Дисертації з теми "Vnf modeling"

Thesis (M.S. in Electrical Engineering) Naval Postgraduate School, December 1995.
Thesis advisor(s): Rasler W. Smith, R.W. Adler. "December 1995." Includes bibliographical references. Also available online.

Widely used design techniques for radio frequency power amplifiers yield results which are approximate; the initial design is usually refined by applying trial-and-error procedures in the laboratory. More accurate design techniques are complicated in their application and have not gained acceptance by practicing engineers. A new design technique for VHF linear power amplifiers using bipolar junction transistors is presented in this report. This design technique is simple in its application but yields accurate results. The design technique is based upon a transistor model which is simple enough to be useful for design, but which is sufficiently accurate to predict performance at high frequencies. Additionally, the model yields insight into many of the processes which take place within the typical RF power transistor. The fundamental aspect of the model is the inclusion of charge storage within the transistor base. This charge storage effect gives rise to a nearly sinusoidal collector current waveform, even in a transistor which ostensibly is biased for class B or nonsaturating class C operation. Methods of predicting transistor input and output impedances are presented. A number of other topics related to power amplifier measurement and design are also included. A unique measurement approach which is ideally suited for use with power amplifiers is discussed. This measurement approach is a hybrid of the common S-parameter measurement technique and the "load-pull" procedure. Practical considerations such as amplifier stability, bias network design, and matching network topology are also included in the report.
Ph. D.

In this project, the flow field characteristics of the simplified 2D rectangular primary sedimentation tank in Syvab wastewater treatment plant were achieved by a transient water-air two phases finite-volume method, applying Volume-Of-Fluid (VOF) model. RNG k-ε turbulence model was also employed to calculate the turbulent kinetic energy and its dissipation rate. The undesired hydraulic phenomenon for solid sedimentation was detected in original tank. To reduce the velocity and turbulence intensity of the influent, two categories of optimization methods were proposed, which are installing the baffle and changing the velocity inlet. The modifying effects of different methods were compared by the velocity profiles and the contours of kinetic energy. It turns out that both ways provide a preferred condition for particle settling. In the end, further research was forecasted and the work direction were given.
I detta projekt uppnåddes 2D modellering av avloppsflödesfältets karaktär hos en förenklade och rektangulär primära sedimenteringsbassäng, Syvab avloppsreningsverk, genom en transient-tvåfas-finita-volymmetoden med vatten och luft, som tillämpades med Volume-Of-Fluid (VOF) modellen. RNG k-ε turbulensmodellen användes även för att beräkna den kinetiska energin av turbulas och dess dissipationshastighet. Detta för att oönskade hydrauliska fenomen har uppmärksammats hos sedimenteringsbassängen hos avloppsreningsverket. För att minska flödeshastigheten och turbulens föreslås två optimeringsmetoder, vilket är att installera skärm och att ändra inloppets hastighet. Eeffekterna av de olika metoderna jämförs med hjälp av hastighets- och kinetisk energiprofiler. Det visar sig att båda metoderna kan ge gynsammare tillstånd för sedimentering av partikelar. Som avslutning ges prognos för den fortsatta forskningen och arbetsriktningen inom ämnet.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely interfacial heat transfer and combustionanalysis approach.After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities,a new solver, including the two developed models for phase change, was implemented under the name ofinterPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physicalbehavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties.It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advancedsolvers within the phase change class.

