Numerical simulation of bubbles and drops in complex geometries by using dynamic meshes

CFD techniques are important tools for the study of multiphase flows, because most of the physical phenomena of these flows often happen on space and time scales where experimental methodologies are impossible in practice. Notwithstanding, numerical approaches are limited by the computational power of the present computers. In this sense, small improvements in the efficiency of the simulations can make the difference between an approachable problem and an unapproachable one. The proposal of this doctoral thesis is focused on developing numerical algorithms to optimize the simulations of multiphase solvers based on single fluids formulations, applied on three-dimensional unstructured meshes, in the context of a finite-volume discretization. In particular, the methods developed in the context of this PhD thesis use a conservative level set technique to deal with the multiphase domain. The work has been organized in five chapters and four appendices. The first chapter constitutes an introduction to the multiphase flows and the different approaches used to study them. The core work of the of this PhD thesis is explained throughout chapters two, three, and four. In those chapters, the improvements performed on the multiphase DNS techniques are addressed in detail, providing results comparisons and discussions on the obtained outcomes. After developing the main ideas of the thesis, a final concluding chapter is presented, summarizing the main findings of this research, and pointing out some future work. Finally, the appendices includes some material that can be useful to understand in depth some specific parts of the thesis but, conversely, they are not essential to follow the main thread. As said before, the core work of this thesis is presented throughout chapters two, three and four. In chapter two, four domain optimization methods are formulated and tested. By using these techniques, small domains can be used in rising bubble simulations, thus saving computational resources. These methods have been implemented in a conservative level set framework. Some of these methods require the use of open boundaries. Therefore, a careful treatment of both inflow and outflow boundaries has been carried out. This includes the development of a new outflow boundary condition as a variation of the classical convective outflow. At this point, a study about the sizing of the computational domain has been conducted, paying special attention to the placement of the inflow and outflow boundaries. Additionally, once the methods are formulated, several validation cases are run to discuss the applicability and robustness of each method. The third chapter present a physical study of a challenging problem: the Taylor bubble. By using the most promising technique from those presented in the previous chapter (i.e. the moving mesh method), the problem of an elongated bubble rising in stagnant liquid is addressed here. A transient study on the velocity field of the problem is provided. Moreover, the study also includes sensitivity analyses with respect to the initial shape of the bubble, the initial volume of the bubble, the flow regime and the inclination of the channel. Chapter number four presents an extension of the developed method to simulate bubbles and drops evolving in complex geometries. The use of an immersed boundary method allows to deal with intricate geometries and to reproduce internal boundaries within an ALE framework. The resulting method is capable of dealing with full unstructured meshes. Different problems are studied here to assert the proposed formulation, both involving constricting and non-constricting geometries. In particular, the following problems are addressed: a 2D gravity-driven bubble interacting with a highly-inclined plane, a 2D gravity-driven Taylor bubble turning into a curved channel, the 3D passage of a drop through a periodically constricted