Auswahl der wissenschaftlichen Literatur zum Thema „Turbulent flow“

An experimental method is proposed to study dispersed two-phase flows at an airwater interface, a family of flows of practical significance in environmental and industrial settings. The applicability of this technique is demonstrated through the study of a lightly-deformed turbulent free-surface laden with floating particles (`floaters'). A low-mean turbulent flow is generated in a turbulence box actuated by a 10×10 synthetic jet array. Using LEDs and a single camera, free-surface flow measurements are carried out by Particle Image Velocimetry (PIV) simultaneously with Lagrangian tracking of the floaters, allowing the potential to characterise the coupling between the floater dynamics and the (sub)surface flow. Discrimination of the dispersed and continuous phases is carried out based on size. Individual floaters and clusters of floaters are successfully tracked throughout the field of view while they navigate through elongated and circular regions of high and low vorticity, characteristic features typically observed when a subsurface turbulent flow interacts with a free surface. Preliminary results of the floater-fluid interactions are presented to highlight the potential of this technique to better our understanding of floaterladen turbulent free surfaces.

Two-phase pipe flow is a common occurrence in many industrial applications such as sewage, water, oil, and gas transportation. Accurate prediction of liquid velocity, holdup and pressure drop is of vast importance to ensure effective design and operation of fluid transport systems. This paper aimed at the simulation of a two-phase flow of air and sewage (water) using an open source software OpenFOAM. Numerical Simulations have been performed using varying dimensions of pipes as well as their inclinations. Specifically, a Standard k- turbulence model and the Volume of Fluid (VOF) free water surface model is used to solve the turbulent mixture flow of air and sewage (water). A two dimensional, 0.5m diameter pipe of 20m length is used for the CFD approach based on the Navier-Stokes equations. Results showed that the flow pattern behaviour is influenced by the pipe diameters as well as their inclination. It is concluded that the most effective way to optimize a sewer network system for Tororo Municipality conditions and other similar situations, is by adjusting sewer diameters and slope gradients and expanding the number of sewer network connections of household and industries from 535 (i.e., 31.2% of total) to at least 1,200 (70% of total).

Three-dimensional flow around a horizontal-axis wind turbine has been investigated with LES method coupled with sliding mesh and experimental measurement. The boundary conditions are set as the same as those of the experiment. The images of the pressure distribution, flow rate distribution, turbulent intensity, velocity vector and vortices of the wind turbine are presented to show the three-dimensional flow characteristics around the wind turbine. The relationship between flow and sound is further studied by analyzing the flow parameters pulsation spectrum to get the sound pressure level. LES results are compared with the wind-tunnel measurements collected with PIV, and good agreement is observed. The results serve as a reference for optimum design of the wind turbine with high functional performance and low level noise generation.

