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Статті в журналах з теми "Explicit diffusive kinetic scheme":
1
Aregba-Driollet, D., R. Natalini, and S. Tang. "Explicit diffusive kinetic schemes for nonlinear degenerate parabolic systems." Mathematics of Computation 73, no. 245 (August 26, 2003): 63–94. http://dx.doi.org/10.1090/s0025-5718-03-01549-7.
2
Dimarco, Giacomo, Lorenzo Pareschi, and Vittorio Rispoli. "Implicit-Explicit Runge-Kutta Schemes for the Boltzmann-Poisson System for Semiconductors." Communications in Computational Physics 15, no. 5 (May 2014): 1291–319. http://dx.doi.org/10.4208/cicp.090513.151113a.
Анотація:
AbstractIn this paper we develop a class of Implicit-Explicit Runge-Kutta schemes for solving the multi-scale semiconductor Boltzmann equation. The relevant scale which characterizes this kind of problems is the diffusive scaling. This means that, in the limit of zero mean free path, the system is governed by a drift-diffusion equation. Our aim is to develop a method which accurately works for the different regimes encountered in general semiconductor simulations: the kinetic, the intermediate and the diffusive one. Moreover, we want to overcome the restrictive time step conditions of standard time integration techniques when applied to the solution of this kind of phenomena without any deterioration in the accuracy. As a result, we obtain high order time and space discretization schemes which do not suffer from the usual parabolic stiffness in the diffusive limit. We show different numerical results which permit to appreciate the performances of the proposed schemes.
3
Boudin, Laurent, Céline Grandmont, Bérénice Grec, Sébastien Martin, Amina Mecherbet, and Frédérique Noël. "Fluid-kinetic modelling for respiratory aerosols with variable size and temperature." ESAIM: Proceedings and Surveys 67 (2020): 100–119. http://dx.doi.org/10.1051/proc/202067007.
Анотація:
In this paper, we propose a coupled fluid-kinetic model taking into account the radius growth of aerosol particles due to humidity in the respiratory system. We aim to numerically investigate the impact of hygroscopic effects on the particle behaviour. The air flow is described by the incompressible Navier-Stokes equations, and the aerosol by a Vlasov-type equation involving the air humidity and temperature, both quantities satisfying a convection-diffusion equation with a source term. Conservations properties are checked and an explicit time-marching scheme is proposed. Twodimensional numerical simulations in a branched structure show the influence of the particle size variations on the aerosol dynamics.
4
Huh, Joo Youl, and Jong Pa Hong. "Influences of Elastic Stress and Interfacial Kinetic Barrier on Phase Evolution Paths of Thin-Film Diffusion Couples." Solid State Phenomena 118 (December 2006): 405–12. http://dx.doi.org/10.4028/www.scientific.net/ssp.118.405.
Анотація:
An explicit, finite difference scheme was used to examine the effects of coherency stresses and interface kinetic barriers on the phase evolution of a binary, thin-film diffusion couple. Thin-films, initially consisting of alternating layers of two terminal phases, α and γ, were held at a temperature at which the formation of an intermediate phase, β, at α/γ interface was thermodynamically probable. When either the coherency stresses or interface kinetic barriers are present, the interfacial compositions become time-dependent and, thus, the formation of the thermodynamically stable β phase can be kinetically prohibited at the early stage of the evolution. Even if the initial α/γ thin-films have the same overall composition, the coherency constraint can result in different final equilibrium states depending on either the initial compositions of the α and γ phases or the relative magnitudes of the interfacial kinetic barriers. When both the coherency constraint and interfacial kinetic barriers are present, an intermediate phase can repeatedly form and disappear during the evolution.
5
Eckermann, Stephen D. "Explicitly Stochastic Parameterization of Nonorographic Gravity Wave Drag." Journal of the Atmospheric Sciences 68, no. 8 (August 1, 2011): 1749–65. http://dx.doi.org/10.1175/2011jas3684.1.
Анотація:
Abstract A straightforward methodology is presented for converting the deterministic multiwave parameterizations of nonorographic gravity wave drag, currently used in general circulation models (GCMs), to stochastic analogs that use fewer waves (in the example herein, a single wave) within each grid box. Deterministic discretizations of source-level momentum flux spectra using a fixed spectrum of many waves with predefined phase speeds are replaced by sampling these source spectra stochastically using waves with randomly assigned phase speeds. Using simple conversion formulas, it is shown that time-mean wave-induced drag, diffusion, and heating-rate profiles identical to those from the deterministic scheme are produced by the stochastic analog. Furthermore, in these examples the need for bulk intermittency factors of small value is largely obviated through the explicit incorporation of stochastic intermittency into the scheme. When implemented in a GCM, the single-wave stochastic analog of an existing deterministic scheme reproduces almost identical time-mean middle-atmosphere climate and drag as its deterministic antecedent but with an order of magnitude reduction in computational expense. The stochastically parameterized drag is also accompanied by inherent variability about the time-mean profile that forces the smallest space–time scales of the GCM. Studies of mean GCM kinetic energy spectra show that this additional stochastic forcing does not lead to excessive increases in dynamical variability at these smallest GCM scales. The results show that the expensive deterministic schemes currently used in GCMs are easily modified and replaced by cheap stochastic analogs without any obvious deleterious impacts on GCM climate or variability, while offering potential advantages of computational savings, reduction of systematic climate biases, and greater and more realistic ensemble spread.
