1,758 research outputs found
Numerical simulations of winds driven by radiation force from the corona above a thin disk
Observations show that winds can be driven from the innermost region (inside
a 50 Schwarschild radius) of a thin disk. It is interesting to study the winds
launched from the innermost region. A hot corona above the black hole (BH) thin
disk is irradiated by the disk. We perform two-dimensional hydrodynamical
simulations to study the winds driven by radiation force from the corona in the
innermost regions. The hard X-ray spectrum from active galactic nuclei (AGNs)
suggests that the corona temperature is about K, so that we mainly
analyze the properties of winds (or outflows) from the K corona. The
disk luminosity plays an important role in driving the outflows. The more
luminous the disk, the stronger the outflows. Mass outflow rate () at a 90 Schwarschild radius depends on disk luminosity, which can be
described as ( is the ratio
of the disk luminosity to the Eddington luminosity). In the case of high
luminosity (e.g. ), the supersonic outflows with maximum speed
Km s are launched at -- and
-- away from the pole axis. The Bernoulli parameter keeps
increasing with the outward propagation of outflows. The radiation force keeps
accelerating the outflows when outflows move outward. Therefore, we can expect
the outflows to escape from the BH gravity and go to the galactic scale. The
interaction between outflows and interstellar medium may be an important AGN
feedback process.Comment: 9 pages, 12 figures, accepted for publication in Ap
Two dimensional numerical simulations of Supercritical Accretion Flows revisited
We study the dynamics of super-Eddington accretion flows by performing
two-dimensional radiation-hydrodynamic simulations. Compared with previous
works, in this paper we include the component of the viscous
stress and consider various values of the viscous parameter . We find
that when is included, the rotational speed of the
high-latitude flow decreases, while the density increases and decreases at the
high and low latitudes, respectively. We calculate the radial profiles of
inflow and outflow rates. We find that the inflow rate decreases inward,
following a power law form of . The value of
depends on the magnitude of and is within the range of .
Correspondingly, the radial profile of density becomes flatter compared with
the case of a constant . We find that the density profile can be
described by , and the value of is almost same for a
wide range of ranging from to . The inward
decrease of inflow accretion rate is very similar to hot accretion flows, which
is attributed to the mass loss in outflows. To study the origin of outflow, we
analyze the convective stability of slim disk. We find that depending on the
value of , the flow is marginally stable (when is small) or
unstable (when is large). This is different from the case of
hydrodynamical hot accretion flow where radiation is dynamically unimportant
and the flow is always convectively unstable. We speculate that the reason for
the difference is because radiation can stabilize convection. The origin of
outflow is thus likely because of the joint function of convection and
radiation, but further investigation is required.Comment: 16 pages, 13 figures, accepted for publication in Ap
Pressure fluctuation signal analysis of pump based on ensemble empirical mode decomposition method
AbstractPressure fluctuations, which are inevitable in the operation of pumps, have a strong non-stationary characteristic and contain a great deal of important information representing the operation conditions. With an axial-flow pump as an example, a new method for time-frequency analysis based on the ensemble empirical mode decomposition (EEMD) method is proposed for research on the characteristics of pressure fluctuations. First, the pressure fluctuation signals are preprocessed with the empirical mode decomposition (EMD) method, and intrinsic mode functions (IMFs) are extracted. Second, the EEMD method is used to extract more precise decomposition results, and the number of iterations is determined according to the number of IMFs produced by the EMD method. Third, correlation coefficients between IMFs produced by the EMD and EEMD methods and the original signal are calculated, and the most sensitive IMFs are chosen to analyze the frequency spectrum. Finally, the operation conditions of the pump are identified with the frequency features. The results show that, compared with the EMD method, the EEMD method can improve the time-frequency resolution and extract main vibration components from pressure fluctuation signals
Estrada index of hypergraphs via eigenvalues of tensors
A uniform hypergraph is corresponding to an adjacency tensor
. We define an Estrada index of by using
all the eigenvalues of as
. The bounds for the Estrada indices of uniform
hypergraphs are given. And we characterize the Estrada indices of -uniform
hypergraphs whose spectra of the adjacency tensors are -symmetric.
Specially, we characterize the Estrada indices of uniform hyperstars
Thermal wind from hot accretion flows at large radii
We study slowly rotating accretion flow at parsec and sub-parsec scale
irradiated by a low luminosity active galactic nuclei. We take into account the
Compton heating, photoionization heating by the central X-rays. The
bremsstrahlung cooling, recombination and line cooling are also included. We
find that due to the Compton heating, wind can be thermally driven. The power
of wind is in the range , with
being the Eddington luminosity. The mass flux of wind is in the range ( is the Eddington
accretion rate, is speed of light). We define the wind generation
efficiency as , with being wind
power, being the mass accretion rate onto the black hole.
lies in the rage . Wind production efficiency
decreases with increasing mass accretion rate. The possible role of the
thermally driven wind in the active galactic feedback is briefly discussed.Comment: 9 pages, 6 figures, accepted by MNRA
What is the real accretion rate onto a black hole for low angular momentum accretion?
Mass accretion rate is a key parameter in accretion disk theory. It
determines black hole accretion mode. In large scale cosmological simulations
studying galaxy formation and evolution, Bondi radius can at most be marginally
resolved. In those simulations, Bondi accretion formula is always used to
estimate black hole accretion rate. Bondi solution is too simple, which cannot
represent the real accretion process. We perform simulations of hot accretion
flow at parsec scale irradiated by a low luminosity active galactic nucleus
(AGN). We perform 77 simulations with varying density and temperature at outer
boundary (10 parsec). Our purpose is to find a formula to calculate real black
hole accretion rate based on the gas density and temperature at parsec scale.
We define Eddington accretion rate to be ,
with and been Eddington luminosity and speed of light
respectively. We set black hole mass to be , is
solar mass. We find that black hole accretion rate can be expressed as
, with and being
density and temperature at parsec scale, respectively. We find the formula can
accurately predict the luminosity of observed low-luminosity AGNs (with black
hole mass ). This formula can be used in the sub-grid
models in large scale cosmological simulations with a black hole mass of .Comment: 12 pages, 9 figures, accepted by MNRA
- β¦