Generation of Quantized Vortices in Alternating Flows of Superfluid Helium 4

几个世纪以来，粘性流体的湍动流动引起了人们对其基本性质和应用的研究兴趣，然而，由于其复杂性，许多悬而未决的问题仍然存在困难。相比之下，超流体中的量子湍流有望成为研究湍流性质的合适目标，因为它的结构简单，只包含量子化的涡旋。在本研究中，我们集中研究了超流氦4中量子化涡旋在极低温度下的运动，阐明了量子湍流的动力学。为此，我们研制了一种由直径2.5μm的超细超导导线制成的振动障碍物。研究结果总结如下：1.极低温度下导线振动的共振频率表明，导线上附着着尺寸为0.11微米量级的量子化涡旋。附着在金属丝上的旋涡的大小随着金属丝的速度而变化：在层流中增加，在湍流中减小。我们已经成功地在导线振动不会产生湍流的条件下，将导线定位在有销孔的盒子中，并在200MK以下充入氦20小时。量子化的涡旋通过超流体转变冷却而成核，并在金属丝和周围的墙之间形成桥梁。桥接旋涡的振动会在超流体中引起湍流。通过比较最近一项研究中的理论预测，我们得出结论：涡旋种子在超流转变过程中成核，通过振动流生长，最终在振动流的临界速度下形成湍流。因此，我们成功地用实验方法解释了量子化涡旋的产生。

Turbulent flow of viscous fluids has attracted research interests on both its basic nature and application for centuries; however, many unresolved problems still remain with difficulties because of complexity. In contrast, quantum turbulence in a superfluid is expected to be a suitable target for studying the nature of turbulence, because of its simple structure consisting only quantized vortices. In the present study, we have concentrated on the motion of quantized vortices in superfluid helium 4 at very low temperature, elucidating dynamics of quantum turbulence. For this purpose, we developed an oscillating obstacle made of a very thin superconducting wire with a 2.5 μm diameter. Results obtained in the study are summarized as follows:1. Resonance frequency of wire vibration at very low temperature suggests that quantized vortices of a size of 0.11.tm order are attached to the wire.2. The size of vortices attached to a wire varies with wire velocity: it increases in a laminar flow and decreases in a turbulent flow.3. We have succeeded in a condition for which vibration of a wire is unable to generate turbulence, by locating a wire in a box with a pin hole and filling helium below 200 mK for 20 hours.4. Quantized vortices are nucleated by cooling through the superfluid transition and form bridges between a wire and a surrounding wall.5. Vibration of the bridged vortices causes turbulence in the superfluid. By comparing a theoretical prediction in a recent study, we conclude that vortex seeds nucleate during the superfluid transition, grow by vibrating flows, and eventually become turbulence at a critical velocity of the vibrating flow. Consequently, we obtained success of elucidating generation of quantized vortices using an experimental method.