Turbulence in a Pure Superfluid

湍流，即流体的混沌旋转运动，通常被描述为经典物理学中最重要的未解决的问题。然而，湍流是每个人都熟悉的东西，从在飞机上飞行时被撞到，到洗澡时水龙头里的水。它发生在所有尺度的流体流动中，从微观到银河系，已经被深入研究了世纪，但完全理解仍然难以捉摸。问题的部分原因是由于组成经典流体（如水）中湍流的涡旋或漩涡的不断变化的性质。纯超流体（如冷却到绝对零度半度以内的4He）中的湍流是研究湍流现象的理想模型系统。量子力学指出，这种流体可以显示旋转运动的唯一方式是通过创建非常细（0.1 nm直径）的相同细丝，其中所有的涡量都集中在这些细丝周围，并且流体循环必须采取特定的固定值。这些物体是量子化的涡旋，湍流完全由这些杂乱的线条描述，我的建议试图回答这种特殊类型的湍流如何衰减的基本问题。由于粘性，没有摩擦机制，例如在经典流体中发生的摩擦机制，因此认为耗散是由于单个涡线上的高频波发出的声音而发生的，但这需要通过实验进行检查。能量如何从流体被搅动的大尺度（厘米）转移到这些小长度（纳米）尺度，人们甚至不太清楚，有几个相互竞争的理论观点。因此，尽管湍流在概念上是一种简单的形式，但在这种流体中有很多新的物理学需要探索，这使得新的实验研究变得成熟。为了大大提高我们的理解，我将进行几种不同类型的实验，这些实验将在巨大的长度尺度范围内探测湍流，跨越六个数量级。这将涉及到通过观察一束微米大小的涡环如何被湍流缠结散射，来监测缠结中涡线的密度如何随时间衰减。均匀湍流将通过突然停止旋转的容器产生，这将诱导超流通过网格。此外，灵敏的量热法将用于测量微观尺度上因耗散而释放的热量，从而深入了解衰变机制。我还将开发技术，以发现在单个线条的尺度上会发生什么，例如通过观察当它们的末端被摇动时，最初直的涡旋线的行为。这将允许一种类型的波湍流，其中能量由于非线性相互作用而转移到较短波长的波中。最后一种实验将检查一种外来的探测粒子，亚稳态氦分子，是否可以被捕获在涡线的核心上。如果是这样的话，那么在未来，这些粒子可以用来可视化湍流涡旋缠结。所有这些实验的共同特点是需要旋转设备以产生直线涡线（在稳定旋转期间）并产生湍流（通过突然停止）。因此，一个新的旋转millikelvin低温恒温器将通过广泛的翻新旧的旋转低温恒温器。这种新的最先进的仪器对于拟议中的量子湍流实验至关重要，但在未来，它也将能够探测我们对液态和固态氦的理解中仍然存在的许多谜团。

Turbulence, the chaotic swirling motion of a fluid, is often described as the most important unsolved problem of classical physics. Yet turbulence is something that everyone has some familiarity with, from getting bumped around while flying on an aircraft to the water gushing out of the taps while running a bath. It occurs in fluid flows on all scales, from the microscopic to the galactic and has been studied intensively for well over a century but a full understanding remains elusive. Part of the problem is due to the constantly changing nature of the vortices, or eddies, that make up the turbulence in a classical fluid (such as water).Turbulent flow in a pure superfluid, such as 4He cooled to within half a degree of absolute zero is an ideal model system for investigating turbulent phenomena. Quantum mechanics dictates that the only way such a fluid can display rotational motion is through the creation of very fine (0.1 nm diameter) identical filaments where all the vorticity is concentrated and around which the fluid circulation has to take a particular fixed value. These objects are quantized vortices and the turbulent flow is completely described by a messy tangle of these lines.My proposal seeks to answer the fundamental question as to how this special type turbulence decays. There is no frictional mechanism due to viscosity, such as occurs in a classical fluid, so it is believed that dissipation occurs due to sound being emitted from high frequency waves on individual vortex lines but this needs to be checked experimentally. How energy can be transferred from the large scales (centimetres) where the fluid is stirred down to these small length (nanometres) scales is even less well understood and there are several competing theoretical ideas. Thus, even though turbulence takes a conceptually simple form in this fluid, there is plenty of new physics to explore, making it ripe for new experimental research.To significantly advance our understanding, I will perform several different types of experiment that will probe the turbulence over a huge range of length scales, spanning six orders of magnitude. This will involve monitoring how the density of vortex lines in the tangle decays with time by observing how a beam of micron-sized vortex rings is scattered by the turbulent tangle. Homogeneous turbulence will be generated by suddenly stopping a rotating container which will induce superfluid flow through a grid. In addition, sensitive calorimetry will be used to measure the heat released due to dissipation at microscopic scales, providing insight into the decay mechanism. I will also develop techniques to discover what happens on the scale of individual lines, such as by looking how initially straight vortex lines behave when their ends are shaken. This will allow a type of wave turbulence, where energy is transferred to shorter wavelength waves due to non-linear interactions, to be probed. The final type of experiment will check whether an exotic type of probe particle, metastable helium molecules, can be trapped on the cores of the vortex lines. If so, then in the future these particles could be used to visualize the turbulent vortex tangles. The common feature of all these experiments is the need to rotate the apparatus to create rectilinear vortex lines (during steady rotation) and generate turbulence (by suddenly stopping). Thus, a new rotating millikelvin cryostat will be constructed through the extensive refurbishment of an old rotating cryostat. The new state-of-the-art instrument will be vital for the proposed experiments on quantum turbulence, but it will also be capable, in the future, of probing many of the mysteries that still remain in our understanding of liquid and solid helium.