Creation and evolution of quantum turbulence in novel geometries

新颖几何形状中量子湍流的产生和演化

• 批准号: EP/X004597/1
• 负责人: Peter Vaughan Elsmere McClintock
• 金额: $ 132.9万
• 依托单位: Lancaster University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FX004597%2F1
• 关键词: Creation; evolution; quantum; turbulence; novel; Creation; evolution; quantum; turbulence; novel

Turbulence is ubiquitous in the real world and affects almost every aspect of our daily lives, including transport, energy production, climate, and biological processes. Despite its universal importance, turbulence is not well understood. Richard Feynman called it the "most important unsolved problem of classical physics". Turbulence is hard to understand at a fundamental level because of the complexity of turbulent motion of the fluid over an extremely wide range of length scales. Quantum mechanics often makes complex problems conceptually simpler, and quantum turbulence (QT) in superfluids is a prime example. At low temperatures, superfluids are the closest attainable approximation to an ideal fluid in that they can flow without friction, are (almost) incompressible, and their vortices are quantised, making all of them identical.Like classical turbulence, QT is a non-equilibrium phenomenon: remove the driving force, and it decays - though perhaps not completely in He II due to residual quantised vortices pinned metastably to the walls. In He II, the creation of QT usually seems to be "seeded" by such remanent vortices. Earlier experiments on oscillating structures have hinted that evolution to fully-developed QT as the oscillatory amplitude increases may occur via at least 2-stages. (i) Above a first critical velocity, shaking of the pinned lines creates a vortex tangle where motion only occurs on length scales comparable with the line spacing. (ii) At a higher critical velocity, a second transition occurs in which laminar flow of the tangle breaks down into turbulence, like flow in a classical fluid. We now propose two closely-related experiments, each utilising novel technology, first to explore the fundamental properties of the remanent vortices and, secondly, to investigate the intrinsic vortex creation process in the absence of remanent vortices.The first set of experiments will explore vortex nucleation in superfluid within a pill-box-shaped cell where there is no flow over convex surfaces when the cell is oscillated about its axis of symmetry. We expect the cell movements to generate Kelvin waves on remanent vortices if they are pinned to the parallel faces, resulting in reconnections above a critical velocity and creation of the quasi-classical vortex tangle in laminar flow. The absence of a convex surface within the superfluid means that the second critical velocity, leading to fully developed QT will be raised, enabling it to be resolved. We will also investigate the pinning of remanent vortices. At finite temperature, we might expect that thermal fluctuations will enable a line to de-pin/re-pin sequentially, sliding its end across the surface whereas, at T=0, the lines would become frozen on pinning sites. However, measurements at UC Davis have questioned this widely-accepted picture, suggesting decreased pinning as the temperature falls, i.e. the opposite of expectation. We will resolve this enigma and will try to account theoretically for what we find. In the second set of experiments, we will study diverse motions of a small, magnetically-levitated, superconducting sphere through the superfluid. Although nothing like this has been attempted previously, we are confident of being able to oscillate the sphere (to make contact with earlier experiments) and, for the first time, to be able to move it in a circle at a steady velocity. Measurements of the drag as a function of time will provide information about the presence/absence of pinned vortex loops and their growth and separation as free vortex rings, as functions of velocity and temperature. We envisage these experiments opening a new chapter in the study of quantized vortex lines in superfluids, quite generally, not just in He-4. The flying sphere experiments will prepare the way for a possible cryogenic "wind tunnel" where a levitated model structure is moved through stationary He-4, with Reynolds numbers up to 100,000,000.

湍流在现实世界中无处不在，几乎影响了我们日常生活的各个方面，包括运输，能源生产，气候和生物过程。尽管它具有普遍的重要性，但湍流尚未得到充分理解。理查德·费曼（Richard Feynman）称其为“古典物理学中最重要的未解决问题”。由于流体在非常宽的长度尺度上的湍流运动的复杂性，在基本层面上很难理解湍流。量子力学通常在概念上使复杂问题变得更简单，而超流体中的量子湍流（QT）是一个很好的例子。 At low temperatures, superfluids are the closest attainable approximation to an ideal fluid in that they can flow without friction, are (almost) incompressible, and their vortices are quantised, making all of them identical.Like classical turbulence, QT is a non-equilibrium phenomenon: remove the driving force, and it decays - though perhaps not completely in He II due to residual quantised vortices pinned metastably to the墙。在He II中，QT的创建通常似乎是由这种不再的涡流“种子”。早期关于振荡结构的实验暗示，随着振荡幅度的增加可能会至少通过至少2个阶段而发生，向完全发达的QT演变。 （i）在第一个临界速度上方，固定线的摇动会产生涡流缠结，其中运动仅在长度尺度上与线间距相当。 （ii）在较高的临界速度下，发生第二个过渡，其中缠结的层流流分解为湍流，例如经典流体中的流动。现在，我们提出了两个密切相关的实验，每种实验都采用新技术，首先探索不一度涡旋的基本属性，其次，在没有偏离的涡流中研究固有的涡旋创造过程。当第一个实验中，当不再造成的涡流中的核心范围内的涡流中的涡流中不存在，而无需过度旋转，而无需过度旋转的核心范围。关于它的对称轴。我们预计，如果细胞的运动将其固定在平行面上，则会在剩余的涡旋上产生开尔文波，从而导致层状流中准经典涡流缠结的临界速度和产生的重新连接。超流体内没有凸面表面，这意味着将提高第二个临界速度，从而使其能够解决。我们还将调查剩余涡旋的固定。在有限温度下，我们可能希望热波动能够顺序脱针/重新钉，并在整个表面上滑动其末端，而在t = 0时，这些线将在固定位点上冻结。但是，加州大学戴维斯分校的测量值质疑这张广泛认可的图片，表明随着温度下降，固定量降低，即预期的相反。我们将解决这个谜团，并尝试从理论上解释我们发现的东西。在第二组实验中，我们将研究通过超流体的小型，磁性的超导球的各种运动。尽管以前没有尝试过这样的尝试，但我们有信心能够振荡球体（与早期实验进行接触），并且第一次能够以稳定的速度以圆圈的速度移动它。阻力随时间的函数的测量将提供有关固定涡流环的存在/不存在的信息，其生长和分离为游离涡旋环，作为速度和温度的功能。我们设想这些实验在超级流体中量化涡流线的研究中开辟了新的章节，这不仅是在HE-4中。飞行球体实验将为可能的低温“风洞”做准备，其中悬浮的模型结构通过固定的HE-4移动，雷诺数的数字高达100,000,000。