Introduction. Mathematical modelling of currents is an urgent research topic in the field of hydrodynamics and oceanography. Despite ongoing research in the field of developing accurate and efficient numerical methods for solving Navier-Stokes equations that take into account vortex viscosity, the problems of accurate prediction and control of turbulence remain unresolved. The influence of nonlinear effects in vortex viscosity models on the accuracy of forecasts and their applicability to various flow conditions also remains relevant. The aim of the study is to study the influence of linearized and quadratic bottom friction and two turbulence models on the numerical solution of stationary and non-stationary periodic flows. Special emphasis is placed on comparing numerical results with analytical solutions within the framework of using various models of bottom friction.Materials and Methods. The computational models used in this study are based on a simplified two-dimensional wave model and full three-dimensional Navier-Stokes equations. The classical model of shallow water motion and the 2D model without taking into account dynamic changes in the geometry of the reservoir surface are derived from a system of equations for a spatially inhomogeneous three-dimensional mathematical model of wave hydrodynamics of a shallow reservoir. Analytical solutions were found by linearization of the equations, which obviously has its limitations. A distinction is made between two types of nonlinear effects – nonlinearities caused by higher-order terms in the equations of motion, i. e. terms of advective acceleration and friction, and nonlinear effects caused by geometric nonlinearities, this is due, for example, to different water depths and reservoir widths, which will be important when modelling a real sea.Results. The results of modeling stationary and non-stationary periodic flows in a schematized rectangular basin using linearized bottom friction are presented. The influence of linearization on the numerical solution is investigated in comparison with analytical profiles using models calculating bottom friction in a quadratic formulation. In combination with quadratic bottom friction, two turbulence models are studied: the constant vortex viscosity and the Prandtl mixing length model. The results obtained as a result of three-dimensional modelling are compared with the results of two-dimensional modeling and analytical solutions averaged in depth.Discussion and Conclusion. New approaches to modelling and studying flows with variable vortex viscosity are proposed, including analysis of the influence of linearization and the use of various turbulence models. For the linearized and quadratic formulations of bottom friction, it is proved that the numerical results for the case of stationary flow show great similarity with analytical solutions, since the surface height is much less than the water depth and advection can be neglected. The numerical results for the unsteady flow also show a good agreement with the theory. Unlike analytical solutions, numerical modelling has minor deviations in the long run. The study of flows, within the framework of using various turbulence models, will make it possible to take into account the influence of nonlinear effects in vortex viscosity models on the accuracy of forecasts and their applicability to various flow conditions. The results obtained make it possible to better understand and describe the physical processes occurring in shallow waters. This opens up new possibilities for applying mathematical modelling to predict and analyze the impact of human activities on the marine environment and to solve other problems in the field of oceanology and geophysics.

The air separators are used to provide safe, clean and appropriate air to the helicopter’s engine. In this operational study, the separation process inside a Ranque-Hilsch air separator cleaning system has been investigated to analyze the impact of choosing the appropriate turbulence model for predicting the separation process inside the air separator. This research is directed towards presenting a computational fluid dynamic explanation performed on a counter-flow air separator using air at different magnitudes of air flow fraction and applying different turbulence models. In a numerical investigation of counter-flow air separator, air has been chosen and its vortex separation phe- nomenon has been analyzed as a function of flow fraction. Furthermore, a numerical analysis to compare the outputs of a seven equation RSM turbulence model applied for the study of vortex separation of a counter-flow air separator with some two-equation turbulence methods, namely, k-ε and k-ω model as well as LES has been presented. All of the turbulence numerical methods are seen to present and predict the same flow pattern inside an air separator, but, with various details. The results show that among the tested methods the RSM creates the most accurate separation pattern. The numerical results are validated by some available experimental data with good agreement.

This study focuses on the analysis of long-exposure-time interferograms obtained with an Interferometric Rayleigh Scattering (IRS) set-up pointing at a high subsonic isothermal jet flow. The objectives are twofold: retrieve from seeding-free optical measurements the mean characteristics of this flow - in particular the mean velocity profile in the jet shear-layer - and determine higher order statistics for the flow velocity in the shear layer. The results are compared to those obtained on the same test bench by use of hot wire anemometry, and conclusions about the relevancy of the approach together with potential improvements are deduced therefrom.

