The objectives of the thesis were, on the one hand, to test the possibilities of a well-known experimental configuration (oscillating grid mixing box), used to study stratified fluids in turbulent flows with zero-mean, on the study of sediment transport, and to find out what aspects involved in this phenomena could be explored from this new perspective. On the other hand, compare a well-established criterion, such as Shields parameter, with the local turbulent energy ( ), as well as to compare the differences between zero-mean and sheared induced lift-off.Verification and adjustments to the parameterizations given in the bibliography for the r.m.s. turbulent velocity, , generated by an oscillating grid were done as well as the calibration of the different mixing boxes and grids to be used through a series of experiments. This gave a clear view of the behaviour of the flow in all the mixing boxes used. A special experimental configuration was designed and performed for the first time, and measurements were taken to get to know the flow properties in the region between the grid and the bottom (solid boundary). Experiments were performed with actual sediment with different characteristics, for instance, different sediment sizes (from clay to medium sands) including samples made of single sediment size and samples extracted from the seabed. An experiment to compare the sediment behaviour under a sheared flow (recirculation tank) and under a no-sheared flow (oscillating grid mixing box) was designed and performed. The present work concluded that the experimental configuration used could be employed in further works to investigate aspects of sediment transport under a turbulent flow with great accuracy. This experimental configuration, in combination with techniques of image analysis, greatly improves the capabilities of the grid stirred experimental configurations, mainly on the study of sediment behaviour. The flow behaviour, between the grid and the free surface, and between the grid and the bottom, is, in general terms, the same. Other important contributions of the present work are the measurements taken near the boundary. Vortices do not distort before impingement. After vortex impingement, no constant mean flow is distinguished. The former enhanced the idea that these experimental configurations are suitable for studying the behaviour of a sediment bed under the influence of a turbulent flow. A smaller magnitude of than of was required to start sediment motion, through comparing the theoretical critical friction velocity, (Critical Shields Parameter), needed to start sediment motion with the measured critical for different sediment sizes. The results obtained show the importance that the time span between extraction and analysis has on the measured value of , and that cohesiveness of sediment play a more important role than sediment size hampering the erosion processes that cause sediment lift off from the sediment bed. Under the same circumstances, a turbulent flow is more efficient than eroding the sediment bed, since it does not depend on the roughness of the sediment bed. It was possible to measure sediment velocities during lift-off and settling, using grid stirred experimental configurations in conjunction with image analysis techniques. It is even possible to determine the sediment size (size mode), even in the range of clays and silts, with an error as small as 6 or 8 percent. It was possible to quantify the sediment flux at a given turbulence intensity. Velocity fluctuations are about the same in the three directions and in a random way above the sediment bed, meaning no preferential direction of stresses. The former hampers the process of particle imbrication making it easier to dislocate the particles from the bed.

.La tesi analitza les dinàmiques no locals i la intermitència en fluids no homogenis i no isotròpics tant en experiments de laboratori com en dades geogràfiques. Després de l'estudi sobre els models que descriuen la intermitència en fluids homogenis i isotròpics, presentem un model per fluids no homogenis i no isotropics. El model es basa en l'anàlisi de la jerarquia de transmissió d'energia. S'utilitza la tècnica de la Extended Self Similarity amb l'objectiu d'estudiar l'escala de lleis de les funcions estructurals de velocitat. Els resultats experimentals de tres tipus de fluids que s'han comparat amb el model per fluids no homogenis i no isotròpics són els següents: turbulència generada per un cilindre en un canal de vent i turbulència generada per una reixa i per un jet en un canal d'aigua. S'analitza la turbulència geogràfica del Delta de l'Ebre i del Knebel Vig i els resultats són comparats amb els del model anteriorment esmentat. La contribució principal és el descobriment d'un paràmetre aparentment relacionat amb la jerarquia de transmissió d'energia en el rang inercial que juga un paper clau en la comprensió de les dinàmiques dels fluids no homogenis. / .The thesis analyze the non-local dynamics and intermittency in non-homogeneous and non-isotropic flows both in the laboratory experiments and geophysical data. After a review on models describing intermittency in homogeneous and isotropic flows, a different model for non-homogeneous and non-isotropic flows is introduced. A model is based on the analysis of the energy transfer hierarchy. The technique of Extended Self Similarity is used to study the scaling laws of the velocity structure functions. Experimental results of three type of flows cylinder wake turbulence in wind tunnel, grid and jet turbulence in water channel are presented and compared with the model. The geophysical turbulence in the Ebro Delta and Knebel Vig are analyzed and the results are compared with the model. The important contribution is the discovery of a new apparently parameter related to the energy transfer hierarchy in the inertial range which plays a key role in the understanding of the dynamics of the non-homogeneous flows.

