A Numerical Study of Nonspherical Black Hole Accretion
Hawley, John Frederick
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https://hdl.handle.net/2142/71601
Description
Title
A Numerical Study of Nonspherical Black Hole Accretion
Author(s)
Hawley, John Frederick
Issue Date
1984
Department of Study
Astronomy
Discipline
Astronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics
Astronomy
Astrophysics
Abstract
This thesis describes in detail a two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. The aim is to study fully dynamic inviscid fluid accretion onto black holes, as well as to study the evolution and development of nonlinear instabilities in pressure supported accretion disks.
In order to fully calibrate and document the code, certain analytic solutions for shock tubes and special accretion flows are derived; these solutions form the basis for code testing. The numerical techniques used are developed and discussed. A variety of alternate differencing schemes are compared on an analytic test bed. Some discussion is devoted to general issues in finite differencing. The working code is calibrated using analytically solvable accretion problems, including the radial accretion of dust and of fluid with pressure (Bondi accretion). Two dimensional test problems include the spiraling infall of low angular momentum fluid, the formation of a pressure supported torus, and the stable evolution of a torus.
A series of numerical models are discussed and illustrated with selected plots. The inflow of constant angular momentum fluid is considered in several limits: angular momentum greater than marginally bound, less than marginally stable, and between marginally bound and stable. Both geometrically thin and thick inflows are computed as well as models with various values of Kerr a.
"All geometrically thick inflows form a funnel, i.e. an evacuated region along the orbital axis. If there is a centrifugal barrier present, an accretion shock can form which creates a hot thick disk. Such a high pressure disk can drive outflows in noncollimated hollow jets. Intermediate angular momentum fluid can flow into the black hole through a ""nozzle"" defined by the centrifugal potential."
In geometrically thin inflows no funnel forms, and, if the angular momentum is less than marginally bound, the fluid flows into the hole. Inflows with greater angular momentum create a hot wind which flows back from the centrifugal wall around the thin inflow creating an effective two-temperature disk. Changes in the Kerr parameter a do not alter the qualitative features of these flows; rather, they merely change the shape and location of the funnel and disk. The values of angular momentum which are marginally bound and marginally stable are similarly altered.
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