Convection And Evolution Of Black Hole Accretion Flows

Gravitational energy is the energy source of many astrophysical systems in the universe, such as quasar and X-ray binaries, etc, and accretion is an effective mechanism of releasing gravitational energy. By the study of accretion disks, people can explain many phenomena on observation, such as luminosity and radiation spectrum, etc. Accretion has become an effective tool for people to understand a variety of high energy phenomena in the universe.The first chapter of this thesis introduces the sorts and formation of black holes; briefly reviews the current situation and progress of the theory of black hole accretion disks. Four known accretion models, namely the Shakura-Sunyaev disk (SSD), the SLE (Shapiro, Lightman & Eardly) disk, the Slim disk and the ADAF, are briefly introduced. We concentrate our attention on the ADAF model, which has become hot in the domain of accretion in recent years, and give the self-similar solutions, global solutions and application of ADAF model. Bimodal black hole accretion disk-SSD+ADAF has been widely used to explain the radiation spectrum of black hole X-ray binaries and galactic nuclei, we describe the transition mechanism of SSD-ADAF from the view of balance analysis for energy, and introduce the time-dependent evolution of accretion disk and try to give some apocalypse for the natural transition of SSD-ADAF. At last, we introduce the current situation and the deficiency of CDAF model.Convection, radiation and conduction are the three ways in transferring energy. Because of the low density of matter in the universe, conduction can be essentially ignored. Nowadays, people have done a lot of works about radiation but only few works for convection. In the second chapter we systematically investigate the CDAF model and give the global solutions of CDAF. This chapter contain two parts: The first part studies the global solutions of CDAF which does not contain radiation, we get three types of global solutions depending on the value of the viscosity parameterα: for largeα(α≥0.1), there is no convection at all and the whole flow is ADAF; we get three zone solution for moderateα(α~0.01), the inner zone is ADAF, the middle zone is CDAF and the outer zone extends outward to match a Keplerian disk, the middle zone of convection-dominated spreads from several tens of gravitational radius (Rg) to several thousands of gravitational radius; When the viscosity parameterαis small (α~0.001), the flow has a two zone structure, the inner zone is ADAF and the outer zone is CDAF, the transition radius is about several tens of gravitational radius. As a validation to the CDAF model of non-radiation, we calculate the global solutions of CDAF including radiation, we find the global dynamical solutions are consistent with the first part, but radiative cooling plays an important role in the convection-dominated zone.In the third chapter we introduce a new numerical code which combines the standard pseudo-spectral method and adaptive domain decomposition together, with this code, we can study the time-dependent evolution of the inner region of accretion disk which is thermally unstable. We made some improvements in the numerical methods: we advance a simple and useful connection technique to ensure a numerically stable continuity for the derivative of a physical quantity across the interface of two conjoint subdomains, and construct a new mapping function to map the CGL collocation points and physical collocation points, the adjustable mapping function enables us to follow adaptively the region that is with the stiff problem and is shifting in space during the time-evolution. We improve the basic equations by including two viscous force terms Frr and Fφφin the radial momentum equation and a corresponding viscous heat term in the energy equation, and we think this is more natural in physics. Then we get limit cycle behavior of accretion disk in the case of moderate viscosity (α~0.1) and give the Bernoulli function. We find the Bernoulli function is negative everywhere in the case of moderate viscosity, so the accretion disk hasn't outflow.The fourth chapter gives summing-up and next work plan for the future.