An embedded cohesive crack model for finite element analysis of quasi-brittle materials

This paper presents a numerical implementation of the cohesive crack model for the anal-ysis of quasibrittle materials based on the strong discontinuity approach in the framework of the finite element method. A simple central force model is used for the stress versus crack opening curve. The additional degrees of freedom defining the crack opening are determined at the crack level, thus avoiding the need for performing a static condensation at the element level. The need for a tracking algorithm is avoided by using a consistent pro-cedure for the selection of the separated nodes. Such a model is then implemented into a commercial program by means of a user subroutine, consequently being contrasted with the experimental results. The model takes into account the anisotropy of the material. Numerical simulations of well-known experiments are presented to show the ability of the proposed model to simulate the fracture of quasibrittle materials such as mortar, concrete and masonry.

Moment transfer and influence of transverse beams in interior waffle flat plate-column connections under lateral loading

This paper addresses two aspects of the behavior of interior reinforced concrete waffle flat plate?column connections under lateral loads: the share of the unbalanced moment between flexure and excentric shear, and the effect of the transverse beams. A non-linear finite element model (benchmark model) was developed and calibrated with the results of quasi-static cyclic tests conducted on a 3/5 scale specimen. First, from this numerical model, the portion cv of the unbalanced moment transferred by the excentricity of shear about the centroid of the critical sections defined by Eurocode 2 (EC-2) and by ACI 318-11 was calculated and compared with the share-out prescribed by these codes. It is found that while the critical section of EC-2 is consistent with the cv provided by this code, in the case of ACI 318-11, the value assigned to cv is far below (about 50% smaller) the actual one obtained with the numerical simulations. Second, from the benchmark model, seven additional models were developed by varying the depth D of the transverse beam over the thickness h of the plate. It was found that the ductility of the connection and the effective width of the plate can respectively be increased up to 50% and 10% by raising D/h to 2 and 1.5.

Theory of intermittency applied to classical pathological cases

The classical theory of intermittency developed for return maps assumes uniform density of points reinjected from the chaotic to laminar region. Though it works fine in some model systems, there exist a number of so-called pathological cases characterized by a significant deviation of main characteristics from the values predicted on the basis of the uniform distribution. Recently, we reported on how the reinjection probability density (RPD) can be generalized. Here, we extend this methodology and apply it to different dynamical systems exhibiting anomalous type-II and type-III intermittencies. Estimation of the universal RPD is based on fitting a linear function to experimental data and requires no a priori knowledge on the dynamical model behind. We provide special fitting procedure that enables robust estimation of the RPD from relatively short data sets (dozens of points). Thus, the method is applicable for a wide variety of data sets including numerical simulations and real-life experiments. Estimated RPD enables analytic evaluation of the length of the laminar phase of intermittent behaviors. We show that the method copes well with dynamical systems exhibiting significantly different statistics reported in the literature. We also derive and classify characteristic relations between the mean laminar length and main controlling parameter in perfect agreement with data provided by numerical simulations

Laminar-turbulent transition induced by a discrete roughness element in a supersonic boundary layer

The linear instability and breakdown to turbulence induced by an isolated roughness element in a boundary layer at Mach 2:5, over an isothermal flat plate with laminar adiabatic wall temperature, have been analysed by means of direct numerical simulations, aided by spatial BiGlobal and three-dimensional parabolized (PSE-3D) stability analyses. It is important to understand transition in this flow regime since the process can be slower than in incompressible flow and is crucial to prediction of local heat loads on next-generation flight vehicles. The results show that the roughness element, with a height of the order of the boundary layer displacement thickness, generates a highly unstable wake, which is composed of a low-velocity streak surrounded by a three-dimensional high-shear layer and is able to sustain the rapid growth of a number of instability modes. The most unstable of these modes are associated with varicose or sinuous deformations of the low-velocity streak; they are a consequence of the instability developing in the three-dimensional shear layer as a whole (the varicose mode) or in the lateral shear layers (the sinuous mode). The most unstable wake mode is of the varicose type and grows on average 17% faster tan the most unstable sinuous mode and 30 times faster than the most unstable boundary layer mode occurring in the absence of a roughness element. Due to the high growthrates registered in the presence of the roughness element, an amplification factor of N D 9 is reached within 50 roughness heights from the roughness trailing edge. The independently performed Navier–Stokes, spatial BiGlobal and PSE-3D stability results are in excellent agreement with each other, validating the use of simplified theories for roughness-induced transition involving wake instabilities. Following the linear stages of the laminar–turbulent transition process, the roll-up of the three-dimensional shear layer leads to the formation of a wedge of turbulence, which spreads laterally at a rate similar to that observed in the case of compressible turbulent spots for the same Mach number.

