Direct forcing immersed boundary method: Improvements to the Ghost Node Method

It has been previously shown that the ghost-cell immersed boundary methods (IBMs) with a maximum stencil size larger than 1 do not yield band matrices and as a result cannot use the more efficient geometric multi-grid algorithms and instead must rely on the more generic and less efficient algebraic multi-grid algorithms. To address these shortcomings and in the pursuit of smaller total run times, smaller memory requirements, and increased accuracy the current article proposes the linear square shifting and the quadratic ghost node methods for the ghost-cell IBM for Cartesian grids. The linear square shifting method guarantees a maximum stencil size of 1 for all immersed boundaries and the increases in accuracy and convergence of the proposed method are comprehensively verified with the canonical verification Poisson test problem. A comprehensive analysis of the effect of the quadratic ghost node method together with the shifting approach for various immersed boundary conditions is also performed with the Poisson test problem. The improved computational efficiency of these methods, and their various combinations, is also verified through the canonical validation test cases of laminar pipe flow and laminar flow past a sphere for various Reynolds numbers, wherein speed-ups of approximately three are achieved

Contribution à la modélisation de la pollution atmosphérique dans les villes

La pollution générée par la circulation automobile au cúur de la ville ainsi que par les activités industrielles à sa périphérie est un problème aigu des grandes cités. Une fois émis dans l'atmosphère, les polluants subissent deux types de contraintes : d'une part ils réagissent chimiquement entre eux donnant naissance à de nouveaux polluants tels que l'ozone, et d'autre part ils sont transportés par les vents. Nous avons choisi de travailler à la fois sur les aspects dynamiques, thermiques et chimiques de la pollution et ce aux différentes échelles des phénomènes : l'échelle locale autour d'un groupe de bâtiments et la méso-échelle pour la modélisation complète d'une ville. Notre ambition n'est pas de réaliser un modèle complet mais d'avancer pas à pas sur chacun de ces aspects. Nous avons participé au développement d'un code de calcul qui pourra ensuite être complété et utilisé pour des études plus concrètes d'épisodes de pollution. Le premier chapitre est consacré à une description de la couche limite atmosphérique. Les chapitres deux et trois présentent les modèles utilisés et le résultat de simulations numériques. Nous insistons particulièrement sur les modèles de turbulence k-e, RNG et v2-f, sur les modèles de rayonnement ainsi que les lois de paroi dynamiques et thermiques permettant la prise en compte de la rugosité de surface. Le dernier chapitre est consacré à la chimie de l'atmosphère. Nous avons également développé un solveur général traitant un nombre quelconque d'équations de transport couplées par une cinétique chimique raide ainsi qu'une cinétique simplifiée de l'ozone.Pollution generated by urban traffic and by industry at its periphery is a serious problem of our cities. Once emitted in the atmosphere, pollutants undergo two types of constraints: on one side they react chemically one with another and, on the other side, they are transported by the wind. We have chosen to work on the dynamical, thermal and chemical aspects of the pollution at different scales of the phenomena: local scale (around a group of buildings) and meso-scale (a whole city). Our ambition is not to realize a complete model but to advance step by step on each aspect. We have contributed to the realization of a numerical tool that will be completed and used for concrete pollution episodes. The first chapter is a description of the atmospheric boundary layer. Chapters two and three present models and results of numerical simulations. We stress on k-e, RNG and v2-f turbulence models, radiative heat transfer models and dynamical and thermal wall laws that allow to take in consideration surface roughness. The last chapter is dedicated to atmospheric chemistry. We have moreover developed a general solver of any number of transport equations coupled by a stiff chemistry and simplified ozone kinetics

Numerical study of the hydrodynamics of regular waves breaking on a sloping beach

International audienceIn the last three decades, great improvements have been brought to the knowledge of the hydrodynamics and general processes occurring in the surf zone, widely affected by the breaking of the waves. Nevertheless, the turbulent flow structure is still very complicated to investigate. The aim of this work is to present and discuss the results obtained by simulating twodimensional breaking waves by solving the Navier-Stokes equations, in air and water, coupled with a dynamic subgrid scale turbulence model (Large Eddy Simulation, LES). First, the ability of the numerical tool to capture the crucial features of this complicated turbulent two-phase flow is demonstrated. Numerical results are compared with experimental observations provided by Kimmoun and Branger [1]. Spilling/plunging breaking regular waves are considered. Generally, there is good agreement and the model provides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore

Modélisation numérique Navier-Stokes/VOF de tsunamis générés par un glissement de terrain aérien

