In turbulent water flows, large quantities of air bubbles are entrained at the free-surfaces. Practical applications of gas-liquid bubbly flows are found in Chemical, Civil, Environmental, Mechanical, Mining and Nuclear Engineering. Air-water flows are observed in small-scale as well as large-scale flow situations. Typical examples include thin circular jets used as mixing devices in chemical plants (Qw = 0.001 L/s, diameter = 1 mm) and spillway flows (Qw larger than 10,000 m3/s, flow thickness over 10 m). In each case, however, the interactions between the entrained air bubbles and the turbulence field are significant. The present manuscript regroups a collection of one book and 43 articles on the study of air bubble entrainment in turbulent flows. The work aims to gain a better understanding of the basic mechanisms of gas entrainment and the nteractions between entrained gas bubbles and the turbulence. It has been the purpose of the research work to assess critically the overall state of this field, to present new analysis and experimental results, to compare these with existing data, and to present new compelling conclusions regarding momentum and void fraction development of air-water gas-liquid bubbly flows. The manuscript presents a comprehensive analysis of the air entrainment processes in free-surface turbulent flows. The air-water flows are investigated as homogeneous mixtures with variable density. The variations of fluid density result from the non-uniform air bubble distributions and the turbulent diffusion process. Several types of air-water free-surface flows are studied : plunging jet flows, open channel flows, and turbulent water jets discharging into air.

Today’s aerospace industry continues to exploit liquid hydrocarbon fossil fuels. Motivated by operational considerations, continued availability and cost, this is likely to be the case for many years, despite the obvious environmental concerns. The interplay of liquid atomisation, spray vaporisation and the combustion process are intricately linked. However, the physical process of fuel injection and its atomisation into tiny droplets prior to combustion remains poorly understood. Because atomisation governs the size of the fuel droplets, and therefore their subsequent evaporation rate, adjusting the injection sequence is of paramount importance and will have far-reaching repercussions on many aspects of the combustion process, for example pollutant formation. In the context of jet engines, kerosene is usually injected in its liquid form via an airblast-type fuel injector. A coflowing high-speed airstream destabilises the liquid fuel, which is thus sprayed into fine droplets into the combustion chamber. The prediction of this phenomenon for various operating conditions relevant to the aeronautical industry requires a deeper understanding of the mechanisms involved in the interaction of the two fluids. A key element in predicting the complex behaviour of spray formation and evolution in jet engines is accurate modelling of fuel atomisation. Atomisation represents one of the key challenges that remains to be undertaken to make predictive computational simulations possible. However, the inherent multi-physics and multi-scale nature of this process limits numerical investigations. Thanks to the steady progress in computer power and Computational Fluid Dynamics (CFD) methods, computational modelling of injection systems emerges as a promising tool that can drive the design of future devices. This research project sets out to investigate the atomisation process in detail, in particular in providing physical insight into the fundamental physics of the phenomenon, in conjunction with an analysis on wetting behaviours and liquid droplet tracking. High-fidelity numerical simulations are performed using a novel in-house state-of-the-art multiphase flow modelling capability, RCLSFoam. The performance of the numerical scheme is demonstrated on typical two-dimensional and three-dimensional benchmark test cases relevant to both multiphase flow modelling and atomisation, and validated against other computational methods. An informed and systematic qualitative assessment of the topological variations of the phase interface during primary atomisation of a liquid film is made through dynamical analysis, while investigating an extensive domain of operating conditions at ambient and aero-engine injection conditions relevant to industry. This analysis demonstrated the influence of shear-driven instabilities on the atomisation process. The shear stress and difference in inertia between liquid and gas are observed to play a significant role in the atomisation process. In addition, the key physical mechanisms and their competing effects have been mapped out in order to predict the evolution of the process according to the operating conditions of the injection system. The proposed cartography gathers four different atomisation mechanisms. In particular, for sufficiently high liquid injection speeds, three-dimensional wave modes were observed to co-exist (the “3-D wave mode” regime). For very low liquid flow rates, accumulated liquid at the atomising edge undergoes deformation by which droplets are generated (the “accumulation” regime). For an increasing gas injection speed and a fixed liquid velocity, the effects of surface tension were observed to result in the generation of streamwise ligaments only, which tend to pair up (the “ligament-merging” regime). Finally, “vortex action” is another observed mechanism by which the liquid film is fragmented. Overall, this research project culminated in (i) the study of dynamic wetting behaviours, with the implementation and validation against experimental data of the Kistler dynamic contact model; and (ii) the demonstration of an algorithm for droplet capture and subsequent post-processing analysis of the droplet characteristics.