Se ha desarrollado una infraestructura numérica, que permite el estudio de problemas donde están presentes fenómenos periódicos espaciales, haciendo uso de dominios computacionales reducidos con condiciones de contorno periódicas. Se enfoca en particular el estudio de la convección natural del aire, numérica y experimentalmente, en estructuras honeycomb de tipo rectangular, de interés aplicativo en aislamientos transparentes para sistemas solares térmicos. Para abordar el estudio, la tesis se ha estructurado en tres bloques, cuyos contenidos abarcan:1) El estudio del tratamiento numérico de condiciones de contorno periódicas, en base al método de volúmenes finitos, estableciéndose estrategias adecuadas de transferencia de información entre contornos periódicos en una forma explícita. Los tratamientos se sustentan en dos procedimientos: (i) en estrategias que tratan directamente con el valor de las variables del problema sobre nodos de los volúmenes de control (ubicados en los contornos periódicos y en posiciones anexas al contorno) y (ii) en interpolaciones sobre los contornos periódicos utilizadas en el método multibloque: interpolaciones de tipo conservativas e interpolaciones matemáticas. Se proponen tres formulaciones: DIF (Direct Interpolation Formulation), EPF (Exact Position Formulation) y CTF (Conservative Treatment Formulation). En la formulación DIF la transferencia de información se basa en la aplicación de interpolaciones Lagrangianas, en la formulación EPF la información se transfiere nodo a nodo entre posiciones geométricamente semejantes, mientras que en la formulación CTF la transferencia de información se basa en forzar la conservación de los flujos físicos (masa, momento y energía). Las formulaciones son comparadas entre si. Para el análisis comparativo se reproduce un caso de la literatura científica, cuyos campos de velocidad y temperatura presentan un comportamiento periódico espacial. Los criterios de comparación se basan en la verificación de las soluciones numéricas obtenidas para cada formulación. Se resuelven otros casos con la finalidad de presentar nuevos detalles acerca del tratamiento de condiciones de contorno periódicas, en base a las metodologías propuestas.2) Se aborda el estudio numérico del comportamiento periódico del aire en cavidades alargadas e inclinadas 45º, en cuyo interior se ubica una estructura honeycomb de tipo rectangular. Se reproducen casos de la literatura científica, numéricos y experimentales, con la finalidad de verificar y validar el código numérico para geometrías específicas. Aprovechando la naturaleza periódica del flujo, se proponen expresiones matemáticas para representar el comportamiento periódico de las variables velocidad, temperatura y presión dinámica. Dichas expresiones fueron utilizadas como modelos para definir las condiciones de contorno periódicas aplicables a una celda de honeycomb. Se establecen estudios comparativos entre los resultados numéricos conseguidos sobre dominios computacionales completos y los obtenidos aplicando condiciones de contorno periódicas sobre dominios computacionales reducidos a una celda de honeycomb. Mediante un estudio específico, se demuestra la utilidad de las simulaciones numéricas para solucionar geometrías de interés tecnológico, próximas al aislamiento transparente para sistemas solares térmicos, aplicando condiciones de contorno periódicas en dominios computacionales reducidos.3) El estudio numérico de la convección natural del aire en una cavidad rectangular, con estructuras honeycomb en su interior. Se lleva a cabo la validación de los resultados numéricos, contrastando sus valores con resultados experimentales obtenidos en base a técnicas PIV (Particle Image Velocimetry), las cuales permitieron visualizar y cuantificar el campo de velocidades.Las simulaciones numéricas de todo el trabajo se basaron en el método de volúmenes finitos. Las ecuaciones gobernantes discretizadas fueron resueltas en forma segregada, utilizándose el algoritmo SIMPLEC. Las geometrías fueron discretizadas en base a mallas Cartesianas desplazadas. Los resultados numéricos fueron verificados a través de herramientas de post - proceso basadas en la extrapolación de Richardson generalizada y en el concepto del GCI (Grid Convergence Index).

