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，几乎影响着我们日常生活的方方面面。尽管湍流具有压倒性的重要性，但人们对它的了解却知之甚少，这主要是因为在很宽的长度尺度范围内湍流运动的复杂性。超流氦中的湍流（称为量子湍流）很特殊，因为量子力学限制所有涡流具有单一固定的循环值。因此，我们正在处理动态缠结的涡线，所有涡线的强度都相同。湍流，包括其量子变体，本质上是一种非平衡现象：去除驱动力，湍流就会衰减。我们的目标是在 T=0 极限下应对量子湍流剩下的两个相互关联的挑战：（i ）观察和研究大块缠结内单个涡流线发生的基本过程； (ii) 探索具有实体边界的涡线的相互作用及其后果。 (i) 低于 0.5K 时涡线运动的阻尼有效地消失。虽然预计大范围长度尺度上的涡旋重联和变形是其动力学的主要成分，但迄今为止尚未在低温下对这些进行直接观察。该程序将在不同类型的湍流中生成涡线、涡线束和缠结的 2D 和 3D 图像序列，通过 He2* 准分子或染色纳米粒子作为示踪剂的荧光进行可视化。因此，我们将获得有关量子湍流不同方面的信息，及其与经典湍流的区别。这项新技术可能会彻底改变量子湍流的研究。由于量子湍流在大尺度上模拟经典湍流，当相干结构导致罕见的大振幅事件时，我们对集中涡度区域的结构和动力学的直接可视化也可能对理解经典湍流的间歇性做出重要贡献。（ ii）理解固体壁附近涡旋缠结的动力学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流“播种”的。有人提出，随着振荡结构振幅的增加，演化到完全发展的量子湍流可能通过两阶段过程发生。首先，线的摇动会脱落小涡环的气体，这些小涡环重新连接形成随机的缠结。这种缠结本身的行为就像经历层流的小粘度流体。然后，在更高的速度下，当流动变成湍流时，会发生第二次转变。我们建议通过实验来测试这张图片。所有早期关于通过振荡结构产生量子湍流的实验都使用具有凸面的物体；它们周围的流动在低速下通常是不稳定的，因此两个假设的转变并没有明显分开。相比之下，我们提出了实验，其中氦气位于绕其轴振荡的药丸盒内，从而消除了凸表面上的所有流动。随着阻尼特性的增加，这两个转变应该被很好地分离和识别。我们还将通过研究残余涡旋对微观突起的钉扎来阐明残余涡旋本身的基本特性。最近的测量表明，涡旋钉扎在低温下会变弱，这可能是通过与残余涡旋网格线的重新连接而实现的。为了测试这些结果，我们建议在球形单元（一种固定涡流环本质上不稳定的几何形状）中进行实验，以及远离和靠近边界的剩余涡流的可视化。

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