arXiv subjects

Akira Kageyama

Publications and source records attributed to Akira Kageyama.

At least 19 recordsLinked to original sources

arXiv2026

Geometry-induced self-excited dynamo in a regular tetrahedron

We present a rotation-free magnetohydrodynamic dynamo driven by laminar thermal convection in a regular tetrahedral cavity. The tetrahedral boundaries organize the convective flow into a robust pattern of helical convection cells without global rotation or turbulence. Direct numerical simulations demonstrate exponential amplification of a weak seed magnetic field followed by nonlinear saturation, with the magnetic energy exceeding the kinetic energy. The velocity field develops $D_4$ dihedral symmetry, while the self-generated magnetic field exhibits a corresponding signed $D_4$ symmetry, including antisymmetry under $π$ rotations about the two horizontal axes. Analysis of the velocity and magnetic-field structures reveals a closed induction cycle sustained by geometry-induced helical convection. This system provides a conceptually simple setting for isolating and understanding the fundamental physical processes underlying magnetohydrodynamic dynamo action.

physics.flu-dyn↗

arXiv2022

Chiral Pattern in Nonrotating Spherical Convection

When the Rayleigh number is low, Rayleigh-Bénard convection in a nonrotating spherical shell with central gravity has symmetric solutions in terms of three-dimensional discrete rotation. All the known patterns with the regular polyhedral symmetries accompany reflection symmetry. We found a new type of steady convection in a nonrotating spherical shell by computer simulations. The pattern has the discrete rotational symmetry of a regular tetrahedron with no reflection symmetry. The convection consists of six pairs of spiral rolls placed on 12 faces of a spherical dodecahedron. Depending on the winding of the spirals, there are two possible configurations that are mirror images of one another.

physics.flu-dyn↗

arXiv2020

Formation of Twisted Liquid Jets

Liquid jets issued from a non-circular orifice exhibit oscillation owing to the surface tension. When the orifice has an $n$-fold rotational symmetry, a material cross section of the jet interchanges two symmetric shapes alternately. This oscillation, called axis switching, is a superposition of two ripples oppositely propagating in the azimuthal direction around the axis. In this study, we used computer simulations to demonstrate that we can pick up one of the two ripples by adjusting the initial velocity profile of the orifice. As a result of the single wave propagation in the azimuth, the jet surface shows a twisted appearance. In contrast to the swirling jets, the twisted jet has no angular momentum around the axis. We numerically demonstrated the formation of twisted jets with various cross sections, including a regular square.

physics.flu-dyn↗

arXiv2016

A Visualization Method of Four Dimensional Polytopes by Oval Display of Parallel Hyperplane Slices

A method to visualize polytopes in a four dimensional euclidian space $(x,y,z,w)$ is proposed. A polytope is sliced by multiple hyperplanes that are parallel each other and separated by uniform intervals. Since the hyperplanes are perpendicular to the $w$ axis, the resulting multiple slices appear in the three-dimensional $(x,y,z)$ space and they are shown by the standard computer graphics. The polytope is rotated extrinsically in the four dimensional space by means of a simple input method based on keyboard typings. The multiple slices are placed on a parabola curve in the three-dimensional world coordinates. The slices in a view window form an oval appearance. Both the simple and the double rotations in the four dimensional space are applied to the polytope. All slices synchronously change their shapes when a rotation is applied to the polytope. The compact display in the oval of many slices with the help of quick rotations facilitate a grasp of the four dimensional configuration of the polytope.

cs.GR↗

arXiv2016

Keyboard Based Control of Four Dimensional Rotations

Aiming at applications to the scientific visualization of three dimensional simulations with time evolution, a keyboard based control method to specify rotations in four dimensions is proposed. It is known that four dimensional rotations are generally so-called double rotations, and a double rotation is a combination of simultaneously applied two simple rotations. The proposed method can specify both the simple and double rotations by single key typings of the keyboard. The method is tested in visualizations of a regular pentachoron in four dimensional space by a hyperplane slicing.

