Publications récentes
HAL : Dernières publications
[hal-02081576] Schéma semi-implicite sur maillages décalés pour le système shallow water multi-couches
21 nov. 2019 03:07
Nous proposons un schéma semi-implicite destiné à un modèle shallow water multicouches 2d avec stratification en densité, formulé sur maillages décalés généraux. Le principal résultat de cette note concerne le contrôle de l'énergie mécanique au niveau discret, qui se base principalement sur des flux advectifs faisant intervenir un terme de diffusion exprimé en fonction du gradient de pression. Le schéma est aussi conçu pour capturer les dynamiques des régimes à faible nombre de Froude et offre d'intéressantes propriétés en termes de positivité et préservation des états d'équilibre. Un test numérique est proposé pour illustrer l'efficacité du schéma dans le cas monocouche.
[hal-01988382] Energy-stable staggered schemes for the Shallow Water equations
19 nov. 2019 03:37
In this work we focus on the development and analysis of staggered schemes for the two-dimensional non-linear Shallow Water equations with varying bathymetry. Semi-implicit and fully explicit time-discretizations are proposed. Particular attention is given on non-linear stability results, principally considered here through discrete energy dissipation arguments. To address such an issue, specific convective fluxes are employed, implying diffusive terms relying on the pressure gradient. In addition of providing an explicit control of the discrete energy budget, the method is shown to preserve motionless steady states as well as the positivity of the water height. These properties are still satisfied in a fully explicit context, provided an appropriate discretization of the pressure gradient. These stability results make the approach particularly robust and efficient, for both coastal flows and low Froude number regimes. As a result, in addition of a great ease of implementation, the presented schemes meet the operational requirements attached to the simulation of large and small scale oceanic flows.
[hal-00764350] Assessing ocean-model sensitivity to wind forcing uncertainties
5 nov. 2019 18:00
In this paper, we assess the short-term forecast error of a mesoscale primitive-equation open-ocean model, induced by uncertainties in wind forcing. Statistics calculated from an ensemble of ocean states show that temperature forecast error is strongest at the top of the ensemble-mean thermocline, as a consequence of vertical displacement of the mixed-layer base around its ensemble mean. Horizontal pattern of the temperature error in the mixed-layer is mainly explained by horizontal advection and surface heat flux fluctuations. These two mechanisms and entrainment through the mixed-layer bottom are presented as the three processes responsible for thermal forecast error growth in the modeled upper ocean.
[hal-01945480] A low-diffusion self-adaptive flux-vector splitting approach for compressible flows
3 juil. 2019 12:48
A low-diffusion self-adaptive flux-vector splitting method is presented for the Euler equations. The flux-vector is here split into convective and acoustic parts following the formulation recently proposed by the authors. This procedure is based on the Zha-Bilgen (or previously Baraille et al. for the Euler barotropic system) approach enriched by a dynamic flow-dependent splitting parameter based on the local Mach number. As a consequence, in the present self-adaptive splitting, the convective and acoustic parts decouple in the low-Mach number regime whereas the complete Euler equations are considered for the sonic and highly subsonic regimes. The low diffusive property of the present scheme is obtained by adding anti-diffusion terms to the momentum and the energy components of the pressure flux in the acoustic part of the present splitting. This treatment results from a formal invariance principle preserving the discrete incompressible phase space through the pressure operator. Numerical results for several carefully chosen one- and two-dimensional test problems are finally investigated to demonstrate the accuracy and robustness of the proposed scheme for a wide variety of configurations from subsonic to highly subsonic flows.
[hal-01680977] Morphology and surface reactivity relationship in the Li1+xMn2–xO4 spinel with x = 0.05 and 0.10: a combined first-principle and experimental study
15 mai 2019 05:52
This article focuses on the surface reactivity of two spinel samples with different stoichiometries and crystal morphologies, namely Li1+xMn2–xO4 with x = 0.05 and 0.10. LiMn2O4 compounds are good candidates as positive electrode of high-power lithium-ion batteries for portable devices. The samples were investigated using both experimental and theoretical approaches. On the experimental point of view, they were characterized in depth from X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) analyses. Then, the reactivity was investigated through the adsorption of (SO2) gaseous probes, in controlled conditions, followed by XPS characterization. First-principle calculations were conducted simultaneously to investigate the electronic properties and the reactivity of relevant surfaces of an ideal LiMn2O4 material. The results allow us to conclude that the reactivity of the samples is dominated by an acido-basic reactivity and the formation of sulfite species. Nonetheless, on the x = 0.05 sample, both sulfite and sulfate species are obtained, the later, in lesser extent, corresponding to a redox reactivity. Combining experimental and theoretical results, this redox reactivity could be associated with the presence of a larger quantity of Mn4+ cations on the last surface layers of the material linked to a specific surface orientation.
