arXiv subjects

Kyo Yoshida

Publications and source records attributed to Kyo Yoshida.

8 recordsLinked to original sources

arXiv2026

Mutual Friction in Dissipative Gross-Pitaevskii Thermal Counterflow Turbulence

We report numerical simulations of the dissipative Gross-Pitaevskii equation for a bulk region of thermal-counterflow turbulence. Quasistationary states are obtained over a range of forcing, damping, and healing-length parameters. The mutual-friction acceleration exhibits cubic scaling with the mean relative velocity between the superfluid and normal-fluid components, and the coefficient of this scaling is linked to the phenomenological damping parameter. The intervortex spacing follows the expected dimensional scaling in the weak-forcing regime. Comparison with a straight-vortex-line model suggests that the vortex-line orientations are nearly isotropic.

cond-mat.other↗

arXiv2025

Numerical search for states with constant enstrophy flux over finite time intervals in two-dimensional turbulence

An ensemble model of turbulence based on states with constant flux in wavenumber space was proposed in [K. Yoshida, Phys. Rev. E, 106, 045106 (2022)]. The justification of this ensemble model relies on the conjecture that almost all states with constant flux correspond to turbulence states. To verify this conjecture, a numerical search for states with constant enstrophy flux in wavenumber space over finite time intervals in two-dimension turbulence is conducted using a Monte Carlo method. Properties of the obtained states, such as energy spectrum and spectra related to fourth-order moments, are examined and compared with those of turbulence states obtained from direct numerical simulations. The dependence of the obtained states on the time interval and the initial conditions used in the numerical search is also discussed.

physics.flu-dyn↗

arXiv2023

Quantum vortex identification method and its application to Gross-Pitaevskii simulation

A method to identify a quantum vortex in a three-dimensional Gross-Pitaevskii simulation has been developed. A quantum vortex was identified by the use of eigenvalues and eigenvectors of the Hessian of the mass density, together with a condition to distinguish a point to constitute a swirling vortex from other confusing data points. This method has been verified to identify vortex axes in a Gross-Pitaevskii simulation appropriately, being useful to elucidate various statistics associated with turbulent quantum vortices. This method provides us with a unified approach to studying vortex statistics in the turbulence of both classic and quantum fluids. Our study reveals that the maximum radius of a swirling region of a quantum vortex can be as large as sixty times the healing length. The characterization of the vortex core radius relative to the healing length is reported for the first time in this paper. Furthermore, the geometrical natures of vortex axes such as the probability density function of the curvature are characterized by the healing length.

physics.flu-dyn↗

arXiv2022

Universal Scaling Bounds on a Quantum Heat Current

We derive new bounds on a heat current flowing into a quantum $L$-particle system coupled with a Markovian environment. By assuming that a system Hamiltonian and a system-environment interaction Hamiltonian are extensive in $L$, we show that the absolute value of the heat current scales at most as $\Theta (L^3)$ in a limit of large $L$. Also, we present an example that saturates this bound in terms of scaling: non-interacting particles globally coupled with a thermal bath. However, the construction of such system requires many-body interactions induced by the environment, which may be difficult to realize with the current technology. To consider more feasible cases, we focus on a class of system where any non-diagonal elements of the noise operator (derived from the system-environment interaction Hamiltonian) become zero in the system energy basis, if the energy difference is beyond a certain value $\Delta E$. Then, for $\Delta E = \Theta (L^0)$, we derive another scaling bound $\Theta (L^2)$ on the absolute value of the heat current, and the so-called superradiance belongs to a class to saturate this bound. Our results are useful to evaluate the best achievable performance of quantum-enhanced thermodynamic devices, which contain far-reaching applications for such as quantum heat engines, quantum refrigerators and quantum batteries.

quant-ph↗

arXiv2022

Quantum battery based on superabsorption

A quantum battery is a device where an energy is charged by using a quantum effect. Here, we propose a quantum battery with a charger system composed of $N$ qubits by utilizing a collective effect called a superabsorption. Importantly, the coupling strength between the quantum battery and charger system can be enhanced due to an entanglement. While the charger time scales as $\Theta\left(N^{-1/2}\right)$ by applying a conventional scheme, we can achieve a charging time $\Theta\left(N^{-1}\right)$ in our scheme. Our results open the path to ultra-fast charging of a quantum battery.

quant-ph↗

arXiv2022

Ensemble model of turbulence based on states of constant flux in wavenumber space

An ensemble model of turbulence is proposed. The ensemble consists of flow fields in which the flux of an inviscid conserved quantity, such as energy (or enstrophy in two-dimensional flow fields), across the wavenumber $k$ is a constant independent of $k$ in an appropriate range. Two-dimensional flow fields of constant enstrophy flux are sampled randomly by a Monte Carlo method. The energy spectra $E_k$ of the flow fields are consistent with the scaling $E_k \propto k^{-3}(\ln(k/k_\mathrm{b}))^{-1/3}$ where $k_\mathrm{b}$ is the bottom wavenumber of the constant-flux range.

physics.flu-dyn↗

arXiv2020

Boltzmann entropy for quantum field systems

A way to construct Boltzmann entropy, i.e., the entropy as a function of a microscopic pure state, for quantum field systems is proposed. Operators that shift the field in wavevector space are used in the construction. By employing an assumption, it is shown that, for almost all states in the ensemble of pure states corresponding to a thermodynamic state, the value of the proposed Boltzmann entropy coincides with that of the thermodynamic entropy for the thermodynamic state. For general self-interacting fields, the Boltzmann entropy evolves with time under Hamiltonian dynamics, so that it is capable of characterizing the thermalization of isolated quantum field systems.

cond-mat.stat-mech↗

arXiv2019

Hessian-based Lagrangian closure theory for passive scalar turbulence

Self-consistent closure theory for passive-scalar turbulence has been developed on the basis of the Hessian of the scalar field. As a primitive indicator of spatial structure of the scalar, we employ the Hessian into the core of the theory to properly characterize the time scale intrinsic to the scalar field itself. The resultant closure model is now endowed with several realistic features, i.e. the scale-locality of the interscale interaction, the detailed conservation, and the memory-fading effect. Applying the current theory to the inertial-convective range eventually leads to self-consistent derivation of the Obukhov-Corrsin spectrum with its universal constant consistent with numerical and experimental data.

physics.flu-dyn↗