Hydrodynamical non-radiative accretion flows in two dimensions     

James M. Stone  James E. Pringle †  Mitchell C. Begelman ‡ 《Monthly notices of the Royal Astronomical Society》1999,310(4):1002-1016

Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν . We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Høiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν ∝ r ^1/2 recovers the widely studied self-similar solution corresponding to an ' α -disc'. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object.  相似文献   

Plenty of nothing: Black hole entropy in induced gravity     

V. P. Frolov  D. V. Fursaev 《Journal of Astrophysics and Astronomy》1999,20(3-4):121-129

We demonstrate how Sakharov's idea of induced gravity allows one to explain the statistical-mechanical origin of the entropy of a black hole. According to this idea, gravity becomes dynamical as the result of quantum effects in the system of heavy constituents of the underlying theory.The black hole entropy is related to the properties of the vacuum in the induced gravity in the presence of the horizon. We obtain the Bekenstein-Hawking entropy by direct counting the states of the constituents.  相似文献   

The distribution of supermassive black holes in the nuclei of nearby galaxies     

Andrea Cattaneo  Martin G. Haehnelt  Martin J. Rees 《Monthly notices of the Royal Astronomical Society》1999,308(1):77-81

The growth of supermassive black holes by merging and accretion in hierarchical models of galaxy formation is studied by means of Monte Carlo simulations. A tight linear relation between masses of black holes and masses of bulges arises if the mass accreted by supermassive black holes scales linearly with the mass-forming stars and if the redshift evolution of mass accretion tracks closely that of star formation. Differences in redshift evolution between black hole accretion and star formation introduce a considerable scatter in this relation. A non-linear relation between black hole accretion and star formation results in a non-linear relation between masses of remnant black holes and masses of bulges. The relation of black hole mass to bulge luminosity observed in nearby galaxies and its scatter are reproduced reasonably well by models in which black hole accretion and star formation are linearly related but do not track each other in redshift. This suggests that a common mechanism determines the efficiency for black hole accretion and the efficiency for star formation, especially for bright bulges.  相似文献   

An application of the Kerr black hole fly-wheel model to statistical properties of QSOs/AGNs     

Shin-ya Nitta 《Monthly notices of the Royal Astronomical Society》1999,308(4):995-1005

The aim of this work is to demonstrate the properties of the magnetospheric model around Kerr black holes (BHs), the so-called fly-wheel (rotation driven) model. The fly-wheel engine of the BH–accretion disc system is applied to the statistics of QSOs/AGNs. In the model, the central BH is assumed to be formed at z ∼10² and obtains nearly maximum but finite rotation energy (∼extreme Kerr BH) at the formation stage. The inherently obtained rotation energy of the Kerr BH is released through a magnetohydrodynamic process. This model naturally leads to a finite lifetime of AGN activity.
Nitta, Takahashi & Tomimatsu clarified the individual evolution of the Kerr BH fly-wheel engine, which is parametrized by BH mass, initial Kerr parameter, magnetic field near the horizon and a dimensionless small parameter. We impose a statistical model for the initial mass function (IMF) of an ensemble of BHs using the Press–Schechter formalism. With the help of additional assumptions, we can discuss the evolution of the luminosity function and the spatial number density of QSOs/AGNs.
By comparing with observations , it is found that a somewhat flat IMF and weak dependence of the magnetic field on the BH mass are preferred. The result explains well the decrease of very bright QSOs and decrease of population after z ∼2.  相似文献   

Computation of mass outflow rate from relativistic quasi-spherical accretion on to black holes     

Tapas K. Das 《Monthly notices of the Royal Astronomical Society》1999,308(1):201-206

We compute the mass outflow rate R _m˙ from relativistic matter that is accreting quasi-spherically on to the Schwarzschild black holes. Taking the pair-plasma pressure-mediated shock surface as the effective boundary layer (of the black hole) from where the bulk of the outflow is assumed to be generated, computation of this rate is done using combinations of exact transonic inflow and outflow solutions. We find that R _m˙ depends on the initial parameters of the flow, the polytropic index of matter, the degree of compression of matter near the shock surface and the location of the shock surface itself. We thus not only study the variation of the mass outflow rate as a function of various physical parameters governing the problem, but also provide a sufficiently plausible estimation of this rate.  相似文献   

