158 research outputs found
The effects of different nano particles of Al2O3 and Ag on the MHD nano fluid flow and heat transfer in a microchannel including slip velocity and temperature jump
No full textThe forced convection of nanofluid flow in a long microchannel is studied numerically according to the finite volume approach and by using a developed computer code. Microchannel domain is under the influence of a magnetic field with uniform strength. The hot inlet nanofluid is cooled by the heat exchange with the cold microchannel walls. Different types of nanoparticles such as Al2O3 and Ag are examined while the base fluid is considered as water. Reynolds number are chosen as Re=10 and Re=100. Slip velocity and temperature jump boundary conditions are simulated along the microchannel walls at different values of slip coefficient for different amounts of Hartmann number. The investigation of magnetic field effect on slip velocity and temperature jump of nanofluid is presented for the first time. The results are shown as streamlines and isotherms; moreover the profiles of slip velocity and temperature jump are drawn. It is observed that more slip coefficient corresponds to less Nusselt number and more slip velocity especially at larger Hartmann number. It is recommended to use Al2O3-water nanofluid instead of Ag-water to increase the heat transfer rate from the microchannel walls at low values of Re. However at larger amounts of Re, the nanofluid composed of nanoparticles with higher thermal conductivity works better.The forced convection of nanofluid flow in a long microchannel is studied numerically according to the finite volume approach and by using a developed computer code. Microchannel domain is under the influence of a magnetic field with uniform strength. The hot inlet nanofluid is cooled by the heat exchange with the cold microchannel walls. Different types of nanoparticles such as Al2O3 and Ag are examined while the base fluid is considered as water. Reynolds number are chosen as Re=10 and Re=100. Slip velocity and temperature jump boundary conditions are simulated along the microchannel walls at different values of slip coefficient for different amounts of Hartmann number. The investigation of magnetic field effect on slip velocity and temperature jump of nanofluid is presented for the first time. The results are shown as streamlines and isotherms; moreover the profiles of slip velocity and temperature jump are drawn. It is observed that more slip coefficient corresponds to less Nusselt number and more slip velocity especially at larger Hartmann number. It is recommended to use Al2O3-water nanofluid instead of Ag-water to increase the heat transfer rate from the microchannel walls at low values of Re. However at larger amounts of Re, the nanofluid composed of nanoparticles with higher thermal conductivity works better
Aero/Hydrodynamics and Symmetry
No full textThis book presents collective works published in the recent Special Issue (SI) entitled "Aero/Hydrodynamics and Symmetry". These works address the existence of symmetry and its breakdown in aero-/hydro-dynamics and their related applications. The presented problems are complex nonlinear, non-Newtonian fluid flow problems that are (in some cases) coupled with heat transfer, phase change, nanofluidic, and magnetohydrodynamics phenomena. The applications vary and range from polymer chain transfer in micro-channel to the evaluation of vertical axis wind turbines, as well as autonomous underwater hovering vehicles. Recent advances in numerical, theoretical, and experimental methodologies, as well as finding new physics, new methodological developments, and their limitations are presented within the scope of the current book. Among others, in the presented works, special attention is paid to validation and improving the accuracy of the presented methodologies. This book brings together a collection of inter-/multi-disciplinary works applied to many engineering applications in a coherent manner
Computational Fluid Dynamics 2020
No full textThis book presents a collection of works published in a recent Special Issue (SI) entitled “Computational Fluid Dynamics”. These works address the development and validation of existent numerical solvers for fluid flow problems and their related applications. They present complex nonlinear, non-Newtonian fluid flow problems that are (in some cases) coupled with heat transfer, phase change, nanofluidic, and magnetohydrodynamics (MHD) phenomena. The applications are wide and range from aerodynamic drag and pressure waves to geometrical blade modification on aerodynamics characteristics of high-pressure gas turbines, hydromagnetic flow arising in porous regions, optimal design of isothermal sloshing vessels to evaluation of (hybrid) nanofluid properties, their control using MHD, and their effect on different modes of heat transfer. Recent advances in numerical, theoretical, and experimental methodologies, as well as new physics, new methodological developments, and their limitations are presented within the current book. Among others, in the presented works, special attention is paid to validating and improving the accuracy of the presented methodologies. This book brings together a collection of inter/multidisciplinary works on many engineering applications in a coherent manner
