Fluid modeling and simulation of the electron population in Hall Effect Thrusters with complex magnetic topologies

Mención Internacional en el título de doctorLa propulsión eléctrica es una tecnología consolidada, utilizada por vehículos espaciales para llevar a cabo maniobras no atmosféricas. Este tipo de motores cohete ha estado presente en numerosas aplicaciones en las últimas décadas y sus usos van desde el mantenimiento de la posición orbital de satélites comerciales a transferencias interplanetarias en misiones de exploración. La mayor ventaja de los numerosos tipos de propulsores eléctricos es su capacidad de proporcionar un determinado impulso a un coste de propelente reducido, en comparación con otros tipos de propulsión. El desarrollo de los motores de plasma, la clase más común de propulsor eléctrico, se ha visto impedido en mayor medida que los cohetes químicos, por ejemplo, debido a la complejidad de la interacción de los fenómenos físicos y a dificultades asociadas con las campañas experimentales. En las últimas dos décadas se ha introducido el uso de simulaciones numéricas para ayudar a la caracterización de estos aparatos. A pesar de que el diseño asistido por ordenador juega aún un papel muy reducido, el incremento de recursos computacionales y la creciente exactitud de los modelos físicos han permitido a estas simulaciones describir numerosos mecanismos físicos, explorar el espacio de diseño de estos aparatos y complementar los ensayos experimentales. Esta tesis está centrada en el estudio numérico de la población de electrones en descargas de plasma poco colisionales, bajo la influencia de campos eléctricos y magnéticos. El trabajo realizado ha contribuido al desarrollo de una nueva herramienta de simulación híbrida, cuasi-neutra, bidimensional y axisimétrica, denominada HYPHEN; su naturaleza híbrida se debe al tratamiento por separado de las especies pesadas, descritas a través de un conocido método de partículas, y de la población de electrones, descrita como un fluido. Una de nuestras mayores contribuciones es la introducciÃsn de un modelo anisotrÃspico de dos temperaturas, que permite capturar los efectos de la falta de uniformidad del campo magnético sobre el transporte de electornes. Esta función abre el camino para la caracterización de nuevos propulsores electromagnéticos. Actualemente, el código está orientado hacia la simulación de las regiones del canal y de la pluma cercana en motores de efecto Hall, en los que se enfoca esta tesis. Parte del trabajo se ha dedicado a dotar al código de las capacidades necesarias para la simulación de topologías magnéticas complejas. El presente documento detalla la motivación detrás de HYPHEN, su metodología de diseño y la influencia de trabajos previos. Se ha prestado una especial atención al modelo fluido propuesto, detallando el uso de una malla alineada con el campo magnético para el tratamiento numérico de la población confinada de electrones, para la cual se han utilizado diversos métodos ad-hoc de discretización temporal y espacial. Varios modelos auxiliares también se han descrito, con el objetivo de caracterizar la respuesta de la capa límite del plasma y de los distintos procesos colisionales en el seno del mismo. Se presenta también el estudio de los aspectos numéricos del modelo fluido, incluyendo la sensibilidad a condiciones iniciales, a los valores del paso temporal, el refinamiento de la malla, etc. Finalmente, HYPHEN se ha testeado para la configuración de un conocido motor Hall. Los resultados demuestran que las propiedades físicas y las actuaciones obtenidas son comparables con resultados provenientes de estudios experimentales. Bajo este contexto, se ha llevado a cabo un estudio paramétrico para determinar la dependencia de la respuesta del motor con algunos de los parámetros más relevantes del modelo, tales como el transporte anómalo de electrones o la fracción de termalización de la capa límite, y con los diferentes modelos colisionales.Electric propulsion is an established technology used for non-atmospheric spacecraft maneuvering. This type of rockets have been present in numerous applications in the last decades, and their uses range from station keeping of commercial satellites to interplanetary transfers in deep space exploration missions. While electric propulsion thrusters are multi-faceted, presenting numerous and distinct types, their best selling point is the capability to deliver a given impulse at much lower propellant cost, in comparison to other types of propulsion. The maturation of plasma thrusters, the most common type of electric propulsion devices, has faced more limitations than chemical rockets, for example, due to the complexity of the physical interactions at play, and the difficulties associated with experimental campaigns. Over the past two decades, numerical simulations were introduced as a novel tool in the characterization of these devices. While true computer-aided-design is not yet a reality, the increment of computational resources and the heightened fidelity of the physical models have allowed to describe numerous physical mechanisms, explore the design space of these devices and complement experimental testing. This thesis focuses on the numerical study of the electron population in weakly collisional plasma discharges, under the influence of applied magnetic and electric fields. The work has been a primary contribution in the development of a new, quasi-neutral, two-dimensional, axisymmetric, hybrid simulation tool, called HYPHEN. Its hybrid nature responds to the different treatment of the heavy species populations, described through a well known discrete-particle approach, and the electron population, described as a fluid. One of our main contributions has been the introduction of a two-temperature anisotropic approach, which allows capturing of the magnetic non-uniformity effects over electron transport; this feature paves the way for the characterization of some novel electromagnetic propulsion technologies. Presently, the code is oriented to the simulation of the channel and near-plume regions in Hall effect thrusters, which have been the main focal point of the thesis. Dedicated efforts have been directed to providing the capabilities for the simulation of the plasma under complex magnetic field topologies. The manuscript details the motivation and design methodology behind HYPHEN, as well as the influence of previous work. Special attention has been given to the particularities of the proposed fluid model; this includes the use of a magnetic field aligned mesh for the numerical treatment of the electron population under magnetic confinement, for which ad-hoc spatial and temporal discretization methods have been proposed. Additional ancillary physical models have also been developed, characterizing the response of plasma boundary layers and the various collisional processes in the plasma. The numerical aspects of the model have been investigated, including the sensitivity to initial conditions, time-step values, mesh refinement, etc. Finally, HYPHEN has been tested in the context of a representative Hall-thruster configuration. The results were found to be in line with experimentally reported thruster performances and plasma discharge quantities. Additionally, a parametric investigation has been carried out in order to investigate the dependency of the thruster response with the most relevant model parameters, such as the anomalous electron transport or the boundary layer thermalization fraction, and the different collisional models.This work has been partially supported by the CHEOPS project, that received funding from the European Union’s Horizon 2020 research and innovation program, under grant agreement No. 730135. Additional support came from Project ESP2016-75887, funded by the National research and development program of Spain.Programa Oficial de Doctorado en Plasmas y Fusión NuclearPresidente: José Javier Honrubia Checa.- Secretario: Mario Merino Martínez.- Vocal: Paul-Quentin Elia