La génération de vagues par des glissements de terrain est étudiée à l'aide d'un modèle basé sur les équations de Navier-Stokes et un suivi d'interface PLIC-VOF ou TVD Lax Wendroff. L'originalité de l'étude repose sur la prise en compte implicite du couplage glissement/fluide. Le modèle est validé dans un premier temps, dans le cas de génération d'un soliton par la chute d'un bloc rigide à l'extrémité d'un canal. Une étude numérique de l'influence du caractère déformable du glissement est ensuite proposée. Ce travail met en évidence l'importance et la complexité du rôle de cette déformation sur les caractéristiques des vagues générées. Il convient donc de prendre en compte de manière plus fine la rhéologie du glissement dans le processus de prédiction des tsunamis

Conditions limites de sortie pour les méthodes de time-splitting appliquées aux équations Navier-Stokes

La simulation d écoulements incompressibles pose de nombreuses difficultés. Une première est la question de savoir comment traiter la contrainte d incompressibilité et le couplage vitesse/pression afin d obtenir une solution précise à moindre coût. Pour cela, nous nous intéressons en particulier à deux méthodes de time splitting : la correction de pression et la correction de vitesse. Une seconde difficulté porte sur des conditions limites de sortie. Nous nous intéressons ici à deux d entre elles : la condition limite de traction et la condition limite d Orlanski. Après avoir détaillé les difficultés pouvant apparaître lors de l implémentation des méthodes de time-splitting, nous proposons une nouvelle implémentation de la condition limite de traction qui permet d améliorer les ordres de convergence obtenus. Nous nous intéressons ensuite à la condition limite d Orlanski qui nécessite une certaine vitesse d advection C dans la direction normale à la limite dont nous proposons ici une nouvelle définition. Nos propositions sont confrontées à de multiples écoulements physiques afin de valider leurs comportements : l écoulement en aval d une marche descendante, l écoulement au niveau d une bifurcation,l écoulement autour d un obstacle et des écoulements de Poiseuille-Rayleigh-Bénard.One of the understudied difficulties in the simulation of incompressible flows is how to treat the incompressibilityconstraint and the velocity/pressure coupling in order to obtain an accurate solution at low computationnalcost. In this context, we develop two methods: pressure-correction and velocity-correction. An anotherdifficulty is due to the boundary conditions. We study here two of them : the traction boundary condition andthe Orlanski boundary condition. After having developed the difficulties that appears when implementing timesplittingmethods, we propose a new way to enforce the traction boundary condition which improves the orderof convergence. Then we propose a new definition of the advective velocity C which is needed for the Orlanskiboundary condition. Our propositions are validated against multiple physical flows: flow over a backward facingstep, flow around a biffurcation, flow around an obstacle and several Poiseuille-Rayleigh-Bénard flows.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Simulation de l'écoulement sous une onde positive ondulée

Un mascaret se forme lors de la propagation d'une onde de marée de grande amplitude dans un estuaire. Le mascaret continue sa course en remontant le cours de la rivière ou du fleuve. Ce phénomène est une onde positive caractérisée par l'évolution d'un front déferlant ou lisse, qui peut être suivi ou non d'ondulations. Cependant, c'est un évènement violent induisant une forte turbulence et de forts mélanges. Les études de terrain sont encore limitées et les effets du mascaret sont encore peu détaillés. Une façon d'apporter plus d'information sur le phénomène des mascarets est d'utiliser la simulation numérique. Le mascaret est étudié dans le cadre du projet ANR MASCARET qui prévoit des études du phénomène sur le terrain, en laboratoire et par des simulations numériques. La simulation des équations de Navier-Stokes est ici réalisée avec des méthodes « VOF » pour le suivi de la surface libre et la méthode de Simulation des Grandes Échelles pour prendre en compte les effets turbulents. Le mascaret peut être modélisé par une surélévation soudaine du niveau d'eau dans un canal, se propageant à contrecourant. Dans le cas de la simulation d'onde positive ondulée, les résultats ont montré l’apparition de recirculations sous les crêtes des ondulations. Ces structures tourbillonnaires semblent induire, dans leur sillage, des fluctuations qui grossissent et se propagent dans la partie supérieure de la colonne d'eau. Ces résultats montrent pour la première fois la complexité de l'écoulement turbulent et permettraient d'expliquer une partie des phénomènes de resuspension et de mélange observés lors de la propagation d'un mascaret ondulé

Supercritical water oxidation using hydrothermal flames at microscale as a potential solution for organic waste treatment in space applications – A practical demonstration and numerical study

