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

无论是从理论角度还是从许多实际应用角度来看，经典流体中的湍流都是一个重要而具有挑战性的问题。这项研究是由佛罗里达大学、耶鲁大学和英国兰开斯特大学、曼彻斯特大学和伯明翰大学合作进行的，旨在了解经典湍流在超流体（如液氦）中是如何被改变的，在这种超流体中，流动受到与角动量量子化和完全缺乏粘度相关的量子条件的严重限制。量子湍流具有依赖于温度的特性。刚好低于超流体转变温度的超流体表现为正常（类似经典）流体和超流体的混合物；其中一种或两种流体都可能是湍流的，这就产生了多种可能的湍流模式。在更低的温度下，只有超流体成分仍然存在，最清楚地展示了量子湍流的基本方面。如果湍流的模式能够可视化，将极大地促进量子湍流的研究。使用微米大小的氢粒子作为示踪剂已经在其他地方开创，但这个项目的研究人员正在实施使用更小的粒子，以亚稳双原子氦分子的形式，这将通过激光荧光观察到。在略低于超流体转变的温度下，这些分子预计会跟随正常流体，而在更低的温度下，它们预计会跟随超流体成分中的量子化漩涡运动。在这项工作的早期阶段，研究人员将把这项技术应用于更容易接近的温度范围，在这个温度范围内，分子可以追踪正常的流体，但在这个温度范围内，量子湍流的许多重要方面仍有待研究和理解。初步实验将为低温应用铺平道路。此外，由于一种非常简单的湍流形式可以在一个稳定移动的网格后产生，研究人员正在开发一种技术，使网格在非常低的温度下通过超流氦移动，而量子湍流只涉及正常流体。由此产生的湍流将在全球范围内以两种方式进行研究：观察超流体因湍流衰减而升温的速率；并利用离子的散射来测量湍流漩涡浓度衰减的速率。拟议实验的成功取决于佛罗里达大学（发展移动网格和研究其背后的能量耗散）、耶鲁大学和曼彻斯特大学（可视化）、伯明翰大学（理论）、兰开斯特大学和曼彻斯特大学（移动网格后湍流漩涡浓度的衰减）之间密切互动合作的发展。所有实验室的学生和博士后同事将在其他实验室工作，以促进整体努力并进一步发展每个研究人员的能力。虽然这项工作由于低温环境而具有挑战性，但可能的回报是高的，因为对量子湍流的理解可能会提供对迄今为止尚未得到普遍理解的经典湍流的见解。

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.