Fracture of concrete structural members subjected to blast

If reinforced concrete structures are to be safe under extreme impulsive loadings such as explosions, a broad understanding of the fracture mechanics of concrete under such events is needed. Most buildings and infrastructures which are likely to be subjected to terrorist attacks are borne by a reinforced concrete (RC) structure. Up to some years ago, the traditional method used to study the ability of RC structures to withstand explosions consisted on a choice between handmade calculations, affordable but inaccurate and unreliable, and full scale experimental tests involving explosions, expensive and not available for many civil institutions. In this context, during the last years numerical simulations have arisen as the most effective method to analyze structures under such events. However, for accurate numerical simulations, reliable constitutive models are needed. Assuming that failure of concrete elements subjected to blast is primarily governed by the tensile behavior, a constitutive model has been built that accounts only for failure under tension while it behaves as elastic without failure under compression. Failure under tension is based on the Cohesive Crack Model. Moreover, the constitutive model has been used to simulate the experimental structural response of reinforced concrete slabs subjected to blast. The results of the numerical simulations with the aforementioned constitutive model show its ability of representing accurately the structural response of the RC elements under study. The simplicity of the model, which does not account for failure under compression, as already mentioned, confirms that the ability of reinforced concrete structures to withstand blast loads is primarily governed by tensile strength.

Behaviour of Concrete Structural Members Subjected to Air Blast Loading

Numerical analysis is a suitable tool in the design of complex reinforced concrete structures under extreme impulsive loadings such as impacts or explosions at close range. Such events may be the result of terrorist attacks. Reinforced concrete is commonly used for buildings and infrastructures. For this reason, the ability to accurately run numerical simulations of concrete elements subjected to blast loading is needed. In this context, reliable constitutive models for concrete are of capital importance. In this research numerical simulations using two different constitutive models for concrete (Continuous Surface Cap Model and Brittle Damage Model) have been carried out using LS-DYNA. Two experimental benchmark tests have been taken as reference. The results of the numerical simulations with the aforementioned constitutive models show different abilities to accurately represent the structural response of the reinforced concrete elements studied.

Accurate Parabolic Navier-Stokes solutions of the supersonic flow around and elliptic cone

Flows of relevance to new generation aerospace vehicles exist, which are weakly dependent on the streamwise direction and strongly dependent on the other two spatial directions, such as the flow around the (flattened) nose of the vehicle and the associated elliptic cone model. Exploiting these characteristics, a parabolic integration of the Navier-Stokes equations is more appropriate than solution of the full equations, resulting in the so-called Parabolic Navier-Stokes (PNS). This approach not only is the best candidate, in terms of computational efficiency and accuracy, for the computation of steady base flows with the appointed properties, but also permits performing instability analysis and laminar-turbulent transition studies a-posteriori to the base flow computation. This is to be contrasted with the alternative approach of using order-of-magnitude more expensive spatial Direct Numerical Simulations (DNS) for the description of the transition process. The PNS equations used here have been formulated for an arbitrary coordinate transformation and the spatial discretization is performed using a novel stable high-order finite-difference-based numerical scheme, ensuring the recovery of highly accurate solutions using modest computing resources. For verification purposes, the boundary layer solution around a circular cone at zero angle of attack is compared in the incompressible limit with theoretical profiles. Also, the recovered shock wave angle at supersonic conditions is compared with theoretical predictions in the same circular-base cone geometry. Finally, the entire flow field, including shock position and compressible boundary layer around a 2:1 elliptic cone is recovered at Mach numbers 3 and 4

An universal system to de-orbit satellites at end of life

A 3-year Project financed by the European Commission is aimed at developing a universal system to de-orbit satellites at their end of life, as a fundamental contribution to limit the increase of debris in the Space environment. The operational system involves a conductive tapetether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. The Project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. Starting at the second year, mission selection is carried out while developing numerical codes to implement control laws on tether dynamics in/off the orbital plane; performing numerical simulations and plasma chamber measurements on tether-plasma interaction; and completing design of subsystems: electronejecting plasma contactor, power module, interface elements, deployment mechanism, and tether-tape/end-mass. This will be followed by subsystems manufacturing and by currentcollection, free-fall, and hypervelocity impact tests.

Evaluación de parámetros que influyen en el transporte de cloruros en hormigón parcialmente saturado

