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We present an hybrid VOF/embedded boundary method allowing to model two-phase flows in presence of solids with arbitrary shapes. The method relies on the coupling of existing methods: a geometric Volume of fluid (VOF) method to tackle the two-phase flow and an embedded boundary method to sharply resolve arbitrary solid geometries. Coupling these approaches consistently is not trivial and we present in detail a quad/octree spatial discretization for solving the corresponding partial differential equations. Modelling contact angle dynamics is a complex physical and numerical problem. We present a Navier-slip boundary condition compatible with the present cut cell method, validated through a Taylor–Couette test case. To impose the boundary condition when the fluid–fluid interface intersects a solid surface, a geometrical contact angle approach is developed. Our method is validated for several test cases including the spreading of a droplet on a cylinder, and the equilibrium shape of a droplet on a flat or tilted plane in 2D and 3D. The temporal evolution and convergence of the droplet spreading on a flat plane is also discussed for the moving contact line given the boundary condition (Dirichlet or Navier) used. The ability of our numerical methodology to resolve contact line statics and dynamics for different solid geometries is thus demonstrated

El estudio de la mecánica de fluidos es uno de los campos más importantes de la ingeniería, ya que permite modelar el comportamiento de líquidos y gases con el objetivo de predecir su respuesta ante situaciones de interés. Este trabajo aborda un aspecto poco estudiado dentro del panorama general de la mecánica de fluidos, el comportamiento del plasma producido por combustión. El objetivo detrás de ello es ver si el efecto producido por un confinamiento magnético tiene o no relevancia en el desarrollo del flujo. Para ello, se configurará un programa para resolver las ecuaciones de comportamiento combinado de Navier-Stokes y del comportamiento general del plasma. Se tomarán una serie de simplificaciones sobre las mismas para poder realizar una primera estimación que permita obtener la influencia que se quiere analizar., Fluid mechanics is one of the most important fields studied in engineering, as they can model liquids and gases behaviour with the aim of predicting their response to several settings of interest. This work addresses one aspect which has not been extensively studied throughout the general fluid mechanics panorama, the behaviour of combustion produced plasma. This project aims at analysing whether the effects of magnetism confinement are signifficant or not to the flow progression. In order to fulfil these objectives the problem will be solved with the usage of a program configured to solve Navier-Stokes equations and the corresponding plasma equations. Some simplifications will be considered to be able to make an adequate first aproach to the problem, which may shed some light on the importance of the influence of magnetic fields in this situation.

Coaxial electrospray is a well established procedure to generate coaxial micro- and nano-structures (microcapsules and coaxial fibers). In this work we show the capabilities of a high-precision numerical code using dynamic mapping, analytic Jacobian formulation and the Taylor–Melcher leaky dielectric model (LDM) to deal with real coaxial electrospray configurations. Since the number of parameters involved in the problem is large, we focus on an experimentally tested liquid pair configuration where the thickness, relative position, and end sharpness of the coaxial feeding tubes are geometrical parameters of interest. The effects of those parameters and the voltage applied at the tubes on the coaxial liquid menisci, liquid streamlines, size of jet and core, and electric current are discussed.

We study experimentally and numerically the onset of tip streaming in an electrified droplet. The experiments show that, for a sufficiently small dimensionless conductivity, the droplet apex oscillates before ejecting a liquid jet. This effect is caused by the limited charge transfer from the bulk to the interface. This reduces the electrostatic pressure at the droplet's stretching tip, preventing liquid ejection. This reduction of the electrostatic pressure is compensated for by the electric shear stress arising during apex oscillations, which eventually leads to the jet formation. The stability limit calculated from the global stability analysis perfectly agrees with experimental results. However, this analysis predicts non-oscillatory, non-localized instability in all the cases, suggesting that both the oscillatory behavior and the small local scale characterizing tip streaming arise during the nonlinear droplet deformation.

The physics of electrospray has been subject to an intense debate for three decades regarding the ultimate electrokinetics that determines the electric current and the size of the emitted droplets in the steady Taylor cone-jet mode (TCJ). In order to solve with a high degree of accuracy the complete electrokinetic structure of the TCJ, in this work, we have used the full Poisson–Nernst–Planck model electrokinetic equations, which have been solved using a high accuracy numerical scheme. We consider a formulation with no interfacial adsorption of ions, as in Mori & Young (J. Fluid Mech., vol. 855, 2018, pp. 67–130). Our simulations corroborate Mori and Young’s conclusion that the classical leaky dielectric model (LDM) recovers the electrodiffusion theory for weak electrolytes when disregarding ion adsorption at the interface. However, for strong electrolytes, our results differ drastically from those provided by the LDM. In this case, we observe that the ion distribution, and consequently the conductivity in the bulk, can be strongly non-homogeneous. Given the rather universal validity of the LDM experimentally observed so far, we postulate that ion interfacial adsorption must be considered in the case of strong, highly dissociated electrolytes to retrieve the LDM limit, mostly for a cone jet operating in the vicinity of the minimum flow rate.