Introduction. The negative consequences that may arise due to an accidental oil spill are difficult to account for, since they disrupt many natural processes and relationships within the ecosystem of the reservoir. After an oil spill, a dense layer of oil film forms on the water surface quite quickly, preventing access to air and light (after a spill of one ton of oil, an oil slick about 10 mm thick forms on the surface of the reservoir after 10 minutes). As a result, the fauna and flora of the reservoir suffer. If the accident occurred in the coastal zone near a populated area, then the toxic effect is enhanced, because petroleum products in combination with various pollutants of human origin can form dangerous compounds. For high-risk areas (the main routes of transportation of petroleum products, places of their bunkering and unloading, etc.), it is necessary to predict various scenarios for the spread and transformation of oil pollution, taking into account their multifractional composition, turbulent diffusion and advective transport, destruction under the influence of natural factors. The aim of the work is to build a linearized non-stationary spatially heterogeneous mathematical model of transport and transformation of oil pollution, taking into account the above factors.Materials and Methods. The oil that has entered the aquatic environment is represented as a surface and suspended substance in the water column. Oil is subject to a variety of transformation processes: advection, gravitational spreading, emulsification, dispersion, dissolution, biodegradation, etc. The study of these processes and their forecasting, as a rule, requires the development of mathematical and software. In mathematical and numerical modeling, one should start from the system of Navier-Stokes equations and continuity equations, as well as introduce additional physical tolerances of the flow geometry, acceptable and justified in each case, as shown by world experience and objective analysis of the physical picture of processes. Mathematical modeling of the oil distribution process in coastal marine systems has been performed.Results. Mathematical oil distribution model has been created, taking into account its multifractional composition. It is assumed that oil fractions can be in water in dissolved or undissolved states. The modeling takes into account such physical characteristics of particles as density, acceleration of gravity, molar mass, etc. After the linearization of the problem under consideration, difference schemes using extended uniform grids were constructed.Discussion and Conclusion. Pollution caused by an oil spill in the aquatic environment occurs very quickly and is often very destructive. An important factor will be prompt response, which plays a crucial role in minimizing its negative consequences. Modeling of the oil spill process can be useful for determining the location and condition of oil at sea, conducting a risk analysis of the spread of the substance and developing measures to localize and eliminate pollution.

In this paper, the development features of a single free jet of hightemperature nitrogen interacting with a flat surface were studied. Calculation of the heat exchange process during heating by the attacking jets is very difficult to implement analytically due to complexity of the gas-dynamic processes occurring both in a single jet and in a system of jets interacting with the metal. The computational difficulties are aggravated by the fact that when interacting with the surface the jet as such disappears. The flat (fan) flow interacts with the surface: form, aerodynamic properties and thermal state of the flow strongly differ from those of the original jet. The studies were conducted on the basis of numerical simulation in the FloEFD software and computing complex for multiphysical simulation based on solution of the equations of gas dynamics and heat transfer. The solved system of equations consisted of Navier-Stokes equations, equations of energy and continuity and was supplemented by k – ε turbulence model. A three-dimensional model was developed for simulation, the necessary properties, initial and boundary conditions were specified. In the study of aerodynamics of a single high-temperature jet interacting with the surface, the main defining values were: nitrogen flow rate from the nozzle U0 , nitrogen temperature T, internal diameter of the nozzle d0 , distance from the nozzle section to the surface h, distance from the critical point (point of intersection of the jet axis with the surface) along the flow radius r. Data on the gas velocity decrease as the jet develops due to the loss of initial energy to engage the motionless surrounding gas in motion, is presented. The studies have shown that increase in the initial velocity of gas outflow brings the area of higher velocities closer to the surface both in the jet itself and in the fan jet. This factor contributes to heat transfer intensification. In addition, high speeds increase the total thickness of the fan flow and reduce the thickness of hydrodynamic boundary layer, which increases with distance from the critical point.

