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

湍流是高速流体流动中普遍存在的现象。它对气象学、船舶、飞机和化学工业几乎具有普遍的重要性。尽管科学家、数学家和工程师做出了许多努力，但这一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。通常，能量以相对较大的长度尺度被馈送到流体中，然后通过非线性过程连续地转化为越来越小的尺度，直到它最终通过耗散过程转化为热。在经典湍流中，这种耗散是由于粘性引起的，而在超流体（4 He低于2K或3 He低于2 mK）中则会出现量子湍流（QT）。在非常低的温度T下，液体不具有正常的流体成分，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，产生由离散涡线组成的湍流，每个涡线具有一个循环量子。由于液体是无粘性的，能量级联不能被粘性终止;在4 He中，能量被认为流过第二个湍流级联，由涡旋上的开尔文波形成，直到通过声子辐射（直接产生热量）在非常小的长度尺度上消散。这个理论的大部分是推测性的，有待实验验证。我们提出的工作是实验性的，在4 He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。第一个涉及调查的大规模性质的特别简单的形式的QT产生的稳定移动的网格在一个非常低的T。它只涉及纯粹的超流体，并且可以预期是近似均匀和各向同性的（参见。最近关于振荡结构产生的复杂和病态QT的工作）。第二个是一个开拓性的项目，旨在在整个超流T范围内的流动可视化（现有的方法不能应用于非常低的T）。第一部分需要一个已经处于高级开发阶段（佛罗里达、兰开斯特和B 'ham）的超导电网牵引电机。这是一项技术上的冒险：承载网格的组件必须悬浮以避免摩擦加热，并且必须以受控的方式移动。可行性研究和测试已经完成。我们现在正在佛罗里达生产第一台工作设备。在佛罗里达，QT的逐渐衰减将通过其产生的热量进行监测，即通过量热法。在兰开斯特，衰变之后将用一束小的带电涡旋环进行反复探测（曼彻斯特最近开发了这项技术，他提供了至关重要的帮助）。这些互补的实验将提供有关的证据都存在一个大的长度尺度范围内QT可能表现的经典，和湍流的性质在较小的（量子）的长度scales.In第二部分，流量将可视化使用He 2 * 准分子作为种子粒子，可以通过激光诱导荧光成像。在4 He中，当温度高于1 K时，它们会跟踪正常流体的运动;低于1 K时，它们可能会被涡线捕获，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4 He中的正常流体在热逆流和高于1 K的网格湍流中的运动的可行性研究（佛罗里达，耶鲁大学，B 'ham的建议），包括能谱的直接观察，我们希望提供近似Kolmogorov标度和由于相干结构的不连续性或形成可能的偏差的直接证据。低于约0.4K的涡流的可视化将最终需要的知识的分子涡流捕获截面，这将是电气测量的曼彻斯特旋转稀释制冷机中的均匀旋转产生的涡流阵列。

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.