Experimental study for the determination of the turbulence onset in natural convection on inclined plates

In June, 8th, 2009 the balloon-borne solar telescope SUNRISE was launched from the Swedish Space Corporation balloon facility Esrange. A telescope with a mirror of 1 m in diameter ob-served the Sun during six days until the mission was terminated in Canada. The design process of SUNRISE and of any optical telescope requires the analysis of the effect of surrounding air on the quality of images. The turbulence encountered in the local telescope environment de-grades its optical performance. This phenomenon called `seeing' consists of optical aberrations produced by density non-homogeneities in the air along the optical path. The refraction index of air changes due to thermal non-uniformities so that the wavefront incident on the mirror is randomly distorted, and therefore, images are altered. When telescope mirrors are heated, as it happens in solar telescopes, and therefore they are at a temperature different from the environment's, natural convection occurs. It is then crucial to know whether the flow in front of the mirror is laminar or turbulent. After reviewing the literature, it was found that the scattering of results about the onset of the transition gives only rough orders of magnitude of the values of the critical Grashof numbers. Aiming to obtain more information about it, the problem of determination of the turbulence onset in natural convection on heated inclined plates in air environment was experimentally revisited. The transition has been determined from hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity perturbations start to grow. Experiments have shown that the onset depends not only on the Grashof number, but also on other parameters as the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting extraordinarily well the experimental data. The results are obtained in terms of non-dimensional numbers, this way they apply to any air pressure and therefore to any floating altitud

Liver Transplantation in Acute Liver Failure: Indications and Outcome

The term acute liver failure (ALF) refers to the acute (<26 weeks) and severe worsening in liver function associated with encephalopathy in a person with no underlying chronic liver disease. ALF constitutes a critical clinical syndrome that is potentially reversible but has a very variable prognosis. No specific treatment is available, and liver transplantation (LT) is the treatment of choice in many cases. However, the challenge remains of identifying those patients with a poor likelihood of spontaneous recovery of liver function and for whom the indication and time of LT in order to guarantee survival (based on identification of prognostic factors) need to be established. In Europe, 8% of LT are due to ALF. Although the results of LT due to ALF have improved over recent years, they are still far from those seen after elective LT

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En este trabajo se describe el subsistema de control térmico de PHI y se presentan las predicciones de temperaturas obtenidas para los distintos casos de carga. Debido a la naturaleza de la órbita seguida por el satélite en el cual PHI va embarcado (Solar Orbiter), el ambiente en el cual va a tener que operar PHI será muy exigente, convirtiendo el diseño térmico en un auténtico desafío. Los resultados obtenidos muestran la viabilidad de PHI desde el punto de vista térmico, aunque indiscutiblemente el instrumento va a operar en un entorno térmico muy hostil