Les émissions de particules hors-échappement, en particulier celles provenant des routes, sont devenues un important contributeur à la pollution de l'air liée au trafic. Ces particules pourraient contribuer à plus de la moitié de la concentration totale dans l'air. Les présents travaux de recherche développent et valident une méthodologie numérique pour analyser la remise en suspension de particules induite par une roue en rotation. Ils se concentrent sur l'identification des zones d'émission ainsi eu sur la compréhension du rôle de l'écoulement de l'air dans le détachement et le transport des particules dans le sillage.L'étude commence par des simulations d'écoulements diphasiques dans des régimes d'écoulement sous-critiques et critiques autour de cylindres statiques et rotatifs. Cette configuration constitue un cas fondamental bien établi, étroitement lié aux écoulements induits par les roues, pour l'étude de la transition de la couche limite, de la séparation, de la topologie des sillages et le transport de particules entraînées par les structures tourbillonnaires. Les résultats mettent en évidence l'influence cruciale du choix du modèle de turbulence dans la capture des transitions laminaires-turbulentes et dans l'amélioration des prédictions de l'écoulement du sillage. La rotation du cylindre affecte de manière significative la topologie du sillage et la dispersion des particules, avec des variations en fonction du régime d'écoulement et de la taille des particules.En s'appuyant sur les résultats de l'étude de l'écoulement autour d'un cylindre, des simulations ont été réalisées pour une roue isolée en rotation sur un sol en mouvement. Les simulations ont permis de capturer les principaux phénomènes d'écoulement, notamment la séparation de la couche limite, le pompage visqueux et les tourbillons de sillage cohérents tels que les structures de jetting, de formes en fer à cheval ou d'arche. Pour la phase particulaire, un modèle de détachement des particules a été introduit afin de simuler le processus de détachement, tandis que le suivi lagrangien des particules a été utilisé pour représenter le transport des particules en interaction avec l'écoulement. Les résultats ont permis d'identifier les zones d'émission prédominantes pour différentes tailles de particules, de quantifier les taux d'émission des particules et de caractériser les trajectoires de dispersion des particules dans le sillage proche et lointain de la roue.Enfin, l'étude a examiné les effets de la vitesse (nombre de Reynolds), du rapport d'aspect de la roue et de la charge surfacique sur la remise en suspension des particules. Des vitesses plus élevées ont impliqué des structures instationnaires du sillage plus intenses, accroissant les émissions et prolongeant le transport des particules en aval de la roue. Les roues plus larges augmentent les zones de détachement et les interactions tourbillonnaires, amplifiant considérablement les émissions. Des charges surfaciques plus élevées ont augmenté la masse des particules remises en suspension tout en modifiant les zones de dépôt au sol. Les résultats de cette étude ont permis de mieux comprendre les interactions entre les particules et les tourbillons, en démontrant la contribution des structures tourbillonnaires au transport des particules dans le sillage proche et lointain de la roue, ainsi qu'au dépôt des particules au sol.Ce travail fournit une bonne compréhension des émissions de particules remise en suspension induites par le passage d'une roue, offrant une approche de simulation validée pour analyser la contribution de la remise en suspension des particules à la pollution de l'air dans divers scénarios urbains
Non-exhaust particulate emissions, particularly from road dust, have emerged as a significant contributor to traffic-related air pollution. These particles could contribute to half of the particulate concentration found in the air. The present research develops and validates a numerical methodology to analyze particle resuspension induced by a rotating wheel. It focuses on identifying emission zones and understanding the role of airflow in particle detachment and transport within the wake flows.The study begins with simulations of particle-laden flows in subcritical and critical flow regimes around static and rotating cylinders. This configuration is a well-established fundamental case closely relevant to wheel-induced flows for investigating boundary-layer transition, flow separation, flow topology, and vortex-driven particle transport. Results highlight the critical influence of turbulence model choice in capturing laminar-to-turbulent transitions and improving wake flow predictions. Cylinder rotation significantly affects wake topology and particle dispersion, with variations depending on flow regime and particle size.Building on insights from the cylinder study, simulations were conducted for an isolated rotating wheel on a moving ground. The simulations captured key flow phenomena, including boundary-layer separation,” viscous pumping'', and coherent wake vortices such as jetting, horseshoe, and arch-shaped structures. For the particle phase, a particle detachment model was introduced to simulate the detachment process, while Lagrangian particle tracking was employed to simulate particle transport within the domain. The results allowed us to identify dominant emission zones for various particle sizes, quantify particle release rates, and characterize particles' dispersion patterns in the wheel's near and far wake.Finally, the investigation has further explored the effects of velocity (Reynolds number), wheel aspect ratio, and ground dust load on particle resuspension. Higher speeds intensified unsteady wake structures, enhancing emissions and extending particle transport downstream the wheel. Wider wheels increased detachment areas and vortex interactions, significantly amplifying emissions. Higher dust loads increased the resuspended particle mass while altering ground deposition patterns. The results of this investigation enhanced the understanding of particle-vortex interactions, demonstrating the contribution of vortical structures to particle transport in the wheel's near and far wake, as well as to particle deposition on the ground.This work provides a comprehensive understanding of wheel-induced particle resuspension emissions, offering a validated simulation approach for analyzing particle resuspension contribution to air pollution across diverse urban scenarios

