Renewable Natural Gas (RNG) is a low-carbon fuel source that is derived from the anaerobic digestion (AD) or thermal gasification (TG) of biomass, or produced using renewable electricity through the methanation of carbon dioxide. This thesis uses a thermodynamic balance to determine the total technical potential of RNG in the United States, as well as the future technical potential of methanation-derived RNG based on growth curves for renewable electricity. Furthermore, this work establishes an analytic decision-making framework for determining on a rolling basis, from an economic standpoint, whether to sell electricity directly to the grid, or produce and sell methanation-derived RNG. This framework is used to establish the economic potential of RNG, based on Texas wind resources. This work details the formulation of a model that determines which production option generates more marginal profit, based on fluctuating electricity and gas prices. The model also aggregates the total amount of electricity and RNG sold, assuming that the main objective is to maximize the marginal profit of integrated wind- and methanation facilities. This work concludes that the annual technical potential of methanation-derived RNG nationally was 1.03 Quads in 2011. The technical potential of biomass-derived RNG was 9.5 Quads. Thus, the total 2011 technical potential of RNG in the United States was 10.5 Quads, or equal to roughly 43% of the total US consumption of natural gas that year. Assuming a constant, 80% electrolyser efficiency, the technical potential of methanation-derived RNG is expected to rise at an average rate of 1.4% per year, following growth curves for renewable power, until the year 2040, when it will be 1.54 Quads. The 2011 economic potential of methanation-derived RNG in Texas was between 2.06×10⁷ MMBTU and 3.19×10⁷ MMBTU, or between 19.4% and 30.1% of the corresponding annual technical potential. Furthermore, the total marginal profit increase from introducing the option of producing and selling methanation-derived RNG was around $366 million, given a ‘best case scenario’ for the state of Texas. / text

This thesis is about linear random number generators with a large integer modulus. It is essential to be able to check that a generator has good properties in Monte-Carlo simulation. The generator family studied here produces points that possess a well known lattice structure that can be studied to assess the uniformity of these generators. The present work is motivated by the update of the LatMRG software which studies the lattice structure of the aforementioned generators. This thesis first presents the different types of generators the software considers. It explains how they can be used to produce random numbers and how to study their period length. It then presents equivalences between some members of this family that are used to simplify LatMRG. It then covers the lattice structure. The thesis describes what it is and how to characterize it. It describes the characterization of the lattice structure for the considered generators. From that, it presents a few algorithms that extract information on the uniformity of generators. At last, the thesis describes the LatMRG software. LatMRG contains both an executable program and a library. The thesis presents both their purpose and their functionalities. It describes upgrades of the software that aim to simplify its usage. Along with the software description are a few examples that serve to illustrate the flexibility and future research avenues. / Ce mémoire s’intéresse au générateurs de nombres aléatoires linéaires modulo un grand entier. Vérifier qu’un générateur possède de bonnes propriétés théoriques est essentiel pour la simulation Monte-Carlo. La famille de générateurs dont il est question produit des points possédant une structure de réseau bien connue pouvant être étudiée pour vérifier l’uniformité de ces générateurs. Le présent travail est motivé par la mise à jour du logiciel LatMRG qui permet d’étudier la structure de réseau de tels générateurs. Ce mémoire présente d’abord les types de générateurs qui sont considérés par le logiciel. Il explique comment ils peuvent être utilisés pour produire des nombres et comment étudier la longueur de leur période. Il présente ensuite des équivalences entre certains membres de la famille dont l’utilisation permet de simplifier le travail dans LatMRG. Il couvre ensuite la structure de réseau. En plus de décrire en quoi elle consiste, il explique comment la caractériser. On décrit une caractérisation de cette structure pour les générateur considérés. À partir de cela, on présente quelques algorithmes permettant d’extraire de l’information sur l’uniformité des générateurs. Le mémoire fait ensuite la description du logiciel LatMRG. LatMRG contient un programme exécutable et une librairie. Ce mémoire présente leur raison d’être et décrit leurs fonctionnalités. Il décrit aussi diverses améliorations qui ont été faites au logiciel avec pour objectif principal de simplifier son utilisation. La description du logiciel s’accompagne de quelques exemples illustrant sa flexibilité et des voies de recherche intéressantes.