channel, and the impingement of a 3D drop on a flat plate.La Mecánica de Fluidos Computacional (CFD) es una importante disciplina para el estudio de flujos multifase. Esto se debe a que, en este tipo de flujos, la mayor parte de los fenómenos físicos ocurren en escalas de tiempo y espacio imposibles de detectar mediante una metodología experimental. Sin embargo, los enfoques numéricos están limitados por la potencia de cálculo de los ordenadores actuales. En este sentido, pequeñas mejoras en la eficiencia de las simulaciones pueden marcar la diferencia entre un problema que puede resolverse mediante CFD o uno que no. En la presente tesis doctoral se propone el desarrollo de varios algoritmos numéricos para optimizar simulaciones de flujos multifase basadas en formulaciones "single fluids", aplicadas en mallas no estructuradas y tridimensionales, en el contexto de discretizaciones "finite-volume". El trabajo se ha organizado en cinco capítulos y cuatro apéndices. El primer capítulo constituye una introducción a los flujos multifase y a los distintos enfoques usados para estudiarlos. El trabajo nuclear de la presente tesis reside en los capítulos tres, cuatro y cinco. En dichos capítulos se presentan las mejoras realizadas en las técnicas de resolución de flujos multifase mediante una metodología "DNS", aportando comparaciones de resultados y discusiones críticas de los resultados obtenidos. Después de desarrollar las ideas centrales de la tesis, se presenta un capítulo final con las conclusiones destacadas de este trabajo, señalando posibles líneas de trabajo futuro. Finalmente, se incluyen varios apéndices con material complementario que puede ser útil para profundizar en algún aspecto concreto del desarrollo, pero que a su vez no es esencial para entender las ideas principales del texto. Como se explica anteriormente, el trabajo central de la tesis se ha desarrollado a lo largo de los capítulos dos, tres y cuatro. En el segundo capítulo se formulan y prueban cuatro métodos de optimización de dominios de cálculo. Mediante la utilización de estos métodos se hace posible usar dominios de cálculo pequeños en problemas de burbujas ascendentes, ahorrando así recursos computacionales. Algunos de estos métodos requieren el uso de fronteras abiertas, por lo que se propone un estudio detallado de las condiciones de contorno de entrada y salida. Esto incluye el desarrollo de una nueva condición tipo "outflow". A continuación se estudia en profundidad el dimensionamiento del dominio de cálculo, prestando una atención especial a la posición de las fronteras de entrada y de salida. Con todo esto, el capítulo se cierra con una comparativa del rendimiento de los distintos métodos propuestos en varios problemas de burbujas ascendentes. El tercer capítulo presenta un estudio físico de un problema clave: la burbuja de Taylor. Usando la técnica con mejor rendimiento del capítulo anterior (es decir, la técnica de malla móvil), se aborda el problema de una burbuja alargada moviéndose en un fluido en reposo. Se lleva a cabo un estudio transitorio de la velocidad del campo fluido. Además, se realizan varios estudios de sensibilidad con respecto a la forma inicial de la burbuja, su volumen inicial, el régimen de flujo y la inclinación del canal. Por último, en el cuarto capítulo se presenta una extensión del método desarrollado para simular gotas y burbujas evolucionando en geometrías complejas. El uso de un método "Immersed Boundary" permite tratar geometrías complejas y reproducir fronteras internas en métodos que utilicen mallas móviles. En este punto, se estudian diversos problemas para validar la formulación propuesta, tanto en geometrías constrictivas como en no constrictivas. En particular, se han resuelto los siguientes problemas: una burbuja 2D interaccionando con un plano inclinado, una burbuja de Taylor 2D girando en un tubo curvo, el ascenso de una gota 3D dentro de un canal corrugado, y el impacto de una gota 3D contra una plaformaPostprint (published version

