Polygonal structures in a gaseous disk: Numerical simulations

The results of numerical simulations of a gaseous disk in the potential of a stellar spiral density wave are presented. The conditions under which straightened spiral arm segments (rows) form in the gas component are studied. These features of the spiral structure were identified in a series of works by A.D. Chernin with coauthors. Gas-dynamic simulations have been performed for a wide range of model parameters: the pitch angle of the spiral pattern, the amplitude of the stellar spiral density wave, the disk rotation speed, and the temperature of the gas component. The results of 2D- and 3D-disk simulations are compared. The rows in the numerical simulations are shown to be an essentially nonstationary phenomenon. A statistical analysis of the distribution of geometric parameters for spiral patterns with rows in the observed galaxies and the constructed hydrodynamic models shows good agreement. In particular, the numerical simulations and observations of galaxies give 〈α〉 }~ 120° for the average angles between straight segments.     

Agn Jet Modeling: Setting the Stage

I review the most recent results obtained in numerical simulations of jets from accretion disks, with specific reference to the phases of acceleration, propagation and termination. The approximations adopted, both in the physics and the numerical methods, are pointed out and possible developments are suggested.     

空间无碰撞激波的数值研究

无碰撞激波是空间等离子体和宇宙等离子体中的重要物理现象。文中评述了数值研究空间无碰撞激波的两种方法－粒子模拟和混合模拟，给出了准垂直和准平行无碰撞激波的数值研究结果。还指出了一些尚未解决的研究问题。     

Barrel-like supernova remnants

The origin of cylindrically-symmetric supernova remnants is discussed. The results of numerical simulations of two most distinguished barrel-like remnants SNR 1006 and G296.5+10.0 are presented.     

A Statistical Study on the Filament Eruption Caused by New Emerging Flux

Observations indicated that solar coronal mass ejections (CMEs) are closely asociated with reconnection-favored new flux emergence. By means of numerial simulations, a physical model of the emerging flux trigger mechanism for CMEs is proposed and explained well the observational results. Based upon this model, leaving the gravity and heat conduction out of consideration, the theoretical results of 2.5 dimensional numerical simulations indicate that whether a CME can be triggered depends on both the amount and the location of an emerging flux, besides its polarity orientation. Furthermore, the eruption and non-eruption regimes are presented in parameter space. By use of 15 filament eruption events in 2002 and 2003 and 44 non-eruption events in 2002, the results of a statistical study on the properties of emerging flux including its polarity orientation, its location and the amount of flux show that not all the emerging flux can make a filament to lose equilibrium and trigger the onset of a CME, The statistic results basically support the theoretical results of numerical simulations. This research provides useful information for the space weather forecast.     

太阳大气中磁重联的MHD数值模拟

回顾了近３０年太阳大气中磁重联过程的ＭＨＤ数值模拟工作取得的进展。着重描述了在验证理论模型,解释观测现象,以及研究各种因素对重联的影响三个方面的成果,如快速磁重联,太阳耀斑机制及色球,日冕中的各种爆发现象等。指出了在数值模拟中应注意的几个问题,并对该领域今后的发展作了简要的展望。     

Numerical Simulation of Accretion Discs in Close Binary Systems andDiscovery of Spiral Shocks

The history of hydrodynamic numerical simulations for accretion disks in close binary systems is reviewed, in which emphasis is placed, in particular, on the facts that spiral shock waves were numerically found in 1986 by researchers including one of the present authors and that spiral structure was discovered in IP Pegasi in 1997 by Steeghs et al. The results of our two and three-dimensional numerical simulations in recent years are then summarized, with comparison being made with observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Numerical modeling of the formation of the Eltanin submarine impact structure

Two-and three-dimensional numerical simulations of the formation of the Eltanin submarine impact structure are described. Based on the numerical results, the impactor size can be estimated, its destruction and the subsequent motion of fragments can be described, and the initial amplitude of the tsunami wave can be determined.     

A numerical and experimental study of the time-evolution of a low Mach number jet

We present a comparison between a plasma-generated 'starting jet' experiment and an axisymmetric numerical simulation of the flow. The experimental flow and the numerical simulation give results that agree both qualitatively and quantitatively, showing that the complex vortical structures arising in the flow are surprisingly well reproduced by the numerical model. This result inspires confidence in the accuracy of astrophysical jet numerical simulations. Also, even though the Mach number of our laboratory jet is somewhat low ( M ∼0.5), the dimensionless parameters of this jet are not very far from those expected for Faranoff–Riley class I radio jets.     