Les polymères jouent un rôle important sur la réduction de la traînée et la modification de la structure des écoulements. Nous avons utilisé le modèle Oldroyd-B pour étudier l’effet de la viscoélasticité de solutions de polymères dilués sur des écoulements en deux dimensions autour d’obstacles dans un canal. Les obstacles sont pris en compte par la méthode de pénalisation volumique et des condition aux limites artificielles sans réflexion sont imposées à la sortie du canal. La discrétisation est effectuée par des schémas performants en différences finies et la résolution par une méthode multigrille. Les simulations numériques sont effectuées pour une large gamme de nombres de Reynolds et de nombres de Weissenberg. Les caractéristiques détaillées des écoulements viscoélastiques sont analysées et comparées entre elles et celles de l’écoulement du fluide sans polymère. En particulier les jeux de paramètres conduisant à une augmentation ou à une baisse de la traînée. Enfin les effets du polymère sur des écoulements turbulents sont aussi analysés
Polymer plays an important role on the drag reduction and modification of the structure of flow. In this thesis, Oldroyd B model is employed to study the effectof viscoelasticity for polymer solutions diluted by two-dimensional direct numerical simulation. The obstacles are taken into account by penalization method. The artificial boundary condition is imposed without any reflection on the channel outlet. Flow pasta cylinder is investigated in detailed by present numerical methods. The numerical codes are valid for predicting the drag force and capturing the important character of viscoelastic flow by comparing with experimental and other numerical results. The drag map of the cylinder is obtained at a wide range of Reynolds number and Weissenberg number space. The detailed characteristics of viscoelastic flow are reported in the thesis. The effects of polymer on two-dimensional turbulent flow are also discussed by grid turbulent flow

Nous nous intéressons au transport turbulent de particules de taille grande devant l'échelle de Kolmogorov. Cette situation se retrouve à la fois dans les écoulements naturels (comme le transport de sédiments) et dans les écoulements industriels (solutés solides dans un mélangeur par exemple). Pour aborder ce problème, nous étudions la dynamique de particules de taille proche de l'échelle intégrale, de densité égale ou légèrement différente de celle du fluide, dans un écoulement turbulent de von Kármán contra-rotatif, à l'aide d'un montage de suivi lagrangien rapide. L'étude de la dynamique rapide des particules montre une diminution forte des fluctuations selon la taille, mais aussi l'apparition d'un phénomène nouveau : à partir d'une certaine taille, les particules n'explorent plus l'écoulement de façon homogène. Cette exploration préférentielle est liée à la structure moyenne de l'écoulement de von Kármán, qui crée une force de piégeage. Cette force devient alors supérieure aux fluctuations des particules quand leur taille dépasse une taille critique. Une étude dans le régime laminaire, où l'écoulement moyen domine largement les fluctuations, a en effet mis en évidence un piégeage fortement accru. Les particules orbitent alors pendant des temps très longs autour des attracteurs stables des particules fluides de l'écoulement laminaire. Même en régime pleinement turbulent, le déplacement des particules entre ces zones s'effectue sur des durées longues, décorrélées des temps de la dynamique turbulente. Nous avons adapté les outils d'analyse pour caractériser cette dynamique et l'avons comparée à celle de particules isodenses dans un écoulement de von Kármán qui possède deux états asymétriques. Nous avons également élaboré un modèle qui reproduit ces caractéristiques dans les cas symétrique et asymétrique. Ces questions sont intimement liées au transfert de masse ou de chaleur entre une particule et l'écoulement. Nous avons donc aussi étudié la fusion de grosses billes de glace en turbulence développée, analysant l'influence de la taille des billes et de la vitesse de glissement sur le transfert thermique, à l'aide d'un montage d'ombroscopie afocale. Nous avons notamment montré que les grosses billes de glace fondent dans un régime ultime de convection forcée lorsqu'elles sont librement advectées par l'écoulement.

