1,449 research outputs found
A three-dimensional full Stokes model of the grounding line dynamics: effect of a pinning point beneath the ice shelf
The West Antarctic ice sheet is confined by a large area of ice shelves, fed by inland ice through fast flowing ice streams. The dynamics of the grounding line, which is the line-boundary between grounded ice and the downstream ice shelf, has a major influence on the dynamics of the whole ice sheet. However, most ice sheet models use simplifications of the flow equations, as they do not include all the stress components, and are known to fail in their representation of the grounding line dynamics. Here, we present a 3-D full Stokes model of a marine ice sheet, in which the flow problem is coupled with the evolution of the upper and lower free surfaces, and the position of the grounding line is determined by solving a contact problem between the shelf/sheet lower surface and the bedrock. Simulations are performed using the open-source finite-element code Elmer/Ice within a parallel environment. The model's ability to cope with a curved grounding line and the effect of a pinning point beneath the ice shelf are investigated through prognostic simulations. Starting from a steady state, the sea level is slightly decreased to create a contact point between a seamount and the ice shelf. The model predicts a dramatic decrease of the shelf velocities, leading to an advance of the grounding line until both grounded zones merge together, during which an ice rumple forms above the contact area at the pinning point. Finally, we show that once the contact is created, increasing the sea level to its initial value does not release the pinning point and has no effect on the ice dynamics, indicating a stabilising effect of pinning points
Avalanche risk evaluation and protective dam optimal design using extreme value statistics
International audienceIn snow avalanche long-term forecasting, existing risk-based methods remain difficult to use in a real engineering context. In this work, we expand a quasi analytical decisional model to obtain simple formulae to quantify risk and to perform the optimal design of an avalanche dam in a quick and efficient way. Specifically, the exponential runout model is replaced by the Generalized Pareto distribution (GPD), which has theoretical justifications that promote its use for modelling the different possible runout tail behaviours. Regarding the defence structure/flow interaction, a simple law based on kinetic energy dissipation is compared with a law based on the volume stored upstream of the dam, whose flexibility allows us to cope with various types of snow. We show how a detailed sensitivity study can be conducted, leading to intervals and bounds for risk estimates and optimal design values. Application to a typical case study from the French Alps, highlights potential operational difficulties and how they can be tackled. For instance, the highest sensitivity to the runout tail type and interaction law is found at abscissas of legal importance for hazard zoning (return periods of 10-1000 a), a crucial result for practical purposes
Growth rate degeneracies in kinematic dynamos
We consider the classical problem of kinematic dynamo action in simple steady flows. Due to the adjointness of the induction operator, we show that the growth rate of the dynamo will be exactly the same for two types of magnetic boundary conditions: the magnetic field can be normal (infinite magnetic permeability, also called pseudovacuum) or tangent (perfect electrical conductor) to the boundaries of the domain. These boundary conditions correspond to well-defined physical limits often used in numerical models and relevant to laboratory experiments. The only constraint is for the velocity field u to be reversible, meaning there exists a transformation changing u into âu. We illustrate this surprising property using S2T2 type of flows in spherical geometry inspired by [Dudley and James, Proc. R. Soc. London A 425, 407 (1989)]. Using both types of boundary conditions, it is shown that the growth rates of the dynamos are identical, although the corresponding magnetic eigenmodes are drastically different
Classification of African ecosystems at 1 km resolution using multiannual SPOT/VEGETATION data and a hybrid clustering approach
Ecosystems classification is the process of allocating vegetation types into groups so that individuals in the same class are similar according to their physiological and phonological characteristics to another one. Over large areas, the only suitable technique to obtain frequent and repetitive data acquisitions over such large areas is the use of observations recorded by sensors of moderate resolution. In order to minimize the role of the analyst and to improve the accuracy of the results, innovative and efficient approaches for the classification of ecosystems continue to appear in the literature. This research developed and implemented a new hybrid unsupervised classification approach to derive ecosystems using multi-annual time series by combining hierarchical and partitioning clustering principles. The latter approach is applied on 8-years time series (2000-2007) of 10-day composite Normalized Difference Vegetation Index (NDVI) recorded by SPOT/VEGETATION. After the first segmentation of the mainland in ecoregions using the Fast Fourier Transform (FFT), successive k-nearest neighbor (k-NN) clustering enhance the discrimination of ecosystems and yields to the production of a new ecosystem map for the African continent. The nomenclature relied on the Land Cover Classification System (LCCS) of the Food and Agricultural Organization (FAO). On the basis of validated continental, a pixel-by-pixel analysis is conducted to assess the accuracy of the new classification. The hybrid clustering facilitates the identification/labeling process and the obtained results which should provide key information needed for management/monitoring of natural resources, biodiversity conservation and biogeochemical studies may also deserve vegetation cover modeling at regional and local scal
High level software for 4.8 GHz LHC Schottky system
The performance of the LHC depends critically on the accurate measurements of
the betatron tunes. The betatron tune values of each LHC beam may be measured
without excitation using a newly installed transverse Schottky monitor. A
high-level software package written in Java has been developed for the Schottky
system. The software allows end users to monitor and control the Schottky
system, and provides them with non-destructive and continuous bunch-by-bunch
measurements for the tunes, momentum spreads, chromaticities and emittances of
the LHC beams. It has been tested with both proton and lead ion beams at the
LHC with very successful results.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