El deterioro del hormigón debido a la presencia del ion cloruro es causa frecuente de problemas en estructuras localizadas en ambiente marino y alta montaña. Su principal efecto consiste en la despasivación del acero de refuerzo embebido en el hormigón y su consecuente inicio de la corrosión del mismo. El ingreso del ion cloruro al interior del hormigón, está condicionado por una serie de parámetros de origen medioambiental e intrínsecos del hormigón. En función de estos parámetros el ingreso de cloruros en el hormigón puede deberse principalmente a los siguientes mecanismos: difusión y succión capilar. El estudio y evaluación de la resistencia del hormigón frente a cloruros, se ha desarrollado principalmente en condiciones saturadas del hormigón. Lo que ha significado que parámetros de importancia no sean considerados. Debido a esto, distintos procesos que suceden en estructuras reales no han sido identificados y estudiados correctamente. En este trabajo, se diseñó un programa de investigación para evaluar los parámetros que influyen en el transporte cloruros en hormigones no saturados. Para esto se diseñaron tres dosificaciones diferentes de hormigón. En la primera se empleó únicamente cemento portland, para el resto se utilizaron adiciones minerales (humo de sílice y escoria de alto horno). El empleo de adiciones se debió a que tienen un papel importante en la durabilidad de hormigones frente a cloruros. Los hormigones fueron dosificados con una relación agua/material cementício de 0,40 para el hormigón elaborado únicamente con cemento portland y 0,45 para las mezclas con adiciones. Para evaluar las propiedades de los hormigones en estado fresco y endurecido se realizaron ensayos vigentes en las normativas. Con los resultados obtenidos se determinaron parámetros de resistencia mecánica, microestructurales, resistencia al transporte de cloruros e higroscópicos. Una vez caracterizados los hormigones, se diseñó una propuesta experimental para estudiar los principales parámetros presentes en estructuras reales con presencia de cloruros. Tanto la concentración de cloruro como las condiciones ambientales se han variado teniendo como referencia las situaciones reales que podrían producirse en ambientes de alta montaña en la zona centro de España. La propuesta experimental consistió en tratar de evaluar la capacidad de los hormigones al transporte de iones en ambientes de alta montaña con presencia de sales fundentes. Para esto se establecieron 5 fases experimentales donde los principales parámetros ambientales y la presencia de iones agresivos sufrieron variaciones. Al término de cada fase se obtuvieron perfiles de penetración de cloruros en los hormigones y se evaluó la influencia de los parámetros presentes en cada fase. Los resultados experimentales se implementaron en un modelo numérico basado en la teoría de elementos finitos, desarrollado por el grupo de investigación del Departamento de Materiales de Construcción. Para esto fue necesario realizar la calibración y validación del modelo numérico para cada hormigón. El calibrado del modelo precisa de datos químicos y microestructurales de cada hormigón, tales como: capacidad de combinación de cloruros y propiedades difusivas e higroscópicas. Para la validación del modelo numérico se realizaron simulaciones de la propuesta experimental. Los resultados obtenidos se compararon con los valores experimentales. Con el objeto de poder estudiar en mayor profundidad la influencia del grado de saturación del hormigón durante la difusión de cloruros, se llevó a cabo una campaña experimental que consideró distintos grados de saturación en los hormigones. Para esto se establecieron en los hormigones cuatro grados de saturación distintos (50%, 60%, 80% y 100%, aproximadamente), posteriormente se expusieron a cloruro de sodio finamente molido. Una vez transcurrido el tiempo necesario se obtuvieron experimentalmente los perfiles de penetración de cloruros para cada grado de saturación y se calcularon los coeficientes de difusión. Los datos obtenidos durante la campaña experimental han demostrado la influencia positiva que ejercen las adiciones en las mezclas de hormigón. Sus principales ventajas son el refinamiento de la red porosa y el aumento en la capacidad de combinación de cloruros, además de mejorar sus propiedades mecánicas. La porosidad total en las mezclas no presentó grandes cambios, sin embargo, el cambio en la distribución del tamaño de poros es importante en las muestras con adiciones. En especial las fabricadas con humo de sílice. Los coeficientes de difusión y migración de cloruros para las mezclas con adiciones disminuyeron significativamente, igual que los valores de resistividad eléctrica. En los ensayos de penetración del agua bajo presión, fueron las muestras con adiciones las que mostraron las menores penetraciones. Los resultados obtenidos al final de la propuesta experimental permitieron estudiar los distintos parámetros involucrados. Se observó claramente que el proceso de difusión provoca el mayor transporte de cloruros hacia el interior del hormigón. Así mismo se comprobó que el lavado superficial y el secado de las probetas, trasladan cloruros hacia las zonas externas del hormigón. El primero debido a una baja concentración de cloruros externa, mientras que el secado provoca el movimiento de la solución de poro hacia las zonas de secado depositando cloruros en ellas. Las medidas higroscópicas permitieron determinar la existencia de dos zonas distintas en el interior del hormigón. La primera se localizó en el rango de 0-10mm, aproximadamente, en ésta se puso de manifiesto una mayor sensibilidad a los cambios experimentados en el exterior de las probetas. La segunda zona se localizó a una profundidad mayor de 10mm, aproximadamente. Se observó claramente una baja influencia de los cambios externos, siendo la difusión de cloruros el principal mecanismo de transporte presente en ella. En cuanto al estudio de la influencia del grado de saturación en la difusión de cloruros, se observó claramente una marcada diferencia entre los coeficientes de difusión de cloruro