This thesis presents the development of GPU accelerated solvers for use in simulation of the primary atomization phenomenon. By using the open source continuum mechanics library, OpenFOAM, as a basis along with the NVidia CUDA API linear system solvers have been developed so that the multiphase solver runs in part on GPUs. This aims to reduce the enormous computational cost associated with modelling primary atomization. The modelling of such is vital to understanding the mechanisms that make combustion efficient. Firstly, the OpenFOAM code is benchmarked to assess both its suitability for atomization problems and to establish efficient operating parameters for comparison to GPU accelerations. This benchmarking then culminates in a comparison to an experimental test case, from the literature, dominated by surface tension, in 3D. Finally, a comparison is made with a primary atomizing liquid sheet as published in the literature. A geometric multigrid method is employed to solve the pressure Poisson equations, the first use of a geometric multigrid method in 3D GPU accelerated VOF simulation. Detailed investigations are made into the compute efficiency of the GPU accelerated solver, comparing memory bandwidth usage to hardware maximums as well as GPU idling time. In addition, the components of the multigrid method are also investigated, including the effect of residual scaling. While the GPU based multigrid method shows some improvement over the equivalent CPU implementation, the costs associated with running on GPU cause this to not be significantly greater.

Understanding the physics governing primary atomization of high pressure fuel sprays is of paramount importance to accurately model combustion in direct injection engines. The small length and time scales of features that characterize this process falls below the resolution power of typical grids in CFD simulations, which necessitates the inclusion of physical models (sub-models) to account for unresolved physics. Unfortunately current physical models for fuel spray atomization used in engine CFD simulations are based on significant empirical scaling because there is a lack of experimental data to understand the governing physics. The most widely employed atomization sub-model used in current CFD simulations assumes the spray atomization process to be dominated by aerodynamically-driven surface instabilities, but there has been no quantitative experimental validation of this theory to date. The lack of experimental validation is due to the high spatial and temporal resolutions required to simultaneously to image these instabilities, which is difficult to achieve. The present work entails the development of a diagnostic technique to obtain high spatial and temporal resolution images of jet breakup and atomization in the near nozzle region of Gasoline Direct Injection (GDI) sprays. It focuses on the optical setup required to achieve maximum illumination, image contrast, sharp feature detection, and temporal tracking of interface instabilities for long-range microscopic imaging with a high-speed camera. The resolution and performance of the imaging system is characterized by evaluating its modulation transfer function (MTF). The setup enabled imaging of GDI sprays for the entire duration of an injection event (several milliseconds) at significantly improved spatial and temporal resolutions compared to historical spray atomization imaging data. The images show that low to moderate injection pressure sprays can be visualized with a high level of detail and also enable the tracking of features across frames within the field of view (FOV)

abstract: Compressible fluid flows involving multiple physical states of matter occur in both nature and technical applications such as underwater explosions and implosions, cavitation-induced bubble collapse in naval applications and Richtmyer-Meshkov type instabilities in inertial confinement fusion. Of particular interest is the atomization of fuels that enable shock-induced mixing of fuel and oxidizer in supersonic combustors. Due to low residence times and varying length scales, providing insight through physical experiments is both technically challenging and sometimes unfeasible. Numerical simulations can help provide detailed insight and aid in the engineering design of devices that can harness these physical phenomena. In this research, computational methods were developed to accurately simulate phase interfaces in compressible fluid flows with a focus on targeting primary atomization. Novel numerical methods which treat the phase interface as a discontinuity, and as a smeared region were developed using low-dissipation, high-order schemes. The resulting methods account for the effects of compressibility, surface tension and viscosity. To aid with the varying length scales and high-resolution requirements found in atomization applications, an adaptive mesh refinement (AMR) framework is used to provide high-resolution only in regions of interest. The developed methods were verified with test cases involving strong shocks, high density ratios, surface tension effects and jumps in the equations of state, in one-, two- and three dimensions, obtaining good agreement with theoretical and experimental results. An application case of the primary atomization of a liquid jet injected into a Mach 2 supersonic crossflow of air is performed with the methods developed. / Dissertation/Thesis / Doctoral Dissertation Aerospace Engineering 2020