6
Chepak-Gizbrekht, M. V., and A. G. Knyazeva. "Two-dimensional model of grain boundary diffusion and oxidation." PNRPU Mechanics Bulletin, no. 1 (December 15, 2022): 156–66. http://dx.doi.org/10.15593/perm.mech/2022.1.12.
Анотація:
The grinding of the structure of materials is accompanied by a change in the physical and mechanical properties. This occurs largely due to the accumulation of energy and defects in the structure, which activates the diffusion of impurities contained in the material. The increase in the number of grain boundaries and joints can cause the inelastic behavior of the material, its additional chemical activation. For some metals and alloys this leads to strengthening, while for others it leads to rapid degradation of mechanical properties. Grain boundary diffusion in such materials is the main mechanism of transport of alloying components or harmful impurities, so its study is important. This paper presents a two-dimensional model of grain boundary diffusion in a material with an explicit structure assignment. The model takes into account the presence of chemical transformations that can determine the corrosion mechanisms under operating conditions. For simplicity of calculation the material structure is taken symmetrical and contains two phases: grains and a boundary phase. Diffusion and kinetic parameters of the phases may differ. The model is represented in dimensionless form so that the distances between neighboring grains or the widths of the boundary phase are the same and the grain sizes can vary. Depending on the ratio of phase sizes we can speak about micro- and nanocrystalline structure. The problem was solved numerically using implicit difference scheme and coordinate splitting. Diffusion and kinetic parameters, which are close to the parameters of oxygen grain boundary diffusion in titanium and titanium oxidation, respectively, were taken for the calculations. Integral concentrations reflect the dynamics (kinetics) of oxygen and oxides accumulation over the calculation area. Results showing the role of changes in the oxidation kinetics due to changes in the reaction constants in the phases and the phase size ratio are presented. The model can be useful for assessing the degree of influence of grain boundary diffusion on the oxidation process and the accompanying change in properties, as well as for setting up appropriate experiments.
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Muñoz-Esparza, Domingo, Robert D. Sharman, and Stanley B. Trier. "On the Consequences of PBL Scheme Diffusion on UTLS Wave and Turbulence Representation in High-Resolution NWP Models." Monthly Weather Review 148, no. 10 (October 1, 2020): 4247–65. http://dx.doi.org/10.1175/mwr-d-20-0102.1.
Анотація:
AbstractMesoscale numerical weather prediction (NWP) models are routinely exercised at kilometer-scale horizontal grid spacings (Δx). Such fine grids will usually allow at least partial resolution of small-scale gravity waves and turbulence in the upper troposphere and lower stratosphere (UTLS). However, planetary boundary layer (PBL) parameterization schemes used with these NWP model simulations typically apply explicit subgrid-scale vertical diffusion throughout the entire vertical extent of the domain, an effect that cannot be ignored. By way of an example case of observed widespread turbulence over the U.S. Great Plains, we demonstrate that the PBL scheme’s mixing in NWP model simulations of Δx = 1 km can have significant effects on the onset and characteristics of the modeled UTLS gravity waves. Qualitatively, PBL scheme diffusion is found to affect not only background conditions responsible for UTLS wave activity, but also to control the local vertical mixing that triggers or hinders the onset and propagation of these waves. Comparisons are made to a reference large-eddy simulation with Δx = 250 m to statistically quantify these effects. A significant and systematic overestimation of resolved vertical velocities, wave-scale fluxes, and kinetic energy is uncovered in the 1-km simulations, both in clear-air and in-cloud conditions. These findings are especially relevant for upper-level gravity wave and turbulence simulations using high-resolution kilometer-scale NWP models.
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Deng, Aijun, and David R. Stauffer. "On Improving 4-km Mesoscale Model Simulations." Journal of Applied Meteorology and Climatology 45, no. 3 (March 1, 2006): 361–81. http://dx.doi.org/10.1175/jam2341.1.
