The Experts below are selected from a list of 56355 Experts worldwide ranked by ideXlab platform
Yanjin Wang - One of the best experts on this subject based on the ideXlab platform.
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the compressible viscous surface Internal wave problem nonlinear rayleigh taylor instability
Archive for Rational Mechanics and Analysis, 2016Co-Authors: Juhi Jang, Ian Tice, Yanjin WangAbstract:This paper concerns the dynamics of two layers of compressible, barotropic, viscous fluid lying atop one another. The lower fluid is bounded below by a rigid bottom, and t he upper fluid is bounded above by a trivial fluid of constant pressure. This is a free boundary problem: the Interfaces between the fluids and above the upper fluid are free to move. The fluids are acted on by gravity in the bulk, and at the free Interfaces we consider both the case of surface tension and the case of no surface forces.We are concerned with the Rayleigh–Taylor instability when the upper fluid is heavier than the lower fluid along the equilibrium Interface. When the surface tension at the free Internal Interface is below the critical value, we prove that the problem is nonlinear unstable.
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the viscous surface Internal wave problem nonlinear rayleigh taylor instability
arXiv: Analysis of PDEs, 2011Co-Authors: Yanjin Wang, Ian TiceAbstract:We consider the free boundary problem for two layers of immiscible, viscous, incompressible fluid in a uniform gravitational field, lying above a rigid bottom in a three-dimensional horizontally periodic setting. The effect of surface tension is either taken into account at both free boundaries or neglected at both. We are concerned with the Rayleigh-Taylor instability, so we assume that the upper fluid is heavier than the lower fluid. When the surface tension at the free Internal Interface is below a critical value, which we identify, we establish that the problem under consideration is nonlinearly unstable.
Juhi Jang - One of the best experts on this subject based on the ideXlab platform.
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the compressible viscous surface Internal wave problem nonlinear rayleigh taylor instability
Archive for Rational Mechanics and Analysis, 2016Co-Authors: Juhi Jang, Ian Tice, Yanjin WangAbstract:This paper concerns the dynamics of two layers of compressible, barotropic, viscous fluid lying atop one another. The lower fluid is bounded below by a rigid bottom, and t he upper fluid is bounded above by a trivial fluid of constant pressure. This is a free boundary problem: the Interfaces between the fluids and above the upper fluid are free to move. The fluids are acted on by gravity in the bulk, and at the free Interfaces we consider both the case of surface tension and the case of no surface forces.We are concerned with the Rayleigh–Taylor instability when the upper fluid is heavier than the lower fluid along the equilibrium Interface. When the surface tension at the free Internal Interface is below the critical value, we prove that the problem is nonlinear unstable.
Ian Tice - One of the best experts on this subject based on the ideXlab platform.
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the compressible viscous surface Internal wave problem nonlinear rayleigh taylor instability
Archive for Rational Mechanics and Analysis, 2016Co-Authors: Juhi Jang, Ian Tice, Yanjin WangAbstract:This paper concerns the dynamics of two layers of compressible, barotropic, viscous fluid lying atop one another. The lower fluid is bounded below by a rigid bottom, and t he upper fluid is bounded above by a trivial fluid of constant pressure. This is a free boundary problem: the Interfaces between the fluids and above the upper fluid are free to move. The fluids are acted on by gravity in the bulk, and at the free Interfaces we consider both the case of surface tension and the case of no surface forces.We are concerned with the Rayleigh–Taylor instability when the upper fluid is heavier than the lower fluid along the equilibrium Interface. When the surface tension at the free Internal Interface is below the critical value, we prove that the problem is nonlinear unstable.
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the viscous surface Internal wave problem nonlinear rayleigh taylor instability
arXiv: Analysis of PDEs, 2011Co-Authors: Yanjin Wang, Ian TiceAbstract:We consider the free boundary problem for two layers of immiscible, viscous, incompressible fluid in a uniform gravitational field, lying above a rigid bottom in a three-dimensional horizontally periodic setting. The effect of surface tension is either taken into account at both free boundaries or neglected at both. We are concerned with the Rayleigh-Taylor instability, so we assume that the upper fluid is heavier than the lower fluid. When the surface tension at the free Internal Interface is below a critical value, which we identify, we establish that the problem under consideration is nonlinearly unstable.
Jonas Faleskog - One of the best experts on this subject based on the ideXlab platform.
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an improved strain gradient plasticity formulation with energetic Interfaces theory and a fully implicit finite element formulation
Computational Mechanics, 2013Co-Authors: Carl F O Dahlberg, Jonas FaleskogAbstract:A fully implicit backward-Euler implementation of a higher order strain gradient plasticity theory is presented. A tangent operator consistent with the numerical update procedure is given. The implemented theory is a dissipative bulk formulation with energetic contribution from Internal Interface to model the behavior of material Interfaces at small length scales. The implementation is tested by solving some examples that specifically highlight the numerics and the effect of using the energetic Interfaces as higher order boundary conditions. Specifically, it is demonstrated that the energetic Interface formulation is able to mimic a wide range of plastic strain conditions at Internal boundaries. It is also shown that delayed micro-hard conditions may arise under certain circumstances such that an Interface at first offers little constraints on plastic flow, but with increasing plastic deformation will develop and become a barrier to dislocation motion.
Michèle Pijolat - One of the best experts on this subject based on the ideXlab platform.