Among the many factors that effect the atomization of a fuel spray in a com- bustion chamber, the flow characteristics of the fuel inside the injector nozzle play significant roles. The enthalpy of the entering fuel can be elevated such that it is higher than the local or downstream saturation enthalpy, which will result in the flash-boiling of the liquid. The phase change process dramatically effects the flow rate and has the potential to cause subsonic two-phase choking. The timescale over which this occurs is comparable to the flow-through time of the nozzle and hence any attempt to model this phenomenon needs to be done as a finite rate process. In the past the Homogeneous Relaxation Model (HRM) has been successfully employed to model the vaporization in one dimension. Here a full three dimensional imple- mentation of the HRM model is presented. Validations have been presented with experiments using water as working fluid. For the external spray modeling, where the fuel is said to be flash boiling, the phase change process plays a role alongside the aerodynamic breakup of the liquid and must be considered for obtaining the fuel spray characteristics. In this study the HRM model is coupled with Linearized Sheet Instability Analysis (LISA) model, for primary atomization, and with Taylor Analogy Breakup (TAB) model for secondary breakup. The aerodynamic breakup model and phase change based breakup model are designed as competing processes. The mechanism which satisfies its breakup criterion first during time integration is used to predict resulting drop sizes.

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The analysis of turbulent flows provides a unique scientific challenge whose solution remains central to unraveling the fundamental nature of all fluid dynamics. Measuring and predicting turbulent flows becomes even more difficult when considering a two-phase flow, which is a commonly encountered engineering problem across many disciplines. One such example, the ingestion of foreign debris into a gas turbine engine, provided the impetus for this study. Despite more than 40 years of research, operation with a particle-laden inlet flow remains a significant problem for modern turbomachines. The purpose, therefore, is to develop experimental methods for investigating multi-phase flows relevant to the cooling of gas turbine components. Initially, several generic components representing turbine cooling designs were evaluated with a particle-laden flow using a special high temperature test facility. The results of this investigation revealed that blockage was highly sensitive to the carrier flowfield as defined by the cooling geometry. A second group of experiments were conducted in one commonly used cooling design using a Time Resolved Digital Particle Image Velocimetry (TRDPIV) system that directly investigated both the carrier flowfield and particle trajectories. Traditional PIV processing algorithms, however, were unable to resolve the particle motions of the two-phase flow with sufficient fidelity. To address this issue, a new Particle Tracking Velocimetry (PTV) algorithm was developed and validated for both single-phase and two-phase flows. The newly developed PTV algorithm was shown to outperform other published algorithms as well as possessing a unique ability to handle particle laden two-phase flows. Overall, this work demonstrates several experimental methods that are well suited for the investigation of wall-bounded turbulent two-phase flows, with a special emphasis on a turbine cooling method. The studies contained herein provide valuable information regarding the previously unknown fluid and particle dynamics within the turbine cooling system. / Ph. D.