Анотація:
Abstract A previous study showed that use of analysis-nudging four-dimensional data assimilation (FDDA) and improved physics in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) produced the best overall performance on a 12-km-domain simulation, based on the 18–19 September 1983 Cross-Appalachian Tracer Experiment (CAPTEX) case. However, reducing the simulated grid length to 4 km had detrimental effects. The primary cause was likely the explicit representation of convection accompanying a cold-frontal system. Because no convective parameterization scheme (CPS) was used, the convective updrafts were forced on coarser-than-realistic scales, and the rainfall and the atmospheric response to the convection were too strong. The evaporative cooling and downdrafts were too vigorous, causing widespread disruption of the low-level winds and spurious advection of the simulated tracer. In this study, a series of experiments was designed to address this general problem involving 4-km model precipitation and gridpoint storms and associated model sensitivities to the use of FDDA, planetary boundary layer (PBL) turbulence physics, grid-explicit microphysics, a CPS, and enhanced horizontal diffusion. Some of the conclusions include the following: 1) Enhanced parameterized vertical mixing in the turbulent kinetic energy (TKE) turbulence scheme has shown marked improvements in the simulated fields. 2) Use of a CPS on the 4-km grid improved the precipitation and low-level wind results. 3) Use of the Hong and Pan Medium-Range Forecast PBL scheme showed larger model errors within the PBL and a clear tendency to predict much deeper PBL heights than the TKE scheme. 4) Combining observation-nudging FDDA with a CPS produced the best overall simulations. 5) Finer horizontal resolution does not always produce better simulations, especially in convectively unstable environments, and a new CPS suitable for 4-km resolution is needed. 6) Although use of current CPSs may violate their underlying assumptions related to the size of the convective element relative to the grid size, the gridpoint storm problem was greatly reduced by applying a CPS to the 4-km grid.
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YOSSIFON, G., I. FRANKEL, and T. MILOH. "Macro-scale description of transient electro-kinetic phenomena over polarizable dielectric solids." Journal of Fluid Mechanics 620 (February 10, 2009): 241–62. http://dx.doi.org/10.1017/s002211200800459x.
Анотація:
We have studied the temporal evolution of electro-kinetic flows in the vicinity of polarizable dielectric solids following the application of a ‘weak’ transient electric field. To obtain a macro-scale description in the limit of narrow electric double layers (EDLs), we have derived a pair of effective transient boundary conditions directly connecting the electric potentials across the EDL. Within the framework of the above assumptions, these conditions apply to a general transient electro-kinetic problem involving dielectric solids of arbitrary geometry and relative permittivity. Furthermore, the newly derived scheme is applicable to general transient and spatially non-uniform external fields. We examine the details of the physical mechanisms involved in the relaxation of the induced-charging process of the EDL adjacent to polarizable dielectric solids. It is thus established that the time scale characterizing the electrostatic relaxation increases with the dielectric constant of the solid from the Debye time (for the diffusion across the EDL) through the ‘intermediate’ scale (proportional to the product of the respective Debye- and geometric-length scales). Thus, the present rigorous analysis substantiates earlier results largely obtained by heuristic use of equivalent RC-circuit models. Furthermore, for typical values of ionic diffusivity and kinematic viscosity of the electrolyte solution, the latter time scale is comparable to the time scale of viscous relaxation in problems concerning microfluidic applications or micro-particle dynamics. The analysis is illustrated for spherical micro-particles. Explicit results are thus presented for the temporal evolution of electro-osmosis around a dielectric sphere immersed in unbounded electrolyte solution under the action of a suddenly applied uniform field, combining both induced charge and ‘equilibrium’ (fixed charge) contributions to the zeta potential. It is demonstrated that, owing to the time delay of the induced-EDL charging, the ‘equilibrium’ contribution to fluid motion (which is linear in the electric field) initially dominates the (quadratic) ‘induced’ contribution.
10
Lu, Jiachen, Negin Nazarian, Melissa Anne Hart, E. Scott Krayenhoff, and Alberto Martilli. "A one-dimensional urban flow model with an eddy-diffusivity mass-flux (EDMF) scheme and refined turbulent transport (MLUCM v3.0)." Geoscientific Model Development 17, no. 7 (April 5, 2024): 2525–45. http://dx.doi.org/10.5194/gmd-17-2525-2024.
Анотація:
Abstract. In recent years, urban canopy models (UCMs) have been used as fully coupled components of mesoscale atmospheric models as well as offline tools to estimate temperature and surface fluxes using atmospheric forcings. Examples include multi-layer urban canopy models (MLUCMs), where the vertical variability of turbulent fluxes is calculated by solving prognostic momentum and turbulent kinetic energy (TKE, k) using mixing length scale (l) and drag parameterizations. These parameterizations are based on the well-established 1.5-order k−l turbulence closure theory and are often informed by microscale fluid dynamics simulations. However, this approach can include simplifications such as assuming the same diffusion coefficient for momentum, TKE, and scalars. In addition, the dispersive stresses arising from spatially averaged flow properties have been parameterized together with the turbulent fluxes despite being controlled by different mechanisms. Both of these assumptions impact the quantification of the turbulent exchange of flow properties and subsequent air temperature predictions in urban canopies. To assess these assumptions and improve corresponding parameterization, we analyzed 49 large-eddy simulations (LES) for idealized urban arrays, encompassing variable building height distributions and a comprehensive range of urban densities (λp∈[0.0625,0.64]) seen in global cities. We find that the efficiency of turbulent transport (numerically described via diffusion coefficients) is similar for scalars and momentum but is 3.5 times higher for TKE. Additionally, parameterizing the dispersive momentum flux using the k−l closure was a source of error, while scaling with the pressure gradient and urban morphological parameters appears more appropriate. In response to these findings, we propose two changes to the previous version of MLUCM: (a) separate characterization for turbulent diffusion coefficient for momentum and TKE and (b) introduction of an explicit physics-based “mass-flux” term to represent the fraction of the dispersive momentum transport directly induced from buildings as an amendment to the existing “eddy-diffusivity” framework. The updated one-dimensional model, after being tuned for building height variability, is further compared against the original LES results and demonstrates improved performance in predicting vertical turbulent exchange in urban canopies.
