Model dependence of transonic properties of accretion flows around black holes     

Sandip K. Chakrabarti †  Santabrata Das 《Monthly notices of the Royal Astronomical Society》2001,327(3):808-812

We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings.  相似文献   

Acoustic-Gravity Waves from Bolide Sources     

Douglas O. ReVelle 《Earth, Moon, and Planets》2008,102(1-4):345-356

We have developed a new approach to modeling the acoustic-gravity wave (AGW) radiation from bolide sources. This first effort involves entry modeling of bolide sources that have available satellite data through procedures developed in ReVelle (Earth Moon Planets 95, 441–476, 2004a; in: A. Milani, G. Valsecchi, D. Vokrouhlicky (eds) NEO Fireball Diversity: Energetics-based Entry Modeling and Analysis Techniques, Near-earth Objects: Our Celestial Neighbors (IAU S236), 2007b). Results from the entry modeling are directly coupled to AGW production through line source blast wave theory for the initial wave amplitude and period at (at 10 blast wave radii and perpendicular to the trajectory). The second effort involves the prediction of the formation and or dominance of the propagation of the atmospheric Lamb, edge-wave composite mode in a viscous fluid (Pierce, J. Acoust. Soc. Amer. 35, 1798–1807, 1963) as a function of the source energy, horizontal range and source altitude using the Lamb wave frequency that was deduced directly during the entry modeling and that is used as a surrogate for the source energy. We have also determined that Lamb wave production by bolides at close range decreases dramatically as either the source energy decreases or the source altitude increases. Finally using procedures in Gill (Atmospheric-Ocean Dynamics, 1982) and in Tolstoy (Wave Propagation, 1973), we have analyzed two simple dispersion relationships and have calculated the expected dispersion for the Lamb edge-wave mode and for the excited, propagating internal acoustic waves. Finally, we have used the above formalism to fully evaluate these techniques for four large bolides, namely: the Tunguska bolide of June 30, 1908; the Revelstoke bolide of March 31, 1965; the Crete bolide of June 6, 2002 and the Antarctic bolide of September 3, 2004. Due to page limitations, we will only present results in detail for the Revelstoke bolide.  相似文献   

Ion acceleration and Alfvén wave generation at the Earth’s bow shock     

E. G. Berezhko  S. N. Taneev 《Astronomy Letters》2007,33(5):346-353

The processes of ion acceleration and Alfvén wave generation by accelerated particles at the Earth’s bow shock are studied within a quasi-linear approach. Steady-state ion and wave spectra are shown to be established in a time of 0.3–4 h, depending on the background level of Alfvénic turbulence in the solar wind. The Alfvén waves produced by accelerated ions are confined within the frequency range 10^?2–1 Hz and their spectral peak with a wave amplitude βB ～ B comparable to the interplanetary magnetic field strength B corresponds to the frequency v = (2–3) × 10^?2 Hz. The high-frequency part of the wave spectrum (v > 0.2 Hz) undergoes damping by thermal ions. The calculated spectra of the accelerated ions and the Alfvén waves generated by them reproduce the main features observed in experiments.  相似文献   

基于能量方程的建筑结构半主动控制   总被引：4，自引：2，他引：2  

涂文戈  邹银生 《地震工程与工程振动》2005,25(3):145-151

基于地震作用下结构的能量响应方程,本文提出根据结构吸收能量、相对动能和变形能的变化(对时间的一次导数)进行结构的“开-关”半主动减震的控制律;在物理坐标系中对这3种控制律的动力特征和控制效果,以及作动器参数的选取进行了分析。本文针对建筑结构进行了数值仿真。计算结果表明,本文提出的控制律可以有效地减小了结构的位移响应,对随机不确定的地震波均具有良好的减震效果,适应性强,而且控制器参数合理。其中根据结构变形能推导的控制律的半主动控制效果优于其它2种控制律。  相似文献   

陨石中林伍德石的形成条件对寻找地球产状林伍德石的启示     

陈鸣 《地学前缘》2005,12(1):23-27

橄榄石高压多形林伍德石被认为是地幔过渡带的主要矿物。天然产状林伍德石主要在发生强烈冲击变质的球粒陨石冲击脉体中出现。目前还没有在地球岩石中发现林伍德石的报告。陨石冲击脉体的温度压力历史和矿物组合特征研究表明,林伍德石形成后,高压下淬火是使林伍德石不发生退变作用的重要条件。陨石中有利于林伍德石保存的淬火时间仅为数秒到十多秒。在地球上任何地质事件中,均难以实现在如此短的时问内使位于地幔过渡带的林伍德石被带往地球表层。寻找地球产状的林伍德石,关键是要在岩石和矿物中存在有利于林伍德石保存的条件,特别是当这些岩石和矿物仍处于高温的环境时。  相似文献   

A kinetic approach to cosmic-ray-induced streaming instability at supernova shocks   总被引：1，自引：0，他引：1  