cs.GR↗

arXiv2015

Differential Rotation in Magnetized and Non-magnetized Stars

Effects of magnetic field on stellar differential rotation are studied by comparing magnetohydrodynamic (MHD) models and their hydrodynamic (HD) counterparts in the broad range of rotation rate and in varying initial rotation profile. Fully-compressible MHD simulations of rotating penetrative convection are performed in a full-spherical shell geometry. Critical conditions for the transition of the differential rotation between faster equator (solar-type) and slower equator (anti-solar type) are explored with focusing on the "Rossby number (${\rm Ro}$)" and the "convective Rossby number (${\rm Ro}_{\rm conv}$)". It is confirmed that the transition is more gradual and the critical value for it is higher in the MHD model than the HD model in the view of the ${\rm Ro}_{\rm conv}$-dependence. The rotation profile shows, as observed in earlier studies, the bistability near the transition in the HD model, while it disappears when allowing the growth of magnetic fields except for the model with taking anti-solar type solution as the initial condition. We find that the transition occurs at ${\rm Ro} \simeq 1$ both in the MHD and HD models independently of the hysteresis. Not only the critical value, the sharpness of the transition is also similar between the two models in the view of the ${\rm Ro}$-dependence. The influences of the dynamo-generated magnetic field and/or the hysteresis on convective motion are reflected in the ${\rm Ro}$. This would be the reason why the transition is unified in the view of the ${\rm Ro}$-dependence. We finally discuss the ${\rm Ro}$-dependence of magnetic dynamo activities with emphasis on its possible relation to the kinetic helicity profile.

astro-ph.SR↗

arXiv2013

An Approach to Exascale Visualization: Interactive Viewing of In-Situ Visualization

In the coming era of exascale supercomputing, in-situ visualization will be a crucial approach for reducing the output data size. A problem of in-situ visualization is that it loses interactivity if a steering method is not adopted. In this paper, we propose a new method for the interactive analysis of in-situ visualization images produced by a batch simulation job. A key idea is to apply numerous (thousands to millions) in-situ visualizations simultaneously. The viewer then analyzes the image database interactively during postprocessing. If each movie can be compressed to 100 MB, one million movies will only require 100 TB, which is smaller than the size of the raw numerical data in exascale supercomputing. We performed a feasibility study using the proposed method. Multiple movie files were produced by a simulation and they were analyzed using a specially designed movie player. The user could change the viewing angle, the visualization method, and the parameters interactively by retrieving an appropriate sequence of images from the movie dataset.

physics.comp-ph↗

arXiv2013

Effects of Penetrative Convection on Solar Dynamo

Spherical solar dynamo simulations are performed. Self-consistent, fully compressible magnetohydrodynamic system with a stably stratified layer below the convective envelope is numerically solved with a newly developed simulation code based on the Yin-Yang grid. The effects of penetrative convection are studied by comparing two models with and without the stable layer. The differential rotation profile in both models is reasonably solar-like with equatorial acceleration. When considering the penetrative convection, a tachocline-like shear layer is developed and maintained beneath the convection zone without assuming any forcing. While turbulent magnetic field becomes predominant in the region where the convective motion is vigorous, mean-field component is preferentially organized in the region where the convective motion is less vigorous. Especially in the stable layer, the strong large-scale field with a dipole symmetry is spontaneously built up. The polarity reversal of the mean-field component takes place globally and synchronously throughout the system regardless the presence of the stable layer. Our results suggest that the stably stratified layer is a key component for organizing the large-scale strong magnetic field, but is not essential for the polarity reversal.

astro-ph.SR↗

arXiv2013

Immersive VR Visualizations by VFIVE. Part 1: Development

We have been developing a visualization application for CAVE-type virtual reality (VR) systems for more than a decade. This application, VFIVE, is currently used in several CAVE systems in Japan for routine visualizations. It is also used as a base system of further developments of advanced visualizations. The development of VFIVE is summarized.

cs.GR↗

arXiv2013

Immersive VR Visualizations by VFIVE. Part 2: Applications

VFIVE is a scientific visualization application for CAVE-type immersive virtual reality systems. The source codes are freely available. VFIVE is used as a research tool in various VR systems. It also lays the groundwork for developments of new visualization software for CAVEs. In this paper, we pick up five CAVE systems in four different institutions in Japan. Applications of VFIVE in each CAVE system are summarized. Special emphases will be placed on scientific and technical achievements made possible by VFIVE.

cs.GR↗

arXiv2013

Applications and a Three-dimensional Desktop Environment for an Immersive Virtual Reality System

We developed an application launcher called Multiverse for scientific visualizations in a CAVE-type virtual reality (VR) system. Multiverse can be regarded as a type of three-dimensional (3D) desktop environment. In Multiverse, a user in a CAVE room can browse multiple visualization applications with 3D icons and explore movies that float in the air. Touching one of the movies causes "teleportation" into the application's VR space. After analyzing the simulation data using the application, the user can jump back into Multiverse's VR desktop environment in the CAVE.

physics.comp-ph↗

arXiv2006

MovieMaker: A Parallel Movie-Making Software for Large Scale Simulations

We have developed a parallel rendering software for scientific visualization of large-scale, three-dimensional, time development simulations. The goal of this software, MovieMaker, is to generate a movie, or a series of visualization images from totally one TB-scale data within one night (or less than 12 hours). The isocontouring, volume rendering, and streamlines are implemented. MovieMaker is a parallel program for the shared memory architecture with dynamic load balancing and overlapped disk I/O.