El método de descomposición de dominios (DD) que se propone en esta tesis pretende resolver flujos incompresibles alrededor de objetos en movimiento relativo. El algoritmo de DD está basado en un acoplamiento del tipo Dirichlet/Neumann(Robin) aplicado a subdominios con solapamiento, y es, por tanto, una extensión del método Dirichlet/Neumann(Robin) clásico con subdominios disjuntos. En realidad, el campo de aplicación de este estudio es mucho más amplio puesto que en el se propone un posible marco teórico para abordar la extensión a subdominios solapados de los métodos mixtos clásicos: métodos Dirichlet/Robin, Dirichlet/Neumann, Robin/Neumann y Robin/Robin. Se observa que los métodos mixtos propuestos heredan propiedades del método de Schwarz y al mismo tiempo conservan el comportamiento de sus equivalentes sin solapamiento cuando este tiende a cero. Se muestra como resultado principal que el solapamiento hace estos métodos más robustos que los métodos sin solapamiento. El método de DD que se estudia es geométrico y algorítmico. Es geométrico en el sentido de que la partición del dominio computacional se lleva a cabo antes del proceso de mallado y de acuerdo con el acoplamiento de DD que se prevé usar. Es también algorítmico porque la solución en cada subdominio se obtiene en procesos diferentes y el intercambio de información entre subdominios se realiza mediante un código maestro. Tal estrategia es muy flexible puesto que requiere muy pocas modificaciones del código numérico original. Por consiguiente, sólo el código maestro necesita ser adaptado a los códigos y estrategias numéricos utilizados en cada subdominio. Se presenta una descripción detallada de la implementación del método de DD propuesto en el contexto numérico de los elementos finitos. Presentamos técnicas de interpolación para los datos de tipo Dirichlet y Neumann y desarrollamos algoritmos de conservación. Una vez el acoplamiento de DD y las interpolaciones definidos, presentamos un método del tipo Chimera para la resolución de flujos alrededor de objetos en movimiento. En particular, definimos transformaciones tensoriales para transformar variables de un subdominio a otro. Finalmente, el algoritmo de DD se aplica a un código implícito para la resolución de las ecuaciones de Navier-Stokes incompresibles y también a las ecuaciones de Navier-Stokes promediadas con un modelo de turbulencia de una ecuación. / The domain decomposition (DD) method we present in this work aims at solving incompressible flows around objects in relative motion. The DD algorithm is based on a Dirichlet/Neumann(Robin) coupling applied to overlapping subdomains. Hence, it is an extension of the classical Dirichlet/Neumann(Robin) method which uses disjoint subdomains. Actually, the field of application of this work is wider as it proposes to set up a possible theoretical framework for studying the overlapping extensions of classical mixed methods: the Dirichlet/Robin, Dirichlet/Neumann, Robin/Neumann and Robin/Robin DD methods. We observe that mixed DD methods inherit some properties of the Schwarz method while they keep the behavior of the classical mixed DD methods when the overlap tends to zero. As a main result, we show that the overlap makes the proposed methods more robust than disjoint mixed DD methods. The DD method we propose is geometric and algorithmic. It is geometric because the partition of the computational domain is performed before the meshing, and in accordance to the DD coupling. It is also algorithmic because the solution on each subdomain is obtained on separate processes and the exchange of information between the subdomains is carried out by a Master code. This strategy is very flexible as it requires almost no modification to the original numerical code. Therefore, only the Master code has to be adapted to the numerical codes and strategies used on each subdomain. We present a detailed description of the implementation of the DD methods in the numerical framework of finite elements. We present interpolation techniques for Dirichlet and Neumann data as well as conservation algorithms. Once the domain decomposition coupling and interpolation techniques are defined, we set up a Chimera method for the solution of the flow over objets in relative movements. Tensorial transformations are introduced to be able to express variables measures in one subdomain. Finally, the DD algorithm is applied to an implicit finite element code for the solution of the Navier-Stokes equations and also of the Reynolds Averaged Navier-Stokes equations together with a one-equation turbulence model.