Northern Sweden is one of the largest hosts for mineral resources in Europe and always has been an interesting area for researchers from various disciplines of Earth sciences. This dissertation is a comprehensive summary of three case study papers on airborne VLF, gravity gradient and magnetic data in the area. In the first paper, tensor VLF data is extracted from an old data set which contains only the total and the vertical magnetic components. The anomalous part of the horizontal magnetic field components is computed by a Hilbert transform of the vertical magnetic field. The normal part of the horizontal magnetic field component is computed as a function of total, vertical and anomalous part of horizontal magnetic fields. The electric field is also calculated for TE mode and impedance tensor and apparent resistivity are computed. In addition tippers are calculated for two transmitters and inverted by a 3D inversion algorithm. Comparison of the estimated model and geology map of bedrock shows that lower resistivity zones are correlated with mineralizations. The second paper deals with the internal consistency of airborne gravity gradient data. The six components of the data are estimated from a common potential function. It is shown that the data is adequately consistent but at shorter land clearances the difference between the estimated data and the original data is larger. The technique is also used for computing the Bouguer anomaly from terrain corrected FTG data. Finally the data is inverted in 3D, which shows that the estimated density model in shallow depth is dominated by short wave length features. Inversion of TMI data is the topic of the third paper where a new type of reference model for 3D inversion of magnetic data is proposed by vertically extending the estimated magnetization of a 2D terrain magnetization model. The final estimated 3D result is compared with the magnetization model where no reference model is used. The comparison shows that using the reference model helps the high magnetization zones in the estimated model at shallow depths to be better correlated with measured high remanent magnetization from rock samples. The high magnetization zones are also correlated with gabbros and volcanic metasediments.

Nowadays there is an increasing demand for compact devices with high performance in heating and cooling applications; a compact geometry permits to have a lower charge of refrigerant as compared to conventional HX. The present research activity is focused on the analysis of condensation phenomena inside a single minichannel and on the application of minichannels technology for condensers and evaporators in heating and cooling systems. When the inner diameter decreases, new parameters can influence in a significant way the two-phase heat transfer. Nowadays powerful numerical tools permit to obtain a detailed analysis of two-phase heat transfer mechanisms inside channels and to model the interaction between the phases without using any empirical closure law. In this way it is possible to have a numerical approach that can be applied for different fluids and operating conditions being independent from the empirical correlations found in the literature. In particular the Volume Of Fluid (VOF) method is used to have investigate the main features of condensation phenomena inside a single minichannel and the numerical results have been validated against experimental data. Since the low mass flux conditions are the least investigated due to the high experimental uncertainly when the heat flow rate is in the order of few watts, numerical simulations have been performed to get an insight into the condensation process at these operating conditions, focusing the investigation on the effect of fluid properties, channel shape, channel size and channel orientation. The VOF simulations were then extended to the study of interfacial instabilities during the R134a annular-wavy downflow condensation inside a vertical 3.4 mm i.d. minichannel. Computational results about the evolution in time of the vapour-liquid interface along the channel have been validated against experimental visualizations and the influence of the waves on heat transfer has been analyzed. Besides the numerical analysis of the heat transfer, the application of minichannel technology in heating and cooling systems has also been investigated. The use of minichannels in heat exchangers allows to significantly decrease the refrigerant charge as compared to conventional finned coil heat exchangers. This is particularly useful in a reversible heat pump when the same heat exchanger can work as a condenser or as an evaporator and it is particularly useful when the refrigerant is flammable or middly flammable. The development of physical models are necessary to the prediction of the performance and the design of completely innovative systems. Computational procedures have been thus developed to evaluate the performance of a reversible water-to-water and air-to-water heat pump using minichannels in the heat transfer with air, considering both the air and the ground as possible heat sources. This work has been possible thanks to the financial support of Riello SpA.
Attualmente vi è una crescente domanda di dispositivi compatti con prestazioni elevate in applicazioni di riscaldamento e raffreddamento; una geometria compatta consente di avere una carica ridotta di refrigerante rispetto agli scambiatori convenzionali. L'attività di ricerca è focalizzata sull'analisi del fenomeni di condensazione all'interno di un singolo minicanale e sull'applicazione della tecnologia a minicanali per condensatori ed evaporatori in sistemi di riscaldamento e raffreddamento. Quando il diametro interno diminuisce, nuovi parametri possono influenzare in modo significativo il trasferimento di calore in deflusso bifase. Oggigiorno potenti strumenti numerici permettono di ottenere un'analisi dettagliata dei meccanismi di trasferimento di calore in deflusso bifase all'interno di canali e di modellare l'interazione tra le fasi senza utilizzare alcuna legge empirica di chiusura. In questo modo è possibile avere un approccio puramente numerico che può essere applicato per diversi fluidi e diverse condizioni operative, risultando indipendente dalle correlazioni empiriche in letteratura. In particolare, il metodo VOF è stato utilizzato per studiare le caratteristiche principali dei fenomeni di condensazione all'interno di un singolo minicanale ed i risultati numerici sono stati poi convalidati con i dati sperimentali. Poiché le condizioni a bassa portata specifica sono le meno indagate a causa della elevata incertezza sperimentale quando il flusso termico è dell'ordine di pochi watt, simulazioni numeriche sono state effettuate allo scopo di ottenere una panoramica del processo di condensazione in tali condizioni operative, concentrando l'indagine sugli effetti della proprietà del fluido, la forma, le dimensioni e l'’inclinazione del canale. Le simulazioni VOF sono state poi estese allo studio delle instabilità all’interfaccia durante il processo di condensazione di R134a per un deflusso anulare-ondoso all'interno di un minicanale verticale con un diametro interno di 3.4 mm. I risultati numerici sull'evoluzione dell’interfaccia liquido-vapore nel tempo sono stati convalidati con visualizzazioni sperimentali e l'influenza delle onde sullo scambio termico è stata analizzata. Oltre all'analisi numerica del trasferimento di calore, l'applicazione della tecnologia a minicanali è stata anche investigata in sistemi di riscaldamento e raffreddamento. L'uso di minicanali negli scambiatori di calore permette di ridurre in modo significativo la carica di refrigerante rispetto ai tradizionali scambiatori di calore a batteria alettata. Questo è particolarmente utile in una pompa di calore reversibile quando lo stesso scambiatore di calore può funzionare come un condensatore o come evaporatore ed è particolarmente utile quando il refrigerante è infiammabile o mediamente infiammabile. Lo sviluppo di modelli fisici è necessario per la previsione delle prestazioni e per la progettazione di sistemi completamente innovativi. Procedure di calcolo sono state così implementate per valutare le prestazioni di una pompa di calore reversibile acqua-acqua e aria-acqua utilizzando i minicanali nel trasferimento di calore con l'aria, considerando sia l'aria che il terreno come possibili fonti di calore. Questo lavoro è stato possibile grazie al sostegno finanziario di Riello SpA .