The new legal regulations derived from climate change dictate that hydraulic structures must be designed to handle flood events associated with return periods up to 10,000 years. This obviously involves adapting the existing infrastructure to meet such requirements. In order to avoid risks in the restitution of the flow discharged to rivers, such as bank overflows or streambed erosion and scour processes, hydraulic design must be supported by reliable tools capable of reproducing the behavior of hydraulic structures. In the work presented herein, a fully three-dimensional CFD model to reproduce the behavior of different types of air-water flow in hydraulic structures is presented. The flow is assumed to be turbulent, isotropic and incompressible. Several RANS turbulence models are tested and structured rectangular meshes are employed to discretize the analyzed domain. The presence of two fluids is modeled using different VOF approaches and simulations are run using the PIMPLE algorithm. The model is implemented using the open-source platform OpenFOAM and its performance is compared to the commercial code FLOW-3D. The analysis is conducted separately on two different parts of hydraulic structures, namely: the spillway and the stilling basin. Additionally, a case of practical application, where the model reproduces the flow of a real-life case, is also presented in order to prove the suitability of the model to actual design cases. Mesh independence and model validation using experimental data are checked in the results of all the case studies. The sensitivity of the presented model to certain parameters is extensively discussed using different indicator variables. Among these parameters are turbulence closure, discretization scheme, surface tracking approach, CFD code or boundary conditions. Pros and contras of each of them are addressed. The analyzed turbulence models are the Standard k ¿ ¿, the Realizable k ¿ ¿, the RNG k ¿ ¿, and the SST k ¿ ¿. The discretization schemes under study are: a first-order upwind method, the second-order limited Van Leer method, and a second-order limited central difference method. The VOF approaches analyzed are the Partial VOF, as implemented in OpenFOAM, and the TruVOF, as implemented in FLOW-3D. In most cases, the Standard k ¿ ¿ model provides the most accurate estimations of water free surface profiles, although the rest of variables, with few exceptions, are better predicted by the RNG k ¿ ¿. The latter model generally requires slightly longer computation times. The SST k ¿ ¿ reproduces correctly the phenomena under study, although it generally turned out to be less accurate than its k ¿ ¿ counterparts. As regards the comparison among VOF approaches and codes, it is impossible to determine which one performs best. E.g. OpenFOAM, using the Partial VOF, managed to reproduce the in- ternal hydraulic jump structure and all derived variables better than FLOW-3D, using the TruVOF, although the latter seems to capture better the momentum transfer and so all derived variables. In the case of flow in stepped spillways, OpenFOAM captures better the velocity profiles, although FLOW-3D is more accurate when estimating the water free surface profile. It is worth remark- ing that not even their response to certain model parameters is comparable. E.g. FLOW-3D is significantly less sensitive to mesh refinement than OpenFOAM. Given the result accuracy achieved in all cases, the proposed model is fully applicable to more complex design cases, where stilling basins, stepped spillways and hydraulic structures in general must be investigated. / Las nuevas disposiciones legales derivadas del cambio climático dictaminan que las estructuras hidráulicas sean capaces de funcionar correctamente con eventos de inundación asociados a periodos de retorno de hasta 10,000 años. Esto, obviamente, implica adaptar la infraestructura existente para satisfacer dichos requerimientos. A fin de evitar riesgos en la restitución de los caudales vertidos al río, como desbordamientos o procesos erosivos y de socavación, el diseño hidráulico ha de sustentarse en herramientas fiables capaces de reproducir el comportamiento de las estructuras hidráulicas. En este trabajo, se presenta un modelo numérico CFD completamente tridimensional para reproducir el comportamiento de diferentes tipos de flujo aire-agua en estructuras hidráulicas. Se asume que el flujo es turbulento, isotrópico e incompresible. Diversos modelos de turbulencia RANS son contrastados y se emplean mallas estructuradas rectanuglares para discretizar el dominio analizado. La presencia de dos fluidos es modelada utilizando diferentes enfoques VOF y las simulaciones son ejecutadas empleando el algoritmo PIMPLE. El modelo es implementado mediante la plataforma de código abierto OpenFOAM y su respuesta es comparada con la del modelo comercial FLOW-3D. El análisis se lleva a cabo sobre dos partes diferentes de una estructura hidráulica, a saber, el aliviadero y el cuenco amortiguador, de forma separada. Además, un caso de aplicación práctica, donde el modelo reproduce el flujo en una estructura real, es presentado también a fin de probar la adecuación del modelo a casos de diseño aplicado. Se comprueban la independencia de la malla y la validación con datos experimentales de los resultados de todos los casos de estudio. La sensibilidad del modelo presentado a ciertos parámetros es analizada de forma exhaustiva empleando diferentes variables indicadoras. Los pros y contras de cada uno de éstos son planteados. Los modelos de turbulencia analizados son el Standard k-epsilon, el Realizable k-epsilon, el RNG k-epsilon y el SST k-omega. Los esquemas de