obtenidos. Para grados de saturación mayores del 80% el mecanismo de penetración de cloruros por difusión existe de forma significativa. Mientras que para valores inferiores los resultados revelaron que las vías de acceso disminuyen (poros conectado con agua) considerablemente limitando en un alto grado la penetración del agresivo. Para grados de saturación inferiores del 50% los valores del coeficiente de difusión son despreciables. The deterioration of concrete due to chloride ions is a frequent problem identified in structures located in marine and high-mountain environments. After entering the outer layer of the concrete, the chlorides tend to penetrate until they reach and then depassivate the steel bars. Subsequently, this induces the deterioration process of the reinforced concrete. This chloride penetration depends on the environmental conditions and intrinsic parameters of the concrete. Several transport mechanisms, such as diffusion, capillary suction and permeability can be present into the concrete. While recent research into the study and evaluation of concretes with chloride presence has been carried out in saturated concrete, it has not considered certain parameters that can modify this condition. Consequently, at the time of writing several processes that take place in real structures have not been identified and studied. In this work a research programme is designed to evaluate the parameters that influence chloride transport into non-saturated concrete. For this, three concrete mixes were designed by using high-early-strength Portland cement and mineral admixtures (silica fume and blast-slag furnace). The water-cement ratio was 0.40 for the concrete made solely with Portland cement and 0.45 for the concretes that used mineral admixtures as a cement replacement. A set of experimental tests were performed to evaluate the concrete properties both in fresh and hardened state. In addition, an experimental simulation was carried out under laboratory conditions in which the main objective was to assess resistance of concrete to chloride penetration under high-mountain conditions with the presence of de-icing salts. The environmental conditions and surface chloride concentration of the concrete used during the experimental simulation were chosen by considering conditions found in the high-mountain environment in central Spain. For the experimental simulation five phases were designed by varying the environmental parameters and concrete surface concentration. At the end of each phase a chloride profile was obtained with the aim of assessing the influence of the parameters on chloride transport. The experimental results were then used to calibrate and validate a numerical model based on finite element theory developed by the research team from the Construction Materials Department in a previous work. In order to carry out model calibration chemical and microstructural data for the concretes was required, such as binding capacity and the diffusive and hygroscopic properties. The experimental results were compared with the numerical simulations and provided a good fit. With the objective of studying the influence of the degree of concrete saturation on chloride diffusion, an experimental programme was designed. This entailed four saturation degrees (50%, 60%, 80% and 100%) being established in several concrete samples. The samples were then exposed to ground sodium chloride. Once the time required was achieved, the chloride profiles and diffusion coefficients were obtained for each saturation degree. The results obtained from the experimental program revealed a positive influence of the mineral admixtures on the concretes. Their effects were reflected in the pore-network refinement and the increase of chloride binding capacity, together with the improvement of the mechanical properties of the concretes. Total porosity did not reveal any notable change, though the pore-size distribution showed a significant degree of change in the concretes with mineral admixtures, specifically the samples prepared through use of silica fume. The chloride diffusion and migration coefficient, as well as the electrical resistivity values, decreased significantly in the concretes with admixtures. In the water penetration under pressure test, the concretes with admixtures presented the lowest penetration depth. The results obtained in the experimental simulation allowed study of the main parameters involved during the chloride penetration processes in non-saturated concretes in the presence of chlorides. According to the results, the diffusion process was the transport mechanism that transferred the greatest amount of chlorides into the concrete samples. In addition, a chloride movement toward external zones of the concrete, caused by the washing of concrete surface and the drying processes, was identified. The washing occurred when the concrete surface came into contact with a low-chloride concentration solution which initiated the outward movement of chloride diffusion. The drying processes corresponded to a movement of pore solution launched by water evaporation from the outer layer. Furthermore, hygroscopic measurements made in the concrete allowed two areas with distinct behavioural patterns to be identified. The first one, located in the range of approximately 0-10mm, showed a greater degree of influence regarding the changes of the external conditions. The second, situated at depths greater than 10mm, displayed a low influence of external conditions. The main process in this area was diffusion. Study of the influence of the degree of concrete saturation on chloride diffusion showed a clear difference among the chloride diffusion coefficients obtained. For degrees of concrete saturation higher than 80%, chloride penetration by diffusion tends to be significant. However, in the case of extent of saturation of lower than 80%, the results revealed that the access zone through which chlorides can penetrate decreased considerably. For degrees of concrete saturation lower than 50%, the chloride diffusion coefficients were negligible.