This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations.
QC 20100820

This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcanic ash plumes. The study starts from the fundamental balance laws for a multiphase gas– particle mixture, reviewing the existing models and developing a new set of Partial Differential Equations (PDEs), well suited for modeling multiphase dispersed turbulence. In particular, a new model generalizing the equilibrium–Eulerian model to two-way coupled compressible flows is developed. The PDEs associated to the four-way Eulerian-Eulerian model is studied, investigating the existence of weak solutions fulfilling the energy inequalities of the PDEs. In particular, the convergence of sequences of smooth solutions to such a set of weak solutions is showed. Having explored the well-posedness of multiphase systems, the three-dimensional compressible equilibrium–Eulerian model is discretized and numerically solved by using the OpenFOAM® numerical infrastructure. The new solver is called ASHEE, and it is verified and validated against a number of well understood benchmarks and experiments. It demonstrates to be capable to capture the key phenomena involved in the dynamics of volcanic ash plumes. Those are: turbulence, mixing, heat transfer, compressibility, preferential concentration of particles, plume entrainment. The numerical solver is tested by taking advantage of the newest High Performance Computing infrastructure currently available. Thus, ASHEE is used to simulate two volcanic plumes in realistic volcanological conditions. The influence of model configuration on the numerical solution is analyzed. In particular, a parametric analysis is performed, based on: 1) the kinematic decoupling model; 2) the subgrid scale model for turbulence; 3) the discretization resolution. In a one-dimensional and steady-state approximation, the multiphase flow model is used to derive a model for volcanic plumes in a calm, stratified atmosphere. The corresponding Ordinary Differential Equations (ODEs) are written in a compact, dimensionless formulation. The six non-dimensional parameters characterizing a multiphase plume are then written. The ODEs is studied both numerically and analytically. Different regimes are analyzed, extracting the first integral of motion and asymptotic solutions. An asymptotic analytical solution approximating the model in the general regime is derived and compared with numerical results. Such a solution is coupled with an electromagnetic model providing the infrared intensity emitted by a volcanic ash plume. Key vent parameters are then retrieved by means of inversion techniques applied to infrared images measured during a real volcanic eruption.

La question de la qualité de l'air est un enjeu de santé publique crucial qui se pose dans les grandes métropoles, à mesure que les activités humaines deviennent de plus en plus polluantes. Selon l’OMS, en diminuant les niveaux de pollution atmosphérique, les pays peuvent réduire la charge de morbidité imputable aux accidents vasculaires cérébraux, aux cardiopathies, au cancer du poumon et aux infections respiratoires. Le transport ferroviaire est à juste titre considéré comme étant une solution de mobilité durable à faible impact en gaz à effet de serre et un faible contributeur aux émissions de polluants dans l'air. Néanmoins, plusieurs études mettent en évidence que les concentrations polluantes dans les enceintes ferroviaires souterraines doivent être considérées comme préoccupantes. Dans certains cas, les concentrations de particules fines peuvent être dix fois plus élevées en intérieur qu'en extérieur. Dans ce contexte, la réduction ou la mitigation des sources d’émissions liées au freinage, principal contributeur dans le secteur ferroviaire, représente un enjeu majeur pour la santé des personnes. Cette thèse prend place dans le cadre du projet BREAQ (BRaking Emissions characterisation and mitigation for Air Quality improvement), mené conjointement par ALSTOM, l’ADEME et plusieurs organismes de recherche. Ce projet a pour vocation de réduire les émissions de particules de freinage à la source et de prédire la diffusion de ces particules dans l’environnement pour développer des solutions de captation efficientes. Dans ce contexte, l’objectif de l’étude consiste à développer et mettre en œuvre une méthode numérique CFD capable de modéliser des écoulements chargés en particules issues du freinage ferroviaire afin de prédire la diffusion de ces particules dans l’environnement proche
The issue of air quality is a crucial public health issue that arises in large cities, as human activities become increasingly polluting. According to WHO, by reducing air pollution levels, countries can reduce the burden of disease from stroke, heart disease, lung cancer and respiratory infections. Rail transport is rightly considered to be a sustainable mobility solution with low greenhouse gas impact and a low contributor to air pollutant emissions. However, several studies highlight that pollutant concentrations in underground railway enclosures must be considered as worrying. In some cases, concentrations of fine particles can be ten times higher indoors than outdoors. In this context, reducing or mitigating sources of emissions linked to braking, the main contributor in the railway sector, represents a major challenge for people's health. This thesis is part of the BREAQ (BRaking Emissions characterization and mitigation for Air Quality improvement) project, jointly conducted by ALSTOM, ADEME and several research organizations. The aim of this project is to reduce braking particle emissions at source and to predict the diffusion of these particles in the environment in order to develop efficient capture solutions. In this context, the objective of the study is to develop and implement a numerical CFD method for modeling particulate flows from railway braking in order to predict the diffusion of these particles in the nearby environment

