Constraints on Slim Accretion Discs

We show that when the gravitational force in the vertical direction is correctly calculated, the well-known S- shaped sequence of thermal equilibrium solutions can be constructed only for small radii of black hole accretion flows, such that slim accretion discs can possibly exist only in the inner regions of these flows.     

Convection-Dominated Accretion Flows with Radiative Cooling

By numerically solving the set of basic equations describing black hole accretion flows with low accretion rates, we show that although the dynamical structure of these flows is essentially unaffected by radiative processes in comparison with the case in which the radiation is not considered, the radiative cooling can be more important than the advective cooling in the flow＇s convection-dominated zone, and this result may have implications to distinguish observationally convection-dominated accretion flows from advection-dominated accretion flows.     

Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j- K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.     

Slim Discs with Varying Accretion Rates

It was revealed in our previous studies that there exists a maximal possible accretion rate for slim discs with constant accretion rates because the correctly calculated vertical gravitational force can only gather some limited amount of accreted matter. Here we show that when the accretion rate is not constant and instead decreases with decreasing radius because of outflows, such that the amount of accreted matter is adjusted to be within the allowed limit, global slim disc solutions can be constructed even for the case that accretion rates at large radii apparently exceed the maximal possible value. This result further demonstrates that outflows seem to be unavoidable for accretion flows with large accretion rates at large radii.     

Energy dissipation and angular momentum transfer within a magnetically torqued accretion disc

We discuss transportation and redistribution of energy and angular momentum in the magnetic connection (MC) process and Blandford-Payne (BP) process. MC results in readjusting the interior viscous torque, and its effects are operative not only in but also beyond the MC region. The BP process is invoked to transfer the “excessive” angular momentum from an accretion disc. In addition, we derive a criterion for the interior viscous torque to resolve the puzzle of the overall equilibrium of angular momentum in disc accretion. It turns out that the efficiency of BP at extracting angular momentum and the intensity of the outflow are required to be greater than some critical values.

     

Time-dependent and outflow boundary conditions for Dissipative Particle Dynamics

We propose a simple method to impose both no-slip boundary conditions at fluid-wall interfaces and at outflow boundaries in fully developed regions for Dissipative Particle Dynamics (DPD) fluid systems. The procedure to enforce the no-slip condition is based on a velocity-dependent shear force, which is a generalized force to represent the presence of the solid-wall particles and to maintain locally thermodynamic consistency. We show that this method can be implemented in both steady and time-dependent fluid systems and compare the DPD results with the continuum limit (Navier-Stokes) results. We also develop a force-adaptive method to impose the outflow boundary conditions for fully developed flow with unspecified outflow velocity profile or pressure value. We study flows over the backward-facing step and in idealized arterial bifurcations using a combination of the two new boundary methods with different flow rates. Finally, we explore the applicability of the outflow method in time-dependent flow systems. The outflow boundary method works well for systems with Womersley number of O(1), i.e., when the pressure and flowrate at the outflow are approximately in-phase.     

Viscous shear in general-relativistic astrophysical flows

Conclusion Introducing the concept of general co-moving frames (gcmf) in [4] we have argued that it may become useful in a number of hydrodynamical and mhd applications. In [5] using thegcmf technique we have constructed a fully covariant, general-relativistic theory of strongly magnetized collisionless plasma. The approach proved itself to be highly convenient-it allowed us to find new equations of state for such a medium.In the present paper we have considered viscous shear in generalrelativistic astrophysical flows as an another example of the effective usage of orthonormal tetrads method. Namely, we have specified general corotating frames (gcrf)-subclass ofgcmf corresponding to the flows being in purely rotational motion. By means of gcrf we have been able to find expressions for nonzero components of shear tensor and turbulent viscosity tensor for the innermost region of a black hole accretion disc.We think that the method may be useful when considering analogous problems with astrophysical flows of more complicated geometry and/or dynamics. In particular, the method may become efficient for jets in active galactic nuclei (agn) and quasars [13], general-relativistic winds of compact objects [14] and the innermost regions of candidates for galactic black hole accretion discs [15]. To be sure, in some of these problems we have to use a more general set ofgcmf instead ofgcrf. Such problems, however, are beyond the scope of this paper, where we have only outlined the main background of the method and demonstrated its productivity in a simple case of quasi-keplerian accretion flow in a general-relativistic standard accretion disc.     

