21,558 research outputs found
Seismic Fault Preserving Diffusion
This paper focuses on the denoising and enhancing of 3-D reflection seismic
data. We propose a pre-processing step based on a non linear diffusion
filtering leading to a better detection of seismic faults. The non linear
diffusion approaches are based on the definition of a partial differential
equation that allows us to simplify the images without blurring relevant
details or discontinuities. Computing the structure tensor which provides
information on the local orientation of the geological layers, we propose to
drive the diffusion along these layers using a new approach called SFPD
(Seismic Fault Preserving Diffusion). In SFPD, the eigenvalues of the tensor
are fixed according to a confidence measure that takes into account the
regularity of the local seismic structure. Results on both synthesized and real
3-D blocks show the efficiency of the proposed approach.Comment: 10 page
Remarks on nonlinear relation among phases and frequencies in modulational instabilities of parallel propagating Alfven waves
Nonlinear relations among frequencies and phases in modulational instability
of circularly polarized Alfven waves are discussed, within the context of one
dimensional, dissipation-less, unforced fluid system. We show that generation
of phase coherence is a natural consequence of the modulational instability of
Alfven waves. Furthermore, we quantitatively evaluate intensity of wave-wave
interaction by using bi-coherence, and also by computing energy flow among wave
modes, and demonstrate that the energy flow is directly related to the phase
coherence generation.Comment: 17 pages, Nonlinear Processes in Geophysics (published), the paper
with full resolution images is
http://www.copernicus.org/EGU/npg/13/npg-13-425.pd
Damping of Growth Oscillations in Molecular Beam Epitaxy: A Renormalization Group Approach
The conserved Sine-Gordon Equation with nonconserved shot noise is used to
model homoepitaxial crystal growth. With increasing coverage the renormalized
pinning potential changes from strong to weak. This is interpreted as a
transition from layer-by-layer to rough growth. The associated length and time
scales are identified, and found to agree with recent scaling arguments. A
heuristically postulated nonlinear term is created
under renormalization.Comment: 17 Pages Late
2D Simulations of the Line-Driven Instability in Hot-Star Winds: II. Approximations for the 2D Radiation Force
We present initial attempts to include the multi-dimensional nature of
radiation transport in hydrodynamical simulations of the small-scale structure
that arises from the line-driven instability in hot-star winds. Compared to
previous 1D or 2D models that assume a purely radial radiation force, we seek
additionally to treat the lateral momentum and transport of diffuse
line-radiation, initially here within a 2D context. A key incentive is to study
the damping effect of the associated diffuse line-drag on the dynamical
properties of the flow, focusing particularly on whether this might prevent
lateral break-up of shell structures at scales near the lateral Sobolev angle
of ca. . We first explore nonlinear simulations that cast the
lateral diffuse force in the simple, local form of a parallel viscosity.
Second, to account for the lateral mixing of radiation associated with the
radial driving, we next explore models in which the radial force is azimuthally
smoothed over a chosen scale. Third, to account for both the lateral line-drag
and the lateral mixing in a more self-consistent way, we explore further a
method first proposed by Owocki (1999), which uses a restricted 3-ray approach
that combines a radial ray with two oblique rays set to have an impact
parameter within the stellar core. From numerical simulations,
we find that, compared to equivalent 1-ray simulations, the high-resolution
3-ray models show systematically a much higher lateral coherence.... (Full
abstract in paper)Comment: Accepted by A&A, 12 pages, 7 figures, 3 only shown in version
available at http://www.mpa-garching.mpg.de/~luc/2778.ps.g
Particle transport and heating in the microturbulent precursor of relativistic shocks
Collisionless relativistic shocks have been the focus of intense theoretical
and numerical investigations in recent years. The acceleration of particles,
the generation of electromagnetic microturbulence and the building up of a
shock front are three interrelated essential ingredients of a relativistic
collisionless shock wave. In this paper we investigate two issues of importance
in this context: (1) the transport of suprathermal particles in the excited
microturbulence upstream of the shock and its consequences regarding particle
acceleration; (2) the preheating of incoming background electrons as they cross
the shock precursor and experience relativistic oscillations in the
microturbulent electric fields. We place emphasis on the importance of the
motion of the electromagnetic disturbances relatively to the background plasma
and to the shock front. This investigation is carried out for the two major
instabilities involved in the precursor of relativistic shocks, the
filamentation instability and the oblique two stream instability. Finally, we
use our results to discuss the maximal acceleration at the external shock of a
gamma-ray burst; we find in particular a maximal synchrotron photon energy of
the order of a few GeV.Comment: 14 pages, 6 figures. Revised versio
On the transition to turbulence of wall-bounded flows in general, and plane Couette flow in particular