Numerical simulation of bubbles and drops in complex geometries by using dynamic meshes

CFD techniques are important tools for the study of multiphase flows, because most of the physical phenomena of these flows often happen on space and time scales where experimental methodologies are impossible in practice. Notwithstanding, numerical approaches are limited by the computational power of the present computers. In this sense, small improvements in the efficiency of the simulations can make the difference between an approachable problem and an unapproachable one. The proposal of this doctoral thesis is focused on developing numerical algorithms to optimize the simulations of multiphase solvers based on single fluids formulations, applied on three-dimensional unstructured meshes, in the context of a finite-volume discretization. In particular, the methods developed in the context of this PhD thesis use a conservative level set technique to deal with the multiphase domain. The work has been organized in five chapters and four appendices. The first chapter constitutes an introduction to the multiphase flows and the different approaches used to study them. The core work of the of this PhD thesis is explained throughout chapters two, three, and four. In those chapters, the improvements performed on the multiphase DNS techniques are addressed in detail, providing results comparisons and discussions on the obtained outcomes. After developing the main ideas of the thesis, a final concluding chapter is presented, summarizing the main findings of this research, and pointing out some future work. Finally, the appendices includes some material that can be useful to understand in depth some specific parts of the thesis but, conversely, they are not essential to follow the main thread. As said before, the core work of this thesis is presented throughout chapters two, three and four. In chapter two, four domain optimization methods are formulated and tested. By using these techniques, small domains can be used in rising bubble simulations, thus saving computational resources. These methods have been implemented in a conservative level set framework. Some of these methods require the use of open boundaries. Therefore, a careful treatment of both inflow and outflow boundaries has been carried out. This includes the development of a new outflow boundary condition as a variation of the classical convective outflow. At this point, a study about the sizing of the computational domain has been conducted, paying special attention to the placement of the inflow and outflow boundaries. Additionally, once the methods are formulated, several validation cases are run to discuss the applicability and robustness of each method. The third chapter present a physical study of a challenging problem: the Taylor bubble. By using the most promising technique from those presented in the previous chapter (i.e. the moving mesh method), the problem of an elongated bubble rising in stagnant liquid is addressed here. A transient study on the velocity field of the problem is provided. Moreover, the study also includes sensitivity analyses with respect to the initial shape of the bubble, the initial volume of the bubble, the flow regime and the inclination of the channel. Chapter number four presents an extension of the developed method to simulate bubbles and drops evolving in complex geometries. The use of an immersed boundary method allows to deal with intricate geometries and to reproduce internal boundaries within an ALE framework. The resulting method is capable of dealing with full unstructured meshes. Different problems are studied here to assert the proposed formulation, both involving constricting and non-constricting geometries. In particular, the following problems are addressed: a 2D gravity-driven bubble interacting with a highly-inclined plane, a 2D gravity-driven Taylor bubble turning into a curved channel, the 3D passage of a drop through a periodically constricted channel, and the impingement of a 3D drop on a flat plate.La Mecánica de Fluidos Computacional (CFD) es una importante disciplina para el estudio de flujos multifase. Esto se debe a que, en este tipo de flujos, la mayor parte de los fenómenos físicos ocurren en escalas de tiempo y espacio imposibles de detectar mediante una metodología experimental. Sin embargo, los enfoques numéricos están limitados por la potencia de cálculo de los ordenadores actuales. En este sentido, pequeñas mejoras en la eficiencia de las simulaciones pueden marcar la diferencia entre un problema que puede resolverse mediante CFD o uno que no. En la presente tesis doctoral se propone el desarrollo de varios algoritmos numéricos para optimizar simulaciones de flujos multifase basadas en formulaciones "single fluids", aplicadas en mallas no estructuradas y tridimensionales, en el contexto de discretizaciones "finite-volume". El trabajo se ha organizado en cinco capítulos y cuatro apéndices. El primer capítulo constituye una introducción a los flujos multifase y a los distintos enfoques usados para estudiarlos. El trabajo nuclear de la presente tesis reside en los capítulos tres, cuatro y cinco. En dichos capítulos se presentan las mejoras realizadas en las técnicas de resolución de flujos multifase mediante una metodología "DNS", aportando comparaciones de resultados y discusiones críticas de los resultados obtenidos. Después de desarrollar las ideas centrales de la tesis, se presenta un capítulo final con las conclusiones destacadas de este trabajo, señalando posibles líneas de trabajo futuro. Finalmente, se incluyen varios apéndices con material complementario que puede ser útil para profundizar en algún aspecto concreto del desarrollo, pero que a su vez no es esencial para entender las ideas principales del texto. Como se explica anteriormente, el trabajo central de la tesis se ha desarrollado a lo largo de los capítulos dos, tres y cuatro. En el segundo capítulo se formulan y prueban cuatro métodos de optimización de dominios de cálculo. Mediante la utilización de estos métodos se hace posible usar dominios de cálculo pequeños en problemas de burbujas ascendentes, ahorrando así recursos computacionales. Algunos de estos métodos requieren el uso de fronteras abiertas, por lo que se propone un estudio detallado de las condiciones de contorno de entrada y salida. Esto incluye el desarrollo de una nueva condición tipo "outflow". A continuación se estudia en profundidad el dimensionamiento del dominio de cálculo, prestando una atención especial a la posición de las fronteras de entrada y de salida. Con todo esto, el capítulo se cierra con una comparativa del rendimiento de los distintos métodos propuestos en varios problemas de burbujas ascendentes. El tercer capítulo presenta un estudio físico de un problema clave: la burbuja de Taylor. Usando la técnica con mejor rendimiento del capítulo anterior (es decir, la técnica de malla móvil), se aborda el problema de una burbuja alargada moviéndose en un fluido en reposo. Se lleva a cabo un estudio transitorio de la velocidad del campo fluido. Además, se realizan varios estudios de sensibilidad con respecto a la forma inicial de la burbuja, su volumen inicial, el régimen de flujo y la inclinación del canal. Por último, en el cuarto capítulo se presenta una extensión del método desarrollado para simular gotas y burbujas evolucionando en geometrías complejas. El uso de un método "Immersed Boundary" permite tratar geometrías complejas y reproducir fronteras internas en métodos que utilicen mallas móviles. En este punto, se estudian diversos problemas para validar la formulación propuesta, tanto en geometrías constrictivas como en no constrictivas. En particular, se han resuelto los siguientes problemas: una burbuja 2D interaccionando con un plano inclinado, una burbuja de Taylor 2D girando en un tubo curvo, el ascenso de una gota 3D dentro de un canal corrugado, y el impacto de una gota 3D contra una plaform

