940 research outputs found
On the decay of turbulence in plane Couette flow
The decay of turbulent and laminar oblique bands in the lower transitional
range of plane Couette flow is studied by means of direct numerical simulations
of the Navier--Stokes equations. We consider systems that are extended enough
for several bands to exist, thanks to mild wall-normal under-resolution
considered as a consistent and well-validated modelling strategy. We point out
a two-stage process involving the rupture of a band followed by a slow
regression of the fragments left. Previous approaches to turbulence decay in
wall-bounded flows making use of the chaotic transient paradigm are
reinterpreted within a spatiotemporal perspective in terms of large deviations
of an underlying stochastic process.Comment: ETC13 Conference Proceedings, 6 pages, 5 figure
Finding instabilities in the community structure of complex networks
The problem of finding clusters in complex networks has been extensively
studied by mathematicians, computer scientists and, more recently, by
physicists. Many of the existing algorithms partition a network into clear
clusters, without overlap. We here introduce a method to identify the nodes
lying ``between clusters'' and that allows for a general measure of the
stability of the clusters. This is done by adding noise over the weights of the
edges of the network. Our method can in principle be applied with any
clustering algorithm, provided that it works on weighted networks. We present
several applications on real-world networks using the Markov Clustering
Algorithm (MCL).Comment: 4 pages, 5 figure
Extension of charge-state-distribution calculations for ion-solid collisions towards low velocities and many-electron ions
Knowledge of the detailed evolution of the whole charge-state distribution of projectile ions colliding with targets is required in several fields of research such as material science and atomic and nuclear physics but also in accelerator physics, and in particular in regard to the several foreseen large-scale facilities. However, there is a lack of data for collisions in the nonperturbative energy domain and that involve many-electron projectiles. Starting from the etacha model we developed [Rozet, Nucl. Instrum. Methods Phys. Res., Sect. B 107, 67 (1996)10.1016/0168-583X(95)00800-4], we present an extension of its validity domain towards lower velocities and larger distortions. Moreover, the system of rate equations is able to take into account ions with up to 60 orbital states of electrons. The computed data from the different new versions of the etacha code are compared to some test collision systems. The improvements made are clearly illustrated by 28.9MeVu-1Pb56+ ions, and laser-generated carbon ion beams of 0.045 to 0.5MeVu-1, passing through carbon or aluminum targets, respectively. Hence, those new developments can efficiently sustain the experimental programs that are currently in progress on the "next-generation" accelerators or laser facilities.Fil: Lamour, E.. Centre National de la Recherche Scientifique; Francia. Universite de Paris; FranciaFil: Fainstein, Pablo Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. ComisiĂłn Nacional de EnergĂa AtĂłmica. Centro AtĂłmico Bariloche; ArgentinaFil: Galassi, Mariel Elisa. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de FĂsica de Rosario. Universidad Nacional de Rosario. Instituto de FĂsica de Rosario; ArgentinaFil: Prigent, C.. Centre National de la Recherche Scientifique; Francia. Universite de Paris; FranciaFil: Ramirez, C. A.. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de FĂsica de Rosario. Universidad Nacional de Rosario. Instituto de FĂsica de Rosario; ArgentinaFil: Rivarola, Roberto Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de FĂsica de Rosario. Universidad Nacional de Rosario. Instituto de FĂsica de Rosario; ArgentinaFil: Rozet, J. P.. Centre National de la Recherche Scientifique; Francia. Universite de Paris; FranciaFil: Trassinelli, M.. Centre National de la Recherche Scientifique; Francia. Universite de Paris; FranciaFil: Vernhet, D.. Centre National de la Recherche Scientifique; Francia. Universite de Paris; Franci
Pattern fluctuations in transitional plane Couette flow
In wide enough systems, plane Couette flow, the flow established between two
parallel plates translating in opposite directions, displays alternatively
turbulent and laminar oblique bands in a given range of Reynolds numbers R. We
show that in periodic domains that contain a few bands, for given values of R
and size, the orientation and the wavelength of this pattern can fluctuate in
