115 research outputs found
Méthode de Prédiction de la Capacité de Conservation des Semences
The invention relates to a method for the early evaluation of the preservation capacity of recently harvested seeds and/or of the resistance capacity thereof to an abiotic stress upon germination by quantifying L-isoaspartate residues in said seeds
The actual impedance of non-reflecting boundary conditions : implications for the computation of resonators
Non-reflecting boundary conditions are essential elements in the computation of many compressible flows: such simulations are very sensitive to the treatment of acoustic waves at boundaries. Non-reflecting conditions allow acoustic waves to propagate through boundaries with zero or small levels of reflection into the domain. However, perfectly non-reflecting conditions must be avoided because they can lead to ill-posed problems for the mean flow. Various methods have been proposed to construct boundary conditions which can be sufficiently non-reflecting for the acoustic field while still making the mean-flow problem well posed. This paper analyses a widely-used technique for non-reflecting outlets (Rudy and Strikwerda, Poinsot and Lele). It shows that the correction introduced by these authors can lead to large reflection levels and non-physical resonant behaviors. A simple scaling is proposed to evaluate the relaxation coefficient used in theses methods for a non-reflecting outlet. The proposed scaling is tested for simple cases (ducts) both theoretically and numerically
Plant Seed : A Pertinent Model to Study Aging Processes
Seeds are the major form of dispersal of plants in
the environment. Seeds of many plant species are exceptionally
adapted to harsh environmental conditions provided they are in a
state of desiccation. Spectacular cases of seed longevity have been
reported. Itâs one of the singular case of pluricellular, differentiate
eukaryotic organ able to survive several years in anhydrobiosis.
Plant scientific community explore these fascinating aspects of
seed aging thanks to the immense possibilities now offered to
create/modify plants at a much faster rate and in a more accurate
way than through classical and molecular genetic approaches
and genomic tools. These investigations allowed unveiling seed
specificities against aging processe
Three-dimensional local anisotropy of velocity fluctuations in the solar wind
We analyse velocity fluctuations in the solar wind at magneto-fluid scales in
two datasets, extracted from Wind data in the period 2005-2015, that are
characterised by strong or weak expansion. Expansion affects measurements of
anisotropy because it breaks axisymmetry around the mean magnetic field.
Indeed, the small-scale three-dimensional local anisotropy of magnetic
fluctuations ({\delta}B) as measured by structure functions (SF_B) is
consistent with tube-like structures for strong expansion. When passing to weak
expansion, structures become ribbon-like because of the flattening of SFB along
one of the two perpendicular directions. The power-law index that is consistent
with a spectral slope -5/3 for strong expansion now becomes closer to -3/2.
This index is also characteristic of velocity fluctuations in the solar wind.
We study velocity fluctuations ({\delta}V) to understand if the anisotropy of
their structure functions (SF_V ) also changes with the strength of expansion
and if the difference with the magnetic spectral index is washed out once
anisotropy is accounted for. We find that SF_V is generally flatter than SF_B.
