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.

湍流本质上是无处不在的，几乎影响了我们日常生活的各个方面。尽管其重要性非常重要，但湍流的理解很少，这主要是由于湍流运动在非常宽的长度尺度上的复杂性。超流体氦气中的湍流（称为量子湍流）是特殊的，因为量子力学限制了所有涡旋，以具有单个循环的固定值。因此，我们正在处理涡旋线的动态缠结，这都是相同的强度。 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）探索具有固体边界的涡流线的相互作用及其后果。 （i）涡流线运动的0.5K阻尼有效地消失了。虽然可以预期，涡旋重新连接和变形在广泛的长度尺度上是其动力学的主要成分，但到目前为止，在低温下未直接观察这些动力学。该程序将在不同类型的湍流中产生涡旋线的2D和3D图像的序列，并通过HE2*精确剂或染色的纳米颗粒作为示踪剂而可视化。因此，我们将获取有关量子湍流不同方面的信息，及其与经典湍流的区别。这种新技术可以彻底改变量子湍流的研究。由于量子湍流模仿了大长度的经典湍流，因此我们对集中涡度区域的结构和动态的直接可视化也可能为理解经典湍流中间歇性的理解做出重要贡献。量子湍流的产生似乎是通过在超流体中预先存在的剩余涡流“种子”的。有人提出，随着振荡结构的振幅增加可能通过2阶段的过程而发生，完全发达的量子湍流的演变。首先，线条摇晃从小涡流环的气体上脱落，然后重新连接以形成随机缠结。这种缠结本身就像经历层流流的小粘度的流体。然后在较高的速度下，当流动变湍流时，会有第二个过渡。我们建议通过实验测试这张照片。关于通过振荡结构产生量子湍流的所有早期实验都使用了具有凸表面的对象。在低速下，它们的流动在经典上是不稳定的，因此两个假定的过渡没有明确分开。相比之下，我们提出了实验，其中氦气位于围绕其轴上振荡的药丸盒内部，从而消除了凸表面上的所有流动。然后，随着阻尼的特征增加，这两个过渡应得到很好的分离和可识别。我们还将通过调查将其固定到微观突起，从而阐明Remanent涡旋本身的基本特性。最近的测量表明，在低温下涡流固定会变得较弱，也许是通过与remanent涡旋的线条重新连接。为了测试这些结果，我们建议在球形细胞中进行实验，该几何形状本质上是不稳定的几何形状，以及远离边界和近乎边界的Remanent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