Numerical simulation of strongly nonlinear and dispersive waves using a Green-Naghdi model

We investigate here the ability of a Green-Naghdi model to reproduce strongly nonlinear and dispersive wave propagation. We test in particular the behavior of the new hybrid finite-volume and finite-difference splitting approach recently developed by the authors and collaborators on the challenging benchmark of waves propagating over a submerged bar. Such a configuration requires a model with very good dispersive properties, because of the high-order harmonics generated by topography-induced nonlinear interactions. We thus depart from the aforementioned work and choose to use a new Green-Naghdi system with improved frequency dispersion characteristics. The absence of dry areas also allows us to improve the treatment of the hyperbolic part of the equations. This leads to very satisfying results for the demanding benchmarks under consideration

The Noble-Abel Stiffened-Gas equation of state

International audienceHyperbolic two-phase flow models have shown excellent ability for the resolution of a wide range of applications ranging from interfacial flows to fluid mixtures with several velocities. These models account for waves propagation (acoustic and convective) and consist in hy-perbolic systems of partial differential equations. In this context, each phase is compressible and needs an appropriate convex equation of state (EOS). The EOS must be simple enough for intensive computations as well as boundary conditions treatment. It must also be accurate , this being challenging with respect to simplicity. In the present approach, each fluid is governed by a novel EOS named 'Noble Abel Stiffened Gas' (NASG), this formulation being a significant improvement of the popular 'Stiffened Gas' (SG) EOS. It is a combination of the so-called 'Noble-Abel' and 'Stiffened Gas' equations of state that adds repulsive effects to the SG formulation. The determination of the various thermodynamic functions and associated coefficients is the aim of this article. We first use thermodynamic considerations to determine the different state functions such as the specific internal energy, enthalpy and entropy. Then we propose to determine the associated coefficients for a liquid in the presence of its vapor. The EOS parameters are determined from experimental saturation curves. Some examples of liquid-vapor fluids are examined and associated parameters are computed with the help of the present method. Comparisons between analytical and experimental saturation curves show very good agreement for wide ranges of temperature for both liquid and vapor

Dynamic relaxation processes in compressible multiphase flows. Application to evaporation phenomena

Phase changes and heat exchanges are examples of physical processes appearing in many industrial applications involving multiphase compressible flows. Their knowledge is of fundamental importance to reproduce correctly the resulting effects in simulation tools. A fine description of the flow topology is thus required to obtain the interfacial area between phases. This one is responsible for the dynamics and the kinetics of heat and mass transfer when evaporation or condensation occurs. Unfortunately this exchange area cannot be obtained easily and accurately especially when complex mixtures (drops, bubbles, pockets of very different sizes) appear inside the transient medium. The natural way to solve this specific trouble consists in using a thin grid to capture interfaces at all spatial scales. But this possibility needs huge computing resources and can be hardly used when considering physical systems of large dimensions. A realistic method is to consider instantaneous exchanges between phases by the way of additional source terms in a full non-equilibrium multiphase flow model [2,15,17]. In this one each phase obeys its own equation of state and has its own set of equations and variables (pressure, temperature, velocity, energy, entropy,...). When enabling the relaxation source terms the multiphase mixture instantaneously tends towards a mechanical or thermodynamic equilibrium state at each point of the flow. This strategy allows to mark the boundaries of the real flow behavior and to magnify the dominant physical effects (heat exchanges, evaporation, drag,...) inside the medium. A description of the various relaxation processes is given in the paper. <br> Les changements de phase et les transferts de chaleur sont des exemples de phénomènes physiques présents dans de nombreuses applications industrielles faisant intervenir des écoulements compressibles multiphasiques. La connaissance des mécanismes associés est primordiale afin de reproduire correctement leurs effets à travers des outils de simulation. Dans ce cadre, une description fine de la topologie d’un écoulement est nécessaire afin de connaître précisément l’aire interfaciale entre toutes les phases. Celle-ci est en effet responsable de la dynamique et de la cinétique des transferts de masse et de chaleur lorsque de l’évaporation et de la condensation se produisent. Malheureusement cette aire interfaciale est difficilement accessible particulièrement lorsque des mélanges complexes se forment (gouttes, bulles, inclusions de différentes tailles) au sein du milieu. La façon la plus naturelle de résoudre ce problème est d’utiliser un maillage suffisamment fin afin de capturer toutes les interfaces présentes à toutes les échelles. Cependant cette possibilité demanderait des ressources informatiques démesurées au vue de certains systèmes pouvant être de très grande taille. Une méthode plus réaliste est de considérer que les échanges entre les phases s’effectuent instantanément. Des termes sources de relaxation liés à ces échanges sont utilisés dans un modèle d’écoulement compressible à phases séparées en déséquilibre [2,15,17]. Dans celui-ci, chaque phase possède son propre jeu d’équations et ses propres variables (pression, vitesse, température, énergie, entropie, ...). Quand les termes de relaxation sont activés, le mélange multiphasique tend instantanément en chaque point de l’écoulement vers un état d’équilibre prédéfini. Cette approche permet également de borner les conditions réelles d’écoulement et de souligner les effets physiques dominants (transfert de chaleur, évaporation, trainée, ...). Une description des différents processus de relaxation est proposée dans ce papier