[ES] La comprensión de los fenómenos físicos que acontecen en la región densa (también conocida como campo cercano) durante la atomización de los sprays ha sido una de las mayores incógnitas a la hora de estudiar sus aplicaciones. En el sector industrial, el rango de interés abarca desde toberas en aplicaciones propulsivas a sprays en aplicaciones médicas, agrícolas o culinarias. Esta evidente falta de conocimiento obliga a realizar simplificaciones en la modelización, provocando resultados poco precisos y la necesidad de grandes caracterizaciones experimentales en la fase de diseño. De esta manera, los procesos de rotura del spray y atomización primaria se consideran problemas físicos fundamentales, cuya complejidad viene dada como resultado de un flujo multifásico en un régimen altamente turbulento, originando escenarios caóticos. El análisis de este problema es extremadamente complejo debido a la ausencia sustancial de teorías validadas referentes a los fenómenos físicos involucrados como son la turbulencia y la atomización. Además, la combinación de la naturaleza multifásica del flujo y su comportamiento turbulento resultan en una gran dificultad para afrontar el problema. Durante los últimos 10 años, las técnicas experimentales han sido finalmente capaces de visualizar la región densa, pero la confianza, análisis y efectividad de dichos experimentos en esta región del spray todavía requiere de mejoras sustanciales. En este contexto, esta tesis trata de contribuir al entendimiento de estos procesos físicos y de proporcionar herramientas de análisis para estos flujos tan complejos. Para ello, mediante Direct Numerical Simulations se ha afrontado el problema resolviendo las escalas de movimiento más pequeñas, y capturando todas las escalas de turbulencia y eventos de rotura. Uno de los objetivos de la tesis ha sido evaluar la influencia de las condiciones de contorno del flujo entrante en la atomización primaria y en el comportamiento turbulento del spray. Para ello, se han empleado dos condiciones de contorno diferentes. En primer lugar se ha empleado una condición de contorno sintética para producir turbulencia homogenea a la entrada, simulando el comporamiento de la tobera. Una de las características más interesantes de este método es la posibilidad de retocar los parámetros dentro del algoritmo. En particular, la escala de longitud integral se ha variado para evaluar la influencia de las estructuras mas grandes de la tobera en la atomización primaria. El análisis de la condición de contorno sintética también ha permitido el diseño óptimo de simulaciones de las cuales se han derivado estadísticas turbulentas significativas. En este escenario, se han llevado a cabo estudios más profundos sobre la influencia de propiedades de las estructuras turbulentas como la homogeneidad y la anisotropía tanto en el espectro de los flujos como en las estadísticas de las gotas. Para tal fin, se han desarrollado metodologías novedosas para computar el análisis espectral y la estadística de las gotas Entre los resultados de este análisis destaca la independencia de la condición de contorno de entrada en las estadísticas de las gotas, mientras que por otra parte, recalca que las características turbulentas desarrolladas en el interior de la tobera afectan a la cantidad total de masa atomizada. Estas consideraciones se encuentran respaldadas por el análisis espectral realizado, mediante el cuál se concluye que la turbulencia multifásica comparte el comportamiento universal descrito por las teorías de Kolmogorov. / [CAT] La comprensió dels fenòmens físics que succeïxen en la regió densa (també coneguda com a camp pròxim) durant l'atomització dels sprays ha sigut una de les majors incògnites a l'hora d'estudiar les seues aplicacions. En el sector industrial, el rang d'interés comprén des de toveres en aplicacions propulsives a sprays en aplicacions mèdiques, agrícoles o culinàries. Esta evident falta de coneixement obliga a realitzar simplificacions en la modelització, provocant resultats poc precisos i la necessitat de grans caracteritzacions experimentals en la fase de disseny. D'esta manera, els processos de ruptura del spray i atomització primària es consideren problemes físics fonamentals, la complexitat dels quals ve donada com resultat d'un flux multifàsic en un règim altament turbulent, originant escenaris caòtics. L'anàlisi d'este problema és extremadament complex a causa de l'absència substancial de teories validades dels fenòmens físics involucrats com són la turbulència i l'atomització. A més, la combinació de la naturalesa multifàsica del flux i el seu comportament turbulent resulten en una gran dificultat per a afrontar el problema. Durant els últims 10 anys les tècniques experimentals han sigut finalment capaces de visualitzar la regió densa, però la confiança, anàlisi i efectivitat dels experiments en esta regió del spray encara requerix de millores substancials. En este context, esta tesi tracta de contribuir