Supercritical water oxidation (SCWO) with hydrothermal flames is well established for the treatment of aqueous organic waste as it not only overcomes the limitations of simple SCWO, such as precipitation of salts, but also exhibits many advantages over other waste treatment processes. Seeking these advantages, we propose to perform SCWO using hydrothermal flames in microfluidic reactors ) for aerospace applications to be used in deep space/ISS missions. The novelty and highlight of this work are successful demonstration of realizing microreactors (channel width 200 ), which can withstand pressure of 250 bar with temperature °C, thereby presenting the feasibility to realize this technology. We present the first evidence of SCWO/hydrothermal in a flow microreactor of sapphire, which is captured through optical visualization. This is followed by a numerical investigation to understand the underlying physics leading to the formation of hydrothermal flame and thus differentiate it from a simple SCWO reaction. The simulations are performed in a 2D domain in a co-flow configuration with equal inlet velocity of fuel and oxidizer at two different inlet temperatures (350 °C and 365 °C), just below the critical temperature of water using ethanol and oxygen, the former acting not only as a model organic matter but also fuel for the formation of hydrothermal flames. It is observed that due to microscale size of the system, hydrothermal flames are formed at low inlet velocities (< 30 mm/s), while reaction at higher ones are characterized as simple SCWO reaction. This upper limit of inlet velocity was found to increase with inlet temperature. Finally, some key characteristics of hydrothermal flames - ignition delay time, flame structure, shape, and local propagation speed are analyzed

The Genome of Borrelia recurrentis, the Agent of Deadly Louse-Borne Relapsing Fever, Is a Degraded Subset of Tick-Borne Borrelia duttonii

In an effort to understand how a tick-borne pathogen adapts to the body louse, we sequenced and compared the genomes of the recurrent fever agents Borrelia recurrentis and B. duttonii. The 1,242,163–1,574,910-bp fragmented genomes of B. recurrentis and B. duttonii contain a unique 23-kb linear plasmid. This linear plasmid exhibits a large polyT track within the promoter region of an intact variable large protein gene and a telomere resolvase that is unique to Borrelia. The genome content is characterized by several repeat families, including antigenic lipoproteins. B. recurrentis exhibited a 20.4% genome size reduction and appeared to be a strain of B. duttonii, with a decaying genome, possibly due to the accumulation of genomic errors induced by the loss of recA and mutS. Accompanying this were increases in the number of impaired genes and a reduction in coding capacity, including surface-exposed lipoproteins and putative virulence factors. Analysis of the reconstructed ancestral sequence compared to B. duttonii and B. recurrentis was consistent with the accelerated evolution observed in B. recurrentis. Vector specialization of louse-borne pathogens responsible for major epidemics was associated with rapid genome reduction. The correlation between gene loss and increased virulence of B. recurrentis parallels that of Rickettsia prowazekii, with both species being genomic subsets of less-virulent strains

numerical simulations of spilling breaking waves

International audienceNumerical simulation of spilling breaking waves is still a very challenging aim to achieve since small interface deformations, air entrainment and vorticity generation are involved during the early stage of the breaking of the wave. High mesh grid resolutions and appropriate numerical methods are required to capture accurately the length scales of the complex mechanisms responsible for the start of the breaking (small plunging jet, white foam, etc.). Numerical works usually showed better agreements when simulating plunging breaking waves than the spilling case compared with available experimental data. Kimmoun and Branger (2007) recently experimented surf-zone breaking waves. Detailed pictures showed a short spilling event occurred at the crest of the waves, before degenerating into strong plunging breaker. This work is devoted to the qualitative comparison of our numerical results with the experimental observations, as we will focus on capturing and describing the spilling phase experimented

A COUPLED NUMERICAL MODEL FOR TSUNAMIS GENERATED BY SUBAERIAL AND SUBMARINE MASS FAILURES

This paper presents a new numerical model simulating tsunamis generation by landslides. Water, air, and the slide (considered either as a viscous fluid or as a rigid bloc using the penalization method) are described by Navier-Stokes equations, expressed in a unified single fluid approach. The PLIC-VOF method is used to describe the motion of fluid interfaces. We present results for two test cases featuring solid blocks. The first one is that of a semi-elliptical body sliding over a plane slope, without surrounding water. We find, the slide motion is accurately reproduced by the model when the slide viscosity is around 10 4 Pa.s. For larger viscosity values, the conditioning of the matrix associated with the Navier-Stokes solver is degraded. In the second case, a falling rectangular block generates waves in a flume. We find, wave height and box velocity are in agreement with experiments by Monaghan and Kos (2000). The simulated flow close to the falling box is similar to that observed in experiment, except that the observed main plunging wave and associated air entrainment.do not occur. In future work, the model thus validated will be applied to simulate more realistic cases of tsunamis generated by subaerial landslides