The focus of this thesis is the study of turbulent solid-liquid stirred suspensions, which are involved in many common unit operations in the chemical, pharmaceutical and food industries. The studies of two-phase flows present a big challenge to researchers due to the complexity of experiments; hence there is a lack of quantitative solid and liquid hydrodynamic measurements. Therefore, an investigation of turbulence modulation by dispersed particles on the surrounding fluid in stirred vessels has been carried out, via two-phase fluorescent Particle Image Velocimetry (FPIV) and micromixing experiments. The main property of interest has been the local dissipation rate, as well as root-mean-square (rms) velocities and turbulent kinetic energy (TKE) of the fluid. Initially a single-phase PIV study was conducted to investigate the flow field generated by a sawtooth (EkatoMizer) impeller. The purpose of this study was to gain insight into various PIV techniques before moving on to more complex two-phase flows. Subsequently stereo-, highspeed and angle-resolved measurements were obtained. The EkatoMizer formed a good case study as information regarding its hydrodynamics is not readily available in literature, hence knowledge has been extended in this area. An analysis of the mean flow field elucidated the general structure of fluid drawn into the impeller region axially and discharged radially; the latter characterised the impeller stream. The radial rms velocity was considered to represent best the system turbulence, even though the tangential rms velocity was greater close to the blade; however the radial component was more prevalent in the discharge stream. Due to differences in rms velocities, TKE estimates obtained from two and three velocity components deviated, being greater in the latter case. Integral (1-D and 2-D) length scales were overestimated by the quantity W / 2 in the impeller region. Ratios of longitudinal-to-lateral length scales also indicated flow anisotropy (as they deviated from 2:1). The anisotropy tensor showed that the flow was anisotropic close to the blade, and returned to isotropy further away from the impeller. Instantaneous vector plots revealed vortices in the discharge stream, but these were not associated with flow periodicity. Alternatively, the vortex structures were interpreted as low frequency phenomena between 0-200 Hz; macro-instabilities were found to have a high probability of occurrence in the discharge stream. Dissipation is the turbulent property of most interest as it directly influences micromixing processes, and its calculation is also the most difficult to achieve. Its direct determination from definition requires highly resolved data. Alternative methods have been proposed in the literature, namely dimensional analysis, large eddy simulation (LES) analogy and deduction from the TKE balance. All methods were employed using 2-D and 3-D approximations from stereo-PIV data. The LES analogy was deemed to provide the best estimate, since it accounts for three-dimensionality of the flow and models turbulence at the smallest scales using a subgrid scale model. (Continues...).

Ce travail s'est intéressé à l?étude de la formation de gouttelettes en microsystèmes à l'aide d'outils de visualisation tels que des caméras rapides et la microvélocimétrie par image de particules (µPIV). La taille des gouttelettes principales et satellites formées en jonction flow-focusing est déterminée pour des fluides newtoniens et non-newtoniens. Les mêmes paramètres critiques sont mis en évidence pour ces jonctions flow-focusing et des jonctions T, illustrant l'influence du confinement sur la formation des gouttelettes. L'évolution de la taille des gouttelettes satellites dépend quant à elle d'un nombre capillaire critique. Les champs de vitesses ont été mesurés dans les gouttelettes en formation ainsi qu'autour de ces gouttelettes, de même que les trois étapes de la dynamique de rupture du cou des gouttelettes. Les liens entre cette dynamique, les champs de vitesse et la taille des gouttelettes ont été déterminés. L'émulsification a également été étudiée dans deux micromélangeurs industriels : le Caterpillar et le StarLaminator, avec des formulations proches de celles cométiques. Les performances énergétiques sont particulièrement prometteuses pour le Caterpillar / This work is focused on droplet formation in microsystems using visualization tools such as high-speed cameras and microparticle image velocimetry (µPIV). The size of main and satellite droplets formed in flow-focusing junctions was determined for both Newtonian and non-Newtonian fluids. The same critical parameters were highlighted for both flow-focusing and T-junctions, illustrating walls? influence on the droplet formation. The size evolution of satellite droplets depends on a critical capillary number. The flow fields were measured inside and outside the forming droplets, as well as the three steps dynamics for droplet neck rupture. This dynamics was straightforwardly linked to the flow fields and the droplet size. Emulsification was also investigated in two industrial micromixers, Caterpillar and StarLaminator respectively, with formulations close to cosmetic ones. The energetic performances of the Caterpillar are particularly promising