E. Amato  P. Blasi 《Monthly notices of the Royal Astronomical Society》2009,392(4):1591-1600

We show that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-Alfvénic fast-growing mode which has the same dispersion relation as that previously found by Bell in 2004 by treating the plasma in the magnetohydrodynamic approximation. The kinetic approach allows us to investigate the dependence of the dispersion relation of these waves on the microphysics of the current which compensates the cosmic ray flow. We also show that a resonant and a non-resonant mode may appear at the same time and one of the two may become dominant on the other depending on the conditions in the acceleration region. We discuss the role of the unstable modes for magnetic field amplification and particle acceleration in supernova remnants at different stages of the remnant evolution.  相似文献   

A structure and energy dissipation efficiency of relativistic reconfinement shocks     

Krzysztof Nalewajko  Marek Sikora 《Monthly notices of the Royal Astronomical Society》2009,392(3):1205-1210

We present a semi-analytical hydrodynamical model for the structure of reconfinement shocks formed in astrophysical relativistic jets interacting with external medium. We take into account exact conservation laws, both across the shock front and in the zone of the shocked matter, and exact angular relations. Our results confirm a good accuracy of the approximate formulae derived by Komissarov & Falle. However, including the transverse pressure gradient in the shocked jet, we predict an absolute size of the shock to be about twice larger. We calculate the efficiency of the kinetic energy dissipation in the shock and show a strong dependence on both the bulk Lorentz factor and opening angle of the jet.  相似文献   

Time-dependent simulations of steady C-type shocks     

S. Van Loo  I. Ashmore  P. Caselli  S. A. E. G. Falle  T. W. Hartquist 《Monthly notices of the Royal Astronomical Society》2009,395(1):319-327

Using a time-dependent multifluid, magnetohydrodynamic code, we calculated the structure of steady perpendicular and oblique C-type shocks in dusty plasmas. We included relevant processes to describe mass transfer between the different fluids, radiative cooling by emission lines and grain charging, and studied the effect of single- and multiple-sized grains on the shock structure. Our models are the first of oblique fast-mode molecular shocks in which such a rigorous treatment of the dust grain dynamics has been combined with a self-consistent calculation of the thermal and ionization structures including appropriate microphysics. At low densities, the grains do not play any significant rôle in the shock dynamics. At high densities, the ionization fraction is sufficiently low that dust grains are important charge and current carriers and, thus, determine the shock structure. We find that the magnetic field in the shock front has a significant rotation out of the initial upstream plane. This is most pronounced for single-sized grains and small angles of the shock normal with the magnetic field. Our results are similar to previous studies of steady C-type shocks showing that our method is efficient, rigorous and robust. Unlike the method employed in the previous most detailed treatment of dust in steady oblique fast-mode shocks, ours allow a reliable calculation even when chemical or other conditions deviate from local statistical equilibrium. We are also able to model transient phenomena.  相似文献   

Some properties of synchrotron radio and inverse-Compton gamma-ray images of supernova remnants     

O. Petruk  V. Beshley  F. Bocchino  S. Orlando 《Monthly notices of the Royal Astronomical Society》2009,395(3):1467-1475

The synchrotron radio maps of supernova remnants (SNRs) in a uniform interstellar medium and interstellar magnetic field (ISMF) are analysed, allowing for different 'sensitivity' of the injection efficiency to the obliquity of the shock. The very-high-energy γ-ray maps arising from inverse Compton processes are also synthesized. The properties of images in these different wavelength bands are compared, with particular emphasis on the location of the bright limbs in bilateral SNRs. Recent High-Energy Stereoscopic System (HESS) observations of SN 1006 show that the radio and inverse Compton γ-ray limbs coincide, and we found that this may happen if (i) injection is isotropic but the variation of the maximum energy of electrons is rather fast to compensate for differences in the magnetic field, or (ii) the obliquity dependence of injection (either quasi-parallel or quasi-perpendicular) and the electron maximum energy are strong enough to dominate the magnetic field variation. In the latter case, the obliquity dependences of the injection and the maximum energy should not be opposite. We argue that the position of the limbs alone, and even their coincidence in radio, X-rays and γ-rays, as discovered by HESS in SN 1006, cannot be conclusive as regards the dependence of the electron injection efficiency, the compression/amplification of the ISMF and the electron maximum energy on the obliquity angle.  相似文献   

The Influence of a New Model of Progenitor Star on Supernova Explosion     

Miao-Jing Zhang  Xiu-Qiang QiuJiang-Hong Pan  Xu-Ning Zhang 《Chinese Astronomy and Astrophysics》2009

By adopting the progenitor model proposed by Woosley in 2002 and using the program WZYW89, the process of explosion of type II supernovae with masses of 11−40M_⊙ is numerically simulated. The results of computation reveal that this new model can in different degrees affect the collapse and explosion of type II supernovae as well as the propagation of shock waves. Besides, the definitions of the primary energy of shock waves are also discussed.  相似文献