physics.geo-ph↗

arXiv2005

Virtual Reality Visualization by CAVE with VFIVE and VTK

The CAVE-type virtual reality (VR) system was introduced for scientific visualization of large scale data in the plasma simulation community about a decade ago. Since then, we have been developing a VR visualization software, VFIVE, for general CAVE systems. Recently, we have integrated an open source visualization library, the Visualization Toolkit (VTK), into VFIVE. Various visualization methods of VTK can be incorporated and used interactively in VFIVE.

physics.geo-ph↗

arXiv2005

Low-degree mantle convection with strongly temperature- and depth-dependent viscosity in a three-dimensional spherical shell

A series of numerical simulations of thermal convection of Boussinesq fluid with infinite Prandtl number, with Rayleigh number $10^7$, and with the strongly temperature- and depth- dependent viscosity in a three-dimensional spherical shell is carried out to study the mantle convection of single-plate terrestrial planets like Venus or Mars without an Earth-like plate tectonics. The strongly temperature-dependent viscosity (the viscosity contrast across the shell is $\geq 10^5$) make the convection under stagnant-lid short-wavelength structures. Numerous, cylindrical upwelling plumes are developed because of the secondary downwelling plumes arising from the bottom of lid. This convection pattern is inconsistent with that inferred from the geodesic observation of the Venus or Mars. Additional effect of the stratified viscosity at the upper/lower mantle (the viscosity contrast is varied from 30 to 300) are investigated. It is found that the combination of the strongly temperature- and depth-dependent viscosity causes long-wavelength structures of convection in which the spherical harmonic degree $\ell$ is dominant at 1--4. The geoid anomaly calculated by the simulated convections shows a long-wavelength structure, which is compared with observations. The degree-one ($\ell = 1$) convection like the Martian mantle is realized in the wide range of viscosity contrast from 30 to 100 when the viscosity is continuously increased with depth at the lower mantle.

physics.geo-ph↗

arXiv2005

Tutorial introduction to Virtual Reality: What possibility are offered to our field?

The virtual reality (VR) provides us a three-dimensional, immersive, and fully interactive visualization environment. To make the best use of the VR's potential in scientific visualization, a VR visualization software named VFIVE has been developed for the CAVE-type VR system. VFIVE enables simulation researchers to analyze three-dimensional scalar and vector fields by various visualization methods including real time volume rendering in the CAVE's room sized booth. Some basic visualization tools of VTK have been integrated to VFIVE, too.

physics.geo-ph↗

arXiv2005

A Note on the Dipole Coordinates

A couple of orthogonal coordinates for dipole geometry are proposed for numerical simulations of plasma geophysics in the Earth's dipole magnetic field. These coordinates have proper metric profiles along field lines in contrast to the standard dipole coordinate system that is commonly used in analytical studies for dipole geometry.

physics.geo-ph↗

arXiv2005

Eulerian Derivation of the Coriolis Force

In textbooks of geophysical fluid dynamics, the Coriolis force and the centrifugal force in a rotating fluid system are derived by making use of the fluid parcel concept. In contrast to this intuitive derivation to the apparent forces, more rigorous derivation would be useful not only for the pedagogical purpose, but also for the applications to other kinds of rotating geophysical systems rather than the fluid. The purpose of this paper is to show a general procedure to derive the transformed equations in the rotating frame of reference based on the local Galilean transformation and rotational coordinate transformation of field quantities. The generality and usefulness of this Eulerian approach is demonstrated in the derivation of apparent forces in rotating fluids as well as the transformed electromagnetic field equation in the rotating system.

physics.geo-ph↗

arXiv2005

Geodynamo and mantle convection simulations on the Earth Simulator using the Yin-Yang grid

We have developed finite difference codes based on the Yin-Yang grid for the geodynamo simulation and the mantle convection simulation. The Yin-Yang grid is a kind of spherical overset grid that is composed of two identical component grids. The intrinsic simplicity of the mesh configuration of the Yin-Yang grid enables us to develop highly optimized simulation codes on massively parallel supercomputers. The Yin-Yang geodynamo code has achieved 15.2 Tflops with 4096 processors on the Earth Simulator. This represents 46% of the theoretical peak performance. The Yin-Yang mantle code has enabled us to carry out mantle convection simulations in realistic regimes with a Rayleigh number of $10^7$ including strongly temperature-dependent viscosity with spatial contrast up to $10^6$.

physics.geo-ph↗