Nonlinear analysis of a simple model of temperature evolution in a satellite

We analyse a simple model of the heat transfer to and from a small satellite orbiting round a solar system planet. Our approach considers the satellite isothermal, with external heat input from the environment and from internal energy dissipation, and output to the environment as black-body radiation. The resulting nonlinear ordinary differential equation for the satellite's temperature is analysed by qualitative, perturbation and numerical methods, which show that the temperature approaches a periodic pattern (attracting limit cycle). This approach can occur in two ways, according to the values of the parameters: (i) a slow decay towards the limit cycle over a time longer than the period, or (ii) a fast decay towards the limit cycle over a time shorter than the period. In the first case, an exactly soluble average equation is valid. We discuss the consequences of our model for the thermal stability of satellites.Comment: 13 pages, 4 figures (5 EPS files

Modelling of atmospheric boundary layer: Generation of shear.

Roughness length, z0 and friction velocity, u* are the defining parameters of wind log profile that must be matched in wind tunnel simulation. To fully understand the role of these parameters, the basics and review from the primitive equations and its relation to the logarithmic profile obtained for wind tunnel conditions were discussed. The problem of roughness, although well known, still needs to be addressed more rigorously especially when determining values of z0 and u* from wind tunnel data and their relation to the roughness element geometry. A review of classic literature and new published material were carried out, focusing on the applicability to wind tunnel modelling

Arenaviruses and lethal mutagenesis. prospects for new ribavirin-based interventions

Lymphocytic choriomeningitis virus (LCMV) has contributed to unveil some of the molecular mechanisms of lethal mutagenesis, or loss of virus infectivity due to increased mutation rates. Here we review these developments, and provide additional evidence that ribavirin displays a dual mutagenic and inhibitory activity on LCMV that can be relevant to treatment designs. Using 5-fluorouracil as mutagenic agent and ribavirin either as inhibitor or mutagen, we document an advantage of a sequential inhibitor-mutagen administration over the corresponding combination treatment to achieve a low LCMV load in cell culture. This advantage is accentuated in the concentration range in which ribavirin acts mainly as an inhibitor, rather than as mutagen. This observation reinforces previous theoretical and experimental studies in supporting a sequential inhibitor-mutagen administration as a possible antiviral design. Given recent progress in the development of new inhibitors of arenavirus replication, our results suggest new options of ribavirin-based anti-arenavirus treatments. © 2012 by the authors; licensee MDPI, Basel, Switzerland.Fundacion Ramón Areces; The Centro de Investigación Biomédica en Red de Enfermedades Hepáticas y Digestivas (CIBERehd); Instituto de Salud Carlos III; Junta de AndalucíaPeer Reviewe

ICT consolidate in European firms despite the economic crisis

Abstract The aims of this study are twofold, first to know if ICT has increased in European firms' despite the recent economic crisis and second, to know the differences between businesses ICT’ implementation by firm size. The focus of the investigation is on ICT use of firms of the EU-29 Member States comparing a crisis period with an increasing economic period, using statistical information from EUROSTAT. The data was analysed with a multiple regression model and a nonparametric correlation coefficient, Kendall's tau. The results show that businesses consider ICT as a key component of their strategy because despite of the crisis, ICT indicators have increased in almost all European countries. The results also indicate that crisis began to affect to eCommerce from 2007 due mainly to the fall of consumer. Finally, related to firm size small firms improved their implementation of information technologies more than large ones but large firms are more active regarding eCommerce. Resumen Esta investigación trata por un lado de saber si el uso de TIC se ha incrementado en las empresas europeas a pesar de la reciente crisis económica y por otro lado, se pretende analizar las diferencias existentes entre las empresas europeas en cuanto al grado de implementación de TIC en función de su tamaño. Se estudia el uso de TIC en las empresas de los 29 Estados miembros de la UE comparando el periodo de crisis y el periodo de crecimiento posterior. Los datos se analizaron con un modelo de regresión múltiple y un coeficiente de correlación no paramétrico, tau de Kendall. Los resultados muestran que las empresas consideran las TIC como un componente clave de su estrategia, porque a pesar de la crisis, todos los indicadores TIC han aumentado en la mayor parte de los países europeos. Los resultados también indican que la crisis comenzó a afectar al comercio electrónico a partir de 2007 debido principalmente a la caída del consumo. Por último, en relación con el tamaño de las empresas, las de menor tamaño mejoraron su implementación de tecnologías de la información más que las grandes, aunque éstas son más activas en el uso del comercio electrónico

On the onset of turbulence in natural convection on inclined plates

The problem of determination of the turbulence onset in natural convection on heated inclined plates in an air environment has been experimentally revisited. The transition has been detected by using hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity fluctuations (measured through turbulence intensity) start to grow. Experiments have shown that the onset depends not only on the Grashof number defined in terms of the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting quite well the experimental data