Aplicación de energía geotérmica de muy baja entalpia para la climatización de un edificio de oficinas en Alcorcón, Madrid

El presente Proyecto tiene como propósito el estudio de la viabilidad de la implantación de energía geotérmica de muy baja entalpia para la climatización de un edificio empresarial en Madrid. Para ello se emplearon cálculos teóricos y simulaciones realizadas mediante programas informaticos especializados con el fin de estimar los parámetros técnicos necesarios para su viabilidad. Se estimo el suministro de la potencia necesaria a través de tres bombas de calor tipo agua-agua que sustituyen a las bombas de calor aire-agua individuales anteriormente instaladas. El intercambio de calor se realiza a través de sondas geotérmicas U-Simple. Se dimensionaron los colectores, bombas de circulación, deposito de inercia y vaso de expansión necesarios para funcionamiento del sistema. El estudio económico determino la necesidad de una alta inversión inicial, con una rentabilidad a medio plazo. Además de los beneficios económicos se realizo un estudio de las emisiones de gases de efecto invernadero demostrando la reducción de los mismos, lo que refleja que además de ser una tecnología económicamente viable es una energía limpia. ABSTRACT The objective of the present project is to evaluate the viability of the imposition of a low-enthalpy geothermal energy system for air conditioning in a business building in Madrid. In this study, both theoretical calculations and numerical simulations in specialized programs were used in order to estimate the technical parameters needed for its viability. We estimated the necessary power supply, which is delivered by three heat water-water pumps that substitute the individual heat air-water pumps previously installed. The heat exchange is achieved through geothermal vertical simple-U probes. Also in this work we have measured the collectors, flow pumps, inertia deposit and the water expansion vessel needed for the proper operation of the system. The economic study determined the need of a high initial investment with mid-term profit. In addition to the economic benefits, the greenhouse gases emissions were evaluated, finding that the geothermal energy system implies a reduction of these emissions. This reflects the fact that, in addition to being economically viable, the geothermal energy is considered a clean energy.

Reproducing the Bolund experiment in wind tunnel

The Bolund experiment has been reproduced in a neutral boundary layer wind tunnel (WT) at scale 1:115 for two Reynolds numbers. All the results have been obtained for an incoming flow from the 270o wind direction (transect B in the Bolund experiment jargon). Vertical scans of the velocity field are obtained using non-time resolved two components particle image velocimetry. Time-resolved velocity time series with a three component hot-wire probe have been also measured for transects at 2 and 5 m height and in the vertical transects at met masts M6, M3 and M8 locations. Special attention has been devoted to the detailed characterization of the inflow in order to reduce uncertainties in future comparisons with other physical and numerical simulations. Emphasis is placed on the analysis of spectral functions of the undisturbed flow and those of the flow above the island. The result?s reproducibility and trustworthiness have been addressed through redundancy measurements using particle image velocimetry, two and three components hot-wire anemometry. The bias in the prediction of the mean speed is similar to the one reported during the Bolund experiment by the physical modellers. However, certain reduction of the bias in the estimation of the turbulent kinetic energy is achieved. TheWT results of spectra and cosprectra have revealed a behaviour similar to the full-scale measurements in some relevant locations, showing that WT modelling can contribute to provide valid information about these important structural loading factors.