Sweden is a country with a relatively high number of private wells, where about 1.2 million inhabitants in permanent housing and an equal amount in summer housing relies on private wells as their drinking water supply. At the same time the market for drinking water treatment products is unregulated regarding quality and requirements are needed. A testing facility with the aim of providing quality certifications as well as sustainability- and efficiency- tests of small scale drinking water treatment techniques is under development and located in a decommissioned groundwater treatment plant in Svanberga, outside Norrtälje, Stockholm County, Sweden. The groundwater treatment plant has three operational bedrock wells connected to it and the groundwater system is rather unknown with only a few previous studies done in the region. Better understanding of the hydrogeological system would aid in further work of evaluating the risks of contaminant spread in the region. Trough fracture outcrop mapping, geophysical Very Low Frequency measurements and groundwater modelling using COMSOL Multiphysics this master thesis have identified several possible flow paths within the bedrock connected to the GWTP. The results indicates there are one or two approximately east-west striking fracture zones that could play a major role in transportation of contaminants related to road traffic and agriculture to the wells, while several north to south striking fracture zones most likely supplies the groundwater basin with water from the thicker soil layers in the northern part of the region. Recommendations of future studies includes conducting a detailed pumping test of the operational wells in Svanberga old GWTP as well as verification of the presumed most important identified fracture zones to the south east and north in the studied area by additional geophysical investigations or  tracer tests.
Testbädd dricksvatten är ett pågående projekt och testanläggning där tester och utveckling av filter och småskaliga tekniker för rening av dricksvatten samt kvalitetscertifiering för effektivitets och hållbarhetstester kommer utföras. Testanläggningen består av ett nedlagt grundvattenverk med tre tillhörande bergborrade brunnar i Svanberga utanför Norrtälje. Grundvattensystemet och flödesvägar för grundvattnet i området är relativt okända och bättre förståelse av hydrogeologin skulle kunna bidra till framtida arbete med att identifiera och bedöma risker för föroreningstransporter i området samt till grundvattenverkets brunnar. Detta examensarbete har genom beprövade metoder i form av sprick-kartering, geofysiska undersökningar (VLF) och grundvattenmodellering med COMSOL Multiphysics identifierat flera möjliga flödesvägar i berggrunden kopplade till grundvattenverket. Resultatet från studien visar att ett antal sprickzoner i öst-västlig riktning utgör en större risk för föroreningstransport av ämnen från trafik och jordbruk samt att en större del av det vatten som tillförs grundvattensystemet troligen har sitt ursprung från tjockare morän-lager i de norra delarna av området. Provpumpningar och verifiering av de identifierade sprick-zonerna genom spårämnestester och geofysiska undersökningar bör utföras för att stärka resultaten från detta arbete och möjligen utöka kunskapen om hydrogeologin i området kring Svanberga gamla grundvattenverk.