​ Nombreuses stratégies de contrôle ont été récemment proposées par la communauté scientifique afin depouvoir réduire la traînée dans les écoulements pariétaux. Entre autres, les Surfaces Superhydrophobes (SHS) ontmontré leurs capacités de pouvoir réduire considérablement le frottement pariétal d’un écoulement liquide grâce à laprésence de microbulles de gaz piégées dans les nano-rugosités de la surface. Dans des conditions géométrique etthermodynamique données pour lesquelles la transition de mouillage est évitée (condition pour laquelle normalementla taille des rugosités qui caractérise la SHS est de plusieurs ordres de grandeur plus petite que l'échellecaractéristique de l'écoulement principal), on peut atteindre ce qu’on appelle ‘l'effet Lotus’, pour lequel l'écoulementglisse à la paroi, avec une vitesse différente de zéro.. Dans ce cadre, nous nous sommes proposés d’étudier, à l’aidede simulations numériques l’influence des SHS sur la transition laminaire-turbulent dans un écoulement de canal.Pour cela, nous avons réalisé une série de simulations numériques directes (DNS), allant de l'état laminaire au casturbulent pleinement développé, en traitant la plupart de scénarios de transition connu en littérature. Des analyses destabilité locale et globale ont aussi été réalisées afin de déterminer l’influence de ces surfaces sur la première phasedu processus de transition. Bien que la procédure de déclenchement de la transition contrôlée (type K, H, C,...) soitbien décrite dans la littérature, cela n’est pas le cas pour les transitions naturelles. À cette fin, une nouvelle méthode aété développée pour déclencher puis étudier la transition naturelle dans des écoulements de type canal. Cette méthodeest basée sur des mécanismes de réceptivité de l'écoulement (resolvent global) permettant de construire un forçagevolumique spécifique. Plusieurs approches pour modéliser les SHS ont été utilisées, de complexités croissantes, touten tenant en compte des caractéristiques physiques de ces surfaces. Dans un premier temps, une condition deglissement homogène a été utilisée et son influence analysée. Chaque rugosité a été ensuite discrétisée spatialement,d’abord avec une alternance de condition limite sur une surface plate, ensuite en tenant compte de la dynamique del’interface gaz-liquide par une méthode Lagrangienne-Eulerienne Arbitraire (ALE). Nous avons montré que les SHSpermettent d’efficacement retarder les transitions contrôlées mais qu’en revanche elles ont peu d’influence sur lestransitions naturelles (développant des stries de vitesse). En effet, ce comportement dérive de l'équilibre entre deuxeffets contradictoires. D’un côté, le glissement pariétal nuit au développement des structures cohérentes de typehairpin ​ , en altérant le processus de ​ vortex stretching-tilting ​ . D’autre part, le mouvement de l’interface gaz-liquideinteragit avec les structures cohérentes de l'écoulement, en produisant des vitesses normales à la paroi favorisantdavantage le processus de ​ sweep-ejection et entraînant le développement de structures en forme d’arche. Nous avonsmontré que les interfaces gaz-liquide statiques retardent la transition de façon analogue à une condition aux limiteshomogène (si l’hétérogénéité pariétale est petite). En revanche la prise en compte de leur dynamique limite le retardde la transition, montrant l’importance du modèle de SHS dans les écoulements transitionnels. / Many passive control strategies have been recently proposed for reducing drag in wall-bounded shearflows. Among them, underwater SuperHydrophobic Surfaces (SHS) have proven to be capable of dramaticallyreducing the skin friction of a liquid flowing on top of them, due to the presence of gas bubbles trapped within thesurface nano-sculptures. In specific geometrical and thermodynamical conditions for which wetting transition isavoided (in particular, when the roughness elements characterizing the SHS are several orders of magnitude smallerthan the overlying flow), the so-called ’Lotus effect’ is achieved, for which the flow appears to slip on the surfacewith a non zero velocity. In this framework, we propose to study, by means of numerical simulations, the influence ofSHS on laminar-turbulent transition in a channel flow. To do so we have performed a series of direct numericalsimulations (DNS), from the laminar to the fully turbulent state, covering the majority of transition scenarios knownin the literature, as well as local and global stability analysis so to determine the influence of SHS onto the initialstages of the process. While the conditions for observing controlled K-type transition in a temporal channel flow arewell defined, this is not the case for uncontrolled ones. To this end, a novel theoretical numerical framework has beendeveloped so to enable the observation of natural transition in wall-bounded flows. This method, similarly to theFree-Stream-Turbulence framework available for the boundary layer flow, is capable of triggering uncontrolledtransition t​ hrough flow receptivity to a purpose-built forcing. Different surface modellings for the superhydrophobicsurfaces are tested. First, homogeneous slip conditions are used. Then, the spatial heterogeneity of the SHS has beenconsidered by modelling it as a flat surface with alternating slip no-slip boundary conditions. Finally, the dynamics ofeach microscopic liquid-gas free-surface has been taken into account by means of a fully coupled fluid-structuresolver, using an Arbitrary Lagrangian Eulerian formulation. We show that while SHS are ineffective in controllingtransition in noisy environment​ , they can strongly delay transition to turbulence for the K-type scenario​ . Thisbehaviour results from the balance of two opposing effects. On one hand slippery surfaces inhibit the development ofcharacteristic hairpin vortices by altering the vortex stretching-tilting process. On the other hand, the movement ofthe gas-liquid free-surfaces interacts with the overlying coherent structures, producing wall-normal velocities thatenhance the sweep-ejection process, leading to a rapid formation of hairpin-like head vortices. Thus, whenconsidering flat interfaces transition time is strongly increased, while taking into account the interface dynamicsinduces smaller changes with respect to the no-slip case, indicating the need for an appropriate modelling of SHS fortransition delay purposes.

