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A04 TSUBOTA, Makoto |Proposed Research Projects (2014-2015)

Paper | Original Paper

2015

*Satoshi Yui, Kazuya Fujimoto, Makoto Tsubota ,
Logarithmic velocity profile of quantum turbulence of superfluid 4He,
Physical Review B 92, 224513/1-5 (2015).

[Summary] The logarithmic velocity profile is the most important statistical law of classical turbulence affected by channelwalls. This paper demonstrates numerically that the logarithmic velocity profile of a superfluid flow appears inquantum turbulence under pure normal flow in a channel. We investigated the configuration and dynamics of aninhomogeneous vortex tangle affected by the walls, and found the characteristic behavior of the log-law.

*Kazuya Fujimoto and Makoto Tsubota,
Bogoliubov-wave turbulence in Bose-Einstein condensates,
Physical Review A 91, 053620/1-12 (2015).

[Summary] We theoretically and numerically study Bogoliubov-wave turbulence in three-dimensional atomic Bose-Einstein condensates with the Gross-Pitaevskii equation, investigating three spectra for the macroscopic wavefunction, the density distribution, and the Bogoliubov-wave distribution. In this turbulence, Bogoliubov wavesplay an important role in the behavior of these spectra, so that we call it Bogoliubov-wave turbulence. In aprevious study [Proment et al., Phys. Rev. A 80, 051603(R) (2009)], a −3/2 power law in the spectrum forthe macroscopic wave function was suggested by using weak wave turbulence theory, but we find that another−7/2 power law appears in both theoretical and numerical calculations.

*Satoshi Yui and Makoto Tsubota,
Counterflow quantum turbulence of He-II in a square channel: numerical analysis with nonuniform flows of the normal fluid,
Physical Review B 91, 184504/1-12 (2015).

[Summary] We perform a numerical analysis of counterflow quantum turbulence of superfluid 4He with nonuniform flows by the vortex filament model. The resulting tangle of quantized vortices makes a characteristic nonlinear oscillation to form inhomogeneous quantum turbulence. We investigated the detail of the state and compare them with some typical experimental results.

Elisa Zemma, Makoto Tsubota, and Javier Luzuriaga,
Possible Visualization of a Superfluid Vortex Loop Attached to an Oscillating Beam,
Journal of Low Temperature Physics 179, 310-319 (2015).

[Summary] Visualization using tracer particles is a relatively new tool available for thestudy of superfluid turbulence and flow, which is applied here to oscillating objectssubmerged in the liquid. We report observations of a structure seen in videos takenfrom outside a cryostat filled with superfluid helium at 2 K, which is possibly a vortexloop attached to an oscillator. The feature, which has the shape of an incomplete arch,is visualized due to the presence of solid H2 tracer particles and is attached to a beamoscillating at 38 Hz in the liquid. It has been recorded in videos taken at 240 frames persecond, fast enough to take ∼6 images per period. This makes it possible to follow thestructure, and to see that it is not rigid. It moves with respect to the oscillator, and itsdisplacement is in phase with the velocity of the moving beam. Analyzing the motion,we come to the conclusion that we may be observing a superfluid vortex attached tothe beam and decorated by the hydrogen particles. An alternative model, consideringa solid hydrogen filament, has also been analyzed, but the observed phase betweenthe movement of the beam and the filamentary structure is better explained by thesuperfluid vortex hypothesis.

2014

*Kazuya Fujimoto, Makoto Tsubota,
Spin-superflow turbulence in spin-1 ferromagnetic spinor Bose-Einstien condensates,
Physical Review A 90, 013629/1-7 (2014).

[Summary] Spin-superflow turbulence (SST) in spin-1 ferromagnetic spinor Bose-Einstein condensates is theoretically and numerically studied. We found that the -5/3 and -7/3 power laws appear in spectra of the superflow kinetic and the spin-dependent interaction energy, respectively.

Ai Nakatsuji, *Makoto Tsubota, and Hideo Yano,
Statistics of vortex loops emitted from quantum turbulence driven by an oscillating sphere,
Physical Review B 89, 174520/1-7 (2014).

[Summary] We perform numerical simulation of quantum turbulence at zero temperature generated by an oscillating sphere. In this simulation, we injected vortices on the sphere to generate turbulence. Although we prepare injected vortex loops of identical length, they are extended by the oscillating sphere to form a tangle through numerous reconnections. The resulting tangle around the sphere is anisotropic and affected by the oscillation. The vortex tangle continues to emit vortex loops, which leave the sphere. The statistics of emitted loops differ significantly from those of the original injected vortices. First, the sizes of the emitted loops are widely distributed, ranging from smaller to much larger than the size of the initial injected loop. Second, the propagation direction of the emitted loops exhibits anisotropy: Small loops move away almost isotropically but large ones do so anisotropically along the oscillation direction of the sphere. Thus, the oscillating object stirs the initial injected vortices to reproduce a group of vortices with different statistics. Such physics is compared with the experiments of vibrating objects.