Au cours des dernières années, la culture de microalgues est largement étudiée pour produire des biocarburants et d’autres produits de valeur en fixant le dioxyde de carbone de l’atmosphère, afin d’atténuer simultanément les effets du changement climatique et de réduire la dépendance à l’égard des carburants fossiles. En comparaison avec les systèmes ouverts, les photobioréacteurs fermés sont davantage utilisés en laboratoire, car ils permettent de contrôler avec précision les facteurs environnementaux tels que le pH, la concentration en éléments nutritifs, etc. Le principe de fonctionnement d’un photobioréacteur repose sur l’injection de bulles dans le milieu de culture pour (i) apporter du dioxyde de carbone aux cellules (ii)agiter le liquide. Par l’apport d’énergie lumineuse les cellules transforment le carbone inorganique en carbone organique par photosynthèse. Ainsi, les phénomènes physiques - l’écoulement, transfert de matière, transfert radiatif - et les phénomènes biologiques - photosynthèse, croissance cellulaire et mort - coexistent dans un photobioréacteur. Plus important encore, tous les phénomènes de base ne sont pas complètement indépendants les uns des autres. Des recherches récentes ont révélé que le comportement des bulles avait également une incidence directe sur le processus biologique. En raison du comportement significatif des bulles sur la productivité d'un photobioréacteur, la génération de bulles a été étudiée dans cette thèse au moyen de méthodes expérimentales et numériques.Dans l'étude expérimentale, nous avons conçu puis fabriqué un nouveau photobioréacteur afin d'étudier le bullage in situ. L’emploi d’une technique d’ombroscopie couplée à une caméra vidéo a permis l’enregistrement de séries de bulles. Les images traitées ont permis de mesurer des caractéristiques de bulles (fréquence, volume, facteur de forme). Le volume moyen de bulle et la fréquence de formation de bulles augmentent avec le débit de gaz. De plus, la distribution volumique monodisperse à faible débit devient de plus en plus polydisperse par l’accroissement de celui-ci. L’évolution de la forme des bulles lors de leur remontée dans le liquide a été évaluée par l’emploi de facteurs de forme. Ces facteurs diminuent avec la remontée des bulles et traduisent une déformation horizontalement. A débit élevé, les formes des bulles oscillent et coalescent plus fréquemment.La simulation du bullage a été réalisé par l’emploi d’une méthode Volume of Fluid (VOF) et d’une bibliothèque open source de mécanique numérique des fluides OpenFOAM. Ces choix de méthodes sont motivés en raison de la robustesse d'OpenFOAM en matière de simulation d'écoulements diphasiques rapportée dans la littérature. Une première étude numérique de simulation 2D a permis de déterminer les valeurs appropriées des paramètres numériques (nombre de Courant et la taille du maillage) tout en minimisant le temps de calcul par rapport à une pré-étude 3D. Sans surprise, nous avons déterminé que la taille des mailles devait être inférieure au diamètre de la buse pour obtenir des résultats significatifs. De façon plus surprenante, nous avons observé que le nombre maximum de Courant n’a pas d’importance particulière pour ces simulations (dans une limite raisonnable : 0 à 1). Les simulations 3D ont été menées sur un supercalculateur. Elles ont montré que le volume des bulles et l’évolution de leur forme calculées numériquement étaient en accord avec les résultats expérimentaux. Cependant, les simulations 3D n’ont pas permis de représenter la polydispersité de la distribution volumique des bulles en raison d’un temps de calcul nécessaire trop important pour générer une population de bulles suffisamment nombreuse. Au final, l'outil numérique a aussi été utilisé avec succès pour explorer plusieurs caractéristiques hydrodynamiques de mélange dans le liquide
The working principle of a typical photobioreactor is to inject gas bubbles into the culture medium, providing CO2 to the cells and also stirring the liquid. Subsequently, the cells convert inorganic carbon into organic carbon through photosynthesis under illumination. Therefore, physical phenomena, e.g. bubbly flow, mass transfer, radiative transfer, and biological phenomena, e.g. photosynthesis, cell growth and death, coexist in a photobioreactor. More importantly, all the basic phenomena are not completely independent to each other. For example, bubble volume and bubble shape can influence gas-liquid mass transfer according to Young-Laplace equation and Henry's law. Moreover, some recent research revealed that bubble behaviors also directly affect the biological process. In view of the important impact of bubble behaviors on productivity of a phototbioreactor, the bubbly flow was investigated in this thesis by both experimental and numerical methods.In the experimental study, we first manufactured a new photobioreactor in order to study the bubbles and other phenomena. Subsequently, the bubbles were captured by high speed camera by virtue of a shadowgraphy technique and bubble behaviors were obtained by processing and analyzing the images. From the experimental results, we found that both averaged bubble volume and bubbling frequency increased with gas flow rate. Furthermore, we also discovered that the distribution of bubble volume was almost monodisperse at low flow rate, and it became more and more polydisperse with increasing flow rate. Regarding bubble shape evolution, we used two shape factors, viz. aspect ration and circularity, to quantitatively study it. We found that both shape factors dropped rapidly during bubble rising (within the limit of the field of view of our video camera), which implied that bubbles were flattened in the course of rising. Nonetheless, bubbles became more vertically elongated at higher flow rate, partially due to the more frequent bubble coalescence at higher flow rate.In the numerical study, we adopted VOF method and OpenFOAM, an open source CFD library, as our numerical tool to represent bubbly flow. First of all, the robustness of OpenFOAM in simulating two-phase flow was validated by literature survey. Subsequently, 2D simulations were carried out for seeking the appropriate and not very time-consuming numerical parameters, i.e. maximum Courant number and mesh size. We found that mesh size should somehow be smaller than the nozzle diameter to have meaningful results. On the other hand, maximum Courant number had no particular importance in the simulations (as long as between 0 and 1). Furthermore, 3D simulations were in good agreement with the experiments in terms of bubble volume and bubble shape evolution. However, 3D simulations were not able to represent the polydispersity of bubble volume due to the limited computing power. In addition, several hydrodynamic characteristics were also explored by the proposed numerical tool, which gave reasonable results.To conclude, bubble behaviors were successfully captured by experimental methods and represented by numerical methods in this thesis, which will help us go further in understanding the complicated physical-biological phenomena of a photobioreactor

