Generation of Quantized Vortices in Alternating Flows of Superfluid Helium 4

几个世纪以来，粘性流体湍流的基本性质和应用一直引起人们的研究兴趣。然而，由于复杂性，许多尚未解决的问题仍然困难重重。相比之下，超流体中的量子湍流由于其结构简单，仅由量子化的漩涡组成，有望成为研究湍流性质的合适目标。在本研究中，我们集中研究了超流体氦4在极低温下的量子化涡的运动，阐明了量子湍流的动力学。为此，我们开发了一种由直径为2.5 μm的超导体制成的振荡障碍。研究结果总结如下：1。极低温下金属丝振动的共振频率表明，量子化涡的大小为0.11。我的订单系在电线上。附着在金属丝上的旋涡的大小随金属丝的速度而变化：在层流中旋涡增大，在湍流中旋涡减小。3 .我们把一根金属丝放在一个有针孔的盒子里，在200 mK以下充氦20小时，成功地达到了金属丝振动不能产生湍流的条件。量子化涡流通过超流体转变冷却而成核，并在导线和周围壁之间形成桥梁。桥接涡的振动引起超流体中的湍流。通过比较最近研究的理论预测，我们得出了涡旋种子在超流体过渡过程中形成核，在振动流中成长，并最终在振动流的临界速度下成为湍流的结论。因此，我们成功地用实验方法解释了量化涡的产生。

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.