Дисертації з теми "Explicit diffusive kinetic scheme":
1
Haghebaert, Marie. "Outils et méthodes pour la modélisation de la dynamique des écosystèmes microbiens complexes à partir d'observations expérimentales temporelles : application à la dynamique du microbiote intestinal." Electronic Thesis or Diss., université Paris-Saclay, 2023. http://www.theses.fr/2023UPASM036.
Анотація:
Cette thèse est issue du projet Européen Homo.symbiosus qui étudie les transitions d'équilibre des interactions entre l'hôte et son microbiote intestinal. Pour étudier les transitions nous suivons deux directions : la modélisation mécaniste des interactions hôte-microbiote et l'analyse de données temporelles de comptage microbien.Nous avons enrichi et simulé un modèle déterministe de la crypte intestinale grâce au schéma numérique EDK, en étudiant notamment l'impact des différents paramètres en utilisant la méthode des effets élémentaires de Morris. Ce modèle s'est avéré capable de simuler d'une part des états symbiotiques et dysbiotiques des interactions et d'autre part des scénarios de transition.En parallèle, un modèle EDO compartimental du colon inspiré de travaux existants a été développé et couplé au modèle de crypte. La thèse a contribué à l'enrichissement de la modélisation du métabolisme bactérien et à la modélisation de l'immunité innée à l'échelle de la muqueuse intestinale. Une exploration numérique nous a permis d'évaluer l'influence de l'alimentation sur l'état stationnaire du modèle et d'étudier l'effet d'un scénario pathologique en mimant une brèche de la barrière épithéliale.De plus, nous avons développé une approche d'analyse des données microbiennes visant à évaluer la déviation des écosystèmes microbiens subissant une forte perturbation de leur environnement par rapport à un état de référence. Cette méthode, basée sur une classification DMM, permet d'étudier les transitions d'équilibre de l'écosystème dans le cas de données avec peu d'individus et peu de points de temps. Par ailleurs, une méthode de classification de courbes utilisant le modèle SBM a été appliquée pour étudier l'effet de différentes perturbations de l'écosystème microbien, des résultats de cette étude ont pu être utilisés pour enrichir le modèle d'interactions hôte-microbioteThis thesis stems from the European project Homo.symbiosus, which investigates the equilibrium transitions of interactions between the host and its intestinal microbiota. To study these transitions, we pursue two directions: the mechanistic modeling of host-microbiota interactions and the analysis of temporal microbial count data.We enriched and simulated a deterministic model of the intestinal crypt using the EDK numerical scheme, particularly studying the impact of different parameters using the Morris Elementary Effects method. This model proved capable of simulating, on one hand, symbiotic and dysbiotic interaction states and, on the other hand, transition scenarios between states of dysbiosis and symbiosis.In parallel, a compartmental ODE model of the colon, inspired by existing studies, was developed and coupled with the crypt model. The thesis contributed to the enhancement of bacterial metabolism modeling and the modeling of innate immunity at the scale of the intestinal mucosa. A numerical exploration allowed us to assess the influence of diet on the steady state of the model and to study the effect of a pathological scenario by mimicking a breach in the epithelial barrier.Furthermore, we developed an approach to analyze microbial data aimed at assessing the deviation of microbial ecosystems undergoing significant environmental disturbances compared to a reference state. This method, based on DMM classification, enables the study of ecosystem equilibrium transitions in cases with few individuals and few time points. Moreover, a curve classification method using the SBM model was applied to investigate the effects of various disturbances on the microbial ecosystem; the results from this study were used to enrich the host-microbiota interaction model
2
Poltorak, Sergej. "Amperometrinio biojutiklio su chemiškai modifikuotu elektrodu vykstančių reakcijos-difuzijos procesų kompiuterinis modeliavimas ir tyrimas." Master's thesis, Lithuanian Academic Libraries Network (LABT), 2014. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2011~D_20140627_170922-37372.