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
O constante crescimento na demanda de combustíveis fósseis e as últimas descobertas de petróleo no Brasil têm tornado a modelagem de escoamentos multifásicos em tubulações e equipamentos na indústria do Petróleo numa tarefa crucial. Dentro as configurações possíveis que o escoamento multifásico pode adotar, o regime das golfadas possui uma enorme relevância devido às suas características intermitentes e seus fortes gradientes de pressão produzidos nas tubulações por onde escoa. Neste trabalho, estuda-se numericamente, com o método de volumes finitos, o regime das golfadas, determinando-se o campo de velocidade e pressão, assim como a forma do nariz da bolha de Taylor e a velocidade de translação de uma unidade básica de golfada tridimensional em uma tubulação horizontal. Considerou-se o escoamento isotérmico, e utilizouse o referencial coincidente com a bolha de Taylor. Para modelar o escoamento turbulento utilizou-se o modelo K−E RNG. A interface foi determinada com o modelo VOF (Volume of Fluid). Os resultados das simulações foram comparados com dados experimentais disponíveis, apresentando uma boa concordância.
Pipeline and equipment multiphase flow modeling in oil industry has become a crucial task due to the constant increasing fossil fuel demand and latest oil discovers in Brazil. Among all possible flow configurations that multiphase flow can take, the slug flow regime plays an important role due to its intermittent characteristics and its strong pressure gradients in pipelines, where this flow regime can be found. In the present work, a numerical study of a slug flow is performed, with the finite volume method. The flow and pressure field are determined, as well as, the Taylor bubble nose shape and the translational velocity of a three dimensional single unit slug in a horizontal pipeline. The flow field was considered isothermal and it was employed a coordinate system coincident with the Taylor bubble. To model the turbulent flow, the RNG K−E model was selected. The interface was determined with the Volume of Fluid models (VOF). The results from the simulations were compared with available experimental data, presenting good agreement.