秦皇岛市洋河、戴河滨海平原海水入侵的控制与治理   总被引：5，自引：0，他引：5  

韩再生 《现代地质》1990,4(2):105-115

秦皇岛市洋河,戴河冲洪积平原,近20年来抽取地下水量迅速增加,80年代初形成了低于海平面的降落漏斗,造成海水从海岸线向内陆和从河流下游河床向两侧同时侵入孔隙含水层。由于水质恶化,北戴河区水源地面临报废的危险。为了保护和开发淡水资源,控制海水入侵,必须对地下水的水质和水量进行综合评价。采用水动力弥散型地下水水质模型来描述滨海含水层中海水和淡水的运动,把氯离子作为水质模型的变量。在水量模型中采用孔隙压力代替水头作为变量。数学模型用有限单元法求解。模拟面积约180km²,剖分为321个单元,184个结点。作为水文地质实体的模型,它被用来模拟和预报不同降水量年份的地下水水位和水质。计算结果表明,如果保持目前的地下水开采量,海水入侵还会深入。为了控制和治理海水入侵,本文探讨了可行措施。根据数学模型模拟的结果,在洋河下游人海口修建防潮闸,同时加强滨海含水层的管理,具有较好的经济效益和环境效益。  相似文献   

海洋监视工作效益和工作质量评价方法探讨     

张士三  陈劲毅 《台湾海峡》1998,17(3):337-341

海洋监视工作的效益和工作质量评价是海洋行政执法管理中对其工作结果进行检查和自检的一种方法。本文根据海洋监视可控度,海上管理目标实现情况及效益判断事项,分别建立了工作效益和工作质量评价的数学公式。该公式表明,效益指数ＶＳ与海洋监视可控度Ｋ呈正比,与权益怀事项数、性事项数及公益性事项数之和也呈正比。工作质量指数ＶＺ与海洋监视可控度Ｋ呈正比,而与对压Ｋ的违法事件发生率、环境质量参数、海洋资源非合理开发项  相似文献   

Multiple Discriminant Analysis of Marine Sediment Data   总被引：2，自引：0，他引：2  

P. K. S. Shin  K. Y. S. Fong 《Marine pollution bulletin》1999,39(1-12):285-294

A multivariate statistical approach was used in the analysis of inter-relationships among marine benthic infaunal structure, physical and chemical characteristics of sediment samples and toxicity data derived from laboratory tests at 16 locations in shallow, inshore waters of Hong Kong. The method involved classification (cluster analysis using the Bray–Curtis similarity index) and ordination (multi-dimensional scaling) of infaunal patterns and the use of multiple discriminant analysis to relate groupings of the locations to the selected environmental and toxicity data. Analysis of the combined sediment physical, biological, chemical and toxicity dataset by stepwise multiple discriminant analysis allowed identification of those variables most sensitive for discriminating location groups. The use of multiple discriminant analysis in sediment quality characterization was evaluated against the Sediment Quality Triad approach and other statistical techniques.  相似文献   

Eutrophication Dynamics of Tolo Harbour, Hong Kong   总被引：8，自引：0，他引：8  

Joseph H. W. Lee  Feleke Arega 《Marine pollution bulletin》1999,39(1-12):187-192

The time and spatial variation of water quality in Tolo Harbour, a eutrophic landlocked semi-enclosed bay frequented by algal blooms, is studied using a dynamic eutrophication model. Hourly changes of tide levels and currents are computed by a link-node model assuming M₂ tidal forcing. Phytoplankton growth is assumed to be limited by solar radiation, nitrogen and temperature. The model incorporates light acclimation by algae, self-shading, photosynthetic production, nutrient uptake, and a dynamic determination of the carbon to chlorophyll ratio. In particular, sediment-water-pollutant interactions are modelled via an anaerobic benthic layer segment. Using recorded pollution loads and environmental forcing as input, the model predictions of daily-averaged water quality are compared with the extensive water quality monitoring data of the Environmental Protection Department (EPD). The predicted spatial distribution and trends of algal biomass, inorganic nitrogen, dissolved oxygen (DO), as well as sediment oxygen demand (SOD), are in general agreement with field observations.  相似文献   

Application of Numerical Models in Marine Pollution Research in China   总被引：6，自引：0，他引：6  

SU Jilan  Dong Lixian 《Marine pollution bulletin》1999,39(1-12):73-79

Like many countries in the world, China is also facing serious problems of marine pollution in the coastal zone. The prevailing concern is eutrophication of the coastal water due to river inputs and, in some bays, also due to the over-expansion of aquaculture. As part of the marine pollution research activities in China, applications of mathematical models are also widely used, both to understand the present and to project the future. In this paper, selected Chinese works are reviewed, especially with respect to the effort in reducing the errors introduced in the numerical treatment of the water quality equation.  相似文献