Los actuadores de plasma, especialmente los Dispositivos de Descarga con Barrera Dieléctrica (DBD), son una tecnología para el control del flujo que está en ferviente desarrollo durante la última década. Esto se debe a su sencillez, ligereza y a la posibilidad de implementar estrategias de control activo de la corriente. Se prevé que los actuadores de plasma DBD tengan un gran impacto en la aeronáutica, ya que son capaces de lograr un control preciso de la dirección del flujo de aire y de ofrecer una gran variedad de aplicaciones prácticas, como el control activo de la carga aerodinámica, la mejora del consumo de combustible, la reducción del ruido de motor y la generación de empuje adicional. Esto se debe, principalmente, a su capacidad de respuesta rápida y precisa ante las variaciones en el flujo de aire. Esta tecnología se ha aplicado con éxito en algunos prototipos de aviones, y se espera que se aplique a gran escala en la industria aeronáutica en los próximos años. Para estudiar el comportamiento del plasma, existen diversas metodologías computacionales, sin embargo, la mayoría presentan modelos físicos complejos que requieren recursos computacionales elevados y tiempo de cálculo. A lo largo de este proyecto, se ha desarrollado un modelo numérico sencillo, basado en el modelo ingenieril de Suzen-Huang, que se puede ejecutar desde un ordenador personal usando el software computacional Basilisk. Este código permite caracterizar el comportamiento de un actuador de plasma DBD ante diversas geometrías, configuraciones y condiciones contorno, en función de parámetros tanto fluidos como eléctricos., Plasma actuators, especially Dielectric Barrier Discharge (DBD) devices, are a flow control technology that has been under intense development during the last decade. This is due to its simplicity, lightness and the possibility of implementing active flow control strategies. It is envisaged that DBD plasma actuators will have a great impact in aeronautics, since they are able to achieve precise control of the air flow direction and offer a wide range of practical applications, such as active load control, fuel consumption improvement, noise reduction of the engine and additional thrust generation. This is mainly due to their fast and precise response to changes in the air flow. This technology has been successfully applied in some aircraft prototypes, and it is expected to be applied on a large scale in the aeronautical industry in the coming years. To study the plasma behavior, there are several computational methodologies available, however, most of them present complex physical models that require high computational resources and calculation time. Throughout this project, a simple numerical model, based on the Suzen-Huang engineering model, has been developed that can be run from a personal computer using the Basilisk computational software. This code allows to characterize the behavior of a DBD plasma actuator under different geometries, configurations and boundary conditions, depending on both fluid and electrical parameters.

Using a dynamical scaling analysis of the flow variables and their evolution due to bubble bursting, here we predict the size and speed of ejected droplets for the whole range of experimental Ohnesorge and Bond numbers where ejection occurs. The transient ejection, which requires the backfire of a vortex ring inside the liquid to preserve physical symmetry, shows a delicate balance between inertia, surface tension and viscous forces around a critical Ohnesorge number, akin to an apparent singularity. Like in other natural phenomena, this balance makes the process extremely sensitive to initial conditions. Our model generalizes or displaces other recently proposed ones, impacting on, for instance, the statistical description of sea spray.

En las próximas olimpiadas de París del 2024, la clase de windsurf tradicional RS:X será sustituida por una nueva clase llamada IQFoil. Es por ello que surge la necesidad de estudiar este nuevo deporte para sacar el máximo rendimiento. Mediante simulaciones fluidodinámicas se ha conseguido obtener el efecto que tiene la posición del piloto en el balance longitudinal. Además, se describe como afecta el trimado del estabilizador del foil a la forma de navegar., In the following Olimpics games, the traditional windsurfing class, RS:X, will be replaced by the new IQfoil class. Therefore, it is necessary to study this new sport in order to get the maximun performance out of it. Using fluidodynamic simulations it has been possible to obtain the effect that the pilot’s position has on longitudinal balance. Moreover, it will be described how the trim of the stabilizer affects on the way of sailing.

