摘要 :Commercial quartz oscillators of the tuning-fork type with a resonant frequency of ∼ 32 kHz have been investigated in helium liquids. The oscillators are found to have at best Q values in the range 105–106, when measured in vacu...
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Commercial quartz oscillators of the tuning-fork type with a resonant frequency of ∼ 32 kHz have been investigated in helium liquids. The oscillators are found to have at best Q values in the range 105–106, when measured in vacuum below 1.5 K. However, the variability is large and for very low temperature operation the sensor has to be preselected. We explore their properties in the regime of linear viscous hydrodynamic response in normal and superfluid 3He and 4He, by comparing measurements to the hydrodynamic model of the sensor.
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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 determi...
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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.
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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 determi...
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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.
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Oscillating objects such as discs, piles of discs, spheres, grids and wires have been widely used in cryogenic fluid dynamics and in quantum fluids research since the discovery of superfluidity. A new addition are quartz tuning fo...
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Oscillating objects such as discs, piles of discs, spheres, grids and wires have been widely used in cryogenic fluid dynamics and in quantum fluids research since the discovery of superfluidity. A new addition are quartz tuning forks, commercially available frequency standards. We review their use as thermometers, pressure- and viscometers as well as their potential as generators and detectors of cavitation and turbulence in viscous and superfluid He liquids.
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The flow of superfluid helium at very low temperatures around an oscillating microsphere is known to be unstable slightly above the critical velocity. The flow pattern switches intermittently between potential flow and turbulence....
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The flow of superfluid helium at very low temperatures around an oscillating microsphere is known to be unstable slightly above the critical velocity. The flow pattern switches intermittently between potential flow and turbulence. From time series recorded at constant temperature and driving force the statistical properties of the switching phenomenon are discussed. Based on our recent understanding of the critical velocity v c of oscillatory superflows, i.e., vc ~ Ö{k w}v_{c}\sim \sqrt{\kappa\,\omega}, where κ is the circulation quantum and ω is the oscillation frequency, the analysis is being refined now. From the exponential distribution of the lifetimes of the turbulent phases quantitative information on the vortex density L can be inferred such as the distribution and the width of the fluctuations of L. The phases of potential flow show a Rayleigh distribution of the excess oscillation amplitude above the amplitude at turbulent flow. The rms value is found to scale as ω −3/2.
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The flow of superfluid helium at very low temperatures around an oscillating microsphere is known to be unstable slightly above the critical velocity. The flow pattern switches intermittently between potential flow and turbulence....
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The flow of superfluid helium at very low temperatures around an oscillating microsphere is known to be unstable slightly above the critical velocity. The flow pattern switches intermittently between potential flow and turbulence. From time series recorded at constant temperature and driving force the statistical properties of the switching phenomenon are discussed. Based on our recent understanding of the critical velocity v_(c) of oscillatory superflows, i.e., v_(c) approx (kappa omega)~(1/2), where kappa is the circulation quantum and omega is the oscillation frequency, the analysis is being refined now. From the exponential distribution of the lifetimes of the turbulent phases quantitative information on the vortex density L can be inferred such as the distribution and the width of the fluctuations of L. The phases of potential flow show a Rayleigh distribution of the excess oscillation amplitude above the amplitude at turbulent flow. The rms value is found to scale as (omega)~(-3/2).
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摘要 :We investigate the statistical properties of single phase slip events observed in vortex nucleation experiments in 4He by the groups in Berkeley and Paris. From the cumulative distribution function of the events we calculate the s...
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We investigate the statistical properties of single phase slip events observed in vortex nucleation experiments in 4He by the groups in Berkeley and Paris. From the cumulative distribution function of the events we calculate the sliprate as a function of flow velocity. The critical velocity is defned as the mean velocity and its statistical width as the standard deviation. From the slip rate and from the observed linear temperature dependence of the critical velocitywe obtain the energy barrier for vortex nucleation which is a quadratic function of the flow velocity. A comparison with the statistical properties of the laminar to turbulent transition in the flow around an oscillating sphere shows strikingly different behaviour.
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We investigate the statistical properties of single phase slip events observed in vortex nucleation experiments in ~4He by the groups in Berkeley and Paris. From the cumulative distribution function of the events we calculate the ...
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We investigate the statistical properties of single phase slip events observed in vortex nucleation experiments in ~4He by the groups in Berkeley and Paris. From the cumulative distribution function of the events we calculate the slip rate as a function of flow velocity. The critical velocity is defined as the mean velocity and its statistical width as the standard deviation. From the slip rate and from the observed linear temperature dependence of the critical velocity we obtain the energy barrier for vortex nucleation which is a quadratic function of the flow velocity. A comparison with the statistical properties of the laminar to turbulent transition in the flow around an oscillating sphere shows strikingly different behaviour.
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The onset of turbulent flow around an oscillating sphere in superfluid \(^4\)He is known to occur at a critical velocity \(v_{\text {c}} \,\sim \sqrt{\kappa \,\omega }\) where \(\kappa \) is the circulation quantum and \(\omega \)...
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The onset of turbulent flow around an oscillating sphere in superfluid \(^4\)He is known to occur at a critical velocity \(v_{\text {c}} \,\sim \sqrt{\kappa \,\omega }\) where \(\kappa \) is the circulation quantum and \(\omega \) is the oscillation frequency. But it is also well known that initially in a first up-sweep of the oscillation amplitude, \(v_{\text {c}}\) can be considerably exceeded before the transition occurs, thus leading to a strong hysteresis in the velocity sweeps. The velocity amplitude \(v_{\text {c}}^* > v_{\text {c}}\) where the transition finally occurs is related to the density \(L_0\) of the remanent vortices in the superfluid. Moreover, at temperatures below ca. 0.5 K and in a small interval of velocity amplitudes between \(v_{\text {c}}\) and a velocity that is about 2 % larger, the flow pattern is found to be unstable, switching intermittently between potential flow and turbulence. From time series recorded at constant temperature and driving force, the distribution of the excess velocities \(\Delta v = v_{\text {c}}^* - v_{\text {c}}\) is obtained and from that the failure rate. Below 0.1 K we also can determine the distribution of the lifetimes of the phases of potential flow. Finally, the frequency dependence of these results is discussed.
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The critical velocity v_(c) for the onset of turbulent drag of small spheres oscillating in superfluid ~(4)He is frequency dependent (omega/2pi from 100 Hz to 700 Hz) and is described by v_(c) velence 2.6((kappa)(omega))~(1/2), wh...
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The critical velocity v_(c) for the onset of turbulent drag of small spheres oscillating in superfluid ~(4)He is frequency dependent (omega/2pi from 100 Hz to 700 Hz) and is described by v_(c) velence 2.6((kappa)(omega))~(1/2), where kappa is the circulation quantum. A qualitative analysis based on a recent theory of the onset of superfluid turbulence gives v_(c) approx= (8(kappa)(omega)/beta)~(1/2), where beta approx 1 depends on the coefficients of mutual friction. This agrees well with the data and implies that v_(c) is a universal critical velocity that is independent of geometry, size, and surface properties of the oscillating body. This is confirmed by comparing our data on spheres with v_(c) obtained with other oscillating structures by other groups. Numerical simulations indicate somewhat larger critical velocity, above which a rapid increase in vortex length is observed.
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