38,975 research outputs found
Linear stability analysis of magnetized relativistic jets: the nonrotating case
We perform a linear analysis of the stability of a magnetized relativistic
non-rotating cylindrical flow in the aproximation of zero thermal pressure,
considering only the m = 1 mode. We find that there are two modes of
instability: Kelvin-Helmholtz and current driven. The Kelvin-Helmholtz mode is
found at low magnetizations and its growth rate depends very weakly on the
pitch parameter. The current driven modes are found at high magnetizations and
the value of the growth rate and the wavenumber of the maximum increase as we
decrease the pitch parameter. In the relativistic regime the current driven
mode is splitted in two branches, the branch at high wavenumbers is
characterized by the eigenfunction concentrated in the jet core, the branch at
low wavenumbers is instead characterized by the eigenfunction that extends
outside the jet velocity shear region.Comment: 22 pages, 13 figures, MNRAS in pres
Performance Analysis and Design of Maximum Ratio Combining in Channel-Aware MIMO Decision Fusion
In this paper we present a theoretical performance analysis of the maximum
ratio combining (MRC) rule for channel-aware decision fusion over
multiple-input multiple-output (MIMO) channels for (conditionally) dependent
and independent local decisions. The system probabilities of false alarm and
detection conditioned on the channel realization are derived in closed form and
an approximated threshold choice is given. Furthermore, the channel-averaged
(CA) performances are evaluated in terms of the CA system probabilities of
false alarm and detection and the area under the receiver operating
characteristic (ROC) through the closed form of the conditional moment
generating function (MGF) of the MRC statistic, along with Gauss-Chebyshev (GC)
quadrature rules. Furthermore, we derive the deflection coefficients in closed
form, which are used for sensor threshold design. Finally, all the results are
confirmed through Monte Carlo simulations.Comment: To appear in IEEE Transactions on Wireless Communication
Linear and nonlinear evolution of current-carrying highly magnetized jets
We investigate the linear and nonlinear evolution of current-carrying jets in
a periodic configuration by means of high resolution three-dimensional
numerical simulations. The jets under consideration are strongly magnetized
with a variable pitch profile and initially in equilibrium under the action of
a force-free magnetic field. The growth of current-driven (CDI) and
Kelvin-Helmholtz (KHI) instabilities is quantified using three selected cases
corresponding to static, Alfvenic and super-Alfvenic jets.
During the early stages, we observe large-scale helical deformations of the
jet corresponding to the growth of the initially excited CDI mode. A direct
comparison between our simulation results and the analytical growth rates
obtained from linear theory reveals good agreement on condition that
high-resolution and accurate discretization algorithms are employed.
After the initial linear phase, the jet structure is significantly altered
and, while slowly-moving jets show increasing helical deformations, larger
velocity shear are violently disrupted on a few Alfven crossing time leaving a
turbulent flow structure. Overall, kinetic and magnetic energies are quickly
dissipated into heat and during the saturated regime the jet momentum is
redistributed on a larger surface area with most of the jet mass travelling at
smaller velocities. The effectiveness of this process is regulated by the onset
of KHI instabilities taking place at the jet/ambient interface and can be held
responsible for vigorous jet braking and entrainment.Comment: 14 pages, 11 figure
On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
We consider the problem of convergence in stratified isothermal shearing
boxes with zero net magnetic flux. We present results with the highest
resolution to-date--up to 200 grid-point per pressure scale height--that show
no clear evidence of convergence. Rather, the Maxwell stresses continue to
decrease with increasing resolution. We propose some possible scenarios to
explain the lack of convergence based on multi-layer dynamo systems.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
Fully Convective Magnetorotational Turbulence in Stratified Shearing Boxes
We present a numerical study of turbulence and dynamo action in stratified
shearing boxes with zero magnetic flux. We assume that the fluid obeys the
perfect gas law and has finite (constant) thermal diffusivity. We choose
radiative boundary conditions at the vertical boundaries in which the heat flux
is propor- tional to the fourth power of the temperature. We compare the
results with the corresponding cases in which fixed temperature boundary
conditions are applied. The most notable result is that the formation of a
fully convective state in which the density is nearly constant as a function of
height and the heat is transported to the upper and lower boundaries by
