Generation of Quantized Vortices in Alternating Flows of Superfluid Helium 4

粘性流体湍流的基本性质和应用研究已引起人们的极大兴趣，但由于其复杂性，至今仍有许多尚未解决的问题。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。在目前的研究中，我们集中在超流氦4在非常低的温度下的量子涡旋的运动，阐明量子湍流的动力学。为此，我们开发了一种由直径为2.5 μm的极细超导导线制成的振荡障碍物。研究结果如下：1.在非常低的温度下的线振动的共振频率表明，量子化的漩涡的大小为0.11.tm顺序连接到线。附在金属丝上的涡的大小随金属丝速度而变化：它在层流中增大，在湍流中减小。我们已经成功地在一个条件下的振动的一根线是无法产生湍流，通过定位一根线在一个盒子与针孔和填充氦低于200 mK的20小时。4.量子化涡旋通过超流过渡冷却而成核，并在金属丝和周围壁之间形成桥。桥涡的振动在超流体中引起湍流。通过比较最近的一项研究中的理论预测，我们得出结论，涡种子成核过程中的超流过渡，增长的振动流，并最终成为湍流的振动流的临界速度。因此，我们成功地用实验方法阐明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.