Microscopic dynamics of quantized vortices in turbulent superfluid in the T=0 limit

T=0极限下湍流超流体中量子化涡旋的微观动力学

• 批准号: EP/P025625/1
• 负责人: Andrei Golov
• 金额: $ 117.55万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FP025625%2F1
• 关键词: Microscopic; dynamics; quantized; vortices; turbulent

Turbulence is ubiquitous in nature and affects almost every aspect of our daily lives. Despite its overwhelming importance, turbulence is poorly understood, mainly because of the complexity of turbulent motion over a very wide range of length scales. Turbulence in superfluid helium, known as quantum turbulence, is special, because quantum mechanics restricts all vortices to have a single fixed value of circulation. Thus we are dealing with a dynamic tangle of vortex lines, all of the same strength. Turbulence, including its quantum variant, is an inherently non-equilibrium phenomenon: remove the driving force, and the turbulence decays.Our goal is to confront the two remaining, mutually interconnected, challenges of quantum turbulence in the T=0 limit: (i) to observe and investigate the elementary processes occurring with individual vortex lines inside bulk tangles; (ii) explore the interaction, and its consequences, of vortex lines with solid boundaries. (i) Below 0.5K damping of the motion of vortex lines effectively vanishes. While it is expected that vortex reconnection and deformation on a broad range of length scales are the main ingredients of their dynamics, no direct observations of these at low temperatures have been made so far. The programme will produce sequences of 2D and 3D images of vortex lines, their bundles and tangles - in different types of turbulent flow, visualized through fluorescence of either He2* excimers or dyed nanoparticles as tracers. Hence, we will obtain information on different aspects of quantum turbulence, and its distinction from classical turbulence. This new technique could revolutionize the study of quantum turbulence. As quantum turbulence mimics classical turbulence on large length scales, our direct visualization of the structure and dynamics of the region of concentrated vorticity might also make an important contribution to the understanding of intermittency in classical turbulence when coherent structures cause rare events of large amplitude.(ii) The understanding of the dynamics of vortex tangles near solid walls is another outstanding fundamental question. The creation of quantum turbulence seems to be "seeded" by remanent vortices pre-existing in the superfluid. It was suggested that the evolution to fully-developed quantum turbulence as the amplitude of an oscillating structure increases may occur via a 2-stage process. First, shaking of the lines sloughs off a gas of small vortex rings, which reconnect to form a random tangle. This tangle itself behaves like a fluid of small viscosity undergoing laminar flow. Then at a higher velocity there is a second transition when the flow turns turbulent. We propose to test this picture experimentally. All earlier experiments on the generation of quantum turbulence by oscillating structures have used objects with convex surfaces; the flow round them is classically unstable at a low velocity, so that the two supposed transitions are not clearly separated. In contrast, we propose experiments where the helium is inside a pill-box that oscillates about its axis, thus eliminating all flow over convex surfaces. The two transitions should then be well separated and identifiable as characteristic increases in damping. We will also illuminate the fundamental properties of the remanent vortices themselves, by investigating their pinning to microscopic protuberance. Recent measurements indicate that vortex pinning get weaker at low temperatures, perhaps through reconnections with lines of the mesh of remanent vortices. To test these results, we propose experiments in a spherical cell, a geometry in which pinned vortex loops are inherently unstable, as well as visualization of remanent vortices, both away from and near boundaries.

湍流在自然界中无处不在，几乎影响到我们日常生活的方方面面。尽管湍流具有压倒性的重要性，但人们对它知之甚少，主要是因为湍流运动在非常广泛的长度尺度上是复杂的。超流氦中的湍流，也就是所谓的量子湍流，是特殊的，因为量子力学限制了所有的涡旋只有一个固定的循环值。因此，我们正在处理的是一个动态缠绕的涡旋线，所有涡线的强度都是相同的。湍流，包括它的量子变体，是一种内在的非平衡现象：去除驱动力，湍流衰减。我们的目标是在T=0极限下面对量子湍流剩下的两个相互关联的挑战：(I)观察和研究个体涡线在主体纠缠中发生的基本过程；(Ii)探索涡线与固体边界的相互作用及其后果。(I)在0.5K以下，涡线运动的阻尼力有效地消失。虽然预计大范围长度尺度上的涡旋重联和变形是它们动力学的主要成分，但到目前为止还没有在低温下对它们进行直接观测。该计划将产生涡旋线、涡旋束和涡旋团的2D和3D图像序列--在不同类型的湍流中，通过He2*准分子或染色纳米颗粒作为示踪剂的荧光可视化。因此，我们将获得有关量子湍流的不同方面的信息，以及它与经典湍流的区别。这项新技术可能会给量子湍流研究带来革命性的变化。由于量子湍流在大尺度上模拟了经典湍流，我们对集中涡量区结构和动力学的直接可视化也可能对理解经典湍流中相干结构导致罕见的大振幅事件时的间歇性做出重要贡献。(Ii)对固体壁附近涡旋纠缠动力学的理解是另一个突出的基本问题。量子湍流的产生似乎是由超流体中预先存在的残余涡旋“播种”的。结果表明，当振荡结构的振幅增大时，向完全发展的量子湍流的演化可能通过一个两阶段的过程发生。首先，摇动这些线会剥离一团小涡环，它们重新连接在一起，形成随机的缠结。这种缠绕本身就像一种小粘度的流体在经历层流流动。然后，在更高的速度下，当流动变成湍流时，会有第二个转变。我们建议对这幅图进行实验测试。所有关于振荡结构产生量子湍流的早期实验都使用了具有凸面的物体；它们周围的流动在低速时经典地是不稳定的，因此两个假设的转变并不是明确分开的。相反，我们提出了这样的实验：氦位于围绕其轴线摆动的药丸盒内，从而消除了在凸面上的所有流动。然后，这两个转变应该被很好地分开，并随着阻尼值的特征增加而被识别。我们还将通过研究它们与微观突起的钉扎来阐明剩余涡旋本身的基本性质。最近的测量表明，涡钉扎在低温下变得更弱，可能是通过重新连接剩余涡网的线条。为了验证这些结果，我们建议在球室中进行实验，在这种几何中，钉扎的涡环本质上是不稳定的，以及在远离和接近边界的情况下显示剩余的涡旋。

项目成果

Quantized Vortex Rings and Loop Solitons

量子化涡环和环孤子

• DOI: 10.1007/s10909-020-02516-0
• 发表时间: 2020
• 期刊: Journal of Low Temperature Physics
• 影响因子: 2
• 作者: Green P

Experimental signature of quantum turbulence in velocity spectra?

• DOI: 10.1088/1367-2630/abfe1f
• 发表时间: 2021-06
• 期刊: New Journal of Physics
• 影响因子: 3.3
• 作者: J. Salort;F. Chillà;E. Rusaouën;P. Roche;M. Gibert;I. Moukharski;A. Braslau;F. Daviaud;B. Gallet;E. Saw;B. Dubrulle;P. Diribarne;B. Rousset;M. B. Mardion;J. Moro;A. Girard;C. Baudet;V. L'vov;A. Golov;S. Nazarenko