Wormlike micelles are viscoelastic fluids that present an intermediate behavior between solids and ordinary liquids since they are elastic at short time scales but flow easily at large time scales. In opposition to Newtonian fluids, which have constant viscosity, these fluids usually exhibit a non-Newtonian response with a rate-dependent shear viscosity. Wall-bounded oscillatory flows of Newtonian and complex fluids are found in many practical situations. Oscillatory pipe flows are especially important in physiology in connection with the circulatory and respiratory systems of human beings, as well as in industrial processes such as fluid pumping, secondary oil recovery or filtration, and in acoustics. Pulsating flows are of particular interest also in the rheological characterization of complex fluids. We analyze the laminar oscillatory flow of viscoelastic fluids using the Maxwell and Oldroyd-B models. We have shown that in wall-bounded oscillatory flows of viscoelastic fluids the two characteristic lengths of the Ferry waves, the damping length and wavelength, together with the characteristic separation of the walls, define all the flow properties for fluid models with a linear shear-stress equation in unidirectional flow. In wall-bounded settings there exists the possibility that shear waves generated at different locations superpose themselves before decaying so that the shear waves interfere, giving rise to a resonant flow at well defined frequencies of driving. The theoretical predictions obtained for the laminar velocity profiles are validated by carrying out time-resolved Particle Image Velocimetry (PIV) experiments in a vertical pipe at small driving amplitudes. The oscillatory pipe flow has been investigated in the whole range of experimentally accessible driving frequencies and amplitudes, and classified in three main flow regimes: laminar, vortical, and non-axisymmetric vortical. By ramping up and down the driving amplitude at constant frequency we have been able to characterize the transition from laminar to more complex flows, under controlled driving conditions. The first hydrodynamic instability occurs when the laminar base flow becomes unstable against the formation of axisymmetric toroidal vortices that appear distributed along the cylinder. The calculation of root-mean-square fluctuations in the vertical direction, of the vertical and radial components of the velocity (averaged in time or over the tube diameter) has allowed to determine the critical amplitude at which the instability sets in with high accuracy. In the vortical flow an abrupt increase of the fluctuations is observed, that accounts for the loss of the vertical translational symmetry and the formation of vortices in the flow. This transition exhibits hysteresis when the driving amplitude is ramped up and down, which makes us presume that the bifurcation from the laminar flow has a subcritical nature. A second hydrodynamic instability occurs when the vortical flow loses the axial symmetry. In this flow regime the vortices are heavily distorted and no longer axisymmetric. The velocity and vorticity maps of the vortical flow measured in a meridional plane of the tube appear periodic in time, on time scales comparable to the driving period. Interestingly, the vortex formation is favored in the acceleration phases of the piston oscillation. Besides, we have uncovered a spatio-temporal dynamics on long time scales (much larger than the relaxation time of the fluid) that substantially modifies the flow organization. This slow dynamics is more effective in the bottom half of the cylinder, specially next to the driving piston. A global inspection of the vortical flow along the tube length reveals that the instability takes place earlier in the bottom part of the tube, in the vicinity of the driving piston. At increasing the driving amplitude the boundary between laminar and vortical flow progressively raises towards the top regions. And above a critical driving amplitude the entire fluid flow is vortical. The mechanism triggering the hydrodynamic instability from the laminar to the axisymmetric vortical flow is not yet clear. / L'objectiu d'aquesta Tesi és estudiar el flux oscil•latori vertical en fluids micel•lars. Els fluids micel•lars són fluids complexos amb propietats viscoelàstiques, de manera que mostren un comportament intermedi entre els sòlids i els líquids: són elàstics a escales de temps curtes però flueixen a escales de temps més llargues. En contraposició als fluids Newtonians, que tenen una viscositat constant, els fluids complexos mostren un comportament no-Newtonià, amb una viscositat que depèn del ritme de deformació. El fluxos oscil•latoris de fluids Newtonians o complexos en geometries confinades són especialment importants en fisiologia, en relació amb el sistema circulatori i respiratori d'éssers humans, i també en processos industrials com el bombejat de fluids, l'extracció de petroli, i en particular són interessants en la caracterització reològica de fluids complexos. Primer estudiem el flux oscil•latori des d'una perspectiva teòrica i analitzem el flux laminar de fluids viscoelàstics utilitzant els models de Maxwell i Oldroyd-B en un tub vertical. Hem mostrat que en fluxos confinats existeix la possibilitat que les ones de cisalla generades a les diferents parets se sobreposin abans d'esmorteir-se i que eventualment donin lloc a un fenomen de ressonància. Les prediccions teòriques obtingudes pel flux laminar són validades duent a terme experiments de Velocimetria d'Imatges de Partícules (PIV) en un tub vertical, per amplituds petites del forçament oscil•latori. Quan s'incrementa l'amplitud de l'oscil•lació el flux laminar evoluciona cap a fluxos que presenten una dependència espai-temporal més complexa. Fent rampes d'amplitud creixent a una freqüència fixada hem pogut caracteritzar experimentalment la transició del flux laminar a aquests fluxos més complexos, sota condicions de forçament ben controlades. La primera inestabilitat apareix quan el flux laminar esdevé inestable amb la corresponent formació d'anells de vorticitat apilats al llarg del tub. Es manifesta una segona inestabilitat per amplituds del forçament més grans, per la qual el flux vortical perd la simetria axial. En aquest nou règim els vòrtex estan fortament distorsionats i no són axisimètrics. Fent rampes d’amplitud creixent i decreixent hem observat que aquestes dues transicions presenten histèresi, i que per tant són de caràcter subcrític.