Анотація:
Magistrinio darbo tikslas yra sukurti kompiuterinį amperometrinio biojutiklio su chemiškai modifikuotu elektrodu modelį ir ištirti jo savybes. Modelis papildo egzistuojančius modelius mediatoriaus sluoksniu. Mediatoriaus koncentracija sluoksnyje nėra konstanta, tirpsta biojutiklio veikimo eigoje. Darbe apžvelgiama medžiaga apie amperometrinius biojutiklius, biojutiklio modeliavimo aspektus, mediatoriaus tirpimo priežastis ir veikimo principus. Vėliau suformuluojamas pasirinkto biojutiklio matematinis modelis. Matematiniame modelyje pateikiamos diferencialinės lygtys su dalinėmis išvestinėmis, aprašančios biojutiklyje vykstančias reakcijas ir difuzijas. Pagal matematinį modelį yra sudaromas skaitinis modelis. Pagal žinomus analizinius sprendimus modelis yra ratifikuojamas. Remiantis skaitiniu modeliu buvo sukurta programinė įranga įgyvendinanti diferencialinių lygčių su dalinėmis išvestinėmis sprendimo metodą (išreikštinę baigtinių skirtumų schemą) ir simuliuojanti biojutiklio veikimą. Programinė įranga yra karkasas, leidžiantis nagrinėti sumodeliuoto biojutiklio savybes bei charakteristikas. Buvo ištirtos substrato ir mediatoriaus koncentracijų, mediatoriaus, fermento, difuzijos sluoksnių, bei mediatoriaus sluoksnio difuzijos koeficiento įtakos biojutiklio generuojamos srovės tankiui. Be to gauti dviejų modelių palyginimo rezultatai parodė sumodeliuoto biojutiklio modelio skirtumus ir panaudojimo prasmingumą.The aim of master thesis is to model amperometric biosensor with chemically modified electrode, introducing into existing model one more additional mediator layer. Mediator concentration inside the layer is changing during the biosensor action. Master thesis content consists of: enzyme kinetics introduction, biosensor modeling peculiarities exploration, chemically modified electrode information, biosensor action aspects description. Following chapter concentrates on mathematical modeling of particular biosensor type. Mathematical model represents differential equations with partial derivatives describing the reactions and diffusion inside biosensor. Next, numerical model is formulated. The explicit method technique was used. Based on numerical model software was made and validated. Using software the biosensor action was simulated in order to investigate biosensor properties and characteristics. In this work several properties were analyzed: impact of substrate and mediator concentrations, size of mediator, enzyme and diffusion layers, mediator layer diffusion coefficient on biosensor response. This model was compared with previous introduced model and it was shown that there is some difference between them and there is a reason to use new model.
3
Devi, K. "Numerical simulation of free surface flow using lax diffusive explicit scheme." Thesis, 2014. http://ethesis.nitrkl.ac.in/6267/1/E-96.pdf.
Анотація:
In an open channel or overland flow of shallow depth, flood wave propagation is the concept which requires governing equations for its solution. The computation of governing equations (both momentum equation and continuity equations) to be solved are generally called the Saint-Venant equations. These equations are highly nonlinear partial differential equations, the solutions of which are very much complex. Numerical approaches are generally employed to solve these equations and proper discretization with proper selection of grid size and time step provides the results more effectively and accurately. In the present research work the Saint –Venant equations are solved through the lax diffusive explicit finite difference scheme. In this the characteristic equations are simultaneously solved in both boundaries for dynamic wave, which leads to give very accurate result. Two types of downstream boundary conditions were considered together with the condition of discharge hydrograph at upstream end. The physical laws which govern two basic principles in the hydraulics of flow of water are principle of conservation of mass and principle of conservation of momentum. These two laws are of mathematical form generally expressed in partial differential equation form known as Saint-Venant equations. Conversion of these equations into ordinary partial differential equation forms and the simple discretization of this equation by explicit scheme using CFD tool are presented in this paper.
Тези доповідей конференцій з теми "Explicit diffusive kinetic scheme":
1
Kern, Matthias, Paris Fokaides, Peter Habisreuther, and Nikolaos Zarzalis. "Applicability of a Flamelet and a Presumed JPDF 2-Domain-1-Step-Kinetic Turbulent Reaction Model for the Simulation of a Lifted Swirl Flame." In ASME Turbo Expo 2009: Power for Land, Sea, and Air. ASMEDC, 2009. http://dx.doi.org/10.1115/gt2009-59435.