Design of a test bench for measuring friction force in a piston-cylinder system

In the present work, design of a first approach of a test bench prototype for measuring friction in a piston-cylinder system is presented. The bench consists of a motor, belt and pulley transmission, crank mechanism, a piston, a cylinder and a lubrication system. Friction will be determined by means of strain gages placed on the connecting rod of the mechanism. The bench also includes a phonic wheel for acquiring angular speed and angular position signals of the crank. In future research, friction will be studied for different surface finishes obtained by means of honing and plateau honing. Honing processes provide a crosshatch pattern that holds oil and helps lubrication of piston and rings. Honing conditions will be selected with the goal of minimizing friction. Friction is directly related to energy consumption of an engine and, thus, to its environmental impact. Keywords: test bench, friction, piston-cylinder system, honing, roughness.Postprint (published version

The scaled-time test as an alternative to the pseudo-dynamic test

The stepped and excessively slow execution of pseudo-dynamic tests has been found to be the source of some errors arising from strain-rate effect and stress relaxation. In order to control those errors, a new continuous test method which allows the selection of a more suitable time scale factor in the response is proposed in this work. By dimensional analysis, such scaled-time response is obtained theoretically by augmenting the inertial and damping properties of the structure, for which we propose the use of hydraulic pistons which are servo-controlled to produce active mass and damping, nevertheless using an equipment which is similar to that required in a pseudo-dynamic test. The results of the successful implementation of this technique for a simple specimen are shown here

Numerical study of binary droplets collision in the main collision regimes

Direct numerical simulation of binary droplets collision is done using a conservative level-set method. The Navier-Stokes and level-set equations are solved using a finite-volume method on collocated grids. A novel lamella stabilization approach is introduced to numerically resolve the thin lamella film appeared during a broad range of collision regimes. This direction-independent method proves to be numerically efficient and accurate compared with experimental data. When the droplets collide, the fluid between them is pushed outward, leaving a thin gas layer bounded by the surface of two droplets. This layer progressively gets thinner and depending on the collision regime, may rupture resulting in coalescence of the droplets or may linger resulting in bouncing-off the droplets. Embedded ghost-nodes layer makes it possible to mimic both bouncing and coalescence phenomena of the droplets collision. The numerical tools introduced are validated and verified against different experimental results for a wide range of collision regimes. A very good agreement is observed between the results of this paper and experimental data available in the literature. A detailed study of the energy budget for different shares of kinetic and dissipation energies inside of the droplet and matrix, in addition to the surface tension energy for studied cases, is provided. Supplementary quantitative values of viscous dissipation rate inside of the matrix and droplet, and also the radial expansion of the droplet are presented as well.Peer ReviewedPostprint (published version