Deep knowledge of combustion phenomena is of great scientific and technological interest. In fact, better design of combustion equipments (furnaces, boilers, engines, etc) can contribute both in the energy efficiency and in the reduction of pollutant formation. One of the limitations to design combustion equipments, or even predict simple flames, is the resolution of the mathematical formulation. Analytical solutions are not feasible, and recently numerical techniques have received enormous interest. Even though the ever-increasing computational capacity, the numerical resolution requires large computational resources due to the inherent complexity of the phenomenon (viz. multidimensional flames, finite rate kinetics, radiation in participating media, turbulence, etc). Thus, development of capable mathematical models reducing the complexity and the stiffness as well as efficient numerical techniques are of great interest.The main contribution of the thesis is the analysis and application of the laminar flamelet concept to the numerical simulation of both laminar and turbulent non-premixed flames. Assuming a one-dimensional behavior of combustion phenomena in the normal direction to the flame front, and considering an appropriate coordinates transformation, flamelet approaches reduce the complexity of the problem.The numerical methodology employed is based on the finite volume technique and a parallel multiblock algorithm is used obtaining an excellent parallel efficiency. A post-processing verification tool is applied to assess the quality of the numerical solutions.Before dealing with flamelet approaches, a co-flow partially premixed methane/air laminar flame is studied for different levels of partial premixing. A comprehensive study is performed considering different mathematical formulations based on the full resolution of the governing equations and their validation against experimental data from the literature. Special attention is paid to the prediction of pollutant formation.After the full resolution of the governing equations, the mathematical formulation of the flamelet equations and a deep study of the hypothesis assumed are presented. The non-premixed methane/air laminar flame is considered to apply the flamelet modelling approach, comparing the results with the simulations obtained with the full resolution of the governing equations. Steady flamelets show a proper performance to predict the main flame features when differential diffusion and radiation are neglected, while unsteady flamelets are more suitable to account for these effects as well as pollutant formation. Assumptions of the flamelet equations, the scalar dissipation rate modelling, and the evaluation of the Lagrangian flamelet time for unsteady flamelets are specially analysed. For the numerical simulation of turbulent flames, the mathematical formulation based on mass-weighted time-averaging techniques, using RANS EVM two-equation models is considered. The laminar flamelet concept with a presumed PDF is taken into account. An extended Eddy Dissipation Concept model is also applied for comparison purposes. A piloted non-premixed methane/air turbulent flame is studied comparing the numerical results with experimental data from the literature. A clear improvement in the prediction of slow processes is shown when the transient term in the flamelet equations is retained. Radiation is a key aspect to properly define the thermal field and, consequently, species such as nitrogen oxides. Finally, the consideration of the round-jet anomaly is of significant importance to estimate the flame front position.In conclusion, flamelet modelling simulations are revealed to be an accurate approach for the numerical simulation of laminar and turbulent non-premixed flames. Detailed chemistry can be taken into account and the stiffness of the chemistry term is solved in a pre-processing task. Pollutant formation can be predicted considering unsteady flamelets.

En este trabajo abordan diversos problemas relacionados con los flujos de fluido en régimen turbulento. El principal problema tratado es el de la dispersión de partículas en tiempos intermedios. Se analizan también las funciones de estructura a pequeña escala.En la primera parte, además de las ecuaciones de fundamentales de la Dinámica de Fluidos y de algunas herramientas estadísticas básicas, se tratan los temas de turbulencia bidimensional, espectro de energía, correlaciones de velocidad, escalas de la turbulencia, intermitencia, métodos numéricos y dipersión de partículas en flujo turbulento.En la segunda parte de la Tesis se analizan dos métodos numéricos de base muy diferente. El primero es una Simulación Numérica Directa (DNS), con la que se resuelve, de forma explícita, todas las escalas de un flujo turbulento a partir de las ecuaciones de la dinámica. Esto limita fuertemente el número de Reynolds de la simulación. El segundo método es conocido como Simulación Cinemática (KS), donde la única ecuación de la dinámica que se considera es la condición de continuidad.En la KS el flujo turbulento es simulado como la superposición de una serie de modos de Fourier. A cada modo le correponde una escala, determinada por un vecor número de onda, y una energía, dada por el coeficiente del modo. Estos coeficientes son calculados a partir del espectro de energía del campo que se pretende simular. Las características más importantes del campo simulado mediante la KS son:"No hay malla de discretización ni extrapolación a pequeñas escalas de espacio y tiempo. El campo es calculado en cualquier instante y cualquier posición. Esto lo hace especialmente atractivo para análisis Lagrangiano."Es posible elegir diferentes parámetros del flujo: espectro de energía, escalas, energía total, número de Reynolds.En la última