81 research outputs found
Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics
We present results of numerical simulation of inviscid thick accretion disks
and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH)
technique for this purpose. Formation of thick disks are found to be preceded
by shock waves travelling away from the centrifugal barrier. For a large range
of the parameter space, the travelling shock settles at a distance close to the
location obtained by a one-and-a-half dimensional model of inviscid accretion
disks. Occasionally, it is observed that accretion processes are aided by the
formation of oblique shock waves, particularly in the initial transient phase.
The post-shock region (where infall velocity suddenly becomes very small)
resembles that of the usual model of thick accretion disk discussed in the
literature, though they have considerable turbulence. The flow subsequently
becomes supersonic before falling into the black hole. In a large number of
cases which we simulate, we find the formation of strong winds which are hot
and subsonic when originated from the disk surface very close to the black hole
but become supersonic within a few tens of the Schwarzschild radius of the
blackhole. In the case of accretion of high angular momentum flow, very little
amount of matter is accreted directly onto the black hole. Most of the matter
is, however, first squeezed to a small volume close to the black hole, and
subsequently expands and is expelled as a strong wind. It is quite possible
that this expulsion of matter and the formation of cosmic radio jets is aided
by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press
Simple absorbing layer conditions for shallow wave simulations with Smoothed Particle Hydrodynamics
We study and implement a simple method, based on the Perfectly Matched Layer approach, to treat non
reflecting boundary conditions with the Smoothed Particles Hydrodynamics numerical algorithm. The
method is based on the concept of physical damping operating on a fictitious layer added to the
computational domain. The method works for both 1D and 2D cases, but here we illustrate it in the case
of 1D and 2D time dependent shallow waves propagating in a finite domain
Numerical Simulations of Standing Shocks in Accretion Flows around Black Holes: A Comparative Study
We compare the results of numerical simulations of thin and quasi-spherical
(thick) accretion flows with existing analytical solutions. We use a Lagrangian
code based on the Smooth Particle Hydrodynamics (SPH) scheme and an Eulerian
finite difference code based on the Total Variation Diminishing (TVD) scheme.
In one-dimensional thin flows, the results of the simulations, with or without
shocks, agree very well with each other and with analytical solutions. In
two-dimensional thick flows, the general features, namely the locations and
strengths of centrifugal and turbulent pressure supported shocks, centrifugal
barriers, and the funnel walls which are expected from analytical models, agree
very well, though the details vary. Generally speaking, the locations of the
shocks may be better obtained by SPH since the angular momentum is strictly
preserved in SPH, but the shocks themselves are better resolved by TVD. The
agreement of these code test results with analytical solutions provides us with
confidence to apply these codes to more complex problems which we will discuss
elsewhere.Comment: ApJ accepted, 16 pages with 7 figures, using aaspp4.sty available
upon request to [email protected] or from
http://astro1.chungnam.ac.kr/Homepage/ryu/mypage.htm
Resonance Oscillation Of Radiative Shock Waves In Accretion Disks Around Compact Objects
We extend our previous numerical simulation of accretion disks with shock
waves when cooling effects are also included. We consider bremsstrahlung and
other power law processes: to mimic cooling
in our simulation. We employ {\it Smoothed Particle Hydrodynamics} technique as
in the past. We observe that for a given angular momentum of the flow, the
shock wave undergoes a steady, radial oscillation with the period is roughly
equal to the cooling time. Oscillations seem to take place when the disk and
cooling parameters (i.e., accretion rate, cooling process) are such that the
infall time from shock is of the same order as the post-shock cooling time. The
amplitude of oscillation could be up to ten percent of the distance of the
shock wave from the black hole when the black hole is accreting. When the
accretion is impossible due to the centrifugal barrier, the amplitude variation
could be much larger. Due to the oscillation, the energy output from the disk
is also seen to vary quasi-periodically. We believe that these oscillations
might be responsible for the quasi periodic oscillation (QPO) behaviors seen in
several black hole candidates, in neutron star systems as well as dwarf novae
outbursts such as SS Cygni and VW Hyi.Comment: 24 pages of text plus 6 figures (in tar-compressed form);
Astrophysical journal (in press
On the Azimuthal Stability of Shock Waves around Black Holes
Analytical studies and numerical simulations of time dependent axially
symmetric flows onto black holes have shown that it is possible to produce
stationary shock waves with a stable position both for ideal inviscid and for
moderately viscous accretion disks.
