Stereoscopic visualization study of turbulence and vortex-tangle dynamics in He II

非技术摘要：当液氦冷却到零下271摄氏度以下时，它就变成了一种超流体（称为He II），它可以做其他流体不能做的事情，比如在没有明显摩擦的情况下流过狭窄的毛细管，以及在重力作用下爬过容器壁。He II迷人的流体力学有许多重要的应用。例如，它支持一种称为热逆流的极其有效的传热模式，因此被广泛用作科学仪器中的冷却剂材料。当被搅拌时，He II也可以产生涡流管，本质上是带有薄空心核心的微型龙卷风。了解涡旋管的动力学有助于研究各种物理系统，如由中子对超流体组成的中子星。在本项目中，研究小组旨在探索逆流湍流中的一些预测异常行为，并使用先进的三维流动可视化技术来可视化河II中的涡管。这项研究可以为更好地应用He II提供必不可少的基础知识。研究团队由博士后和研究生组成。通过该项目，团队成员有望获得流体动力学、低温学和激光技术方面的宝贵知识。他们通过研究培训学到的技能对他们在今天的就业市场上取得成功也至关重要。此外，研究小组计划在国家高磁场实验室的各种推广项目中进行涉及低温流体的教育演示。PI还致力于通过组织研讨会和会议吸引初级研究人员来为研究界服务。技术摘要：目前的研究工作包括：1)用立体分子标记速度法（MTV）研究He II型逆流湍流中的异常各向异性；2)用扫描粒子跟踪速度法（PTV）探索漩涡缠结动力学。该立体MTV系统采用了独特的双线光学装置，是量子湍流研究领域的首个此类设备。它提供了在耦合双流体系统中对正流体速度场及其空间相关性进行明确测量的能力，这为解决各种具有挑战性的问题开辟了道路。对He II逆流湍流异常各向异性的研究有望产生实验数据，这对于发展对这一复杂双湍流系统的完整理论认识至关重要。这样的理解不仅提高了紊流的知识，而且还有助于低温工程师设计更有效的基于He II的冷却系统。所开发的扫描PTV系统能够实现三维空间中量子化涡旋缠结的实时成像，为涡旋研究提供了一个很好的机会。在探索涡旋-重连尺度、涡旋-速度相关性和涡旋轨迹统计方面的计划研究将为开发先进的量子湍流统计模型提供重要的新知识。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。

Non-Technical Abstract: When liquid helium is cooled to below about -271 degrees Celsius, it becomes a superfluid (called He II), which can do things that other fluids cannot, such as flowing through a narrow capillary without apparent friction and climbing over container walls despite the gravity. The fascinating hydrodynamics of He II has many important applications. For instance, it supports an extremely efficient heat-transport mode called thermal counterflow and therefore has been widely utilized as a coolant material in scientific instruments. When stirred, He II can also create vortex tubes which are essentially tiny tornadoes with thin hollow cores. Understanding the dynamics of a tangle of vortex tubes can benefit the study of various physical systems such as neutron stars that consist of neutron-pair superfluid. In this project, the research team aims to explore some predicted anomalous behaviors in counterflow turbulence and to visualize the vortex tubes in He II using advanced three-dimensional flow visualization techniques. This research can produce fundamental knowledge indispensable for better applications of He II. The research team consists of both postdoctoral researchers and graduate students. Through the project, the team members are expected to gain valuable knowledge in fluid dynamics, cryogenics, and laser technologies. The skills they learn through the research training are also crucial for their success in today's job market. In addition, the research team plans to conduct educational demonstrations involving cryogenic fluids in various outreach programs at the National High Magnetic Field Laboratory. The PI is also committed to serve the research community by organizing workshop and conferences to engage junior researchers. Technical Abstract:The current research work consists of: 1) a stereoscopic molecular tagging velocimetry (MTV) study of the anomalous anisotropy in He II counterflow turbulence, and 2) an exploration of the vortex tangle dynamics using a scanning particle tracking velocimetry (PTV) method. The stereoscopic MTV system, which incorporates a unique double-line optical setup, is developed as the first-of-its-kind equipment in quantum turbulence research. It offers the capability of making unambiguous measurements of the normal-fluid velocity field and its spatial correlations in the coupled two-fluid system, which breaks the ground for solving various challenging problems. The study of the anomalous anisotropy in He II counterflow turbulence is expected to produce experimental data that are critical for the development of a complete theoretical understanding of this complex doubly turbulent system. Such an understanding not only advances the knowledge of turbulence in general but also aids the cryogenic engineers in designing more efficient He II based cooling systems. The scanning PTV system to be developed enables real-time imaging of quantized vortex tangles in three-dimensional space, which offers a great opportunity for vortex research that is not possible otherwise. The planned research on exploring the vortex-reconnection scaling, vortex-velocity correlations, and vortex trajectory statistics should produce important new knowledge needed for the development of an advanced statistical model of quantum turbulence.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.