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

非技术摘要：当液氦冷却到大约零下271摄氏度时，它就变成了一种超流体(称为He II)，它可以做其他流体做不到的事情，比如流过狭窄的毛细管而没有明显的摩擦，以及在重力的作用下爬过容器壁。He II迷人的流体力学有许多重要的应用。例如，它支持一种名为热逆流的极其高效的热传输模式，因此已被广泛用作科学仪器的冷却剂材料。当搅拌时，He II还可以产生涡流管，本质上是带有薄薄中空核心的微小龙卷风。了解涡旋管纠缠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。在这个项目中，研究小组的目标是探索逆流湍流中一些预测的异常行为，并使用先进的三维流动可视化技术来可视化He II中的涡管。这项研究可以产生更好地应用He II所必需的基础知识。研究团队由博士后研究人员和研究生组成。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也是他们在今天的就业市场上取得成功的关键。此外，研究小组计划在国家强磁场实验室的各种推广计划中进行涉及低温液体的教育演示。国际和平研究所还致力于通过组织研讨会和会议来吸引初级研究人员，为研究界服务。技术摘要：目前的研究工作包括：1)氦II逆流湍流中反常各向异性的立体分子标记测速(MTV)研究；2)用扫描粒子跟踪测速(PTV)方法探索涡旋缠绕动力学。立体MTV系统结合了独特的双线光学装置，是量子湍流研究中的第一台设备。它提供了对耦合两流体系统中的法向流体速度场及其空间相关性进行明确测量的能力，为解决各种具有挑战性的问题开辟了新的道路。对He II逆流湍流中反常各向异性的研究有望产生实验数据，这些数据对于发展对这一复杂的双湍流系统的完整的理论理解是至关重要的。这样的理解不仅提高了对湍流的总体认识，而且有助于低温工程师设计更有效的基于He II的冷却系统。即将开发的扫描PTV系统可以在三维空间中实时成像量化的涡旋纠缠，这为涡旋研究提供了一个很好的机会，否则是不可能的。计划中的关于探索涡旋重联比例、涡流速度关联和涡旋轨迹统计的研究应该会产生开发先进的量子湍流统计模型所需的重要新知识。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.