Thermique et Solidification
International audienceNous montrons l'importance des phĂ©nomĂšnes thermiques dans tous les aspects de la solidification: - sur le plan thĂ©orique, en dĂ©montrant que l'Ă©cart Ă la tempĂ©rature d'Ă©quilibre est le moteur de la solidification - sur le plan expĂ©rimental, en prĂ©sentant les diverses mesures thermiques qui permettent de contrĂŽler la croissance - sur le plan technique, oĂč les calculs de thermique sont Ă la base du dimensionnement des appareillages - au niveau de la simulation numĂ©rique des procĂ©dĂ©s oĂč les codes de thermique sont trĂšs largement employĂ©
Microsecond time-resolved X-ray diffraction for the investigation of fatigue behavior during ultrasonic fatigue loading
International audienceA new method based on time-resolved X-ray diffraction is proposed in order to measure the elastic strain and stress during ultrasonic fatigue loading experiments. Pure Cu was chosen as an example material for the experiments using a 20â
kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. A two-dimensional hybrid pixel X-ray detector (XPAD3.2) was triggered by the strain gage signal in a synchronous data acquisition scheme (pumpâprobe-like). The method enables studying loading cycles with a period of 50â
”s, achieving a temporal resolution of 1â
”s. This allows a precise reconstruction of the diffraction patterns during the loading cycles. From the diffraction patterns, the position of the peaks, their shifts and their respective broadening can be deduced. The diffraction peak shift allows the elastic lattice strain to be estimated with a resolution of âŒ10â5. Stress is calculated by the self-consistent scale-transition model through which the elastic response of the material is estimated. The amplitudes of the cyclic stresses range from 40 to 120â
MPa and vary linearly with respect to the displacement applied by the ultrasonic machine. Moreover, the experimental results highlight an increase of the diffraction peak broadening with the number of applied cycles
Quasi-static magnetohydrodynamic turbulence at high Reynolds number
We analyse the anisotropy of homogeneous turbulence in an electrically
conducting fluid submitted to a uniform magnetic field, for low magnetic
Reynolds number, in the quasi- static approximation. We interpret disagreeing
previous predictions between linearized theory and simulations: in the linear
limit, the kinetic energy of transverse velocity components, normal to the
magnetic field, decays faster than the kinetic energy of the axial component,
along the magnetic field (Moffatt (1967)); whereas many numerical studies
predict a final state characterised by dominant energy of transverse velocity
components. We investigate the corresponding nonlinear phenomenon using Direct
Numerical Simulations of freely-decaying turbulence, and a two-point
statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian
model. The transition from the three-dimensional turbulent flow to a
"two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of
the combined effects of short-time linear Joule dissipation and longer time
nonlinear creation of polarisation anisotropy. It is this combination of linear
and nonlinear effects which explains the disagreement between predictions from
linearized theory and results from numerical simulations. The transition is
characterized by the elongation of turbulent structures along the applied
magnetic field, and by the strong anisotropy of directional two-point
correlation spectra, in agreement with experimental evidence. Inertial
equatorial transfers in both DNS and the model are presented to describe in
detail the most important equilibrium dynamics. Spectral scalings are
maintained in high Reynolds number turbulence attainable only with the EDQNM
model, which also provides simplified modelling of the asymptotic state of
quasi-static MHD turbulence.Comment: Journal of Fluid Mechanics, 201
On the existence and structure of a mush at the inner core boundary of the Earth
It has been suggested about 20 years ago that the liquid close to the inner
core boundary (ICB) is supercooled and that a sizable mushy layer has developed
during the growth of the inner core. The morphological instability of the
liquid-solid interface which usually results in the formation of a mushy zone
has been intensively studied in metallurgy, but the freezing of the inner core
occurs in very unusual conditions: the growth rate is very small, and the
pressure gradient has a key role, the newly formed solid being hotter than the
adjacent liquid. We investigate the linear stability of a solidification front
under such conditions, pointing out the destabilizing role of the thermal and
solutal fields, and the stabilizing role of the pressure gradient. The main
consequence of the very small solidification rate is the importance of
advective transport of solute in liquid, which tends to remove light solute
from the vicinity of the ICB and to suppress supercooling, thus acting against
the destabilization of the solidification front. For plausible phase diagrams
of the core mixture, we nevertheless found that the ICB is likely to be
morphologically unstable, and that a mushy zone might have developed at the
ICB. The thermodynamic thickness of the resulting mushy zone can be
significant, from km to the entire inner core radius, depending on
the phase diagram of the core mixture. However, such a thick mushy zone is
predicted to collapse under its own weight, on a much smaller length scale
( km). We estimate that the interdendritic spacing is probably
smaller than a few tens of meter, and possibly only a few meters
Defects in TRPM7 channel function deregulate thrombopoiesis through altered cellular Mg2+ homeostasis and cytoskeletal architecture
Mg2+ plays a vital role in platelet function, but despite implications for life-threatening conditions such as stroke or myocardial infarction, the mechanisms controlling [Mg2+](i) in megakaryocytes (MKs) and platelets are largely unknown. Transient receptor potential melastatin-like 7 channel (TRPM7) is a ubiquitous, constitutively active cation channel with a cytosolic alpha-kinase domain that is critical for embryonic development and cell survival. Here we report that impaired channel function of TRPM7 in MKs causes macrothrombocytopenia in mice (Trpm7(fl/fl-Pf4Cre)) and likely in several members of a human pedigree that, in addition, suffer from atrial fibrillation. The defect in platelet biogenesis is mainly caused by cytoskeletal alterations resulting in impaired proplatelet formation by Trpm7(fl/fl-Pf4Cre) MKs, which is rescued by Mg2+ supplementation or chemical inhibition of non-muscle myosin IIA heavy chain activity. Collectively, our findings reveal that TRPM7 dysfunction may cause macrothrombocytopenia in humans and mice
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