discretización estudiados son: un método de primer orden upwind, uno de Van Leer de segundo orden y un esquema de segundo orden limitado de diferencias centradas. Los enfoques VOF analizados son el Partial VOF, implementado en OpenFOAM, y el TruVOF, implementado en FLOW-3D. En la mayoría de casos, el modelo k-epsilon aporta las estimaciones más precisas de perfiles de lámina libre de agua, pese a que el resto de variables, con alguna excepción, son mejor predichas por el RNG k-epsilon. Este modelo generalmente requiere mayores tiempos de cálculo. El k-omega reproduce correctamente los fenómenos bajo estudio, pese a que su precisión es generalmente más baja que la de los modelos k-epsilon. En lo que respecta a la comparación entre enfoques VOF y códigos, es imposible determinar cuál es el mejor. Por ejemplo, OpenFOAM, empleando el Partial VOF, logra reproducir la estructura interna del resalto hidráulico y todas las variables derivadas mejor que FLOW-3D, empleando el TruVOF, a pesar de que este último parece capturar mejor la transferencia de cantidad de movimiento y, por tanto, todas las variables derivadas. En el caso del flujo en aliviaderos escalonados, OpenFOAM captura mejor los perfiles de velocidad, pese a que FLOW-3D es más preciso en la estimación de los perfiles de lámina libre de agua. Conviene recalcar que ni tan sólo su respuesta a ciertos parámetros del modelo es comparable. Por ejemplo, FLOW-3D es significativamente menos sensible al refinado de malla que OpenFOAM. A la luz de la precisión de los resultados obtenidos en todos los casos, el modelo propuesto es completamente aplicable a casos de diseño más complejos, donde cuencos amortiguadores, aliviaderos escalonados y estructuras hidráulicas en general han de ser investigadas. / Les noves disposicions legals derivades del canvi climàtic dictaminen que cal que les estructures hidràuliques siguen capaces de funcionar correctament amb esdeveniments d'inundació associats a períodes de retorn de fins a 10,000 anys. Això, òbviament, implica adaptar la infraestrctura existent per satisfer aquests requeriments. A fi d'evitar riscs en la restitució dels cabals vessats al riu, com desbordaments o processos erosius i de socavació, el disseny hidràulic ha de recolzar-se en ferramentes fiables capaces de reproduir el comportament de les estructures hidràuliques. En aquest treball, es prsenta un model numèric CFD completament tridimensional per a reproduir el comportament de diferents tipus de flux aire-aigua en estructures hidràuliques. S'assumeix que el flux és turbulent, isotròpic i incompressible. Diferents models de turbulència RANS són contrastats i s'empren malles estructurades rectangulars per discretitzar el domini analitzat. La presència de dos fluids és modelada utilitzant diferents enfocaments VOF i les simulacions són executades emprant l'algorisme PIMPLE. El model és implementat mitjançant la plataforma de codi obert OpenFOAM i la seua resposta és comparada amb la del codi comercial FLOW-3D. L'anàlisi es du a terme sobre les diferents parts d'una estructura hidràulica, a saber, sobreeixidors esgraonats i vas esmorteïdor, de forma separada. A més, un cas d'aplicació pràctica, on el model reprodueix el flux a una estructura real, és presentat també a fi de provar l'adequació del model a casos de disseny aplicat. Es comproven la independència de la malla i la validació amb dades experimentals dels resultats de tots els casos d'estudi. La sensibilitat del model presentat a certs paràmetres és analitzada de forma exhaustiva emprant diferents variables indicadores. Els pros i contres de cadascun d'aquests són plantejats. Els models de turbulència analitzats són l'Standard k-epsilon, el Realizable k-epsilon, el RNG k-epsilon i l'SST k-omega. Els esquemes de discretització estudiats són: un mètode de primer ordre upwind, un de Van Leer de segon ordre i un esquema de segon ordre limitat de diferències centrades. Els enfocaments VOF analitzats són el Partial VOF, implementat en OpenFOAM, i el TruVOF, implementat en FLOW-3D. En la majoria de casos, el model Standard k-epsilon aporta les estimacions més precises de perfils de làmina lliure d'aigua, tot i que la resta de variables, amb alguna excepció, són millor predites pel RNG k-epsilon. Aquest model generalment requereix majors temps de càlcul. El k-omega reprodueix correctament els fenòmens sota estudi, tot i que la seua precisió és generalment més baixa que la dels models k-epsilon. Pel que fa la comparació entre enfocaments VOF i codis, és impossible determinar quin és el millor. Per exemple, OpenFOAM, emprant el Partial VOF, aconsegueix reproduir l'estructura interna del ressalt hidràulic i totes les variables derivades millor que FLOW-3D, emprant el TruVOF, tot i que aquest últim pareix capturar millor la transferència de quantitat de moviment i, per tant, totes les variables derivades. En el cas del flux en sobreeixidors esgraonats, OpenFOAM captura millor els perfils de velocitat, tot i que FLOW-3D és més precís en estimar els perfils de làmina lliure d'aigua. Cal deixar palès que ni tan sols la seua resposta a certs paràmetres del model és comparable. Per exemple, FLOW-3D és significativament menys sensible al refinament de malla que OpenFOAM. En base a la precisió dels resultats obtinguts en tots els casos, el model proposat és completament aplicable a casos de disseny més complexos, on vassos esmorteïdors, sobreeixidors esgraonats i estructures hidràuliques en general han de ser investigades. / Bayón Barrachina, A. (2017). Numerical analysis of air-water flows in hydraulic structures using computational fluid dynamics (CFD) [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90440 / TESIS