Numerical simulation of compressible flows by lattice Boltzmann method
No full textInternational audienceIn this article, we propose a numerical framework based on multiple relaxation time lattice Boltzmann (LB) model and novel discretization techniques for simulating compressible flows. Highly efficient finite difference lattice Boltzmann methods are employed to simulate one- and two-dimensional compressible flows. These numerical techniques are applied on the single- and multiple-relaxation-time on the 16-discrete-velocity (Kataoka and Tsutahara, Phys. Rev. E, 69(5):056702, 2004) compressible lattice Boltzmann model. The Boltzmann equation is discretized via modified Lax-Wendroff and modified total variation diminishing schemes which have ability to damps oscillations at discontinuities, effectively. The results of compressible models are compared and validated with the well-known inviscid compressible flow benchmark test cases, so called Riemann problems. The proposed method shows its superiority over available techniques when compared to the analytical solutions. It is then used to solve two-dimensional inviscid compressible flow benchmarks, including regular shock reflection and Richtmyer–Meshkov instability problems to ensure its applicability for more complex problems. It is found that, the applied discretization techniques improve the stability of original LB models and enhance the robustness of compressible flow problems by preventing the formation of oscillation
Numerical simulation of compressible flows by lattice Boltzmann method
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Etude et analyse de la régénération des filtres à suie à l'aide de la méthode Lattice Boltzmann
No full textLa maîtrise des émissions de noir de carbone est une tâche importante dans de nombreux domaines d'application, le secteur des transports étant l'un des domaines les plus importants. Les moteurs diesel, encore largement utilisés dans le monde entier, sont l'une des principales sources d'émissions anthropiques de noir de carbone. Afin de contrer l'effet néfaste du noir de carbone sur la santé humaine, le traitement des gaz d'échappement est au centre de la recherche depuis de nombreuses décennies. Les filtres à suie de pointe utilisent une structure en nid d'abeille en céramique, agissant comme des filtres à flux mur. Ces filtres nécessitent une régénération périodique une fois qu'une contre-pression de filtre critique est atteinte. La régénération est effectuée soit sous forme de régénération active à des températures élevées (>600 °C), soit en continu, sous forme de régénération passive à des températures à partir de 300 °C. La température nécessaire des gaz d'échappement pour la régénération active entraîne une pénalité en carburant, rendant le contrôle précis du processus de régénération impératif. Des travaux antérieurs ont suggéré que la morphologie mésoscopique de la suie et son évolution pendant la combustion de la suie influencent la réactivité, affectant ainsi le processus de régénération. Par conséquent, le contrôle du système de régénération nécessite une connaissance précise des phénomènes physiques et chimiques en jeu, nécessitant des simulations du processus de régénération. Dans cette thèse, un cadre de simulation pour modéliser l'écoulement de gaz, composé des différentes espèces réactives, en tenant compte des interactions solide-gaz, est créé. De plus, le transfert de chaleur conjugué, les réactions hétérogènes et la libération de chaleur de réaction à l'interface entre les phases solide et gazeuse sont traités. À cette fin, la méthode de Boltzmann sur réseau (LBM), en raison de sa nature mésoscopique, est choisie comme un excellent outil pour modéliser la combustion hétérogène à l'échelle des pores. Dans cette thèse, un cadre LBM est créé et des méthodes appropriées pour modéliser la combustion de la suie sont choisies et largement validées. Une procédure d'utilisation des données de microscopie électronique à balayage par faisceau ionique focalisé (FIB-SEM) de véritables échantillons de suie pour la simulation de combustion est mise en œuvre. De plus, les régimes de combustion sont analysés en fonction de la variation du nombre de Péclet, du nombre de Damköhler et de la fraction molaire d'oxygène dans le flux gazeux d'entrée. Des simulations avec des géométries de suie réalistes sont réalisées et les résultats sont comparés avec des résultats