Modelling of the dispersed phase motion in free-surface flows with the two-fluid smoothed particle hydrodynamics approach

The sediment transport is an important problem in hydro-engineering. Accurate numerical modelling of this complex phenomenon remains a challenging task. In the present study we employ the Smoothed Particle Hydrodynamics (SPH) approach in the two-fluid formulation to compute the interactions between the carrier and dispersed phases. The main goal is to test this rather uncharted SPH variant for simple cases and to find problematic points that require further improvement. We present initial results of validation with experiment involving a vertical sheet of sand entering the water tank through free surface, as well as results from a simplified quasi-2D study of sedimentation

Particles in wall-bounded turbulent flows deposition, re-suspension and agglomeration

The book presents an up-to-date review of turbulent two-phase flows with the dispersed phase, with an emphasis on the dynamics in the near-wall region. New insights to the flow physics are provided by direct numerical simuation and by fine experimental techniques. Also included are models of particle dynamics in wall-bounded turbulent flows, and a description of particle surface interactions including muti-layer deposition and re-suspension

Development of Smoothed Particle Hydrodynamics approach for modelling of multiphase flows with interfaces

L'approche Smoothed Particle Hydrodynamics (SPH) est une méthode de calcul pour simuler des écoulements fluides avec une méthode Lagrangienne de type suivi de particules. A l'inverse des méthodes Euleriennes, ce type d'approche ne nécessite pas de maillage. C'est là l'un des atouts majeurs de l'approche SPH puisqu'elle permet de s'affranchir des méthodes de suivi d'interfaces couramment utilisées dans les approches Euléeriennes (par exemple Volume-of-Fluid, Level-Set ou Front-Tracking). L'approche SPH est donc de plus en plus utilisée dans les domaines de l'hydro-ingénierie et de la géophysique notamment de part le traitement naturel des écoulements à surface libre dans la méthode SPH. Cependant, l'approche SPH n'est utilisée que depuis peu pour simuler des écoulements multiphasiques complexes et de nombreux problèmes restent en suspens, notamment concernant une formulation adéquate ou le micro-mélange aux interfaces. L'un des principaux enjeux de ces travaux de thèse est d'analyser de façon objective les différentes approches de type SPH existantes et d'évaluer leur capacité à simuler des écoulements multiphasiques complexes. Ainsi, la modélisation des phénomènes liés à la tension de surface a été réalisée et validée via l'utilisation de techniques de type Continuum Surface Force. Les phénomènes de convection naturelle ont quant à eux été modélisés grâce à une nouvelle formulation plus générale (non-Boussinesq). Une partie de ces travaux est dédiée à l'étude des problèmes de micro-mélange aux interfaces: les effets indésirables (notamment la fragmentation de l'interface) sont analysés et des solutions sont proposées. Une autre part de travail porte sur la modélisation des mouvements ascendants de bulles dans des liquides, avec l'inclusion des interactions entre bulles. Des simulations SPH ont été réalisées pour différents régimes d'écoulement, chacun d'eux correspondant à un ratio spécifique entre la tension de surface, la viscosité et la flottabilité. Les prédictions numériques de la topographie des bulles, de leur vitesse ainsi que de leur coefficient de trainée ont été validées. Pour ce faire, les résultats numériques ont été comparés non seulement aux données expérimentales de référence mais également à d'autres simulations numériques de bulles ascendantes. Dans ces travaux de thèse, une étude détaillée des concepts liés aux contraintes d'incompressibilité a été réalisée. Dans cet objectif, deux traitements différents ont été comparés: l'approche faiblement compressible (où une équation d'état adéquate est choisie) et l'approche incompressible (où une projection des champs de vitesse sur un espace sans divergence est réalisée de deux facons différentes). La pertinence de ces modèles pour des simulations d'écoulements multiphasiques est également évaluée. Les problèmes associés aux paramètres numériques sont discutés et un choix approprié de ces paramètres est proposé. Pour ce faire, de nombreux calculs de validation en deux et trois dimensions ont été réalisés. Enfin, une extension est proposée pour simuler les phénomènes liés à l'ébullition via une approche SPH. Ce sujet étant encore en friche, de nouvelles idées et schémas sont proposés pour le changement de phase liquide-vapeur dans l'approche SPHSmoothed Particle Hydrodynamics (SPH) is a fully Lagrangian, particle based approach for fluid-flow simulations. One of its advantages over Eulerian techniques is no need of a numerical grid. Therefore, there is no necessity to handle the interface shape as it is done in Volume-of-Fluid, Lavel-Set or Front-Tracking methods. Thus, the SPH approach is increasingly used for hydro-engineering and geophysical applications involving free-surface flows where the natural treatment of evolving interfaces makes it an attractive method. However, for real-life multi-phase simulations this method has only started to be considered and many problems like a proper formulation or a spurious fragmentation of the interface remain to be solved. One of the aims of this work is to critically analyse the existing SPH variants and assess their suitability for complex multi-phase problems. For modelling the surface-tension phenomena the Continuum Surface Force (CSF) methods are validated and used. The natural convection phenomena are modeled using a new, more general formulation, beyond the Boussinesq approximation. A substantial part of the work is devoted to the problem of a spurious fragmentation of the interface (the micro-mixing of SPH particles). Its negative effects and possible remedies are extensively discussed and a new variant is proposed. Contrary to general opinion, it is proven that the micro-mixing is not only the problem of flows with neglegible surface tension. A significant part of this work is devoted to the modelling of bubbles rising through liquids, including bubble-bubble interactions. The SPH simulations were performed for several flow regimes corresponding to different relative importance of surface tension, viscosity and buoyancy effects. The predicted topological changes, bubble terminal velocity and drag coefficients were validated with respect to reference experimental data and compared to other numerical methods. In the work, fundamental concepts of assuring the incompressibility constraint in SPH are also recalled. An important part of work is a thorough comparison of two different incompressibility treatments: the weakly compressible approach, where a suitably chosen equation of state is used, and truly incompressible method (in two basic variants), where the velocity field is projected onto a divergence-free space. Their usefulness for multi-phase modelling is discussed. Problems associated with the numerical setup are investigated, and an optimal choice of the computational parameters is proposed and verified. For these purposes the study is supported by many two- and three-dimensional validation cases. In addition, the present work opens new perspectives to future simulations of boiling phenomena using the SPH method. First ideas and sketches for the implementation of the liquid-vapour phase change are presentedNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Modelling of the dispersed phase motion in free-surface flows with the two-fluid smoothed particle hydrodynamics approach