en l'enteniment d'estos processos físics i de proporcionar ferramentes d'anàlisi per a estos fluxos tan complexos. Per a això, per mitjà de Direct Numerical Simulations s'ha afrontat el problema resolent les escales de moviment més menudes, al mateix temps que es capturen totes les escales de turbulència i esdeveniments de ruptura. Un dels objectius de la tesi ha sigut avaluar la influència que les condicions de contorn del flux entrant tenen en l'atomització primària i en el comportament turbulent del spray. Per a això, s'han empleat dos condicions de contorn diferents. En primer lloc s'ha empleat una condició de contorn sintètica per a produir turbulència homogènia a l'entrada, simulant el comportament de la tovera. Una de les característiques més interessants d'este mètod és la possibilitat de retocar els paràmetres dins de l'algoritme. En particular, l'escala de longitud integral s'ha variat per a avaluar la influència de les estructures mes grans de la tovera en l'atomització primària. L'anàlisi de la condició de contorn sintètica també ha permés el disseny òptim de simulacions de les quals s'han derivat estadístiques turbulentes significatives. En este escenari, s'han dut a terme estudis més profunds sobre la influència de propietats de les estructures turbulentes com l'homogeneïtat i l'anisotropia tant en l'espectre dels fluxos com en les estadístiques de les gotes. Per a tal fi, s'han desenrotllat metodologies noves per a computar l'anàlisi espectral i l'estadística de les gotes. Entre els resultats d'esta anàlisi destaca la independència de la condició de contorn d'entrada en les estadístiques de les gotes, mentres que d'altra banda, es recalca que les característiques turbulentes desenrotllades en l'interior de la tovera afecten a la quantitat total de massa atomitzada. Estes consideracions es troben recolzades per l'anàlisi espectral realitzat, per mitjà del qual es conclou que la turbulència multifásica compartix el comportament universal descrit per les teories de Kolmogorov. / [EN] The understanding of the physical phenomena occurring in the dense region (also known as near field) of atomizing sprays has been long seen as one of the biggest unknown when studying sprays applications. The industrial range of interest goes from nozzles in combustion and propulsion applications to medical sprays, agricultural and food process applications. This substantial lack of knowledge is responsible for some important simplification in modeling, that often result to be inaccurate or simply partial, leading to the evident need of large experimental characterization during the design phase. In fact, the spray breakup and primary atomization processes are indeed fundamental problems of physics, which complexity results from the combination of a multiphase flow in a highly turbulent regime that leads to chaotic scenarios. The analysis of this problem is extremely problematic, due to a substantial lack of definitive theories about the physical phenomena involved, namely turbulence and atomization. Furthermore, the combination of the multiphase nature of the flow and its turbulent behavior makes substantially difficult to address the problem. Only within the last 10 years, experimental techniques have been capable of visualizing the dense region, but the experiments reliability, analysis and effectiveness in this region still requires vast improvements. In this scenario, this thesis aims to contribute in the understanding of these physical process and to provide analysis tools for these complex flows. In order to do so, Direct Numerical Simulations have been used for addressing the problem at its smallest scale of motion, while reliably capturing all turbulence scales and breakup events. The multiphase nature of the flow is accounted for by using the Volume of Fluid method. One of the goal of the thesis was to assess the influence of the inflow boundary conditions on the primary atomization and on the spray's turbulence behavior. In order to do so, two different boundary conditions were used. In a first place, a synthetic inflow boundary condition was used in order to produce a homogeneous turbulence inflow, simulating the nozzle behavior. One of the interesting features of this method was the possibility of tweaking the parameters within the algorithm. In particular, the integral length scale was varied in order to assess the influence of nozzle larger turbulent structures on the primary atomization. The analysis on the synthetic boundary condition also allowed to optimally design simulations from which derive meaningful turbulence statistics. On this framework, further studies were carried over on the influence of turbulent structures properties, namely homogeneity and anisotropy, on both the flows spectra and droplets statistics. In order to achieve this goal, novel procedures for both computing the flow spectra and analyzing droplets were developed and are carefully addressed in the thesis. The results of the analysis highlight the independence of droplets statistics from the inflow boundary condition, while, on the other hand, remarking how the total quantity of atomized mass is significantly affected by the turbulence features developed within the nozzle. This considerations are supported by the spectrum analysis performed, which also highlighted how multiphase turbulence shares the universal features described in Kolmogorov theories. / Crialesi Esposito, M. (2019). Analysis of primary atomization in sprays using Direct Numerical Simulation [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/133975 / TESIS