time. A procedure is defined to detect well-oriented episodes and to determine
the statistics of their lifetimes. The latter turn out to be distributed
according to exponentially decreasing laws. This statistics is interpreted in
terms of an activated process described by a Langevin equation whose
deterministic part is a standard Landau model for two interacting complex
amplitudes whereas the noise arises from the turbulent background.Comment: 13 pages, 11 figures. Accepted for publication in Journal of
statistical physic
Transient growth in Taylor-Couette flow
Transient growth due to non-normality is investigated for the Taylor-Couette
problem with counter-rotating cylinders as a function of aspect ratio eta and
Reynolds number Re. For all Re < 500, transient growth is enhanced by
curvature, i.e. is greater for eta < 1 than for eta = 1, the plane Couette
limit. For fixed Re < 130 it is found that the greatest transient growth is
achieved for eta between the Taylor-Couette linear stability boundary, if it
exists, and one, while for Re > 130 the greatest transient growth is achieved
for eta on the linear stability boundary. Transient growth is shown to be
approximately 20% higher near the linear stability boundary at Re = 310, eta =
0.986 than at Re = 310, eta = 1, near the threshold observed for transition in
plane Couette flow. The energy in the optimal inputs is primarily meridional;
that in the optimal outputs is primarily azimuthal. Pseudospectra are
calculated for two contrasting cases. For large curvature, eta = 0.5, the
pseudospectra adhere more closely to the spectrum than in a narrow gap case,
eta = 0.99
Modeling transitional plane Couette flow
The Galerkin method is used to derive a realistic model of plane Couette flow
in terms of partial differential equations governing the space-time dependence
of the amplitude of a few cross-stream modes. Numerical simulations show that
it reproduces the globally sub-critical behavior typical of this flow. In
particular, the statistics of turbulent transients at decay from turbulent to
laminar flow displays striking similarities with experimental findings.Comment: 33 pages, 10 figure
Electronic temperatures, densities and plasma X-ray emission of a 14.5 GHz Electron-Cyclotron Resonance Ion Source
We have performed a systematic study of the Bremsstrahlung emission from the
electrons in the plasma of a commercial 14.5 GHz Electron-Cyclotron Resonance
Ion Source. The electronic spectral temperature and the product of ionic and
electronic densities of the plasma are measured by analyzing the Bremsstrahlung
spectra recorded for several rare gases (Ar, Kr, Xe) as a function of the
injected power. Within our uncertainty, we find an average temperature of ? 48
keV above 100W, with a weak dependency on the injected power and gas
composition. Charge state distributions of extracted ion beams have been
determined as well, providing a way to disentangle the ionic density from the
electronic density. Moreover X-ray emission from highly charged argon ions in
the plasma has been observed with a high-resolution mosaic crystal
spectrometer, demonstrating the feasibility for high-precision measurements of
transition energies of highly charged ions, in particular of the magnetic
dipole (M1) transition of He-like of argon ions
Global downscaling of remotely sensed soil moisture using neural networks
Characterizing soil moisture at spatiotemporal scales relevant to land surface processes (i.e.,
of the order of 1 km) is necessary in order to quantify its role in regional
feedbacks between the land surface and the atmospheric boundary layer.
Moreover, several applications such as agricultural management can benefit
from soil moisture information at fine spatial scales. Soil moisture
estimates from current satellite missions have a reasonably good temporal
revisit over the globe (2–3-day repeat time); however, their finest spatial
resolution is 9 km. NASA's Soil Moisture Active Passive (SMAP) satellite has
estimated soil moisture at two different spatial scales of 36 and 9 km since
April 2015. In this study, we develop a neural-network-based downscaling
algorithm using SMAP observations and disaggregate soil moisture to 2.25 km
spatial resolution. Our approach uses the mean monthly Normalized Differenced
Vegetation Index (NDVI) as ancillary data to quantify the subpixel
heterogeneity of soil moisture. Evaluation of the downscaled soil moisture
estimates against in situ observations shows that their accuracy is better
than or equal to the SMAP 9 km soil moisture estimates.</p
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