parte de la Tesis se muestran los resultados de dos análisis diferentes realizados con la KS.En primer lugar se realiza una comparación exhaustiva de un flujo bidimensional generado mediante KS con otro, con idéntico espectro de energía, simulado con una DNS. Sobre ambos se realiza un análisis comparativo tanto Euleriano como Lagrangiano. El análisis Euleriano muestra bastantes diferencias entre ambos flujos. Las funciones de estructura, sin embargo, con muy semejantes a pequeña escala. El análisis Lagrangiano ofrece resultados sorprendentes en cuanto a semejanza entre un flujo y otro. El valor de la escala integral Lagrangiana de tiempo permite ajustar la evolución temporal de la KS. La dispersión absoluta es afectada por las estructuras coherentes debidas a la dinámica presente en el flujo simulado mediante la DNS, y ausentes en la KS. Estas estructuras coherentes dinámicas no parecen afectar a la evolución temporal de la dispersión relativa.En segundo lugar se realizan dos series de experimentos con la KS, analizando diferentes aspectos de la variación de la ley espectral del flujo turbulento. En la primera serie de experimentos se varia la ley espectral manteniendo uniforme la energia total del campo. Se analizan, por un lado, la estructura de la turbulencia para diferente ley espectral de potencia, y se propone una expresión para la función de estructura de orden p en función de la escala y de la ley espectral, para escalas menores que la microescala de Taylor. Por otro lado, se analiza también el efecto sobre la dispersión relativa.En la segunda serie de experimentos se varia la ley espectral manteniendo uniformes las escalas características del campo, de forma que se obtienen fuertes variaciones de energía. Se analiza el efecto sobre la dipersión relativa, observándose que se cumple la ley de Richardson generalizada, independientemente de la energia del campo, para espectros con potencia menor que 2. / In this work we study different problems related to the flow of fluids in turbulent motion. The main problem on which we focus is the particle dispersion for middle times. Structure functions on a small scale are analised as well.In the first part, besides the fundamental dynamic equations and some basic statistical tools, the following points are discussed: bidimensional turbulence, energy spectrum, velocity correlations, scales of turbulence, intermitence, numerical methods and particle dispersion in turbulent flow.In the second part, two numerical methods with very different bases are analised. The first one is a Direct Numerical Simulation (DNS), where all the scales of the flow are explicitly resolved from the dynamic equations. The second model is known as Kinematic Simulation (KS), where the only dynamic equation involved is the continuity condition.With the KS the turbulent flow is simulated as a superposition of a Fourier modes series. Every mode is related to a scale which is brougth about by a wavenumber vector, and an energy which is given by the mode coefficient. These coefficients are calculated from the energy spectrum of the simulated field.The most important characteristics of the KS simulated field are:"There is no discretitation grid nor small time and space scales interpolation. The field can be calculated in any time and any position. This makes this model specially attractive for Lagrangian analysis."It is possible to choose between different parameters: energy spectrum, scales, total energy, Reynolds number.In the last part of the thesis, the results of two different analysis carried out with KS are shown.In first place, an exhaustive comparison of a bidimensional flow generated by KS and another with an identical energy spectrum generated by DNS, is performed. With both a comparative analysis Eulerian and Lagrangian is carried out. The Eulerian analysis shows various differences between both flows. However, the structure functions are very similar on small scales. The Lagrangian analysis gives surprising results with regard to the similarity between both flows. The value of the Lagrangian integral time scale allows the KS temporary evolution to be adjusted. The absolute dispersion is affected by the coherent structures due to the dynamic present in the DNS simulated flow, which are absent in the KS. These dynamic coherent structures don't seem to affect the temporary evolution of the relative dispersion.In second place, two experimental series with KS are carried out, analysing different aspects of the variation of the spectral law of turbulent flow. In the first experimental series, the spectral law is modified keeping the total energy of the field uniform. On one hand, the structure of the turbulence for different spectral laws are analysed and an expression for the p-order structure function for scales smaller than the Taylor microscale, is proposed. On the other hand, the effect on the relative dispersion is analysed.In the second experimental series, the spectral law is modified keeping the field characteristic scales uniform, obtaining strong energy variations. The effect on the relative dispersion is analysed, observing that the generalized Richardson law is fulfilled, independently of the field energy, and for spectra with