Afin de se rapprocher des conditions du mélange homogène du moteur essence, plusieurs fluides sont injectés dans l'atmosphère à une pression amont et une température variées. Cinq prototypes d'injecteurs réels trois-trous de Continental ont été utilisés. En augmentant la pression d'injection, l'écoulement passe par quatre régimes où le niveau de développement de cavitation varie. Le coefficient de décharge Cd dépend essentiellement du nombre de cavitation. Au point critique de cavitation, deux corrélations ont été obtenues reliant respectivement Cd et le nombre de cavitation critique au nombre de Reynolds correspondant. Le jet en champ proche est gouverné par trois nombres sans dimensions : celui de Weber, de Reynolds et de cavitation. L'effet de chacun d'eux sur l'angle du jet à la sortie a été obtenu. La comparaison des résultats entre deux injecteurs a montré que le rapport entre la longueur et le diamètre de l'orifice est d'une influence d'ordre 1 sur l'angle du jet. / In order to get closer to the homogeneous mixture conditions of a gasoline engine, different fluids are injected into the atmosphere at varying upstream pressure and temperature. Five three-hole real injector prototypes from Continental were used. When injection pressure is increased, the internal flow goes through four regimes where the cavitation development level varies from one to another. The discharge coefficient Cd was found mainly dependent on the cavitation number. At the cavitation critical point, two correlations between Cd and the critical cavitation number on one side respectively, and the correspondent Reynolds number on the other side were found. The near field jet is ruled by three dimensionless numbers : Weber, Reynolds and cavitation. The effect of each one of them on the jet angle at the orifice outlet was obtained. By comparing the results of two injectors, it was found that the length over diameter ratio has a first order influence on the jet angle.

La réduction des émissions polluantes et l'amélioration des performances des turboréacteurs nécessitent une connaissance détaillée des phénomènes physiques mis en jeu dans une chambre de combustion. L'atomisation du carburant résulte du cisaillement engendré par un fort écoulement d'air généré dans l'injecteur. La simulation numérique directe d'écoulements avec interface permet de simuler l'ensemble du processus d'atomisation. L'utilisation de maillages Cartésiens permet la réalisation de calculs HPC efficaces et précis. Mais, une des complexités de l'atomisation vient d'une interaction forte entre le comportement de la nappe liquide et l'écoulement gazeux dans les conduites de l'injecteur, rendant impératif la simulation de l'injecteur complet. Ceci étant impossible avec des maillages Cartésiens structurés, l'objectif de cette thèse est de développer une méthode d'interface immergée permettant l'inclusion d'objets solides dans un domaine de calcul, indépendamment du maillage, afin de réaliser des DNS du système d'injection complet. Les équations de Navier-Stokes incompressibles diphasiques sont résolues à l'aide d'un algorithme de projection, l'interface liquide-gaz étant transportée avec une méthode CLSVOF conservative en masse et quantité de mouvement. La présence du solide est prise en compte grâce à la méthode d'interface immergée. Cette méthode a été appliquée à la simulation numérique de nappes liquides cisaillées pour une configuration d'injecteur utilisée en essais à l'ONERA et a permis une meilleure prédiction de la fréquence de battement de la nappe. / The reduction of polluting emissions and improvement of aeronautical engines efficiency depends on the detailed knowledge of the physical phenomena encountered in a combustion chamber. Fuel atomization results from the shearing effect induced by the high velocity airflow generated inside the injector. The Direct Numerical Simulation of interfacial flows allows the simulation of the whole atomization process, while Cartesian structured meshes allows efficient and accurate HPC computations. However, the complexity of atomization comes from a strong interaction between the jet behavior and the injector internal flow, which makes essential to simulate the whole injector system. As that is impossible on Cartesian structured grids, the main objective of this thesis is to develop an Immersed Interface Method (IIM) allowing the inclusion of solid objects in the computational domain, independently of the mesh. The incompressible two-phases Navier-Stokes equations are solved using a projection algorithm with the CLSVOF method, conservative in mass and momentum. The solid presence is taken into account using the IIM. The proposed IIM has been applied to the numerical simulation of sheared liquid sheets corresponding to an ONERA experimental configuration and allows a better prediction of the flapping frequencies of the liquid sheet.