When expansion passes from strong to weak, a further flattening of the
perpendicular SF_V occurs and the small-scale anisotropy switches from
tube-like to ribbon-like structures. These two types of anisotropy, common to
SF_V and SF_B, are associated to distinct large-scale variance anisotropies of
{\delta}B in the strong- and weak-expansion datasets. We conclude that SF_V
shows anisotropic three-dimensional scaling similar to SF_B, with however
systematic flatter scalings, reflecting the difference between global spectral
slopes.Comment: accepted in MNRA
The interplay between helicity and rotation in turbulence: implications for scaling laws and small-scale dynamics
Invariance properties of physical systems govern their behavior: energy
conservation in turbulence drives a wide distribution of energy among modes,
observed in geophysical or astrophysical flows. In ideal hydrodynamics, the
role of helicity conservation (correlation between velocity and its curl,
measuring departures from mirror symmetry) remains unclear since it does not
alter the energy spectrum. However, with solid body rotation, significant
differences emerge between helical and non-helical flows. We first outline
several results, like the energy and helicity spectral distribution and the
breaking of strict universality for the individual spectra. Using massive
numerical simulations, we then show that small-scale structures and their
intermittency properties differ according to whether helicity is present or
not, in particular with respect to the emergence of Beltrami-core vortices
(BCV) that are laminar helical vertical updrafts. These results point to the
discovery of a small parameter besides the Rossby number; this could relate the
problem of rotating helical turbulence to that of critical phenomena, through
renormalization group and weak turbulence theory. This parameter can be
associated with the adimensionalized ratio of the energy to helicity flux to
small scales, the three-dimensional energy cascade being weak and self-similar
Energy spectrum of turbulent fluctuations in boundary driven reduced magnetohydrodynamics
The nonlinear dynamics of a bundle of magnetic flux ropes driven by
stationary fluid motions at their endpoints is studied, by performing numerical
simulations of the magnetohydrodynamic (MHD) equations. The development of MHD
turbulence is shown, where the system reaches a state that is characterized by
the ratio between the Alfven time (the time for incompressible MHD waves to
travel along the field lines) and the convective time scale of the driving
motions. This ratio of time scales determines the energy spectra and the
relaxation toward different regimes ranging from weak to strong turbulence. A
connection is made with phenomenological theories for the energy spectra in MHD
turbulence.Comment: Published in Physics of Plasma
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
Transition to Chaos in a Shell Model of Turbulence
We study a shell model for the energy cascade in three dimensional turbulence
at varying the coefficients of the non-linear terms in such a way that the
fundamental symmetries of Navier-Stokes are conserved. When a control parameter
related to the strength of backward energy transfer is enough small,
the dynamical system has a stable fixed point corresponding to the Kolmogorov
scaling. This point becomes unstable at where a stable
limit cycle appears via a Hopf bifurcation. By using the bi-orthogonal
decomposition, the transition to chaos is shown to follow the Ruelle-Takens
scenario. For the dynamical evolution is intermittent
with a positive Lyapunov exponent. In this regime, there exists a strange
attractor which remains close to the Kolmogorov (now unstable) fixed point, and
a local scaling invariance which can be described via a intermittent
one-dimensional map.Comment: 16 pages, Tex, 20 figures available as hard cop
Spontaneous non-steady magnetic reconnection within the solar environment
This work presents a 2.5-dimensional simulation study of the instability of
current-sheets located in a medium with a strong density variation along the
current layer. The initial force-free configuration is observed to undergo a
two-stage evolution consisting of an abrupt regime transition from a slow to a
fast reconnection process leading the system to a final chaotic configuration.
Yet, the onset of the fast phase is not determined by the presence of any
anomalous enhancement in plasma's local resistivity, but rather is the result
of a new mechanism discovered in Lapenta (2008)* and captured only when
sufficient resolution is present. Hence, the effects of the global resistivity,
the global viscosity and the plasma beta on the overall dynamics are
considered. This mechanism allowing the transition from slow to fast
reconnection provides a simple but effective model of several processes taking
place within the solar atmosphere from the high chromosphere up to the low
corona. In fact, the understanding of a spontaneous transition to a
self-feeding fast reconnection regime as well as its macroscopic evolution is
the first and fundamental step to produce realistic models of all those
phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008,
Phys. Rev. Lett., 100, 235001).Comment: 29 pages, 10 figure
MADOR: A NEW TOOL TO CALCULATE DECREASE OF EFFECTIVE DOSES IN HUMAN AFTER DTPA THERAPY
Abstract models have been developed to describe dissolution of Pu/Am/Cm after internal contamination by inhalation or wound, chelation of actinides by diethylene triamine penta acetic acid (DTPA) in different retention compartments and excretion of actinide-DTPA complexes. After coupling these models with those currently used for dose calculation, the modelling approach was assessed by fitting human data available in IDEAS database. Good fits were obtained for most studied cases, but further experimental studies are needed to validate some modelling hypotheses as well as the range of parameter values. From these first results, radioprotection tools are being developed: MAnagement of DOse Reduction after DTPA therapy
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