a steadily moving grid, the investigators are developing techniques for moving a grid through superfluid helium at a very low temperature where the quantum turbulence involves only the normal fluid. The resulting turbulence will be studied on a global scale in two ways: observing the rate at which the superfluid heats up as a result of the decay of the turbulence; and using the scattering of ions to measure the rate at which the concentration of turbulent eddies decays.
Success of the proposed experiments is dependent on the development of close interactive collaboration between Florida (development of the moving grid and study of energy dissipation behind it), Yale and Manchester (visualization), Birmingham (theory), Lancaster and Manchester (decay of the concentration of turbulent eddies behind a moving grid). Students and postdoctoral associates from all labs will work in the other labs both to promote the overall effort and further to develop each researcher's capability. While the work is challenging because of the low temperature environment, the possible pay off is high, as an understanding of quantum turbulence may offer insights into classical turbulence which have heretofore escaped common understanding.
