Stability of Laminar and Turbulent Flow of Superfluid 4He at mK Temperatures Around an Oscillating Microsphere
Stability of Laminar and Turbulent Flow of Superfluid 4He at mK Temperatures Around an Oscillating Microsphere
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振荡微球周围 mK 温度下超流体 4He 的层流和湍流稳定性
DOI：
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发表时间：
2004
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影响因子：
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通讯作者：
W. Schoepe
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作者：
M. Niemetz;W. Schoepe
在1K到25MK温度范围内，研究了超流氦绕振动微球的流动。在小振幅下观察到纯势流，球上的线性阻力仅由0.7K以下的弹道准粒子散射决定，声子只贡献0.5K以下。在较大的振幅下，如果至少0.6PW从球体转移到湍流超流体中，则强烈的非线性阻力证明了稳定的湍流流动。在振幅(或驱动力)的中间范围内，这两种流型都是不稳定的，并且在0.5K以下观察到两者之间的间歇性切换。我们记录了在恒定驱动和持续36小时的温度下这种切换现象的时间序列。运用可靠性理论对时间序列进行了统计分析。湍流相的寿命随着驱动力的增加而增长，并在一个临界值(或至少变得不可测量的长)处发散。中间区域层流相的稳定性取决于球体在临界速度以上的过剩速度。在平均寿命限制为25分钟的临界速度以上观察到亚稳层流相，这是由于自然本底放射性，有时由于液体的电离而产生局部涡旋。最后指出，潜在流量的崩溃属于可靠性测试中众所周知的“系统故障”实验，其统计特性可用极值理论来描述。PACS编号：67.40.vs，47.27.Cn。
The flow of superfluid helium around a vibrating microsphere is investigated at temperatures between 1 K and 25 mK. At small oscillation amplitudes pure potential flow is observed, the linear drag force on the sphere being determined only by ballistic quasiparticle scattering below 0.7 K with phonons contributing exclusively below 0.5 K. At larger oscillation amplitudes a strongly nonlinear drag force gives evidence of stable turbulent flow if at least 0.6 pW are transferred from the sphere to the turbulent superfluid. In an intermediate range of amplitudes (or driving forces) both flow patterns are unstable and intermittent switching between both is observed below 0.5 K. We have recorded time series of this switching phenomenon at constant drives and temperatures lasting up to 36 hours. We have made a statistical analysis of the times series by means of reliability theory. The lifetime of the turbulent phases grows with increasing drive and diverges at a critical value (or at least becomes unmeasurably long). Stability of the laminar phases in the intermediate regime depends on the excess velocity of the sphere above the critical velocity. Metastable laminar phases are observed above the critical velocity having a mean lifetime limited to 25 minutes by natural background radioactivity which occasionally produces local vorticity due to ionization of the liquid. Finally, it is suggested that the breakdown of potential flow belongs to the class of “system failure” experiments which is well known in reliability testing and whose statistical properties are described by extreme-value theory.PACS numbers: 67.40.Vs, 47.27.Cn.