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Why reaction rate does not always obey Arrhenius law during gas-solid reactions?
2016Co-Authors: Loïc Favergeon, Michèle PijolatAbstract:Gas-solid reactions are extensively studied by thermal analysis. To describe the kinetics of these reactions, the reaction rate dα/dt (i.e. the derivative of the extent of conversion vs. time) is almost exclusively expressed by the product of a temperature function k(T) by a mathematical function f(α) depending on the kinetic model. In general, the temperature function k(T) is supposed to follow Arrhenius law with a pre-exponential term and the apparent activation energy Eapp according to : cf. abstract file. Non-Arrhenius behavior has been observed in many cases such as for example CaO hydroxylation and carbonation. Simon suggested that other k(T) functions than Arrhenius equation may be equally used. Moreover, the use of the Arrhenius equation in model-free methods when such equation is not relevant will necessarily induce Eapp variation with the extent of conversion. Galwey has recently stated that such variations are inconsistent with the Arrhenius activation model and he wonders how and why this variation. To precise the origin of such a complexity, it is necessary to come back to the mechanism of growth of the solid product phase which can be decomposed into a series of elementary steps: adsorption/desorption, external Interface reaction, Internal Interface reaction, diffusion of species transferring from an Interface to the other. Several reasons may be invoked to explain the non-Arrhenius behavior of k(T) function (in case of single reaction): when the reaction conditions are far from equilibrium, and if a rate-determining step i governs the kinetics, the rate equation involves concentration terms which can be expressed by means of the equilibrium constants Kj (i and j are elementary steps of the growth mechanism, i ≠ j). Due to adsorption and/or desorption steps, it comes out that the rate may depend of temperature through terms deriving from Langmuir isotherm equation, and thus that it will not follow Arrhenius equation, when the reaction conditions are near the equilibrium, the rate will never follow Arrhenius equation since the rate of the opposite reaction of the elementary rate-determining step cannot be neglected compared to that of the direct one. This is why we generally propose to write the rate equation using a function which accounts for possible complexity of the rate with thermodynamic variables. Examples of rate equations where the temperature term is complex will be presented to illustrate these theoretical considerations.
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New insight into the ZnO sulfidation reaction: mechanism and kinetics modeling of the ZnS outward growth.
Physical chemistry chemical physics : PCCP, 2013Co-Authors: Laure Neveux, Anne-Sophie Gay, Javier Pérez-pellitero, Loïc Favergeon, David Chiche, Michèle PijolatAbstract:Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fischer-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the transformation at the crystal scale, its detailed mechanism and kinetics. A model ZnO material with well-determined characteristics (particle size and shape) has been synthesized to perform this study. Characterizations of sulfided samples (using XRD, TEM and electron diffraction) have shown the formation of oriented polycrystalline ZnS nanoparticles with a predominant hexagonal form (wurtzite phase). TEM observations also have evidenced an outward development of the ZnS phase, showing zinc and oxygen diffusion from the ZnO-ZnS Internal Interface to the surface of the ZnS particle. The kinetics of ZnO sulfidation by H(2)S has been investigated using isothermal and isobaric thermogravimetry. Kinetic tests have been performed that show that nucleation of ZnS is instantaneous compared to the growth process. A reaction mechanism composed of eight elementary steps has been proposed to account for these results, and various possible rate laws have been determined upon approximation of the rate-determining step. Thermogravimetry experiments performed in a wide range of H(2)S and H(2)O partial pressures have shown that the ZnO sulfidation reaction rate has a nonlinear variation with H(2)S partial pressure at the same time no significant influence of water vapor on reaction kinetics has been observed. From these observations, a mixed kinetics of external Interface reaction with water desorption and oxygen diffusion has been determined to control the reaction kinetics and the proposed mechanism has been validated. However, the formation of voids at the ZnO-ZnS Internal Interface, characterized by TEM and electron tomography, strongly slows down the reaction rate. Therefore, the impact of the decreasing ZnO-ZnS Internal Interface on reaction kinetics has been taken into account in the reaction rate expression. In this way the void formation at the Interface has been modeled considering a random nucleation followed by an isotropic growth of cavities. Very good agreement has been observed between both experimental and calculated rates after taking into account the decrease in the ZnO-ZnS Internal Interface.
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Nucleation and anisotropic growth model for isothermal kaolinite dehydroxylation under controlled water vapour pressure
Physical Chemistry Chemical Physics, 2002Co-Authors: Kais Nahdi, Michèle Pijolat, Stéphane Perrin, Françoise Rouquerol, Najia Ariguib, Malika AyadiAbstract:A new kinetic model of kaolinite dehydroxylation is proposed in order to take into account data obtained by gravimetry at 450°C under controlled water vapour pressure ranging from 2.5 to 10 mbar. This model involves a process of random nucleation and anisotropic growth of nuclei. Under the above experimental conditions, the growth of nuclei on the surface of the particles is rapid, whereas the growth towards the inside of the particles is limited by two-dimensional diffusion. This model allowed us to evaluate the 'areic frequency of nucleation' and the 'areic reactivity of growth' for our experiments. The variation of the latter quantity with the water vapour pressure was also obtained by a method based upon a sudden pressure change during isothermal experiments. It is this variation which allows us to conclude that the growth of nuclei into the particle is limited, under our experimental conditions, by the diffusion of hydroxyl groups from the Internal Interface to the external one.