On the effects of resolution in dissipationless cosmological simulations

We present a study of numerical effects in dissipationless cosmological simulations. The numerical effects are evaluated and studied by comparing the results of a series of 64³-particle simulations of varying force resolution and number of time-steps, performed using three of the N -body techniques currently used for cosmological simulations: the Particle–Mesh (PM), the Adaptive Particle–Particle–Particle–Mesh (AP³M) and the newer Adaptive Refinement Tree (ART) codes. This study can therefore be interesting both as an analysis of numerical effects and as a systematic comparison of different codes.
We find that the AP³M and the ART codes produce similar results given that convergence is reached within the code type. We also find that numerical effects may affect the high-resolution simulations in ways that have not been discussed before. In particular, our study revealed the presence of two-body scattering, the effects of which can be greatly amplified by inaccuracies in time integration. This process appears to affect the correlation function of matter, the mass function, the inner density of dark matter haloes and other statistics at scales much larger than the force resolution, although different statistics may be affected in a different fashion. We discuss the conditions for which strong two-body scattering is possible and discuss the choice of the force resolution and integration time-step. Furthermore, we discuss recent claims that simulations with force softening smaller than the mean interparticle separation are not trustworthy and argue that this claim is incorrect in general, and applies only to the phase-sensitive statistics. Our conclusion is that, depending on the choice of mass and force resolution and the integration time-step, a force resolution as small as 0.01 of the mean interparticle separation can be justified.     

Implications of recent numerical calculations for the fission theory of the origin of the Moon

Recent numerical simulations of fission instabilities in rotating fluid masses give results which are consistent with some requirements of the fission hypothesis for the origin of the moon.     

A-star envelopes: a test of local and non-local models of convection

We present results of a fully non-local, compressible model of convection for A-star envelopes. This model quite naturally reproduces a variety of results from observations and numerical simulations which local models based on a mixing length do not. Our principal results, which are for models with T _eff between 7200 and 8500 K, are the following. First, the photospheric velocities and filling factors are in qualitative agreement with those derived from observations of line profiles of A-type stars. Secondly, the He  ii and H  i convection zones are separated in terms of convective flux and thermal interaction, but joined in terms of the convective velocity field, in agreement with numerical simulations. In addition, we attempt to quantify the amount of overshooting in our models at the base of the He  ii convection zone.     

MHD simulations of accretion disks and jets: strengths and limitations

Observations are providing increasingly detailed quantitative information about the accretion flows that power such high energy systems as X-ray binaries and Active Galactic Nuclei. These observations have been modeled in some detail by a variety of accretion scenarios, but such models rely on unavoidable assumptions such as regular flow geometry and a simple, parameterized stress. Global numerical simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions. We are now carrying out fully three-dimensional general relativistic magnetohydrodynamic simulations of time-dependent inflows into Kerr black holes. The results from recent global simulations of black hole accretion disks will be reviewed, and some implications of those results for observations will be discussed.     

Numerical Simulations of Vertical Oscillations of a Curved Coronal Loop

We consider an impulsively-started, vertical excitation of a solar coronal loop that is embedded into a potential arcade. The two-dimensional numerical model we implement includes the effects of line curvature and allows us to explore the effect of varying the initial pulse position. The results of the numerical simulations reveal kink mode oscillations with waveperiods that are reasonably close to the observational findings of Wang and Solanki (2004).     

The loss of water from Mars: Numerical results and challenges

The simulation of Mars is a very challenging effort. However, simulations are a major method of addressing the issues of the solar wind interaction with Mars. Further, it is via simulations that issues such as water loss from Mars via solar wind pick up of ionospheric ions will be addressed. This paper discusses some of the issues raised during the Chapman Conference on Solar Wind Interactions with Mars, SWIM. It also addresses numerical issues and the authors attempts to address them, coupled with results of preliminary simulations of Mars.     

新浮磁流触发暗条爆发的统计研究

观测研究表明有利于磁重联的新浮磁流与日冕物质抛射(CME)有密切关系．利用数值模拟的方法,新浮磁流触发CME的物理模型对观测结果进行了物理解释．基于这种模型,不考虑重力和热传导, 2．5维的数值模拟的理论结果显示:是否能够触发暗条爆发及CME,取决于新浮磁流磁通量的大小、浮现的位置以及其磁极走向,并给出了能够触发暗条爆发与不能触发爆发的参数空间．利用2002年和2003年的15个暗条爆发事例以及2002年的44个非爆发事例,对新浮磁流磁通量的大小、浮现的位置以及磁极走向进行了统计研究．结果表明并非所有的新浮磁流都能够使暗条失去平衡,形成CME．统计结果基本上支持了数值模拟的理论结果．这个结果可为空间天气预报研究提供有用的参考信息．     

Material flows of an evolving magnetic field

The possibility of material flows to trace out the magnetic field configuration is examined through numerical simulations. In particular, the evolution of a magnetic arcade due to differential motions of its footpoints is considered. With the use of numerical scheme based on the method of projected characteristics and newly derived proper boundary conditions, it is shown that material flows develop to outline the configuration of evolving magnetic field. Physical implications of the results are discussed.     

Classical Calculations of Scattering Signatures from a Gravitational Singularity or the Scattering and Absorption Cross-Sections of a Black Hole

Within the context of general relativity theory we calculate, analytically, scattering signatures around a gravitational singularity: angular and time distributions of scattered massive objects and photons and the time and space modulation of Doppler effects. Additionally, the scattering and absorption cross sections for the gravitational interactions are calculated. The results of numerical simulations of the trajectories are compared with the analytical results.