Chip scale refrigeration system is critical for the development of electronics with the rapid increase of power consumption and substantial reduction of device size, resulting in an emergent demand on novel cooling technologies with a high efficiency for the thermal management. In this thesis, active refrigeration devices based on Stirling cycle and an electrocaloric material, are designed and investigated to achieve a high cooling performance. Firstly, a new Stirling micro-refrigeration system composed of arrays of silicon MEMS cooling elements is designed and evaluated. The cooling elements are fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. A first-order of thermodynamic analysis is performed to study the effect of geometric parameters. Losses due to regenerator non-idealities and chamber heat transfer limitation are estimated. A multiphysics computational approach for analyzing the system performance that considers compressible flow and heat transfer with a large deformable mesh is demonstrated. The optimal regenerator porosity for the best system COP (coefficient of performance) is identified. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model is used to allow for modeling of a full element. The analysis indicates the work recovery of the system and the diaphragm actuation are main challenges for this cooler design.The pressure drop and friction factor of gas flow across circular silicon micro pillar arrays fabricated by deep reactive ion etch (DRIE) process are investigated. A new correlation that considers the coupled effect of pillar spacing and aspect ratio, is proposed to predict the friction factor in a Reynolds v number range of 1-100. Silicon pillars with large artificial roughness amplitudes is also fabricated, and the effect of the roughness is studied in the laminar flow region. The significant reduction of pressure drop and friction factor indicates that a large artificial roughness could be built for pillar arrays in the regenerator to enhance the micro-cooler efficiency. The second option is to develop a fluid-based refrigeration system using an electrocaloric material poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer. Each cooling element includes two diaphragm actuators fabricated in the plane of a silicon wafer, which drive a heat transfer fluid back and forth across terpolymer layers that are placed between them. Finite element simulations with an assumption of sinusoidal diaphrahm motions are conducted to explore the system performance detailedly, including the effects of the applied electric field, geometric dimensions, operating frequency and externally-applied temperature span. Multiphysics modeling coupled with solid-fluid interaction, heat transfer, electrostatics, porous medium and moving mesh technique is successfully performed to verify the thermal modeling feasibility. The electrocaloric effect in thin films of P(VDF-TrFE-CFE) terpolymer is directly measured by infrared imaging at ambient conditions. At an electric field of 90 V/μm, an adiabatic temperature change of 5.2 °C is obtained and the material performance is stable over a long testing period. These results suggest that application of this terpolymer is promising for micro-scale refrigeration.