Las situaciones transitorias son muy comunes en las instalaciones hidráulicas y pueden conllevar graves consecuencias en estas debido a las situaciones oscilatorias que se forman que pueden tener altos picos de presión y altas frecuencias. En este trabajo se ha desarrollado una herramienta de simulación de instalaciones hidráulicas para situaciones estacionarias y transitorias, que permite calcular presión, caudal y velocidad en un punto cualquiera y un instante cualquiera de la instalación hidráulica. Esta herramienta permitiría realizar el diseño de una instalación y comprobar su comportamiento ante determinadas situaciones. Las ecuaciones de flujos en conductos en el caso transitorio son ecuaciones hiperbólicas, por lo tanto han de ser resueltas por métodos numéricos como el método de las características, que es el más popular, o el método de diferencias finitas. Sin embargo, para la resolución de flujos transitorios la herramienta implementa el método de Godunov, el método se basa en un método de resolución del Problema de Riemann, divide los conductos en celdas y calcular los flujos en las interfaces de la celda. La implementación de condiciones de contorno, como válvulas, uniones de tuberías y depósitos, dentro del enfoque de Godunov es similar a la del enfoque del método de características. Se aplica la herramienta una serie de casos tanto estacionarios como transitorios., Transient situations are very common in hydraulic installations and can have serious consequences in these due to the oscillatory situations that are formed, which can have high pressure peaks and high frequencies. In this work we have developed a simulation tool for hydraulic installations for stationary and transient situations, which allows to calculate pressure, flow and velocity at any point and at any time of the hydraulic installation. This tool would allow the design of an installation and check its behavior in certain situations. The equations of flow in conduits in the transient case are hyperbolic equations, therefore they have to be solved by numerical methods such as the method of characteristics, which is the most popular, or the finite difference method. However, for the resolution of transient flows the tool implements Godunov’s method, the method is based on a method of solving the Riemann Problem, divide the ducts into cells and calculate the flows at the cell interfaces. The implementation of boundary conditions, such as valves, pipe joints and tanks, within the Godunov approach is similar to that of the method of characteristics approach. The tool is applied to a number of both stationary and transient cases.

Despite their fundamental and applied importance, a general model to predict the natural breakup length of steady capillary jets has not been proposed yet. In this work, we derive a scaling law with two universal constants to calculate that length as a function of the liquid properties and operating conditions. These constants are determined by fitting the scaling law to a large set of experimental and numerical measurements, including previously published data. Both the experimental and numerical jet lengths conform remarkably well to the proposed scaling law. This law is explained in terms of the growth of perturbations excited by the jet breakup itself.

A novel swirl flow-focusing microfluidic axisymmetric device for the generation of monodisperse microbubbles at high production rates to be used as in-line contrast agents for medical applications is presented. The swirl effect is induced upstream of the discharge orifice by a circular array of microblades which form a given angle with the radial direction. The induced vortical component on the focusing liquid stabilizes the gas meniscus by the vorticity amplification due to vortex stretching as the liquid is forced through the discharge orifice. The stabilized meniscus tapers into a steady gas ligament that breaks into monodisperse microbubbles. A reduction up to 57% in the microbubble diameter is accomplished when compared to conventional axisymmetric flow-focusing microdevices. An exhaustive experimental study is performed for various blade angles and numerous gas to liquid flow rate ratios, validating previous VoF numerical simulations. The microbubbles issued from the stabilized menisci verify prior scaling law of flow-focusing.

El proyecto se centrará en la utilización de RETScreen con tres centrales hidroeléctricas andaluzas: la antigua central de Casas Nuevas (Jaén), central de El Carpio (Córdoba) y la central de Iznájar (Málaga). RETScreen se trata de un software que simula instalaciones de generación eléctrica (en nuestro caso instalaciones hidráulicas) y que permite de una forma sencilla realizar el prediseño de la central, así como estudiar la viabilidad del proyecto (con ratios financieros) en función de unos datos de partida. En el primer caso se estudiará la posibilidad de recuperar la central abandonada para ponerla en marcha de nuevo, utilizando el antiguo emplazamiento para instalar nuevos equipos de turbinas. En los otros casos se comparará los resultados obtenidos con la utilización de RETScreen con la realidad, para comprobar el alcance de esta herramienta., The project will focus on the use of RETScreen with three Andalusian hydroelectric plants: the old central of Casas Nuevas (Jaén), central of El Carpio (Córdoba) and the central Iznájar (Málaga). RETScreen is a software that simulates electrical generation facilities (in our case hydraulic installations) and that allows in a simple way to carry out the pre-design of the plant, as well as to study the viability of the project (with financial ratios) based on some data of departure. In the first case, the possibility of recovering the abandoned plant to start it up again will be studied, using the old site to install new turbine equipment. In the other cases, the results obtained will be compared with the use of RETScreen with reality, to check the scope of this tool.