High reynolds numbers turbulent boundary layers

Esta tesis estudia las similitudes y diferencias entre los flujos turbulentos de pared de tipo externo e interno, en régimen incompresible, y a números de Reynolds moderada¬mente altos. Para ello consideramos tanto simulaciones numéricas como experimentos de capas límites con gradiente de presiones nulo y de flujos de canal, ambos a números de Reynolds en el rango δ+ ~ 500 - 2000. Estos flujos de cortadura son objeto de numerosas investigaciones debido a la gran importancia que tienen tanto a nivel tecnológico como a nivel de física fundamental. No obstante, todavía existen muchos interrogantes sobre aspectos básicos tales como la universalidad de los perfiles medios y de fluctuación de las velocidades o de la presión, tanto en la zona cercana a la pared como en la zona logarítmica, el escalado y el efecto del número de Reynolds, o las diferencias entre los flujos internos y externos en la zona exterior. En éste estudio hemos utilizado simulaciones numéricas ya existentes de canales y capas límites a números de Reynolds δ+ ~ 2000 y δ+ ~ 700, respectivamente. Para poder comparar ambos flujos a igual número de Reynolds hemos realizado una nueva simulación directa de capa límite en el rango δ+ ~ 1000-2000. Los resultados de la misma son presentados y analizados en detalle. Los datos sin postprocesar y las estadísticas ya postprocesadas están públicamente disponibles en nuestro sitio web.162 El análisis de las estadísticas usando un único punto confirma la existencia de perfiles logarítmicos para las fluctuaciones de la velocidad transversal w'2+ y de la presión p'2+ en ambos tipos de flujos, pero no para la velocidad normal v'2+ o la velocidad longitudinal u'2+. Para aceptar o rechazar la existencia de un rango logarítmico en u'2+ se requieren números de Reynolds más altos que los considerados en éste trabajo. Una de las conse¬cuencias más importantes de poseer tales perfiles es que el valor máximo de la intensidad, que se alcanza cerca de la pared, depende explícitamente del número de Reynolds. Esto ha sido confirmado tras analizar un gran número de datos experimentales y numéricos, cor¬roborando que el máximo de u'2+, p/2+, y w'2+ aumenta proporcionalmente con el log(δ+). Por otro lado, éste máximo es más intenso en los flujos externos que en los internos. La máxima diferencia ocurre en torno a y/δ ~ 0.3-0.5, siendo esta altura prácticamente independiente del número de Reynolds considerado. Estas diferencias se originan como consecuencia del carácter intermitente de las capas límites, que es inexistente en los flujos internos. La estructura de las fluctuaciones de velocidad y de presión, junto con la de los esfuer¬zos de Reynolds, se han investigado por medio de correlaciones espaciales tridimensionales considerando dos puntos de medida. Hemos obtenido que el tamaño de las mismas es gen¬eralmente mayor en canales que en capas límites, especialmente en el caso de la correlación longitudinal Cuu en la dirección del flujo. Para esta correlación se demuestra que las es¬tructuras débilmente correladas presentan longitudes de hasta 0(75), en el caso de capas límites, y de hasta 0(185) en el caso de canales. Estas longitudes se obtienen respecti-vamente en la zona logarítmica y en la zona exterior. Las longitudes correspondientes en la dirección transversal son significativamente menores en ambos flujos, 0(5 — 25). La organización espacial de las correlaciones es compatible con la de una pareja de rollos casi paralelos con dimensiones que escalan en unidades exteriores. Esta organización se mantiene al menos hasta y ~ 0.65, altura a la cual las capas límites comienzan a organi¬zarse en rollos transversales. Este comportamiento es sin embargo más débil en canales, pudiéndose observar parcialmente a partir de y ~ 0.85. Para estudiar si estas estructuras están onduladas a lo largo de la dirección transver¬sal, hemos calculado las correlaciones condicionadas a eventos intensos de la velocidad transversal w'. Estas correlaciones revelan que la ondulación de la velocidad longitudinal aumenta conforme nos alejamos de la pared, sugiriendo que las estructuras están más alineadas en la zona cercana a la pared que en la zona lejana a ella. El por qué de esta ondulación se encuentra posiblemente en la configuración a lo largo de diagonales que presenta w'. Estas estructuras no sólo están onduladas, sino que también están inclinadas respecto a la pared con ángulos que dependen de la variable considerada, de la altura, y de el contorno de correlación seleccionado. Por encima de la zona tampón e independien¬temente del número de Reynolds y tipo de flujo, Cuu presenta una inclinación máxima de unos 10°, las correlaciones Cvv y Cm son esencialmente verticales, y Cww está inclinada a unos 35°. Summary This thesis studies the similitudes and differences between external and internal in¬compressible wall-bounded turbulent flows at moderately-high Reynolds numbers. We consider numerical and experimental zero-pressure-gradient boundary layers and chan¬nels in the range of δ+ ~ 500 — 2000. These shear flows are subjects of intensive research because of their technological importance and fundamental physical interest. However, there are still open questions regarding basic aspects such as the universality of the mean and fluctuating velocity and pressure profiles at the near-wall and logarithmic regions, their scaling and the effect of the Reynolds numbers, or the differences between internal and external flows at the outer layer, to name but a few. For this study, we made use of available direct numerical simulations of channel and boundary layers reaching δ+ ~ 2000 and δ+ ~ 700, respectively. To fill the gap in the Reynolds number, a new boundary layer simulation in the range δ+ ~ 1000-2000 is presented and discussed. The original raw data and the post-processed statistics are publicly available on our website.162 The analysis of the one-point statistic confirms the existence of logarithmic profiles for the spanwise w'2+ and pressure p'2+ fluctuations for both type of flows, but not for the wall-normal v'2+ or the streamwise u'2+ velocities. To accept or reject the existence of a logarithmic range in u'2+ requires higher Reynolds numbers than the ones considered in this work. An important consequence of having such profiles is that the maximum value of the intensities, reached near the wall, depends on the Reynolds number. This was confirmed after surveying a wide number of experimental and numerical datasets, corrob¬orating that the maximum of ul2+, p'2+, and w'2+ increases proportionally to log(δ+). On the other hand, that maximum is more intense in external flows than in internal ones, differing the most around y/δ ~ 0.3-0.5, and essentially independent of the Reynolds number. We discuss that those differences are originated as a consequence of the inter¬mittent character of boundary layers that is absent in internal flows. The structure of the velocity and pressure fluctuations, together with those of the Reynolds shear stress, were investigated using three-dimensional two-point spatial correlations. We find that the correlations extend over longer distances in channels than in boundary layers, especially in the case of the streamwise correlation Cuu in the flow direc-tion. For weakly correlated structures, the maximum streamwise length of Cuu is O(78) for boundary layers and O(188) for channels, attained at the logarithmic and outer regions respectively. The corresponding lengths for the transverse velocities and for the pressure are shorter, 0(8 — 28), and of the same order for both flows. The spatial organization of the velocity correlations is shown to be consistent with a pair of quasi-streamwise rollers that scales in outer units. That organization is observed until y ~ 0.68, from which boundary layers start to organize into spanwise rollers. This effect is weaker in channels, and it appears at y ~ 0.88. We present correlations conditioned to intense events of the transversal velocity, w', to study if these structures meander along the spanwise direction. The results indicate that the streamwise velocity streaks increase their meandering proportionally to the distance to the wall, suggesting that the structures are more aligned close to the wall than far from it. The reason behind this meandering is probably due to the characteristic organization along diagonals of w'. These structures not only meander along the spanwise direction, but they are also inclined to the wall at angles that depend on the distance from the wall, on the variable being considered, and on the correlation level used to define them. Above the buffer layer and independent of the Reynolds numbers and type of flow, the maximum inclination of Cuu is about 10°, Cvv and Cpp are roughly vertical, and Cww is inclined by 35°.