La régulation des polluants a mené à la création de nouveaux systèmes de combustion. Le carburant étant stocké sous forme liquide, sa transformation jusqu’à sa combustion est complexe. La capacité de la Simulation aux grandes échelles à simuler des écoulements turbulents réactifs a été montrée sur des cas académiques comme sur des configurations industrielles, tout en prenant en compte les phénomènes multiphysiques intervenant dans ces configurations, mais les études sur la structure de flamme diphasique sont encore trop peu nombreuses. La présence de deux solveurs pour la simulation d’une phase liquide étant disponible dans le code AVBP, leur utilisation permet une comparaison et une compréhension des phénomènes en jeu combinant dispersion, évaporation, et combustion. La première partie de l’étude relate la validation du modèle d’injection FIM-UR. Ce modèle est capable de reconstruire les profils de vitesses et de granulométrie à l’injecteur sans avoir à simuler les phénomènes d’atomisation primaire et secondaire. Une validation en régime turbulent avait déjà été réalisée, et on propose ici de valider le modèle dans un cas laminaire. Des comparaisons entre simulations monodisperses et polydisperse et des expériences sont effectuées. La simulation monodisperse Lagrangienne donne une bonne structure globale mais la simulation polydisperse Lagrangienne permet de retrouver le comportement au centre du cône avec la présence des petites gouttes et à la périphérie du cône par la présence des grosses gouttes. De plus, des améliorations sont apportées au modèle pour le formalisme Eulérien et montrent de bons résultats. La partie suivante s’intéresse à caractériser un spray polydisperse par une distribution monodisperse. En effet, au cas où une approche polydisperse n’est pas possible, le choix du diamètre moyen à prendre pour une simulation monodisperse est délicat. On propose donc d’analyser le comportement d’un spray polydisperse en le comparant à ceux de sprays monodisperses. Deux configurations académiques sont choisies : des cas de Turbulence Homogène Isotrope chargée en particules pour étudier la dynamique, et des calculs d’évaporation 0D. Trois paramètres sont étudiés pour la dynamique : la concentration préférentielle (ou ségrégation), la traînée moyenne et la traînée réduite moyenne. Cette dernière et la ségrégation de la distribution polydisperse semblent affectées par les tailles de goutte les plus faibles, et la concentration préférentielle apparait alors comme la moyenne des ségrégations des classes qui la composent pondérées par l’inverse du nombre de Stokes associé à chacune de ces classes. La traînée moyenne de la simulation polydisperse possède un comportement proche des diamètres moyens D10 et D20. Ces analyses nous poussent donc à choisir le D10 pour caractériser la dynamique d’un spray polydisperse. Les calculs d’évaporation 0D ne permettent pas dans un premier temps de caractériser efficacement la masse évaporée d’un spray polydisperse par celle d’un spray monodisperse équivalent, mais la définition de nouveaux diamètres issus de la littérature des lits fluidisés comme le D50% le permet, ce qui le place autour du D32. On propose donc de caractériser l’évaporation d’un spray polydisperse par ce diamètre. Enfin, la dernière partie étudie la structure de flamme diphasique dans la chambre MERCATO, à l’aide du formalisme Lagrangien, monodisperse et polydisperse, mais aussi en utilisant le formalisme Eulérien. La validation du modèle FIM-UR du premier chapitre et ses améliorations sont utilisées pour représenter les conditions d’injection liquide. En plus d’un calcul polydisperse, deux simulations monodisperses Lagrangiennes sont réalisées en prenant les diamètres moyens D10 et D32, suite à la partie précédente. Des comparaisons qualitatives et des validations sont réalisées, en comparant des profils de vitesses gazeuses axiale et fluctuante et vitesse axiale liquide issus de l’expérience