expérimentaux. Il est constaté que l'évolution de la surface réactive spécifique, telle que reçue des simulations LBM, n'est pas comparable aux résultats expérimentaux. L'analyse par microscopie électronique à transmission (TEM) et les spectres Raman de la suie avant et après les expériences de combustion ont révélé que la combustion affecte les particules primaires à l'échelle nanométrique. Pour cette raison, un modèle séparé pour décrire les particules primaires hétérogènes et leur combustion a été créé. Ensuite, les premières simulations avec couplage d'échelle ont été menées, en reliant les simulations LBM mésoscopiques avec la conception des particules primaires à l'échelle nanométrique. Il est démontré qu'une augmentation plus réaliste de la surface spécifique peut être obtenue dans les simulations en couplant le modèle LBM mésoscopique avec un modèle de particules primaires à l'échelle nanométrique.The control of the emission of carbon black is an important task in many fields of application, with the transport sector being one of the most important domains. Diesel engines, still being extensively used worldwide, are one of the main contributors to the anthropogenic emission of carbon black. In order to counteract the detrimental effect of carbon black on human health, exhaust gas treatment has been the focal point of research for many decades.State of the art soot filters use a ceramic honey-comb structure, acting as wall flow filters. These filters require periodic regeneration once a critical filter back-pressure is reached. Regeneration is conduced either as active regeneration at elevated temperatures (>600 °C) or continuously, as passive regeneration at temperatures starting from 300 °C. The necessary exhaust gas temperature of active regeneration results in a fuel penalty, making the precise control of the regeneration process imperative. Previous works suggested that the mesoscopic morphology of soot and its evolution during soot combustion influence the reactivity, thus affecting the regeneration process. Hence, the control of the regeneration system requires precise knowledge of the physical and chemical phenomena at hand, necessitating simulations of the regeneration process.In this thesis, a simulation framework to model gas flow, consisting of the different reactive species, taking into account solid-gas interactions, is created. Furthermore, conjugate heat transfer, heterogeneous reactions and the release of reaction heat at the interface between the solid and gas phases is treated. For this purpose, the lattice Boltzmann method (LBM), due to its mesoscopic nature, is chosen as an excellent tool to model the heterogeneous combustion on the pore scale. Within this thesis, a LBM framework is created and appropriate methods to model soot combustion are chosen and extensively validated. A procedure to use focused ion beam scanning electron microscopy (FIB-SEM) data of realistic soot samples for the combustion simulation is implemented. Furthermore, the combustion regimes are analysed based on variation of Péclet number, Damköhler number, and oxygen mass fraction in the inlet gas stream. Simulations with realistic soot geometries are performed and the results are compared with experimental results. It is found that the evolution of the specific reactive surface, as received from LBM simulations, is not comparable to the experimental results. Transmission electron microscopy (TEM) analysis and Raman spectra of the soot before and after combustion experiments revealed that combustion affects the primary particles on the nano-scale. For this reason, a separate model to describe the heterogeneous primary particles and their combustion was created. Subsequently, first simulations with scale-coupling were conducted, by connecting the mesoscopic LBM simulations with the primary particle design on the nano-scale. It is shown that a more realistic increase in specific surface could be achieved in simulations by coupling the mesoscopic LBM model with a nano-scale primary particle model
Insights into transitional supersonic boundary layers : DNS investigations and streak control strategies
No full textDans les écoulements à haute vitesse, une traînée visqueuse élevée et des charges thermiques importantes sont des conséquences inhérentes sur les corps aérodynamiques. Ces effets augmentent de manière significative pendant la phase de transition lorsque la couche limite devient turbulente. Afin de réduire les risques de dommages mécaniques et de défaillances liées à la fatigue, des systèmes de protection thermique sont intégrés aux véhicules, ajoutant de la complexité aux aspects techniques et économiques de la conception. La solution réside dans l’acquisition d’une compréhension approfondie des mécanismes de transition et le développement de systèmes de contrôle pour prolonger la couche limite laminaire le long de