A review of electrodynamic tethers for science applications

A bare electrodynamic tether (EDT) is a conductive thin wire or tape tens of kilometres long, which is kept taut in space by gravity gradient or spinning, and is left bare of insulation to collect (and carry) current as a cylindrical Langmuir probe in an ambient magnetized plasma. An EDT is a probe in mesothermal flow at highly positive (or negative) bias, with a large or extremely large 2D sheath, which may show effects from the magnetic self-field of its current and have electrons adiabatically trapped in its ram front. Beyond technical applications ranging from propellantless propulsion to power generation in orbit, EDTs allow broad scientific uses such as generating electron beams and artificial auroras; exciting Alfven waves and whistlers; odifying the radiation belts; and exploring interplanetary space and the Jovian magnetosphere. Asymptotic analysis, numerical simulations, laboratory tests, and planned missions on EDTs are reviewed

Caracterización dinámica del glaciar Hurd combinando observaciones de campo y simulaciones numéricas

El objetivo fundamental de esta tesis es la caracterización de la morfología y del estado de deformaciones y tensiones del Glaciar Hurd (Isla Livingston, Archipiélago de las Shetland del Sur, Antártida), mediante una combinación de observaciones de campo, registros de georradar y simulaciones numéricas. La morfología y el estado de deformaciones y tensiones actuales son la expresión de la evolución dinámica del glaciar desde tiempos pretéritos hasta recientes, y su análisis nos dará las pautas con las cuales ser capaces de predecir, con el apoyo de las simulaciones numéricas, su evolución futura. El primer aspecto que se aborda es el estudio de las estructuras que pueden observarse en la superficie del glaciar. Describimos las distintas técnicas utilizadas (medidas de campo, fotointerpretación de ortofotografías, análisis geoquímico de cenizas volcánicas, etc.) y presentamos el análisis e interpretación de los resultados morfo-estructurales, así como la correlación, mediante análisis geoquímicos (fluorescencia de rayos X), entre las cenizas volcánicas que extruyen en la superficie del Glaciar Hurd y las del volcán Decepción, origen de las cenizas. Esto nos permite realizar una datación de las mismas como Tefra 1, correspondiente a la erupción de 1970, Tefra 2, correspondiente a las erupciones pre-1829, y el conjunto Tefra 3, asociado a las erupciones más antiguas. En segundo lugar nos ocupamos de las estructuras presentes en el interior del glaciar, cuya herramienta de detección fundamental es el georradar. Identificadas estas estructuras internas, las vinculamos con las observadas en la superficie del glaciar. También hemos estudiado la estructura hidrotérmica del glaciar, obteniendo una serie de evidencias adicionales de su carácter politérmico. Entre éstas se contaban, hasta ahora, las basadas en el valor del parámetro de rigidez de la relación constitutiva del hielo determinada por ajuste de modelos dinámicos y observaciones realizados por Otero (2008) y las basadas en las velocidades de las ondas de radar en el hielo determinadas con el método de punto medio común por Navarro y otros (2009). Las evidencias adicionales que aportamos en esta tesis son: 1) la presencia de estructuras típicas de régimen compresivo en la zona terminal del glaciar y de cizalla en los márgenes del mismo, y 2) la presencia de un estrato superficial de hielo frío (por encima de otro templado) en la zona de ablación de los tres lóbulos del Glaciar Hurd –Sally Rocks, Argentina y Las Palmas–, que alcanzan espesores de 70, 50 y 40 m, respectivamente. Este estrato de hielo frío está probablemente congelado al lecho subglaciar en la zona terminal (Molina y otros, 2007; esta tesis). Por último, nos ocupamos de la simulación numérica de la dinámica glaciar. Presentamos el modelo físico-matemático utilizado, discutimos sus condiciones de contorno y cómo éstas se miden en los trabajos de campo, y describimos el procedimiento de resolución numérica del sistema de ecuaciones parciales del modelo. Presentamos los resultados para los campos de velocidades, deformaciones y tensiones, comparando estos resultados con las estructuras observadas. También incluimos el análisis de las elipses de deformación acumulativa, que proporcionan información sobre las estructuras a las que puede dar lugar la evolución del estado de deformaciones y tensiones a las que se ve sometido el hielo según avanza, lentamente, desde la cabecera hasta la zona terminal del glaciar, con tiempos de tránsito de hasta 1.250 años, recogiendo así la historia de deformaciones en el glaciar. Concluyendo, ponemos de manifiesto en esta tesis que las medidas de campo de las estructuras y niveles de cenizas, las medidas de georradar y