Анотація:
Lifted diffusion flames are an interesting topic due to many reasons. Mainly, lifting the reaction zone provides explicit time for mixing and avoids, therefore, hot spots at near stoichiometric conditions. Hence, they promise low emissions and make them auspicious for industrial application. In comparison to lean premixed flames, which are promising in terms of emissions as well, they distinguish themselves in the nonexistence of the risk of flashback by concurrently nearly premixed flame conditions. From exploratory considerations they are an excellent case for the investigation of flame stability. However, especially this kind of flame is challenging for the reaction model due to its high turbulence and nearly premixed burning state. CFD is a powerful tool to get a clear insight in complex mechanisms, as it delivers detailed information of the flow field. Although the contribution of highly sophisticated models like LES is steadily growing in current research, fast models as RANS are most important. Solely they provide the feasibility of extensive parametric studies or the application in industrial design processes. Therefore, appropriate reaction models are needed. The applicability of two different reaction models for non-premixed flames to predict structure and stability of such flames has been investigated in this work. A stable confined diffusion flame produced by a double swirler airblast nozzle has been chosen as test case. Leaving the secondary air stream non-swirled creates a flame which stabilizes in a lifted state. The turbulent flamelet model as proposed by Peters in the early 90th basically models the impact of the turbulent strain rate on the diffusion flame. The local state of mixing is characterized by the mixture fraction, whereas the interaction of flame and turbulence is described by the mean scalar dissipation rate. The fact that the strain rate is the only non-equilibrium parameter describing the state of the reaction permits the use of detailed chemical mechanisms. The presumed jpdf model based on a 2-domain-1-step kinetic scheme has its focus on the interaction of mixing and reaction progress and uses a presumed shape for the joint probability density function. The reaction is characterized by a single variable describing the mixing state and one single additional variable, describing the state of reaction progress. In this paper assets and drawbacks of both models and their applicability to lifted flames have been discussed in detail. Furthermore, conclusions on the stability mechanism of a lifted swirl flame are taken.
2
Rizales, Johnny J. M., Paulo T. T. Esperanc¸a, and Andre´ Belfort Bueno. "Simulation of Flow Around Circular Cylinder Using a Collocation Spectral Method." In ASME 2005 24th International Conference on Offshore Mechanics and Arctic Engineering. ASMEDC, 2005. http://dx.doi.org/10.1115/omae2005-67152.
Анотація:
The purpose of this paper is to develop a Fourier-Chebyshev collocation spectral method for computing unsteady two-dimensional viscous incompressible flow past a circular cylinder for low Reynolds numbers. The incompressible Navier-Stokes equations (INSE) are formulated in terms of the primitive variables, velocity and pressure. The incompressible Navier-Stokes equations in curvilinear coordinates are spectrally discretized and time integrated by a second-order mixed explicit/implicit time integration scheme. This scheme is a combination of the Crank-Nicolson scheme operating on the diffusive term and Adams-Bashforth scheme acting on the convective term. The projection method is used to split the solution of the INSE to the solution of two decoupled problems: the diffusion-convection equation (Burgers equation) to predict an intermediate velocity field and the Poisson equation for the pressure, it is used to correct the velocity field and satisfy the continuity equation. Finally, the numerical results obtained for the drag and lift coefficients around the circular cylinder are compared with results previously published.
3
Marti´nez R., Johnny J., and Paulo T. T. Esperanc¸a. "Numerical Simulation of Flow Around a Circular Cylinder Using a Fourier-Chebyshev Collocation Spectral Method." In ASME 2003 22nd International Conference on Offshore Mechanics and Arctic Engineering. ASMEDC, 2003. http://dx.doi.org/10.1115/omae2003-37241.
Анотація:
Vortex-induced vibrations (VIV) of slender structural elements (marine cables, pipes and risers) are very important aspects to be considered in the design stage of many offshore structures, because the fatigue life of risers installed in deep water are often dominated by VIV effects. Despite the relatively fundamental nature of the problem, a small amount is known about the nature of the fluid-structure interaction. The purpose of this paper is to develop a Fourier-Chebyshev collocation spectral method for computing unsteady two-dimensional viscous incompressible flow past a circular cylinder for moderate Reynolds numbers. The incompressible Navier-Stokes equations (INSE) are formulated in terms of the primitive variables, velocity and pressure. The incompressible Navier-Stokes equations in curvilinear coordinates are spectrally discretized and time integrated by a second-order mixed explicit/implicit time integration scheme. This scheme is a combination of the Crank-Nicolson scheme operating on the diffusive term and Adams-Bashforth scheme acting on the convective term. The projection method is used to split the solution of the INSE to the solution of two decoupled problems: the diffusion-convection equation (Burgers equation) to predict an intermediate velocity field and the Poisson equation for the pressure, it is used to correct the velocity field and satisfy the continuity equation. Finally, the numerical results obtained for the drag and lift coefficients around the circular cylinder are compared with results previously published.
4
Burbelko, Andriy A., Daniel Gurgul, Edward Fras´, and Edward Guzik. "Multiscale Modeling of Ductile Iron Solidification With Continuous Nucleation by a Cellular Automaton." In ASME 2010 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. ASMEDC, 2010. http://dx.doi.org/10.1115/detc2010-28764.
