25 research outputs found
Growing pseudo-eigenmodes and positive logarithmic norms in rotating shear flows
Rotating shear flows, when angular momentum increases and angular velocity
decreases as functions of radiation coordinate, are hydrodynamically stable
under linear perturbation. The Keplerian flow is an example of such systems
which appears in astrophysical context. Although decaying eigenmodes exhibit
large transient energy growth of perturbation which could govern nonlinearity
into the system, the feedback of inherent instability to generate turbulence
seems questionable. We show that such systems exhibiting growing
pseudo-eigenmodes easily reach an upper bound of growth rate in terms of the
logarithmic norm of the involved nonnormal operators, thus exhibiting feedback
of inherent instability. This supports the existence of turbulence of
hydrodynamic origin in the Keplerian accretion disc in astrophysics. Hence,
this enlightens the mismatch between the linear theory and
experimental/observed data and helps in resolving the outstanding question of
origin of turbulence therein.Comment: 12 pages including 4 figures; to appear in New Journal of Physic
Growing hydrodynamic modes in Keplerian accretion disks during secondary perturbations: Elliptical vortex effects
The origin of hydrodynamic turbulence, and in particular of an anomalously
enhanced angular momentum transport, in accretion disks is still an unsolved
problem. This is especially important for cold disk systems which are
practically neutral in charge and therefore turbulence can not be of
magnetohydrodynamic origin. While the flow must exhibit some instability and
then turbulence in support of the transfer of mass inward and angular momentum
outward, according to the linear perturbation theory, in absence of
magnetohydrodynamic effects, it should always be stable. We demonstrate that
the three-dimensional secondary disturbance to the primarily perturbed disk,
consisting of elliptical vortices, gives significantly large hydrodynamic
growth in such a system and hence may suggest a transition to an ultimately
turbulent state. This result is essentially applicable to accretion disks
around quiescent cataclysmic variables, in proto-planetary and star-forming
disks, the outer region of disks in active galactic nuclei, where the gas is
significantly cold and thus the magnetic Reynolds number is smaller than 10^4.Comment: 21 pages including 4 figures, aastex format; Accepted for publication
in The Astrophysical Journa
Bypass to Turbulence in Hydrodynamic Accretion: Lagrangian Analysis of Energy Growth
Despite observational evidence for cold neutral astrophysical accretion
disks, the viscous process which may drive the accretion in such systems is not
yet understood. While molecular viscosity is too small to explain the observed
accretion efficiencies by more than ten orders of magnitude, the absence of any
linear instability in Keplerian accretion flows is often used to rule out the
possibility of turbulent viscosity. Recently, the fact that some fine tuned
disturbances of any inviscid shear flow can reach arbitrarily large transient
growth has been proposed as an alternative route to turbulence in these
systems. We present an analytic study of this process for 3D plane wave
disturbances of a general rotating shear flow in Lagrangian coordinates, and
demonstrate that large transient growth is the generic feature of
non-axisymmetric disturbances with near radial leading wave vectors. The
maximum energy growth is slower than quadratic, but faster than linear in time.
The fastest growth occurs for two dimensional perturbations, and is only
limited by viscosity, and ultimately by the disk vertical thickness.
After including viscosity and vertical structure, we find that, as a function
of the Reynolds number, R, the maximum energy growth is approximately 0.4
(R/log R)^{2/3}, and put forth a heuristic argument for why R > 10^4 is
required to sustain turbulence in Keplerian disks. Therefore, assuming that
there exists a non-linear feedback process to replenish the seeds for transient
growth, astrophysical accretion disks must be well within the turbulent regime.
However, large 3D numerical simulations running for many orbital times, and/or
with fine tuned initial conditions, are required to confirm Keplerian
hydrodynamic turbulence on the computer.Comment: 25 preprint pages, 2 figures, some modifications mainly to the
Discussions section, Accepted for publication in Ap
Description of Pseudo-Newtonian Potential for the Relativistic Accretion Disk around Kerr Black Holes
We present a pseudo-Newtonian potential for accretion disk modeling around
the rotating black holes. This potential can describe the general relativistic
effects on accretion disk. As the inclusion of rotation in a proper way is very
important at an inner edge of disk the potential is derived from the Kerr
metric. This potential can reproduce all the essential properties of general
relativity within 10% error even for rapidly rotating black holes.Comment: 5 Latex pages including 1 figure. Version to appear in Astrophysical
Journal, V-581, N-1, December 10, 200
Dynamics of electromagnetic waves in Kerr geometry
Here we are interested to study the spin-1 particle i.e., electro-magnetic
wave in curved space-time, say around black hole. After separating the
equations into radial and angular parts, writing them according to the black
hole geometry, say, Kerr black hole we solve them analytically. Finally we
produce complete solution of the spin-1 particles around a rotating black hole
namely in Kerr geometry. Obviously there is coupling between spin of the
electro-magnetic wave and that of black hole when particles propagate in that
space-time. So the solution will be depending on that coupling strength. This
solution may be useful to study different other problems where the analytical
results are needed. Also the results may be useful in some astrophysical
contexts.Comment: 15 Latex pages, 4 Figures; Accepted for publication in Classical and
Quantum Gravit
Behaviour of spin-1/2 particle around a charged black hole
Dirac equation is separable in curved space-time and its solution was found
for both spherically and axially symmetric geometry. But most of the works were
done without considering the charge of the black hole. Here we consider the
spherically symmetric charged black hole background namely Reissner-Nordstrom
black hole. Due to presence of the charge of black-hole charge-charge
interaction will be important for the cases of incoming charged particle (e.g.