We perform several two dimensional numerical simulations of accretion flows
in the equatorial plane to study shock stability against non-axisymmetric
azimuthal perturbations. We find a peculiar new result. A very small
perturbation seems to produce an instability as it crosses the shock, but after
some small oscillations, the shock wave suddenly transforms into an asymmetric
closed pattern, and it stabilizes with a finite radial extent, despite the
inflow and outflow boundary conditions are perfectly symmetric. The main
characteristics of the final flow are: 1) The deformed shock rotates steadily
without any damping. It is a permanent feature and the thermal energy content
and the emitted energy vary periodically with time. 2) This behavior is also
stable against further perturbations. 3) The average shock is still very strong
and well defined, and its average radial distance is somewhat larger than that
of the original axially symmetric circular shock. 4) Shocks obtained with
larger angular momentum exhibit more frequencies and beating phenomena. 5) The
oscillations occur in a wide range of parameters, so this new effect may have
relevant observational consequences, like (quasi) periodic oscillations, for
the accretion of matter onto black holes. Typical time scales for the periods
are 0.01 and 1000 seconds for black holes with 10 and 1 million solar mass,
respectively.Comment: 15 pages, 7 figures, accepted by the Astrophysical Journa
Zero-energy rotating accretion flows near a black hole
We characterize the nature of thin, axisymmetric, inviscid accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hold by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum (lambda less than or similar to lambda(b)), most of the material is accreted into the black hold, forming a quasi-spherical flow or a simple disklike structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound, value, lambda greater than or similar to(mb)), almost no accretion into the black hole occurs. Instead, the flow produces a stable shock with one or more vortices behind it and is deflected away at the shock as a conical, outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than lambda(mb) (lambda less than or similar to lambda less than or similar to lambda(mb)), a fraction (typically 5%-10%) of the incoming material is accreted into the black hole, but the flow structure formed is similar to that for lambda greater than or similar to lambda(mb). Some of the deflected material is accreted back into the black hole while the rest is blown away as an outgoing wind. These two cases with lambda greater than or similar to lambda(u) correspond those studied in the previous works by Molteni, Lanzafame, & Chakrabarti, and Ryu et al. However, the flow with angular momentum close to the marginally stable value (lambda(ms)) is found to be unstable. More specifically, if lambda(b) less than or similar to lambda similar to lambda(ms) less than or similar to lambda(u), the flow displays a distinct periodicity in the sense that the inner part of the disk is built and destroyed regularly. The period is roughly equal to (4-6) x 10(3) R(g)/c, depending on the angular momentum of the flow. In this case, the internal energy of the flow around the black hold becomes maximum when the structure with the accretion shock and vortices is fully developed. But the mass accretion rate into the black hole reaches a maximum value when the structure collapses. Averaged over periods, more than half the incoming material is accreted into the black hole. We suggest the physical origin of these separate regimes from a global perspective. Then we discuss the possible relevance of the instability work to quasi-periodic oscillationsopen716
Modelos de desarrollo de las políticas públicas educativas : Presupuesto, planes estratégicos y metas educativas en la Educación Superior
La perspectiva es comprender el desarrollo de las políticas de planeamiento en la Educación Superior desde el análisis de los presupuestos y los planes estratégicos en función de metas educativas globales que han sido consensuadas y aceptadas para constituir el norte macroinstitucional del sistema.
En el desarrollo de los estudios sobre la Educación Superior, hay una preeminencia de indagaciones o ensayos referidos centralmente a la historicidad del campo, a la organización de datos económicos y financieros en tablas comparativas sin mucho contexto de sus respectivas aplicaciones, a la presentación de indicadores o metas sólo educativas o sólo institucionales. Esta constatación sobre los antecedentes se origina, por supuesto, en una apreciación crítica pero también tributaria de dichos aportes y desarrollos, porque consideramos necesario hacer un aporte que vincule las tres fuentes principales de datos concretos para lograr una propuesta integrada hacia el planeamiento prospectivo de la Educación Superior: los presupuestos (fuentes presupuestarias, fondos materiales, ejecuciones coyunturales), los planes estratégicos (los deseos y planes institucionales sobre la actividad inmediata y mediata de la organización) y las metas educativas globales (los acuerdos y consensos de organizaciones supraeducativas o interinstitucionales). La formulación investigativa también es precisa acerca del sentido del análisis y los posibles aportes: al final del proceso, el trabajo pondrá en circulación una reflexión compleja acerca del lugar de la UNLP en el sistema de Educación Superior argentino y la discusión por los modelos de desarrollo integrales de las políticas públicas en el contexto de las intencionalidades internacionalistas, globalizadoras y cooperativo-interinstitucionalistas.Dirección de Asuntos Municipale
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