Анотація:
The solidification of metals and alloys is a typical example of multiphysics and multiscale engineering systems. The phenomenon of different time and spatial scales should be taken into consideration in the modeling of a microstructure formation: heat diffusion, the components diffusion in the liquid and solid phases, the thermodynamics of phase transformation under a condition of inhomogeneous chemical composition of growing and vanishing phases, phase interface kinetics, and grains nucleation. The results of a two-dimensional modeling of the microstructure formation in a ductile cast iron are presented. The cellular automaton model (CA) was used for the simulation. The model takes into account the nucleation of two kinds of grains that appear inside of the liquid during solidification: austenite and graphite. The six states of CA cells correspond to the above-mentioned three phases (liquid, austenite and graphite) and to the three two-phase interfaces. A numerical solution was used for the modeling of concentration and temperature fields. The parabolic nonlinear differential equations with a source function were solved by using the finite element method and explicit scheme. In the mono-phase cells the source function is equal to zero. In the interface cells the value of the source function varies depending on the local undercooling. The undercooling value depends on the front curvature, the local temperature and the local chemical composition of the phases. Overlapping lattices with the same spatial step were used for concentration field modeling and for the CA. The time scale of the temperature field for this lattice is about 104 times shorter. Due to the above reasons, another lattice was used with a multiple spatial step and the same time step. The new grain nucleation of solid phases from a liquid is a phenomenon which must be taken into account for correct simulation of a polycrystalline structure formation. The cumulative distribution curve approach was used to calculate the number of substrates on which nucleation takes place as a function of under-cooling below the equilibrium temperature. An algorithm of continuous nucleation modeling during solidification is presented. The undercooling of solid phase grain nucleation was calculated on the basis of the inverse function of the above-mentioned cumulative distribution curve (fractile) with the argument equal to the random number generated in the interval 0…1 with uniform density. The domain of correct usage of this algorithm was analyzed.
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Hayatdavoodi, Masoud. "Boundary Layer and Wake Region Simulation for Low Reynolds Number Flows Around Bluff Bodies Using the Lattice Boltzmann Method." In ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering. ASMEDC, 2009. http://dx.doi.org/10.1115/omae2009-79037.
Анотація:
Two and three-dimensional flows around solid boundaries are interesting and important subjects to both scientists and engineers. Lattice Boltzmann Method (LBM) is a relatively new computational method to simulate fluid flows by tracking the collision, advection and propagation of mesoscopic fluid particles. LBM is originated from the Cellular automata combined with kinetic theory and the Boltzmann equation. The method solves the explicit finite difference scheme lattice Boltzmann equations which are second order in space and first order in time. LBM does not attempt to solve the Navier-Stokes equations directly, however, it obeys them. The two-dimensional flows around square and circular cylinders are simulated with uniform and nonuniform grid structures using LBM. The boundary-layer growth and wake region physics are captured with small scale details, and the results are discussed in comparison with the available references for Reynolds numbers between 50 and 350. The compatibility of the method to simulate a flow around ship-shaped geometries and a combination of objects is also provided.
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Escobar, Jose, Ismail Celik, and Donald Ferguson. "Development of a Log-Time Integration Method for Reactive Flows." In ASME 2012 Fluids Engineering Division Summer Meeting collocated with the ASME 2012 Heat Transfer Summer Conference and the ASME 2012 10th International Conference on Nanochannels, Microchannels, and Minichannels. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/fedsm2012-72090.
Анотація:
In reactive flow simulations integration of the stiff species transport equations consumes most of the computational time. Another important aspect of combustion simulation is the need to simulate at least tens of species in order to accurately predict emissions and the related combustion dynamics. Small time scales and systems with tens of species lead to very high computational costs. Classic integration methods such as Euler method are restricted by the smallest characteristic time scale, and explicit Runge-Kutta methods require intermediate predictor corrector steps which make the problem computationally expensive. On the other hand, implicit methods are also computationally expensive due the calculation of the Jacobian. This work presents a strategy to significantly reduce computational time for integration of species transport equations using a new explicit integration scheme called Log-Time Integration Method (LTIM). LTIM is fairly robust and can compete with methods such as the 5th order Runge-Kutta method. Results showed that LTIM applied to the solution of a zero dimensional reactive system which consists of 4 chemical species obtains the solution around 4 times faster than 5th order Runge-Kutta method. LTIM was also applied to the solution of a one dimensional methane-air flame. The chemical reactions were modeled using a reduced chemical mechanism ARM9 that consists of 9 chemical species and 5 global reactions. The solution was carried out for 9 species transport equations along with the energy equation. Governing equations were decoupled into flow and chemical parts and were solved separately using a split formulation. Thermodynamic properties were obtained using NASA format polynomials and transport properties using kinetic-theory formulation. It is shown that the new one dimensional flame code is able to calculate the adiabatic flame temperature of the system and corresponding flame speed for the methane-air flame thus validating its robustness and accuracy.
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Felten, Fre´de´ric N., and Gregory M. Laskowski. "Large Eddy Simulations of Fully Developed Flow Through a Spanwise Rotating Infinite Serpentine Passage." In ASME Turbo Expo 2007: Power for Land, Sea, and Air. ASMEDC, 2007. http://dx.doi.org/10.1115/gt2007-27423.