The thesis deals with experimental research of mechanism of liquid breakup at twin-fluid atomizers. Four different atomizers were examined at the beginning of the research. Two of them were of standard design (Y-jet and effervescent nozzles), and the rest two atomizers were developed as a part of the thesis (so called CFT and inversed effervescent atomizers). Results show that only the inversed effervescent atomizer was capable of generating stable spray under examined conditions due to the specific breakup mechanism. This mechanism is similar to what was observed in effervescent atomizers. However, the mixing process inside the inversed effervescent atomizer is very different. The specific breakup mechanism was then defined as the main scope of the thesis. It was investigated by the high-speed imaging. The images were then processed by proper orthogonal decomposition and by fast Fourier transformation. Spray spatial development was examined using phase Doppler anemometer. The data was analyzed to describe the dynamics of the spray. A detailed description of the breakup mechanism is made from the combination of the experimental and post-processing techniques. The thesis brings new insight into the understanding of the liquid breakup mechanism and shows a potential for a further development of the inversed effervescent atomizer.

Les écoulements diphasiques jouent un rôle prépondérant dans les moteurs-fusées à ergols liquides cryogéniques, équipant par exemple les lanceurs de la famille Ariane. L'étude expérimentale de tels engins propulsifs étant complexe et onéreuse, disposer d'outils numériques à même de simuler fidèlement leur fonctionnement se révèle être un objectif aussi important qu'ambitieux. La difficulté majeure réside dans le caractère fortement multi-échelles du problème, si bien qu’aucune approche numérique existante n'est capable à elle seule de décrire parfaitement l'ensemble des échelles liquides. Partant de ce constat, les travaux présentés dans cette thèse visent à mettre en place une stratégie de couplage entre des modèles bien adaptés aux différentes topologies d'écoulement diphasique, et ce dans le cadre de la plateforme logicielle multi-physique CEDRE développée par l'ONERA. La démarche adoptée consiste précisément à coupler un modèle à interface diffuse de type ``4 équations'' pour les zones à phases séparées, et un modèle cinétique eulérien pour la phase dispersée, rendant ainsi possible la description de l’atomisation primaire. Par ailleurs, les conditions sévères qui règnent dans les moteurs cryotechniques, où de forts gradients de température, vitesse et densité sont rencontrés, mettent à l'épreuve la robustesse des méthodes numériques. Une nouvelle méthode MUSCL multipente pour maillages non structurés généraux a ainsi été développée, permettant d’améliorer la robustesse et la précision des schémas de discrétisation spatiale. L’ensemble de la stratégie de couplage est finalement appliquée à la simulation du banc Mascotte de l'ONERA pour la combustion cryotechnique. / Two-phase flows play a significant role for the proper functioning of cryogenic liquid-propellant rocketengines, such as those that equip the launchers of the Ariane family. Since the experimental investigationof such propulsion devices is complex and expensive, developing numerical tools able to accuratelysimulate their functioning, is a crucial but nonetheless ambitious objective. The major difficulty is due tothe multiscale nature of the problem, as a result of which there is currently no numerical approach ableto perfectly describe all the liquid scales on its own. Based on this observation the work presented in thisthesis aims at setting up a coupling strategy between models well-adapted to each two-phase flowtopology, in the framework of the ONERA’s multiphysics CEDRE software. The approach adoptedprecisely consists in coupling a 4-equation diffuse interface model for the separated phases and aeulerian kinetic model for the dispersed phase, thus making it possible to describe primary atomization.Besides, the harsh conditions within cryogenic rocket engines, where large temperature, velocity anddensity gradients are encountered, severely challenge the robustness of numerical methods. A newmultislope MUSCL method for general unstructured meshes is thus developed in order to improve therobustness and accuracy of space discretization schemes. The whole coupling strategy is finally appliedto the numerical simulation of the ONERA’s Mascotte test bench for cryogenic combustion research.