las simulaciones numéricas de la dinámica glaciar, realizadas de forma combinada, permiten caracterizar el régimen actual de velocidades, deformaciones y tensiones del glaciar, entender su evolución en el pasado y predecir su evolución futura. ABSTRACT The main objective of this thesis is to characterize the morphology and the state of strains and stresses of Hurd Glacier (Livingston Island, South Shetland Islands archipelago, Antarctica) through a combination of field observations, ground-penetrating radar measurements and numerical simulations. The morphology and the current state of strain and stresses are the expression of the dynamic evolution of the glacier from the past to recent times, and their analysis gives us the guidelines to be able to predict, with the support of numerical simulations, its future evolution. The first subject addressed is the study of structures that can be observed on the glacier surface. We describe the different techniques used (field measurements, photointerpretation of orthophotos, geochemical analysis of volcanic ashes, etc.) and we present the analysis and interpretation of the morpho-structural results, as well as the correlation with geochemical analysis (XRF) between the volcanic ashes extruded to the surface of Hurd Glacier and those of Deception Island volcano, from which the ashes originate. This allows us dating the ashes as Tephra 1, corresponding to the 1970 eruption, Tephra 2, corresponding to the pre-1829 eruptions, and the Tephra 3 group, associated with older eruptions. Secondly we focus on the study of the structures present within the glacier, which are detected with the help of ground-penetrating radar. Once identified, we link these internal structures with those observed on the glacier surface. We also study the hydrothermal structure of the glacier, getting a series of additional evidences of its polythermal structure. Among the evidences available so far, we can mention those based on the value of the stiffness parameter of the constitutive relation of ice, determined by fitting dynamic models to observations, as done by Otero (2008), and those based on the velocity of propagation of the radar waves through the glacier ice, measured using the common midpoint method, as done by Navarro et al. (2009). The additional evidences that we provide in this thesis are: 1) the presence of structures typical of compressive regime in the terminal zone of the glacier, together with shear at its margins, and 2) the presence of a surface layer of cold ice (overlying a layer of temperate ice) in the ablation zone of the three lobes of Hurd Glacier –Sally Rocks, Argentina and Las Palmas–, reaching thicknesses of 70, 50 and 40 m, respectively. This cold layer is probably frozen to the subglacial bed in the terminal zone (Molina and others 2007; this thesis). Finally, we deal with the numerical simulation of glacier dynamics. We present the physical-mathematical model, discuss its boundary conditions and how they are measured in the field work, and describe the method of numerical solution of the model’s partial differential equations. We present the results for the velocity, strain and stress fields, comparing these results with the observed structures. We also include an analysis of the ellipses of cumulative deformation, which provide information about the structures that can result from the evolution of the strain and stress regime of the glacier ice as it moves slowly from the head to the snout of the glacier, with transit times of up to 1,250 years, so picking the history of deformation of the glacier. Summarizing, we show in this thesis that field measurements of structures and ash layers, ground-penetrating radar measurements and numerical simulations of glacier dynamics, performed in combination, allow us to characterize the current regime of velocities, strains and stresses of the glacier, to understand its past evolution and to predict its future evolution.

Lagrangian tools and the assessment of the predictive capacity of geophysical data sets

We examine, with recently developed Lagrangian tools, altimeter data and numerical simulations obtained from the HYCOM model in the Gulf of Mexico. Our data correspond to the months just after the Deepwater Horizon oil spill in the year 2010. Our Lagrangian analysis provides a skeleton that allows the interpretation of transport routes over the ocean surface. The transport routes are further verified by the simultaneous study of the evolution of several drifters launched during those months in the Gulf of Mexico. We find that there exist Lagrangian structures that justify the dynamics of the drifters, although the agreement depends on the quality of the data. We discuss the impact of the Lagrangian tools on the assessment of the predictive capacity of these data sets.