Cryptographically secure random number generators usually require an outside seed to be initialized. Other solutions instead use a continuous entropy stream to ensure that the internal state of the generator always remains unpredictable. This thesis analyses four such generators with entropy inputs. Furthermore, different ways to estimate entropy is presented and a new method useful for the generator analysis is developed. The developed entropy estimator performs well in tests and is used to analyse entropy gathered from the different generators. Furthermore, all the analysed generators exhibit some seemingly unintentional behaviour, but most should still be safe for use. / Kryptografiskt säkra slumptalsgeneratorer behöver ofta initialiseras med ett oförutsägbart frö. En annan lösning är att istället konstant ge slumptalsgeneratorer entropi. Detta gör det möjligt att garantera att det interna tillståndet i generatorn hålls oförutsägbart. I den här rapporten analyseras fyra sådana generatorer som matas med entropi. Dessutom presenteras olika sätt att skatta entropi och en ny skattningsmetod utvecklas för att användas till analysen av generatorerna. Den framtagna metoden för entropiskattning lyckas bra i tester och används för att analysera entropin i de olika generatorerna. Alla analyserade generatorer uppvisar beteenden som inte verkar optimala för generatorns funktionalitet. De flesta av de analyserade generatorerna verkar dock oftast säkra att använda.

This study presents air distribution systems that are based on confluent jets; this system can be of interest for the establishment of indoor environments, to fulfill the goals of indoor climate and energy-efficient usage. The main objective of this study is to provide deeper understanding of the flow field development of a supply device that is designed based on wall confluent jets and to investigate the ventilation performance by experimental and numerical methods. In this study, the supply device can be described as an array of round jets on a flat surface attached to a side wall. Multiple round jets that issue from supply device apertures are combined at a certain distance downstream from the device and behave as a united jet or so-called confluent jets. Multiple round jets that are generated from the supply device move downward and are attached to the wall at the primary region, due to the Coanda effect, and then they become wall confluent jets until the floor wall is reached. A wall jet in a secondary region is formed along the floor after the stagnation region. The characteristics of the flow field and the ventilation performance of conventional wall confluent jets and modified wall confluent jets supply devices are investigated experimentally in an office test room. The study of the modified wall confluent jets is intended to improve the efficiency of the conventional one while maintaining acceptable thermal comfort in an office environment. The results show that the modified wall confluent jets supply device can provide acceptable thermal comfort for the occupant with lower airflow rate compared to the conventional wall confluent jets supply device. Numerical predictions using three turbulence models (renormalization group (RNG k– ε), realizable (Re k– ε), and shear stress transport (SST k– ω) are evaluated by measurement results. The computational box and nozzle plate models are used to model the inlet boundary conditions of the nozzle device. In the isothermal study, the wall confluent jets in the primary region and the wall jet in the secondary region, when predicted by the three turbulence models, are in good agreement with the measurements. The non-isothermal validation studies show that the SST k– ω model is slightly better at predicting the wall confluent jets than the other two models. The SST k– ω model is used to investigate the effects of the nozzle diameter, number of nozzles, nozzle array configuration, and inlet discharge height on the ventilation performance of the proposed wall confluent jets supply device. The nozzle diameter and number of nozzles play important roles in determining the airflow pattern, temperature field, and draught distribution. Increased temperature stratification and less draught distribution are achieved by increasing the nozzle diameter and number of nozzles. The supply device with smaller nozzle diameters and fewer nozzles yields rather uniform temperature distribution due to the dominant effect of mixing. The flow behavior is nearly independent of the inlet discharge height for the studied range. The proposed wall confluent jets supply device is compared with a mixing supply device, impinging supply device and displacement supply device. The results show that the proposed wall confluent jets supply device has the combined behavior of both mixing and stratification principles. The proposed wall confluent jets supply device provides better overall ventilation performance than the mixing and displacement supply devices used in this study. This study covers also another application of confluent jets that is based on impinging technology. The supply device under consideration has an array of round jets on a curve. Multiple jets issue from the supply device aperture, in which the supply device is positioned vertically and the jets are directed against a target wall. The flow behavior and ventilation performance of the impinging confluent jets supply device is studied experimentally in an industrial premise. The results show that the impinging confluent jets supply device maintains acceptable thermal comfort in the occupied zone by creating well-distributed airflow during cold and hot seasons.