Simulações de escoamentos multifásicos são de grande interesse em aplicações práticas na indústria, em particular na indústria petrolífera, entre outras. Vários processos dependem do entendimento físico de escoamentos envolvendo iteração com partículas, sedimentação e separação de fluidos. Dos muitos métodos existentes para a simulação dos processos acima descritos, há um crescente interesse no aumento de precisão, o que levou ao desenvolvimento de estratégias que utilizam esquemas de elementos finitos discretizados em malhas dinâmicas e adaptativas, usando uma formulação ALE (do inglês, Arbitrary Lagrangian-Eulerian), juntamente com uma representação geométrica da interface. Neste sentido, este trabalho tem o objetivo de estudar e implementar estratégias robustas de controle e adaptação de malhas, em situações onde a malha dinâmica é sujeita a grandes deformações. Uma biblioteca de algoritmos e rotinas foi então desenvolvida para este fim, implementando técnicas de controle e otimização da qualidade dos elementos da malha, técnicas de adaptação da interface entre fluidos com esquemas de conservação de massa, técnicas de mudanças topológicas e preservação de propriedades materiais, além de uma comunicação facilitada destas rotinas com códigos de simulação numérica de escoamentos multifásicos existentes / Multiphase flow simulations are of great interest in practical applications, particularly in the oil industry. Several processes depend on understanding physical aspects of flows with particle interaction, sedimentation and fluid separation. Among the several existing methods to simulate the processes described above, theres a growing interest in achieving higher precision, which led to the development of strategies that use finite element discretization in adaptive, dynamic meshes, using the ALE formatulation along with a geometrical representation of the interface. In this context, this thesis aims to study and implement robust strategies for mesh adaptation, for cases where the dynamic mesh is subject to large deformations. A library of routines and algorithms was developed, implementing mesh elements control and quality optimization techniques, fluid interface adaptation techniques with a mass conservation scheme, topological modifications and material properties preservation techniques, and also a decoupled, simplified communication between these routines with existing multiphase flow numerical simulation code

Direct numerical simulations, where both the large and small scales in the flow are fully resolved, provide an excellent instrument to validate multiphase flow processes and also further our understanding of it. Three multiphase systems are studied using a finite difference/front-tracking method developed for direct numerical simulations of time-dependent system¬¬s. The purpose of these studies is to demonstrate the benefit in developing accurate sub-grid models that can be coupled with the direct numerical simulations to reduce the computational time. The primary reason to use the models is that the systems under consideration are sufficiently large that resolving the smallest scales is impractical. The processes that are examined are: (1) droplet motion and impact (2) nucleate boiling and (3) convective mass transfer. For droplet impact on solid walls and thin liquid films, the splash characteristics are studied. The collision of a fluid drop with a wall is examined and a multiscale approach is developed to compute the flow in the film between the drop and the wall. By using a semi-analytical model for the flow in the film we capture the evolution of films thinner than the grid spacing reasonably well. In the nucleate boiling simulations, the growth of a single vapor from a nucleation site and its associated dynamics are studied. The challenge here is the accurate representation of the nucleation site and the small-scale motion near the wall. To capture the evaporation of the microlayer left behind as the base of the bubble expands we use a semi-analytical model that is solved concurrently with the rest of the simulations. The heat transfer from the heated wall, the evolution of the bubble size and the departure diameter are evaluated and compared with the existing numerical results. The mass transfer near the interface, without fully resolving the layer by refining the grid is accommodated by using a boundary layer approximation to capture it. The behavior of the concentration profile is taken to be self-similar. A collection of potential profiles is tested and the accuracy of each of these models is compared with the full simulations.