Regulations on pollutants have led to the creation of new combustion systems. Giving that fuel is stored in a liquid form, its evolution until combustion is complex. The ability of Large Eddy Simulation has been demonstrated on academic cases, as well as on industrial configurations, by taking into account the multi-physics phenomena, but there is a lack of studies about two-phase flow flame structures. Two solvers for the simulation of two-phase flows are available in the AVBP code, hence both simulations are performed to compare and increase understanding of the phenomena involved such as dispersion, evaporation and combustion. The first part of the study focuses on the validation of the FIM-UR injection model. This model is able to build velocity and droplet profiles at the injector, without simulating primary and secondary break up. A validation in a turbulent case has already been done, and this study validates the model in a laminar case. Comparisons between monodisperse and polydisperse simulations, and experiments are performed. The monodisperse Lagrangian simulation shows good results but the polydisperse simulation is able to represent profiles in the center of the cone by small droplets and at the peripheral part of the cone, by big ones. Moreover, improvements in the Eulerian model exhibit good results. The next section tries to evaluate the impact of polydispersion. Indeed, when a polydisperse approach is not available, choosing the mean diameter can be tricky. A comparison between the behavior of polydisperse spray and monodisperse sprays ones is realised. Two academic cases are studied: Homogeneous Isotropic Turbulence with particles to analyze the dynamics, and 0D evaporation cases. For the dynamics, preferential concentration, mean drag and reduced mean drag are studied. The latter and preferential concentration are affected by small droplets, and the preferential concentration of a polydisperse spray is equivalent to the average of preferential concentration of classes, extracted from the polydisperse distribution, weighted by the inverse of the Stokes number of each class. The mean drag behaves like the D10 and D20 mean drags. This analysis allows us to choose the D10 to characterize a polydisperse distribution for the dynamics. Zero-D evaporation simulations cannot characterize the polydisperse spray evaporated mass by the evaporated mass of monodisperses sprays. New definitions of diameters from fluidized bed literature enable the use of D50%, which is close to D32. We propose to use this diameter to characterize the evaporation of a polydisperse spray. Finally, the last section studies the structure of two-phase flames in the MERCATO bench, using the Lagrangian formalism, monodisperse and polydisperse but also using the Eulerian formalism. The validation of FIM-UR model and improvements from the first section are used to represent liquid injection conditions. A polydisperse simulation is realized and two monodisperse simulations are computed using mean diameters D10 and D32, thanks to the previous section. Qualitative comparisons and validations are realized, comparing gaseous velocity profiles and liquid velocity profiles. Good agreements are found and the mean diameter D32 seems to be close to the polydisperse spray. A comparison between mean flames is done with an Abel transform of the flame from the experiments. The flame has an "M shape", anchored by small recirculation zones out of the swirler, and by a point at the tip of the central recirculation zone. Then, the impact of droplet distributions is analyzed. Even if few bigger droplets from the polydisperse distribution are convected in the hot gases due to bigger particular time and evaporation time, two-phase flow flame structures are equivalent. Different combustion regimes appeared with premixed flames and pockets of fuel burning in the hot gases