la surface du véhicule. De nombreuses techniques de contrôle actives et passives peuvent être utilisées pour le contrôle de la transition, parmi lesquelles la méthode de l’emploi de stries émerge comme une approche particulièrement prometteuse. Cette méthode consiste à générer des stries étroitement espacées dans la direction de l’envergure, créant des zones alternées de haute et basse vitesse dans le champ d’écoulement. Bien que la méthode ait été testée récemment dans des écoulements supersoniques, démontrant son efficacité pour retarder la transition, sa pertinence doit être évaluée plus avant. Dans ce travail de recherche, des cas de DNS sont réalisés dans des régimes supersoniques et près-hypersoniques. Les stries sont introduites à l’aide d’une bande de soufflage/aspiration placée sur la paroi avant celle de la perturbation qui est utilisée pour déclencher la transition de manière “contrôlée”, forcée par une perturbation à une seule fréquence et longueur d’onde. L’enquête à Mach 2.0 confirme que les stries avec cinq fois la longueur d’onde fondamentale sont les plus bénéfiques pour le contrôle de la transition. De plus, le refroidissement améliore l’efficacité de la méthode, tandis que le chauffage détériore considérablement la capacité de contrôle des stries. La condition murale isotherme n’altère pas l’impact stabilisateur comparable de la déformation du flux moyen (DFM) et de la partie 3D du contrôle à Mach 2.0. Cependant, à Mach 4.5, tant le type d’instabilité que les caractéristiques des stries changent de manière significative. L’impact stabilisateur de la DFM devient presque absent, et la partie 3D du contrôle prédomine, les caractéristiques des stries n’étant plus considérées comme indépendantes de leur amplitude de perturbation initiale.In high-speed flows, elevated viscous drag and thermal loads are inherent outcomes over aerodynamic bodies. These effects escalate substantially during the transition phase when the boundary layer becomes turbulent. To mitigate potential mechanical damage and fatigue-related failures, thermal protection systems are integrated into vehicles, adding complexity to the technical and economic aspects of design. The solution lies in gaining a comprehensive understanding of transition mechanisms and developing control systems to prolong laminar boundary layer along the vehicle’s surface. Numerous active and passive control techniques can be employed for transition control, with the streak employment method emerging as a particularly promising approach. This method involves generating narrowly spaced streaks in the spanwise direction, creating alternating high and low-speed regions in the flow field. Although the method has only recently been tested in supersonic flows, demonstrating its effectiveness in delaying transition, its suitability needs to be assessed further. In this research work, direct numerical simulations are performed in supersonic and near-hypersonic regimes. Streaks are introduced through a blowing/suction strip placed at the wall prior to that of the perturbation which is used to trigger transition in a “controlled” fashion, forced by a single frequency and wavenumber disturbance. The investigation at Mach 2.0 confirms that streaks with five times the fundamental wavenumber are most beneficial for transition control. Additionally, cooling enhances the method’s effectiveness, while heating severely deteriorates the capability of control streaks. The isothermal wall condition does not alter the comparable stabilizing impact of the mean flow deformation (MFD) and the 3-D part of the control at Mach 2.0. However, at Mach 4.5, both the type of instability and the characteristics of the streaks change significantly. The stabilizing impact of the MFD becomes nearly absent, and the 3-D part of the control predominates, with the characteristics of the streaks no longer considered independent of their initial disturbance amplitude
Simulation of Rayleigh-Taylor instability by smoothed particle hydrodynamics: advantages and limitations
No full textA Smoothed Particle Hydrodynamics (SPH) solution to the Rayleigh-Taylor Instability (RTI) problem in an incompressible viscous two-phase immiscible fluid with surface tension is presented. To validate the numerical model for the RTI problem, simulation results are quantitatively compared with analytical solutions in linear regime. It is found that the SPH method slightly overestimates the border of instability. The long time evolution of simulations is presented for investigating changes in the topology of rising bubbles and falling spike in RTI. It is shown that the numerical algorithm used in this work is capable of capturing the interface evolution and growth rate in RTI accurately
Numerical simulation of shock/droplets interaction with poly-dispersion of cloud particles
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