overturning motions is robust and persists even in cases with radiative
boundary conditions. Interestingly, in the convective regime, although the
diffusive transport is negligible the mean stratification does not relax to an
adiabatic state.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence
We examine the relationship between magnetic flux generation, taken as an
indicator of large-scale dynamo action, and magnetic helicity, computed as an
integral over the dynamo volume, in a simple dynamo. We consider dynamo action
driven by Magneto-Rotational Turbulence (MRT) within the shearing-box
approximation. We consider magnetically open boundary conditions that allow a
flux of helicity in or out of the computational domain. We circumvent the
problem of the lack of gauge invariance in open domains by choosing a
particular gauge -- the winding gauge -- that provides a natural interpretation
in terms of average winding number of pairwise field lines. We use this gauge
precisely to define and measure the helicity and helicity flux for several
realizations of dynamo action. We find in these cases, that the system as a
whole does not break reflectional symmetry and the total helicity remains small
even in cases when substantial magnetic flux is generated. We find no
particular connection between the generation of magnetic flux and the helicity
or the helicity flux through the boundaries. We suggest that this result may be
due to the essentially nonlinear nature of the dynamo processes in MRT.Comment: 26 pages, 10 figures, ApJ accepte
Feasibility experiments on time-resolved fluorosensing applied to oil slicks
The introduction of time resolved observations can provide a very penetrating tool in the practice of laser fluorosensing. The investigations have demonstrated a relevance of multispectral, time resolved analysis for oil fingerprinting. By comparative studies on a variety of crude oils and their most significant fractions, it was found that the process of time decay in a composite oil is characterized by a few steps, which are associated with specific components in the medium light range. The average decay times of these pure fractions are markedly differentiated as to absolute values and spectral spread; as a consequence, the corresponding parameters in the resultant crude are quite sensitive to the particular mixture of these components. Measurements of the time response give then a finer discrimination between oil classes, depending on the relative content of certain fractions. Experiments were pursued with an improved fluorosensor facility, in order to test the application of time resolved fluorosensing to remote samples on water
Making Fanaroff-Riley I radio sources. Numerical Hydrodynamic 3D Simulations of Low Power Jets
Extragalactic radio sources have been classified into two classes,
Fanaroff-Riley I and II, which differ in morphology and radio power. Strongly
emitting sources belong to the edge-brightened FR II class, and weakly emitting
sources to the edge-darkened FR I class. The origin of this dichotomy is not
yet fully understood. Numerical simulations are successful in generating FR II
morphologies, but they fail to reproduce the diffuse structure of FR Is.
By means of hydro-dynamical 3D simulations of supersonic jets, we investigate
how the displayed morphologies depend on the jet parameters. Bow shocks and
Mach disks at the jet head, which are probably responsible for the hot spots in
the FR II sources, disappear for a jet kinetic power L_kin < 10^43 erg/s. This
threshold compares favorably with the luminosity at which the FR I/FR II
transition is observed.
The problem is addressed by numerical means carrying out 3D HD simulations of
supersonic jets that propagate in a non-homogeneous medium with the ambient
temperature that increases with distance from the jet origin, which maintains
constant pressure.
The jet energy in the lower power sources, instead of being deposited at the
terminal shock, is gradually dissipated by the turbulence. The jets spread out
while propagating, and they smoothly decelerate while mixing with the ambient
medium and produce the plumes characteristic of FR I objects.
Three-dimensionality is an essential ingredient to explore the FR I evolution
because the properties of turbulence in two and three dimensions are very
different, since there is no energy cascade to small scales in two dimensions,
and two-dimensional simulations with the same parameters lead to FRII-like
behavior.Comment: 11 pages, 12 figures, to appear on A&
The Rainbow Prim Algorithm for Selecting Putative Orthologous Protein Sequences
We present a selection method designed for eliminating species redundancy in clusters of putative orthologous sequences, to be applied as a post-processing procedure to pre-clustered data obtained from other methods. The algorithm can always zero-out the cluster redundancy while preserving the number of species of the original cluster
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