Reconnaissance de composés volatils du colza (Brassica napus L var oleifera) par l'abeille

National audienc

Long-term effects of soybean protease inhibitors on digestive enzymes, survival and learning abilities of honeybees

28 ref.International audienc

Proton damage effects on GaAs/GaAlAs vertical cavity surface emitting lasers

International audienceA series of proton irradiations of GaAs/GaAlAs vertical cavity surface emitting lasers (VCSELs) has been carried out for the purpose of assessing the suitability of these devices for space applications. The irradiations were performed on biased and unbiased devices at energies of 30, 40, 50, and 60 MeV. Both current versus voltage (I–V) and optical power versus current (P–I) characteristics were measured before and after each irradiation phase. A simple circuit equivalent model for the VCSEL has been developed to analyze proton damage effects through the extraction of electrical parameters. The current threshold of VCSEL is shown to be the only important parameter modified by a high fluence (up to 1012 protons/cm2) irradiation. Changes in the threshold current show radiation generated recombination centers to be the main cause of degradation. Due to carrier injection annealing related effects, we observed that unbiased devices show the greatest relative threshold increase (between 15% and 20% at 1013 protons/cm2). The threshold current damage factor was also calculated. The analysis of the I–V characteristics shows that in the range of low fluences (1010–1012 protons/cm2) radiation induced ordering effects may compete with the usual radiation degradation that we observed at higher fluences. Consequently, the nonionizing energy loss approach, which is extensively used to predict the degradation of electronic devices under a full spectrum of energetic particles, is deemed to be not yet applicable for prediction of end-of-life performances of VCSELs

Complex odour discrimination in honeybee: combined behavioural, electrophysiological and chemical approaches

National audienc

Proton damage effects on GaAs/GaAlAs vertical cavity surface emitting lasers

International audienceA series of proton irradiations of GaAs/GaAlAs vertical cavity surface emitting lasers (VCSELs) has been carried out for the purpose of assessing the suitability of these devices for space applications. The irradiations were performed on biased and unbiased devices at energies of 30, 40, 50, and 60 MeV. Both current versus voltage (I–V) and optical power versus current (P–I) characteristics were measured before and after each irradiation phase. A simple circuit equivalent model for the VCSEL has been developed to analyze proton damage effects through the extraction of electrical parameters. The current threshold of VCSEL is shown to be the only important parameter modified by a high fluence (up to 1012 protons/cm2) irradiation. Changes in the threshold current show radiation generated recombination centers to be the main cause of degradation. Due to carrier injection annealing related effects, we observed that unbiased devices show the greatest relative threshold increase (between 15% and 20% at 1013 protons/cm2). The threshold current damage factor was also calculated. The analysis of the I–V characteristics shows that in the range of low fluences (1010–1012 protons/cm2) radiation induced ordering effects may compete with the usual radiation degradation that we observed at higher fluences. Consequently, the nonionizing energy loss approach, which is extensively used to predict the degradation of electronic devices under a full spectrum of energetic particles, is deemed to be not yet applicable for prediction of end-of-life performances of VCSELs