Flow Visualization Study of Quantum Hydrodynamics in Superfluid Helium-4

当液氦冷却到零下271摄氏度时，它就变成了一种无粘性的超流体，可以做其他流体不能做的事情，比如渗透过超薄的裂缝，爬过容器壁。超流氦令人着迷的流体力学有许多重要的科学和工程应用。例如，它支持最有效的传热机制，作为冷却材料，它也允许在飞机和船舶模型测试的小型实验室设备中产生剧烈的湍流。然而，缺乏这种冷流体的定量流量测量工具阻碍了对其流体动力学的理解和利用。在这个项目中，研究小组旨在通过采用一种新开发的分子标记流动可视化技术来阐明超流氦中各种湍流流动的紧急性质。这项研究有望为超流氦的更好应用提供必不可少的基础知识。研究团队由研究生和本科生组成。这些学生可以获得流体动力学、低温学和先进激光技术方面的经验。这些技能使学生具备了在当今以科技为主导的市场中脱颖而出所必需的技术灵活性。此外，研究小组计划在国家高磁场实验室的各种教育和推广项目中进行涉及超流氦的演示，向公众介绍深刻的科学概念。研究工作的目的是应用一种新开发的分子标记测速（MTV）技术来解决超流氦中两种流动形式的突出问题：由外加热流产生的热逆流和通过机械强迫产生的准经典流动。在首席研究员的实验室里对逆流的初步研究揭示了一种新的湍流形式。理解这种湍流是目前量子湍流研究中最具挑战性的问题之一。利用MTV技术，研究小组计划对逆流湍流的能谱如何在广泛的温度范围内随热通量变化进行系统的研究。这些信息可以形成对有趣的逆流湍流的理论理解发展的基础。在研究拖曳网格产生的准经典湍流的实验中，将流动可视化与二次声衰减方法相结合，探测超流氦中两种流体组分的运动。研究小组计划研究双流体耦合模型，并测量过去无法可靠确定的紧急流动特性。这项工作有望为超流氦在未来湍流研究中的各种令人兴奋的应用铺平道路。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

When liquid helium is cooled to about -271 degrees Celsius, it becomes an inviscid superfluid and can do things that other fluids cannot, such as seeping through ultra-thin cracks and climbing over container walls. The fascinating hydrodynamics of superfluid helium has many important scientific and engineering applications. For instance, it supports the most efficient heat-transport mechanism as a coolant material, and it also allows the generation of violent turbulent flows in compact laboratory equipment for model testing of airplanes and ships. However, the lack of quantitative flow measurement tools in this cold fluid has impeded progress in understanding and utilizing its hydrodynamics. In this project, the research team aims to elucidate the nature of emergent properties of various turbulent flows in superfluid helium by employing a newly developed molecular-tagging flow visualization technique. This research is expected to produce fundamental knowledge indispensable for better applications of superfluid helium. The research team is composed of graduate and undergraduate students. These students can gain experience in fluid dynamics, cryogenics, and advanced laser technologies. These skills give the students the technical dexterity necessary to excel in today's science- and technology-dominated market. In addition, the research team plans to conduct demonstrations involving superfluid helium in various educational and outreach programs at the National High Magnetic Field Laboratory to introduce profound scientific concepts to the general public.The objective of the research work is to apply a newly developed molecular tagging velocimetry (MTV) technique to tackle outstanding problems in two forms of flows in superfluid helium: thermal counterflow that can be produced by an applied heat current and quasiclassical flow that can be generated via mechanical forcing. Preliminary study on counterflow in the principle investigator's lab has revealed a novel form of turbulence. Understanding this turbulence is now regarded as one of the most challenging problems in quantum turbulence research. Using the MTV technique, the research team plans to conduct systematic study on how the energy spectrum of the counterflow turbulence may vary with heat flux in a wide range of temperatures. This information can form the base for the development of a theoretical understanding of the intriguing counterflow turbulence. In the experiment on studying towed-grid generated quasiclassical turbulence, flow visualization is combined with second sound attenuation method for probing the motion of the two fluid components in superfluid helium. The research team plans to examine the two-fluid coupling model and measure emergent flow properties that cannot be reliably determined in the past. This work is expected to pave the way for various exciting applications of superfluid helium in future turbulence research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.