The sediment transport is an important problem in hydro-engineering. Accurate numerical modelling of this complex phenomenon remains a challenging task. In the present study we employ the Smoothed Particle Hydrodynamics (SPH) approach in the two-fluid formulation to compute the interactions between the carrier and dispersed phases. The main goal is to test this rather uncharted SPH variant for simple cases and to find problematic points that require further improvement. We present initial results of validation with experiment involving a vertical sheet of sand entering the water tank through free surface, as well as results from a simplified quasi-2D study of sedimentation

Generalised Eddy Dissipation Concept for MILD combustion regime at low local Reynolds and Damköhler numbers. Part 1: Model framework development

Moderate or Intense Low Oxygen Dilution (MILD) combustion is a fuel-flexible combustion technology featuring high efficiency and low pollutant emissions. Fundamental studies reveal that turbulence-chemistry interactions are extremely complex in MILD conditions and reactor-type approaches seem to be the adequate modelling choice. In this work we develop a generalised Eddy Dissipation Concept (EDC) adapted to MILD combustion regime accounting for finite rate chemistry. We examine two recent modifications of the standard EDC and present a generalised model. It is based on functional expressions where the model parameters are adjusted to the local conditions in terms of Reynolds and Damköhler numbers, contrary to the usually proposed ad hoc tuning of the global EDC constants. Numerical results reveal that previously presented corrections are indeed suitable for specific conditions; their appropriate combination, guided by physical premises and a scrutiny of computation results, leads to a reformulation of the EDC framework. The study consists of two parts: the model development is described here (Part 1); in a companion paper (Part 2), we present a thorough validation process performed against twelve flames issuing from the jet-in-hot-coflow burners from Delft and Adelaide, representing a wide range of operating conditions. The new, generalised model can serve as a plug and play engineering tool without complex pre- or post-processing treatment.SCOPUS: ar.jinfo:eu-repo/semantics/publishe