Effects of the energy equation in studies of limit-cycle behaviors of black hole accretion disks

The inconsistency of the energy equation used in the literature is pointed out and a new consistent energy equation is given. With this new energy equation, calculations are made for the limit-cycle behaviors of thermally unstable accretion disks around black holes. From the comparison of our numerical results with those obtained using the inconsistent energy equation, it is found that the inconsistent energy equation undervalues the temperature and overvalues the effective optical depth when the accreted gas becomes effectively optically thin. Thus, it is dangerous if the inconsistent energy equation is used in the studies of very hot and optically thin accretion flows such as advection-dominated accretion flows (ADAFs), and our new energy equation is likely to be a better alternative. Supported by the National Basic Research Program of China (Grant No. 2009CB824800), the National Natural Science Foundation of China (Grant Nos. 10673009 and 10833002), and the Natural Science Foundation of Fujian Province of China (Grant No. V0750001)     

Experimental observation and characterization of the magnetorotational instability

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational instability. Our system consists of liquid sodium between differentially rotating spheres, with an imposed coaxial magnetic field. We characterize the observed patterns, dynamics, and torque increases, and establish that this instability can occur from a hydrodynamic turbulent background.     

Some developments of hot accretion flow theory in the past ten years

The hot accretion flow model was re-discovered in 1994 by Narayan and collaborators. Intensive theoretical works have been conducted and significant progresses have been achieved. In this paper, we review several developments in the past ten years. This mainly includes the finding of outflow and convection and its dynamical effect on inflow; the direct electron heating by viscous dissipation; the effect of large scale toroidal magnetic fields in the inner region of the accretion flow; and the effect of global Compton scattering. Their observational applications are also introduced very briefly.     

Lattice Boltzmann simulations of 2D laminar flows past two tandem cylinders

We apply the lattice Boltzmann equation (LBE) with multiple-relaxation-time (MRT) collision model to simulate laminar flows in two-dimensions (2D). In order to simulate flows in an unbounded domain with the LBE method, we need to address two issues: stretched non-uniform mesh and inflow and outflow boundary conditions. We use the interpolated grid stretching method to address the need of non-uniform mesh. We demonstrate that various inflow and outflow boundary conditions can be easily and consistently realized with the MRT-LBE. The MRT-LBE with non-uniform stretched grids is first validated with a number of test cases: the Poiseuille flow, the flow past a cylinder asymmetrically placed in a channel, and the flow past a cylinder in an unbounded domain. We use the LBE method to simulate the flow past two tandem cylinders in an unbounded domain with Re = 100. Our results agree well with existing ones. Through this work we demonstrate the effectiveness of the MRT-LBE method with grid stretching.     

天体物理学讲座第二讲 活动星系核物理

星系的活动是星系核心大质量黑洞吸积周围的气体释放巨大的辐射功率的过程,它是强引力场物理,高能物理和辐射流体物理的天然实验室,文章介绍了活动星系核中黑洞吸积,发射线形成和外流的观测事实和基本物理过程等,指出了现有理论存在的一些问题。     

Instabilities, bifurcations, and multiple solutions in expanding channel flows

We present an experimental realization of the classical Jeffery-Hamel flows inside a wedge-shaped channel. We compare the measured velocity fields with the predictions of Jeffery-Hamel theory. A detailed experimental study of bifurcation diagrams for the solutions reveals the absolute stability of the pure outflow solution and an interesting hysteretic structure for bifurcations. We also observe a multiple-vortex flow regime predicted earlier numerically and analytically.     