Анотація:
The ability to predict turbine blade heat transfer is an important problem and is strongly dependant on the ability to predict the turbulent flow-field. Turbine blade internal coolant flow is often subjected to strong curvature and possibly rotational effects, two flow characteristics that make RANS modeling efforts difficult. An assessment of Large Eddy Simulation (LES) has been conducted for a flow with strong streamline curvature and rotational effects. Simulations of fully developed turbulent flow in an isothermal, smooth-wall, serpentine passage have been performed and compared to the Direct Numerical Simulation (DNS) data of Laskowski (2004). The flow is periodic in the stream-wise and span-wise directions and simulations were conducted for span-wise rotation numbers Rob = 0.0 and Rob = 0.32. The geometry has dimensions 12πδ × 2δ × 3πδ, in the stream-wise, wall-normal and span-wise directions, respectively, where δ is half the passage height. The inner radius of the bends is δ. The Reynolds number based on the bulk velocity and H = 2δ is Reb = 5600. A kinetic-energy conserving, finite-volume, collocated-mesh scheme (Felten and Lund, 2006) is applied to treat the streamwise and wall-normal directions, while Fourier collocation is used in the spanwise direction. A third-order Runge-Kutta explicit marching scheme is used to advance the solution in time and the pressure Poisson Equation is solved using a multigrid technique. The LES results are presented and close agreement with the DNS is noted for both the stationary and rotating cases.
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Das, Shankhadeep, Battalgazi Yildirim, Sanjay R. Mathur, Alina Alexeenko, and Jayathi Y. Murthy. "A Parallel Coupled Ordinates Method for Rarefied Gas Dynamics Simulations." In ASME 2012 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/imece2012-89256.
Анотація:
Deterministic solution techniques for non-equilibrium rarefied flows in RF MEMS switches are frequently based on the ellipsoidal statistical Bhatnagar-Gross-Krook (ESBGK) form of the Boltzmann kinetic equation. These numerical schemes involve the sequential solution of the distribution function in velocity space. However, these schemes have poor convergence rates, especially at low Knudsen numbers, because of the explicit coupling of the distribution functions in velocity space. Furthermore, parallel implementation of these schemes is inefficient, making simulation of real-life devices practically impossible. In this paper we describe the parallel performance of a recently-developed numerical procedure called the coupled ordinates method (COMET) to solve ESBGK equations. In this method, the distribution functions for all velocity ordinates are strongly coupled at each physical point, resulting in an implicit solution procedure in velocity space. The coupled procedure is used as a relaxation sweep in a geometric multigrid scheme to promote spatial coupling. Results show that COMET gives excellent CPU scaling on multiple processors even for very small workload per processor. The solver is also shown to have very good strong and weak scaling characteristics. The parallel COMET solver also gives significantly faster solutions than the parallel implementation of the conventional sequential solution procedure. It is believed that the parallel COMET solver can become an efficient tool to model real-life RF MEMS switches.
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Jiang, Jiamin. "Efficient Localized Nonlinear Solution Strategies for Unconventional-Reservoir Simulation with Complex Fractures." In SPE Reservoir Simulation Conference. SPE, 2021. http://dx.doi.org/10.2118/203987-ms.
Анотація:
Abstract It is very challenging to simulate unconventional reservoirs efficiently and accurately. Transient flow can last for a long time and sharp solution (pressure, saturation, compositions) gradients are induced because of the severe permeability contrast between fracture and matrix. Although high-resolution models for well and fracture are required to achieve adequate resolution, they are computationally too demanding for practical field models with many stages of hydraulic fracture. The paper aims to innovate localization strategies that take advantage of locality on timestep and Newton iteration levels. The strategies readily accommodate to complicated flow mechanisms and multiscale fracture networks in unconventional reservoirs. Large simulation speed-up can be obtained if performing localized computations only for the solution regions that will change. We develop an a-priori method to exploit the locality, based on the diffusive character of the Newton updates of pressure. The method makes adequate estimate of the active computational gridblock for the next iterate. The active gridblock set marks the ones need to be solved, and then the solution to local linear system is accordingly computed. Fully Implicit Scheme is used for time discretization. We study several challenging multi-phase and compositional model cases with explicit fractures. The test results demonstrate that significant solution locality of variables exist on timestep and iteration levels. A nonlinear solution update usually has sparsity, and the nonlinear convergence is restricted by a limited fraction of the simulation model. Through aggressive localization, the proposed methods can prevent overly conservative estimate, and thus achieve significant computational speedup. In comparison to a standard Newton method, the novel solver techniques achieve greatly improved solving efficiency. Furthermore, the Newton convergence exhibits no degradation, and there is no impact on the solution accuracy. Previous works in the literature largely relate to the meshing aspect that accommodates to horizontal wells and hydraulic fractures. We instead develop new nonlinear strategies to perform localization. In particular, the adaptive DD method produces proper domain partitions according to the fluid flow and nonlinear updates. This results in an effective strategy that maintains solution accuracy and convergence behavior.