The understanding and modelling of heat and radiation transport in tokamak plasmas is essential in order to progress in the development of thermonuclear fusion towards a practical energy source which meets all the future needs of environment, safety, and fuel inexhaustibility. This activity enables prospective studies and design to be carried out for next step tokamaks. Due to the complexity of the exact calculation, synchrotron losses are usually estimated in such studies, with expressions derived from a plasma description using simplifying assumptions on the geometry, radiation absorption, density and temperature profiles. In this thesis, a complete formulation of the transport of synchrotron radiation is performed for realistic conditions of toroidal plasma geometry with elongated cross-section, using a precise method for the calculation of the absorption coefficients, and for arbitrary shapes of density and temperature profiles. In particular, this formulation is able to describe plasmas with arbitrary aspect ratios and with temperature profiles obtained in internal transport barrier regimes, which cannot be described accurately with the present expressions. As an illustration, we show that in the case of an advanced high-temperature plasma envisaged for a steady state D-T commercial reactor, synchrotron losses represent approximately 20% of the total losses. Considering the quantitative importance of the above effects and the significant magnitude of synchrotron losses in the thermal power balance of a D-T tokamak reactor plasma, a new fit for the fast calculation of the synchrotron radiation loss is proposed. Using this improved model in the thermal balance, prospective and sensitivity studies are performed for future tokamak projects, and the key issues which limit the performance are isolated. It is shown that, the most restrictive constraint for achieving higher plasma performance is the peak heat flux on the divertor plates. In non-inductive steady-state operation, advanced tokamak regimes are required to achieve relevant thermonuclear plasma performance for next step tokamaks and for a commercial reactor. In the frame of a multi-step strategy towards a commercial reactor, a superconducting next step tokamak compatible with the European budget possibilities is optimized. Considering both the plasma physics and the magnetic system technology and for a given aspect ratio, the smallest machine meeting the physical and technological requirements is determined. In a steady state tokamak commercial reactor, we show that there is an optimal value for the confinement enhancement factor which maximizes the plasma performance, for a given and also for the highest electrical power into the network. This highest electrical power meeting the stability requirements steadily decreases with the confinement enhancement factor. This effect is crucial because both a high plasma performance and a high enough electrical power into the network are required to minimize the cost of electricity, and consequently to make fusion energy more competitive.

A T E N C I Ó ! ! !Per consultar aquesta tesi cal navegar partint del fitxer 0INICI.pdfRESUMENLas variables físicas que describen la mezcla, son perfectamente comparables para experimentos de laboratorio estudiados por un nutrido grupo de investigadores en la dinámica de fluidos medioambientales. En este trabajo se relacionan variables de tipo físicas y biológicas y se destaca el uso de las primeras como ayuda en la interpretación de las variables biológicas, demostrándose su utilidad. Se utilizan además distintos experimentos de laboratorio enfocados al estudio de la dinámica de la mezcla turbulenta en un flujo estratificado inducida por cizalla y a las escalas de las estructuras inducidas por dinámica de la pluma en el delta. Este objetivo se lleva a cabo describiendo las características de un ambientes estuarino y de la pluma por medio de indicadores como el número de Richardson y de Reynolds. Para ello se emplea una metodología avanzada de toma de muestras, técnicas experimentales de laboratorio como procesamiento de imágenes y técnicas numéricas para la modelización de la pluma en la desembocadura del estuario.El estuario bajo estudio se encuentra presente en el delta del Ebro, donde existe la mayor parte de año una cuña estable que es controlada principalmente por los caudales locales del río. Con la finalidad de observar una cuña estable, se realizo una campaña en el mes de julio de 1997. Adicionalmente para la obtención de variable a partir de las cuales puede hacerse una descripción de la dinámica de la mezcla y la obtención de variables biológicas, se desarrollaron una serie de campañas donde se buscaron diferentes condiciones del flujo del río que determina la mezcla en el estuario; estas campañas se realizaron las fechas del 5 de abril de 1998, 12 de julio, 5 de octubre de 1999 y 5 de febrero de 2000. En este trabajo junto al trabajo de campo, se realizan experimentos de laboratorio y simulaciones numéricas; se establece además un criterio cuantitativo del inicio y final de una capa de una estratificación en la columna de agua, se establecen zonas características de la mezcla en el estuario, la distancia de la cabeza de la pluma en cualquier punto del estuario incluyendo la boca a partir de la medida de la densidad mínima en la columna de agua, que la medida de los caudales locales maneja dinámica de la mezcla tanto en el cuerpo del estuario como en la pluma, además de medidas de eficiencia de mezcla, relaciones de la abundancia de organismos típicos de agua marina en el estuario de acuerdo a uno de los descriptores de la mezcla como el número de Reynolds, se describe la dinámica general de la circulación superficial inducida por la pluma en la zona costera adyacente del delta del Ebro por medio del seguimiento de partículas trazadoras y de la circulación en la columna de agua por seguimiento de colorante, por último se comparan las estructuras y escalas dominantes observadas experimentalmente con estructuras observadas por medio de percepción remota y simulaciones numéricas obteniéndose una buena correlación con los tendencias las medidas naturales y experimentales de laboratorio y numéricas. / Physical variables that describe mixing are compared with experimental laboratory results from a large number of studies of mixing dynamics in environmental fluids to find habitat regimes for primary production. The work relates physical and biological variables, and emphasises the utility of laboratory studies. Several laboratory experiments that focused on turbulent mixing dynamics in stratified shear flows are used to describe (a) mixing in the estuary and (b) induced circulation in the river plume. Mixing descriptors as entrainment, Richardson number and Reynolds number and field data were employed, and advanced techniques of laboratory simulations, image processing and numerical modelling were used to match (a) to (b). Four kinds of experiments were used to describe the dynamics in the whole estuary. 