Y. Nago

Turbulence in boundary flow of superfluid 4He triggered by free vortex rings.

The transition to turbulence in the boundary flow of superfluid 4He is investigated using a vortex-free vibrating wire. At high wire vibration velocities, we found that stable alternating flow around the wire enters a turbulent phase triggered by free vortex rings. Numerical simulations of vortex dynamics demonstrate that vortex rings can attach to the surface of an oscillating obstacle and expand unstably due to the boundary flow of the superfluid, forming turbulence. Experimental investigations indicate that the turbulent phase continues even after stopping the injection of vortex rings, which is also confirmed by the simulations.

Hydrodynamic Property of Oscillating Superfluid 3He in Aerogel

The investigation of the superfluidity of liquid 3He in aerogel of 97.5% and 98.5% porosities using the fourth sound resonance technique revealed two distinct observations. First, the superfluid transition temperature TC and the superfluid density ρs/ρ of 3He in aerogel are greatly suppressed. Second, the sound attenuation does not depend on temperature at higher temperatures, but monotonically diminishes with decreasing temperature at lower temperatures.

Vortex emission by a low-frequency vibrating wire in superfluid 3He-B

We report the investigation of vortex emission by a vibrating wire in superfluid 3He-B at a pressure of 28 bar. We used two vibrating wires with 47 Hz and 183 Hz resonance frequencies as a generator and a detector of vortices. The onset velocity of vortex emission for the 183 Hz vibrating wire is higher than a pair-breaking velocity, indicating that pair breaking causes vortex generation. In contrast, the onset velocity for the 47 Hz vibrating wire is lower than the pair-breaking velocity. This result suggests that another mechanism of vortex emission arises for the lower-frequency vibrating wire: pair breaking due to local flow enhanced by protuberances on the surface of the wire, or instability of remanent vortices attached to the wire by vibration.

Anomalous sound absorption of finite amplitude sound in liquid 4He

We report the nonlinear and unsteady response of the finite amplitude pressure wave in superfluid 4He, using a classical vibrating liquid column standing wave resonator. The frequency was kept at the resonance frequency of the liquid column, so that the maximum of the pressure field always came to the surface of the driver. As a result, in the low driving-amplitude regime, I/O relations were linear. However, in the high driving-amplitude regime, anomalous responses were found; The amplitude of the signals suddenly reduced and gradually recovered; they were repeated with random intervals. Moreover, in the highly suppressed region in the waveform, we found a generation of new type of turbulence. These absorptions are expected to be the result of the vapor bubble ejection from the surface of the driver transducer.

Motion of 3He quasiparticles in aerogel driven by fourth sound

While the shear viscous motion of the normal fluid component causes the energy loss of the fourth sound resonance in superfluid 3He confined in narrow pores, it is not yet clear what causes the energy loss of the fourth sound in the aerogel system. We prepared two aerogel samples of 99.0% and 97.5% porosities, which were directly grown in sintered silver sponges and performed the fourth sound resonance experiments. We found that the energy loss of the fourth sound resonance in aerogel is much smaller than that in pure liquid 3He, showing that the mean free path of 3He quasiparticles is strongly suppressed by aerogel strands. Such behavior is explained with the frictional flow. The temperature dependence of the frictional relaxation time will be discussed.

Laminar-turbulent transition generated by an oscillating object in superfluid 4He

The transition to turbulence in boundary flow of superfluid 4He is investigated using a vibrating wire. An object in a superfluid can easily generate the turbulence by oscillation even at very low temperatures, because remanent vortices are attached to it from the beginning. However, these vortices affect the transition to turbulence, occasionally causing intermittent switchings between laminar and turbulent flows. Recently, we have successfully devised a condition of vortex-free superfluid, in which a vibrating wire cannot generate turbulence. This condition prevents an oscillator from the switchings, enabling us to study the transition from turbulent to laminar flows. Using a vortex-free vibrating wire and a generator of vortex rings, we find that a turbulent state has a lifetime at driving forces near the transition, while it seems to be permanent at high driving forces. Lifetime data scatter widely, suggesting that the transition from turbulent to laminar flows is a probability event.

Generation and detection of vortex rings in superfluid 4He at very low temperature

Motions of vortices are fundamental characteristics of quantum turbulence. These motions are expected to be governed only by quantized circulations in superfluids at the zero temperature limit. In the present paper, we report the motions of vortex rings emitted from a quantum turbulence in superfluid 4He, by detecting vortex rings using a vortex-free vibrating wire as a detector. The time of flights of vortex rings are distributed, because vortex rings are emitted in any direction from a turbulent region and the detector can respond only to a reachable vortex ring. By measuring time-of-flights many times, we find an exponential distribution of time-of-flights with a non-detection period, which corresponds to the fastest time of flights of vortex rings. For a larger generation power of vortex rings, a distribution of time-of-flights still shows a single exponential distribution, but a non-detection period becomes shorter. This result implies that sizes of emitted vortex rings are distributed dependently on the generation power of turbulence. The observed exponential distributions are confirmed by numerical simulations of the dynamics of vortex rings.