The work presented in this thesis can be summarized as a compilation of five different and comprehensive studies in the field of microuidic ows, related to the formation of jets, drops and bubbles; where the surface tension plays a major role. The topics studied are classified in chapters where the problem formulation, procedures, and results are individually presented. Chapter 2 is devoted to understand the evolution of Newtonian capillary jets and to study the instability transition of viscoelastic jets under axisymmetric perturbations. A mathematical model has been used to determine the parameter conditions for which the convective to absolute instability transition takes place, playing special attention to the role played by unrelaxed elastic axial stress. Chapter 3 presents results of a numerical study of rivulets in microchannels in order to characterize stable and unstable regimes. The theoretical frame work and stability analysis are presented in detail. It was found that a basic ow can become unstable when that quantity exceeds a certain critical value, while the rest of governing parameters remain constant. Chapter 4 discuses a ubiquitous process in science and technology - the dissolution of microbubbles. As in the previous chapters, detailed theoretical and numerical approaches are developed from scratch, culminating in a set of carefully performed experiments. Numerical and experimental results agree well and complement each other. We move then onto Chapter 5 which studies the electrical disruption of pendant liquid drops. The focus of the study here is the behaviour of suddenly electrified pendant droplets in dielectric liquid. Supported by numerical and experimental results, we argue that the viscosity of the surrounding uid is responsible for the development of more complex jetting processes such as what is called splashing in which the tip of the cone explodes onto a mushroom-like structure, and splitting regimes. Moreover, when the cone evolves int

The disintegration of liquid drops with low electrical conductivity and subject to an electric field is investigated both theoretically and experimentally. This disintegration takes place through the development of a conical cusp that eventually ejects an ultrathin liquid ligament. A first tiny drop is emitted from the end of this ligament. Due to its exceptionally small size and large electric charge per unit volume, that drop has been the object of relevant recent studies. In this paper, universal scaling laws for the diameter and electric charge of the first issued droplet are proposed and validated both numerically and experimentally. Our analysis shows how charge relaxation is the mechanism that differentiates the onset of electrospray, including the first droplet ejection, from the classical steady cone-jet mode. In this way, our study identifies when and where charge relaxation and electrokinetic phenomena come into play in electrospray, a subject of live controversy in the field.

The breakup of a charged liquid column is studied numerically using Volume-OF-Fluid (VOF) for a range of timescales where electrokinetic phenomena may become significant, i.e when the time to breakup becomes comparable or shorter than the diffusion and the electroosmotic migration times of charged species. Here we propose a conservative method to deal with the diffusion of a tracer in VOF schemes when the diffusion is limited to one of the phases. The method consists in weighing the diffusivity with the value of the volume fraction computed from the analytically reconstructed interface. In this way, the interface is made impermeable to the tracer, which is conservatively kept within one of the phases. The performance of this method is first tested by comparing simple configurations with existing analytical solutions. In the cases when the diffusion, electroosmotic motion and hydrodynamic singularities com pete, the results indicate that, after breakup, charges distribute between droplets differently from models assuming homogeneous and constant electrical conductivities (i.e. no electrokinetic effects). However, such departure does not alter the main hydrodynamic balances leading to well-established scaling laws of breakup.

The absolute and convective instability of an electrically charged viscoelastic liquid jet is studied. The liquid is assumed to be (i) a dilute polymer solution described by the Oldroyd-B viscoelastic model, and also and (ii) a leaky dielectric defined by the Taylor-Melcher leaky dielectric theory. A generalized eigenvalue equation is obtained and solved numerically. Two different viscoelastic liquids, i.e. a PEO aqueous solution and a PIB Boger fluid, are taken as examples to study the effect of electric field and elasticity on the absolute and convective instability characteristic of the axisymmetric and first non-axisymmetric modes of a viscoelastic jet. The analysis shows that normal electric field may induce absolute instability of both axisymmetric and non-axisymmetric modes, being the effect of electric field larger on the latter. Elasticity has a profound destabilizing effect on the absolute and convective instability of the axisymmetric mode while its effect on the non-axisymmetric mode is quite limited. Strategies for suppressing absolute instability of an electrically charged viscoelastic jet are explored. The result indicates that increasing jet velocity or decreasing jet radius may effectively avoid the occurrence of absolute instability

The numerical study of the steady axisymmetric air-water flow in a vertical sealed cylinder, driven by the rotating top disk, describes topological transformations as the rotation intensifies. The air meridional flow (AMF) and swirl induce meridional motions of opposite directions in water. For slow (fast) rotation, the effect of AMF (swirl) dominates. For very fast rotation, large-scale regions of clockwise meridional circulation in air and water are separated by a thin layer of anticlockwise circulation adjacent to the interface in water. This pattern develops for other fluids as well. Physical reasoning behind the flow evolution is provided.