1) Mixing turbulence across a density interface generated by an oscillating grid inside a mixing-box. 2) The horizontal advance of a turbulent front in a stratified system with a lateral current, inside a 1 m x 1 m square box. 3) Induced circulation in the delta del Ebro slope and shelf that were performed with an experimental model in a 2 m x 4 m rectangular tank on a 5-m diameter turntable. 4) Dispersion simulations in the river plume with the OCK3D code. Experiments 1 and 2 were developed in the UPC applied physic laboratories in Barcelona, experiments 3 were performed in the SINTEF laboratories in Trondheim and numerical experiments 4 were realized in the LSEET laboratories in Toulon. The estuary under study is the delta of the del Ebro, where a stable saline wedge is present the greater part of the year, driven by the river flow. A field campaign was carried out to observe a complete large stable saline wedge during the month of July of 1997. Additionally, to obtain the physical descriptors, cruises were made on 5 April, 1998, 12 July and 5 October, 1999 and 5 February, 2000 to observe several flow conditions in the river estuary. Particle tracking and optical measurements in the water column were used to study the induced length scale dynamics in the shelf and slope of the delta del Ebro. The dominant structures and scales from SAR images and numerical and laboratory simulations were compared with the field data.With the help of laboratory and field experiments and numerical simulations, qualitative criteria were suggested to determine the extent of the interface ending in a stratified water column, the different mixing characteristic zones within the estuary, and the distance of the estuary head, since elsewhere points in the estuary (including the river mouth) are obtained by means of the minimum density in the water column. Both estuary mixing and plume dynamics are driven by the local flow. In the present analysis of mixing efficiency, the phytoplankton abundance is in accord with the Reynolds number as a mixing descriptor.

Path integral Monte Carlo (PIMC) is a method suitable for quantum liquid simulations at finite temperature. We present in this thesis a study of PIMC dealing with the theory and algorithms related to it, and then two applications of PIMC to current research problems of quantum fluids in the Bolzmann regime. The first part encompasses a study of the different ingredients of a PIMC code: action, sampling and physical property estimators. Particular attention has been paid to Li-Broughton's higher order approximation to the action. Regarding sampling, several collective movement methods have been derived, including the bisection algorithm, that has been thoroughly tested. We also include a study of estimators for different physical properties, such as, the energy (through the thermodynamic and virial estimators), the pair distribution function, the structure factor, and the momentum distribution. In relation to the momentum distribution, we have developed a novel algorithm for its estimation, the trail method. It surmounts some of the problems exposed by previous approaches, such as the open chain method or McMillan's algorithm.The Richardson extrapolation used within PIMC simulations, is another contribution of this thesis. Up until now, this extrapolation has not been used in this context. We present studies of the energy dependence on the number of "beads", along with the betterment provide by the Richardson extrapolation. Inasmuch as our goal is to perform research of quantum liquids at finite temperature, we have produced a library of codes, written from scratch, that implement most of the features theoretically developed. The most elaborated parts of these codes are included in some of the appendixes.The second part shows two different applications of the algorithms coded. We present results of a PIMC calculation of the momentum distribution of Ne and normal 4He at low temperatures. In the range of temperatures analysed, exchanges can be disregarded and both systems are considered Boltzmann quantum liquids. Their quantum character is well reflected in their momentum distributions witch show clear departures from the classical limit. The PIMC momentum distributions which show clear departures from the classical limit. The PIMC momentum distributions are sampled using the trail method. Kinetic energies of both systems, as a function of temperature and at a fixed density, are also reported. Finally, the solid-liquid neon phase transition along the 35 K isotherm has been characterized.While thermodynamic properties of the solid phase are well known the behaviour of some properties, such as the energy or the dessity, during the trasition presen6 some uncertainties For example, experimental data for the place diagram, which determines solid and liquid boundaries, present sizeable differences. The temperature chosen is high enough so that Bose or Fermi statistics corrections are small, although the system is strongly quantum mechanical. The results obtained show a discontinuity in the kinetic energy during the transition.