Black hole binaries and microquasars

This is a general review Oil the observations and physics of black hole X-ray binaries and microquasars, with the emphasize on recent developments in the high energy regime. The focus is put on understanding the accretion flows and measuring the parameters of black holes in them. It includes mainly two parts： i） Brief review of several recent review article on this subject; ii） Further development on several topics, including black hole spin measurements, hot accretion flows, corona formation, state transitions and thermal stability of standard think disk. This is thus not a regular bottom-up approach, which I feel not necessary at this stage. Major effort is made in making and incorporating from many sources useful plots and illustrations, in order to make this article more comprehensible to non-expert readers. In the end I attempt to make a unification scheme on the accretion-outflow （wind/jet） connections of all types of aecreting BHs of all accretion rates and all BH mass scales, and finally provide a brief outlook.     

Stability and angular-momentum transport of fluid flows between corotating cylinders

Turbulent transport of angular momentum is a necessary process to explain accretion in astrophysical disks. Although the hydrodynamic stability of disklike flows has been tested in experiments, results are contradictory and suggest either laminar or turbulent flow. Direct numerical simulations reported here show that currently investigated laboratory flows are hydrodynamically unstable and become turbulent at low Reynolds numbers. The underlying instabilities stem from the axial boundary conditions, affect the flow globally, and enhance angular-momentum transport.     

How fast can a black hole eat?

We construct stationary spherically symmetric solutions of the equations for accretion of large mass flows onto a black hole, including the interaction of matter and radiation due to Thomson scattering in diffusion approximation. We discuss the relevance of these solutions for a decision on the following question: Does the limitation of the luminosity (Eddington limit) also imply an upper bound to the possible rate of mass flow? The question remains open until all instabilities have been studied. At the moment we still tend to a negative answer.     

Equilibrium of a gravitating plasma in a dipolar magnetic field

The equilibria of plasma in a dipolar magnetic field under the gravitational influence of a massive body (a star or black hole) and a self gravitating plasma are considered. Analytical solutions are found that can be useful for understanding the physics of plasma flows in accretion disks and star formation.     

Mkn 501 X射线和TeV射线准周期震荡的可能机制

我们提出了对流占主导的吸积流（ADAF）盘的辐射线是Mkn 501中从X射线到Tevγ射线的源光子；而且此盘的不稳定性可以解释Mkn 501中在X射线和Tevγ射线光变曲线23天的准周期光变。在这个模型中,光学到X射线辐射进入喷流，后由于喷流内相对论电子的作用而转化成高能光子。在这个过程中，发生在ADAF盘中的不稳定性导致源光子的准周期变化，从而导致了X射线和γ射线的变化。     

On the effect of injection of gas in the numerical simulation of accretion flows

We investigate the effects of various ways of injection of gas at the outer boundary in the numerical simulations of non-viscous accretion flows. We study three models. In Model A, we inject material around the equatorial plane. In Models B and C, fullrange θ injection is used (we employ spherical coordinates). In all three models, the injected material has the same density distribution with polar angle θ. From the equatorial region to the polar regions, angular momentum of the injected material of Model B decreases faster than that in Model C. For all of the models, after a transient episode of infall at the beginning of the simulations, the gas piles up in the equatorial regions outside the black hole and forms a thick torus bounded by a centrifugal barrier. We find that the accretion rates of Models B and C are more than ten times higher than that in Model A. In Model A, there is weak accretion only in the torus and outflows are found on the surface of the torus. In Model B, we find strong inflows on the surface of its torus, and the accretion in the torus is weak. In Model C, strong inflows also occur on the surface of its torus, but the accretion regions are narrower and there are strong outflows in its torus. In all of our models, the time-averaged density, pressure and angular momentum in the equatorial region can be described by a radial power law, with P ∝r ^−3/2, P ∝r ⁻² and l∝r ⁰.