The main part of this contribution to the special issue of EJM-B/Fluids
dedicated to Patrick Huerre outlines the problem of the subcritical transition
to turbulence in wall-bounded flows in its historical perspective with emphasis
on plane Couette flow, the flow generated between counter-translating parallel
planes. Subcritical here means discontinuous and direct, with strong
hysteresis. This is due to the existence of nontrivial flow regimes between the
global stability threshold Re_g, the upper bound for unconditional return to
the base flow, and the linear instability threshold Re_c characterized by
unconditional departure from the base flow. The transitional range around Re_g
is first discussed from an empirical viewpoint ({\S}1). The recent
determination of Re_g for pipe flow by Avila et al. (2011) is recalled. Plane
Couette flow is next examined. In laboratory conditions, its transitional range
displays an oblique pattern made of alternately laminar and turbulent bands, up
to a third threshold Re_t beyond which turbulence is uniform. Our current
theoretical understanding of the problem is next reviewed ({\S}2): linear
theory and non-normal amplification of perturbations; nonlinear approaches and
dynamical systems, basin boundaries and chaotic transients in minimal flow
units; spatiotemporal chaos in extended systems and the use of concepts from
statistical physics, spatiotemporal intermittency and directed percolation,
large deviations and extreme values. Two appendices present some recent
personal results obtained in plane Couette flow about patterning from numerical
simulations and modeling attempts.Comment: 35 pages, 7 figures, to appear in Eur. J. Mech B/Fluid
Improving Fiber Alignment in HARDI by Combining Contextual PDE Flow with Constrained Spherical Deconvolution
We propose two strategies to improve the quality of tractography results
computed from diffusion weighted magnetic resonance imaging (DW-MRI) data. Both
methods are based on the same PDE framework, defined in the coupled space of
positions and orientations, associated with a stochastic process describing the
enhancement of elongated structures while preserving crossing structures. In
the first method we use the enhancement PDE for contextual regularization of a
fiber orientation distribution (FOD) that is obtained on individual voxels from
high angular resolution diffusion imaging (HARDI) data via constrained
spherical deconvolution (CSD). Thereby we improve the FOD as input for
subsequent tractography. Secondly, we introduce the fiber to bundle coherence
(FBC), a measure for quantification of fiber alignment. The FBC is computed
from a tractography result using the same PDE framework and provides a
criterion for removing the spurious fibers. We validate the proposed
combination of CSD and enhancement on phantom data and on human data, acquired
with different scanning protocols. On the phantom data we find that PDE
enhancements improve both local metrics and global metrics of tractography
results, compared to CSD without enhancements. On the human data we show that
the enhancements allow for a better reconstruction of crossing fiber bundles
and they reduce the variability of the tractography output with respect to the
acquisition parameters. Finally, we show that both the enhancement of the FODs
and the use of the FBC measure on the tractography improve the stability with
respect to different stochastic realizations of probabilistic tractography.
This is shown in a clinical application: the reconstruction of the optic
radiation for epilepsy surgery planning
Spatio-temporal evolution of the nonresonant instability in shock precursors of young supernova remnants
A nonresonant cosmic-ray-current-driven instability may operate in the shock
precursors of young supernova remnants and be responsible for magnetic-field
amplification, plasma heating and turbulence. Earlier simulations demonstrated
magnetic-field amplification, and in kinetic studies a reduction of the
relative drift between cosmic rays and thermal plasma was observed as
backreaction. However, all published simulations used periodic boundary
conditions, which do not account for mass conservation in decelerating flows
and only allow the temporal development to be studied. Here we report results
of fully kinetic Particle-In-Cell simulations with open boundaries that permit
inflow of plasma on one side of the simulation box and outflow at the other
end, hence allowing an investigation of both the temporal and the spatial
development of the instability. Magnetic-field amplification proceeds as in
studies with periodic boundaries and, observed here for the first time, the
reduction of relative drifts causes the formation of a shock-like compression
structure at which a fraction of the plasma ions are reflected. Turbulent
electric field generated by the nonresonant instability inelastically scatters
cosmic rays, modifying and anisotropizing their energy distribution. Spatial CR
scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads
to significant nonadiabatic heating of the background plasma maintaining bulk
equipartition between ions and electrons. The highest temperatures are reached
at sites of large-amplitude electrostatic fields. Ion spectra show
supra-thermal tails resulting from stochastic scattering in the turbulent
electric field. Together, these modifications in the plasma flow will affect
the properties of the shock and particle acceleration there.Comment: Accepted for publication in MNRAS. 16 pages, 15 figure
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