Materials World Network: Collaborative Research on Simple Forms of Quantum Turbulence - Production, Decay and Visualization

湍流在高速流体流动中是普遍存在的。它对气象学、船舶、飞机和化学工业具有几乎普遍的重要性。尽管科学家、数学家和工程师付出了很多努力，但这种现象的复杂性仍然没有得到很好的理解，并提出了重大的理论挑战。一般来说，能量以相对较大的长度尺度输入流体，然后通过非线性过程依次转化为越来越小的尺度，直到最终通过耗散过程转化为热量。在经典湍流中，这种耗散是由粘度引起的。量子湍流（QT）出现在超流体中（4He低于2K或3He低于2mk）。在非常低的温度T下，液体没有正常的流体成分，这可能是一个更简单的现象，因为流体运动受到量子效应的约束，产生由离散涡旋线组成的湍流，每个涡旋线都有一个循环量子。能量级联不能被粘度终止，因为液体是无粘性的；在4He中，能量被认为流经第二次湍流级联，由旋涡上的开尔文波形成，直到声子辐射（直接产生热量）在非常小的长度尺度上消散。这一理论的大部分都是推测性的，有待实验验证。我们提出的工作是实验性的，在4He上。它的两个相互联系的部分都依赖于合作伙伴之间的密切合作。第一个涉及到在非常低的t下稳定移动的网格产生的特别简单形式的QT的大规模性质的研究。它只涉及到纯超流体，并且可以预期是近似均匀和各向同性的（参见最近关于由振荡结构产生的复杂和特征不佳的QT的许多工作）。第二个是一个开创性的项目，旨在实现整个超流体T范围内的流动可视化（现有方法无法应用于非常低的T）。第一部分需要超导并网电机，该电机已经处于开发的高级阶段（佛罗里达，兰开斯特和B'ham）。这在技术上是一项困难的冒险：承载电网的组件必须悬浮起来，以避免摩擦加热，并且必须以可控的方式移动。可行性研究和试验已经完成。我们现在正在佛罗里达州生产第一个工作设备。在佛罗里达，QT的逐渐衰减将通过它产生的热量来监测，即量热法。在兰开斯特，衰变之后将用一束带电的小漩涡环重复探测（在曼彻斯特的重要帮助下，他最近开发了这项技术）。这些补充性的实验将提供证据，证明在大长度尺度上QT可能表现出经典的行为，以及在较小（量子）长度尺度上湍流的性质。在第二部分中，将使用He2*准分子作为种子粒子，通过激光诱导荧光成像来可视化流动。在4He中高于1K时，它们有望跟踪正常流体的运动；在1K以下，它们很可能被涡旋线困住，因此可以跟踪它们的位置和运动。这项技术的发展可能会彻底改变QT的研究。目前的资助将涵盖4He中正常流体在热逆流和1K以上网格湍流中的运动的可行性研究（佛罗里达，耶鲁，来自B'ham的建议），包括对能量谱的直接观察，我们希望提供近似Kolmogorov标度的直接证据，以及由于间歇性或形成相干结构而可能出现的偏差。要使低于0.4K的涡流可视化，最终需要了解分子涡捕获截面的知识，这将通过在曼彻斯特旋转稀释冰箱中均匀旋转产生的涡流阵列进行电测量。

Turbulence is universal in high velocity fluid flow. It is of near universal importance for meteorology, ships, aircraft, and the chemical industry. Despite much effort by scientists, mathematicians and engineers, the complexity of the phenomenon is such that it is still not well understood and presents major theoretical challenges. Quite generally, energy is fed into the fluid on a relatively large length scale, and is then successively transformed by non-linear processes to smaller and smaller scales, until it is ultimately converted to heat by dissipative processes. In classical turbulence, this dissipation is due to viscosity.Quantum turbulence (QT) arises in superfluids (4He below 2K or 3He below 2 mK). At very low temperatures T, where the liquid possesses no normal fluid component, it may be a simpler phenomenon because the fluid motion is constrained by quantum effects, yielding turbulence consisting of discrete vortex lines each with one quantum of circulation. The energy cascade cannot be terminated by viscosity because the liquid is inviscid; in 4He the energy is believed to flow through a second turbulent cascade, formed from Kelvin waves on the vortices, until dissipated on very small length scales by phonon radiation (direct generation of heat). Much of this theory is speculative and awaits experimental verification.Our proposed work is experimental, on 4He. Its two linked parts each depend on close collaboration between the partners. The first involves investigation of large-scale properties of the especially simple form of QT produced by a steadily moving grid at a very low T. It involves only the pure superfluid and can be expected to be approximately homogeneous and isotropic (cf. much recent work on the complex and ill-characterized QT produced by oscillating structures). The second is a pioneering project aimed at a flow visualization over the whole superfluid T-range (existing methods cannot be applied at very low T). The first part requires a superconducting grid-drawing motor that is already at an advanced stage of development (Florida, Lancaster and B'ham). It is a technically difficult venture: the component carrying the grid must be levitated to avoid frictional heating, and it must be moved in a controlled way. Feasibility studies and tests have already been completed. We are now moving to production of the first working device, in Florida. In Florida, the gradual decay of the QT will be monitored from the heat it produces, i.e. calorimetrically. In Lancaster the decay will be followed by repeated probing with a beam of small charged vortex rings (with crucial help from Manchester, who recently developed the technique). These complementary experiments will provide evidence relating both to the existence of a range of large length scales at which QT may behave classically, and to the nature of the turbulence on smaller (quantum) length scales.In the second part, the flow will be visualized by use of He2* excimer molecules as seed particles that can be imaged by laser-induced fluorescence. Above 1K in 4He they are expected to track the motion of the normal fluid; below 1K they are likely to be trapped by vortex lines and so can track their positions and motion. The development of this technique could revolutionize the study of QT. The present grant will cover feasibility studies of the motion of the normal fluid in 4He both in thermal counterflow and in grid turbulence above 1K (Florida, Yale, advice from B'ham), including direct observation of the energy spectrum providing, we hope, direct evidence for approximate Kolmogorov scaling and of possible deviations due to intermittency or formation of coherent structures. Visualization of vortices below about 0.4K will ultimately require knowledge of the molecule-vortex capture cross-section, which will be measured electrically for a vortex array produced by uniform rotation in the Manchester rotating dilution refrigerator.