electron, proton etc.). Therefore both gravitational and electromagnetic gauge
fields should be introduced. Naturally behaviour of the particle will be
changed from that in Schwarzschild geometry. We compare both the solutions. In
the case of Reissner-Nordstrom black hole there is a possibility of
super-radiance unlike Schwarzschild case. We also check this branch of the
solution.Comment: 8 Latex pages and 4 Figures; RevTex.style; Accepted for Publication
in Classical and Quantum Gravit
Bypass to Turbulence in Hydrodynamic Accretion Disks: An Eigenvalue Approach
Cold accretion disks such as those in star-forming systems, quiescent
cataclysmic variables, and some active galactic nuclei, are expected to have
neutral gas which does not couple well to magnetic fields. The turbulent
viscosity in such disks must be hydrodynamic in origin, not
magnetohydrodynamic. We investigate the growth of hydrodynamic perturbations in
a linear shear flow sandwiched between two parallel walls. The unperturbed flow
is similar to plane Couette flow but with a Coriolis force included. Although
there are no exponentially growing eigenmodes in this system, nevertheless,
because of the non-normal nature of the eigenmodes, it is possible to have a
large transient growth in the energy of perturbations. For a constant angular
momentum disk, we find that the perturbation with maximum growth has a
wave-vector in the vertical direction. The energy grows by more than a factor
of 100 for a Reynolds number R=300 and more than a factor of 1000 for R=1000.
Turbulence can be easily excited in such a disk, as found in previous numerical
simulations. For a Keplerian disk, on the other hand, similar vertical
perturbations grow by no more than a factor of 4, explaining why the same
simulations did not find turbulence in this system. However, certain other
two-dimensional perturbations with no vertical structure do exhibit modest
growth. For the optimum two-dimensional perturbation, the energy grows by a
factor of ~100 for R~10^4.5 and by a factor of 1000 for R~10^6. It is
conceivable that these two-dimensional disturbances might lead to
self-sustained turbulence. The Reynolds numbers of cold astrophysical disks are
much larger even than 10^6, therefore, hydrodynamic turbulence may be possible
in disks.Comment: 39 pages including 9 figures; Final version to appear in The
Astrophysical Journa
Stability of accretion disk around rotating black holes: a pseudo-general-relativistic fluid dynamical study
We discuss the solution of accretion disk when the black hole is chosen to be
rotating. We study, how the fluid properties get affected for different
rotation parameters of the black hole. We know that no cosmic object is static
in Universe. Here the effect of the rotation of the black hole to the
space-time is considered following an earlier work of the author, where the
pseudo-Newtonian potential was prescribed for the Kerr geometry. We show that,
with the inclusion of rotation of the black hole, the valid disk parameter
region dramatically changes and disk becomes unstable. Also we discuss about
the possibility of shock in accretion disk around rotating black holes. When
the black hole is chosen to be rotating, the sonic locations of the accretion
disk get shifted or disappear, making the disk unstable. To bring it in the
stable situation, the angular momentum of the accreting matter has to be
reduced/enhanced (for co/counter-rotating disk) by means of some physical
process.Comment: 24 Latex pages including 7 figures; Accepted for publication in
Astrophysical Journa
Scalar and Spinor Perturbation to the Kerr-NUT Spacetime
We study the scalar and spinor perturbation, namely the Klein-Gordan and
Dirac equations, in the Kerr-NUT space-time. The metric is invariant under the
duality transformation involving the exchange of mass and NUT parameters on one
hand and radial and angle coordinates on the other. We show that this
invariance is also shared by the scalar and spinor perturbation equations.
Further, by the duality transformation, one can go from the Kerr to the dual
Kerr solution, and vice versa, and the same applies to the perturbation
equations. In particular, it turns out that the potential barriers felt by the
incoming scalar and spinor fields are higher for the dual Kerr than that for
the Kerr. We also comment on existence of horizon and singularity.Comment: 31 pages including 20 figures, RevTeX style: Final version to appear
in Classical and Quantum Gravit
Stability of Accretion Disks in Presence of Nucleosynthesis
We study the effect of nuclear reaction on a thin, axisymmetric,
differentially rotating, inviscid, steady accretion flow around a black hole
from an analytical point of view. We find that for most of the reasonable disk
parameters, when -reaction, dissociation of deuterium and helium are taken
into account, the transonic region of the disk continues to have the inner
sonic point and if the temperature of the flow at the injection sonic point
could be raised (by say, some heating processes) the flow would to pass through
this inner sonic point. Otherwise, the flow may be unstable. We use the sonic
point analysis to study the solution. In the rest of the disk parameters the
inner sonic point is absent altogether and the flow will definitely be
unstable.Comment: 18 Latex pages, 6 figures; Accepted for publication in ApJ;
aaspp4.st