We present a robust and computationally efficient numerical scheme for simulating steady electrohydrodynamic atomization processes (electrospray). The main simplification assumed in this scheme is that all the free electrical charges are distributed over the interface. A comparison of the results with those calculated with a volume-of-fluid method showed that the numerical scheme presented here accurately describes the flow pattern within the entire liquid domain. Experiments were performed to partially validate the numerical predictions. The simulations reproduced accurately the experimental shape of the liquid cone jet, providing correct values of the emitted electric current even for configurations very close to the cone-jet stability limit.

The dynamics of micro-bubble formation in an axisymmetric T-junction for a gas-liquid system is analyzed. The approach adopted involves the creation of a tapering gas-liquid meniscus from which a steady gas ligament issues by the introduction of a coaxial swirl in the liquid stream. A simple and easy geometry (an axisymmetric T-junction) suffices to introduce the swirl and to stabilize the meniscus, leading to the formation of small monodisperse bubbles. Full threedimensional simulations (3D) have also been conducted to show that, even when the liquid injection is not perfectly axisymmetric, the bubbles generated under conditions of some focusing swirl are distinctively smaller than bubbles created in the absence of swirl. In such cases, the bubbles, produced at the trail of the vortex axis, become a serendipitous tool to visualize the nonaxisymmetrical behavior of the vortex core, as shown by the simulations. VC 2011 American Institute of Physics

A discussion is presented on the role of limited conductivity and permittivity on the behavior of electrified jets. Under certain conditions, significant departures with respect to the perfect-conductor limit are to be expected. In addition, an exploration is undertaken concerning the validity of one-dimensional average models in the description of charged jets. To that end, a temporal linear modal stability analysis is carried out of poor-conductor viscous liquid jets flowing relatively to a steady radial electric field. Only axisymmetric perturbations, leading to highest quality aerosols, are considered. A grounded coaxial electrode is located at variable distance. Most available studies in the literature are restricted to the perfect-conductor limit, while the present contribution is an extension to moderate and low electrical conductivity and permittivity jets, in an effort to describe a situation increasingly prevalent in the sector of small-scale free-surface flows. The influence of the electrode distance b, a parameter a defined as the ratio of the electric relaxation time scale to the capillary time scale, and the relative permittivity b on the growth rate has been explored yielding results on the stability spectrum. In addition, arbitrary viscosity and electrification parameters are contemplated. In a wide variety of situations, the perfect-conductor limit provides a good approximation; however, the influence of a and b on the growth rate and most unstable wavelength cannot be neglected in the general case. An interfacial boundary layer in the axial velocity profile occurs in the low-viscosity limit, but this boundary layer tends to disappear when a or b are large enough. The use of a one-dimensional s1Dd averaged model as an alternative to the 3D approach provides a helpful shortcut and a complementary insight on the nature of the jet’s perturbative behavior. Lowest-order 1D approximations saverage modeld, of widespread application in the literature of ele

El método que se describe en esta invención se aplica a la fabricación de fibras mediante la combinación de dos acciones superpuestas. Por un lado, se conforma el material de la fibra (a partir de aquí, material de la fibra o fluido constituyente) en su fase fluida o dúctil por medio de otro fluido (a partir de aquí, fluido enfocante) inmiscible con dicha fase fluida o dúctil, por ejemplo un gas. Para ello se hace fluir en régimen subsónico al fluido enfocante concéntricamente con el material de la fibra a través de una pequefia tobera convergente. Por otro lado, y simultáneamente al citado flujo coaxial, se propicia la solidificación del material de la fibra, que sale en forma de chorro de la tobera, por uno de los tres procedimientos siguientes: enfriamiento, evaporación del solvente, o reacción de componentes. Es característica esencial de la invención el que el fluido enfocante circula por la tobera en régimen subsónico, especificándose los límites precisos de velocidad permitidos para el fluido enfocante en la tobera si se desean evitar las inestabilidades del chorro.