Goal-driven agent-oriented software processes

The quality of software processes is acknowledged as a critical factor for delivering quality software systems. Any initiative for improving the quality of software processes requires their explicit representation and management. A current representational metaphor for systems is agent orientation, which has become one of the recently recognized engineering paradigms. In this article, we argue for the convenience of representing the software process using an agent-oriented language to model it and a goal-driven procedure to design it. Particularly we propose using the i* framework which is both an agent- and a goal-oriented modeling language. We review the possibilities of i* as a software process modeling language, and we also show how success factors can be made explicit in i* representations of the software processes. Finally, we illustrate the approach with an example based on the development of a set of ergonomic and safety software tools.Peer ReviewedPostprint (published version

Inspección de trabajo y seguridad social: criterios de actuación y fundamentación jurídica (caso práctico)

El presente caso práctico reproduce el enunciado del supuesto referido a la actividad de la Inspección de Trabajo y Seguridad Social que se planteó como cuarto ejercicio en la convocatoria de la oposición para el ingreso en el Cuerpo Superior de Inspectores de Trabajo y Seguridad Social correspondiente a 2003. En él se efectúa un análisis de las cuestiones derivadas del planteamiento, incorporando la fundamentación jurídica de la respuesta

La seguridad no se garantiza con la guerra

P. 19-20Las guerras y quienes las financian son la causa principal de los atentados y del drama de los refugiados y refugiadas. Federico Mayor Zaragoza, ex director general de la Unesco, considera que no rebelarnos contra esa barbarie es un “delito de silencio”, porque la indiferencia, asevera, equivale a complicidadN

Influencia de la poda en la producción de la bellota y el crecimiento en dehesas de la provincia de Huelva

Las dehesas son sistemas agrosilvopastorales de gran importancia económica ecológica y social en la provincia de Huelva. Su gestión debe considerarse de forma integral sin olvidar las buenas prácticas selvícolas, en un momento en que muchas de estas formaciones están sufriendo la influencia del fenómeno de la “seca”, que afecta de forma dramática al estado sanitario de estas formaciones y a su supervivencia. En este trabajo se evalúa la influencia de las podas, uno de los tratamientos más frecuentes en las dehesas, en la producción de bellota y en el crecimiento diamétrico de los pies. De acuerdo con los resultados obtenidos se realizan recomendaciones de gestión referentes a este tratamiento._________________________________The “dehesas” are agro-silvicultural systems with a huge economical, ecological and social interest for the province of Huelva. Its management must be considered in a global way without forgetting the good silvicultural practices, specially when this forests are dramatically affected by the oak decline. In this paper the influence of pruning on acorn production and diametric growing is evaluated, and starting from the obtained results, management options in relation with pruning are discussed

Geospatial mapping of carbon estimates for forested areas using the InVEST model and Sentinel-2: A case study in Galicia (NW Spain)

CO2 emissions have increased exponentially in recent years, so measuring and quantifying carbon sequestration is a step towards sustainable forest management and combating climate change. The overall goal of this study is to develop an accurate model for estimating carbon storage and sequestration for forest areas of the Atlantic Biogeographic Region. Specifically, the modelling and field sampling are carried out in the municipality of Baiona (Galicia, NW Spain), which was selected as a representative biome of this region. The methodology consists of carrying out two object-based image analysis (OBIA) classifications in spring and autumn to observe possible stocks of seasonal differences. Two carbon storage and sequestration models are built up (model 1 and model 2): model 1 for forest areas only and model 2 including all other land cover in the study area. Sentinel-2 geospatial data for 2021, Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) tools and geographic information systems (GIS) are used. A Kappa index of 0.92 is obtained for both classifications, thus ruling out any notable seasonal differences in the images used. The results from both models indicate that it is land covers associated with forest uses which store the most carbon in the study area, accounting for >50 % more than the other land covers. It is concluded that the methodology and data used are very useful for quantifying ecosystem services, which will help the governance of the region by implementing measures to mitigate some of the effects of climate change and help to create silvicultural models for the sustainable management of the Atlantic Biogeographic Region.Agencia Estatal de Investigación | Ref. TED2021-130241A-I00Xunta de Galicia | Ref. GPC-ED431B 2022/12Xunta de Galicia | Ref. ED481B-2023-042Universidade de Vigo/CISU