Due to the high-speed flow in a chute spillway, cavitation damages often occur. This undesired phenomenon threatens the safety of the structure. For the purpose of eliminating the damages, an aerator is often installed in the spillway. To understand its characteristics, physical model tests are a popular method. To complement the model tests, computation fluid dynamics (CFD) simulations are used to study aerator flows. To represent the two-phase flows, multiphase models should be employed. This thesis examines two of them, namely, the Volume-Of-Fluid model (VOF) and Two-Fluid model. Based on the background of the Bergeforsen dam, the aerator flow is modelled by means of the VOF model. The simulated spillway discharge capacity is in accordance with the experimental data. Compared with the results, empirical formulas fail to evaluate the air supply capacity of aerator as it is wider than the conventional width. A hypothetical vent modification is proposed. For the original and proposed layouts, the study illustrates the difference in the air-flow conditions. The results show that a larger vent area is, for a large-width aerator, preferable in the middle of the chute. To study the flip bucket-shaped aerators in the Gallejaur dam, physical model tests and prototype observations are conducted. The results lead to contradicting conclusions in terms of jet breakup and air entrainment. A CFD model is, as an option, employed to explain the reason of the discrepancy. The numerical results coincide with the prototype observations. The jet breakup and air entrainment are evaluated from air cavity profiles; the air-pressure drops are small in the cavity. The discrepancy is due to overestimation of the surface-tension effect in the physical model tests. Based on the experimental data of an aerator rig at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, the Two-Fluid model is used to predict air concentration distributions in the aerated flow. The model includes relevant forces governing the motion of bubbles and considers the effects of air bubble size. The numerical results are conformable to the experiments in the air cavity zone. Downstream of the cavity, the air concentration near the chute bottom is higher, which is presumably caused by the fact that the interfacial forces in the Two-Fluid model are underestimated.

QC 20170224

Biological nitrogen fixation is accomplished in the bacterium Azotobacter vinelandii by means of three metalloenzymes: The molybdenum, vanadium, and iron-only nitrogenase. The knowledge regarding biological nitrogen fixation has come from studies on the Mo-dependent reaction. However, the V- and Fe-only-dependent reduction of nitrogen remains largely unknown. By using homology modeling techniques, the protein folds that contain the metal cluster active sites for the V- and Fe-only nitrogenases were constructed. The models uncovered similarities and differences existing among the nitrogenases regarding the identity of the amino acid residues lining pivotal structural features for the correct functioning of the proteins. These differences, could account for the differences in catalytic properties depicted by these enzymes. The quaternary structure of the dinitrogenases also differs. Such component in the Mo-nitrogenase is an α2β2 tetramer while for the V- an Fe-only nitrogenase is an α2β2δ2 hexamer. The latter enzymes are unable to reduce N2 in the absence of a functional δ subunit, yet they reduce H+ and the non-physiological substrate C2H2. Therefore, the δ subunit is essential for V- and Fe-only dependent nitrogen fixation by a mechanism that still remains unknown. In attempt to understand why the δ subunit is essential for V-dependent N2 reduction from a structural stand point, this work presents the strategy followed to clone the vnfG gene and purify its expression product, the δ subunit. The purified protein was subjected to crystallization trials and used to stabilize a histidine-tagged VFe protein that would otherwise purify with low Fe2+ content and poor H+ and C2H2 reduction activities. The VFe preparation was used to conduct substrate reduction assays to assess: i) The electron allocation patterns to each of the reduction products of the substrates C2H2, N2, N2H4, and N3−; and ii) Inhibition patterns among substrate and inhibitor of the nitrogenase reaction. This work also reports on the effect N2H4 and N3− has on the electron flux to the products of the C2H2 reduction. The work presented herein provides information with which to compare and contrast biological nitrogen fixation as catalyzed by the Mo- and V-nitrogenases from Azotobacter vinelandii.

CFD is today a big part of the design process in hydraulic engineering and is more economical and time efficient than traditional scale models. But, there are still issues concerning the agreement with scale models in large and complex geometries. In this degree project a high head, five channeled, slotted flip bucket spillway system is analyzed with the CFD software FLUENT and compared with existing scale model results. The sought hydraulic parameters in each channel were the discharge capacity, the pressure distribution and the throw distance from the flip buckets. The discharge capacity and pressure distribution was practically equal for all five channels and only the throw distance from Channel 1 deviated from the others. The agreement with data from the scale model is quite low. The biggest error sources behind the bad agreement may depend on the lack of computational power which led to bad choice of cell size, model delimitations and simplifications. CFD models can easily be built up by people without experience in hydraulics which can lead to fatal errors when building up the model and interpreting results. Hence, long experience in CFD or verification of the numerical results with several different hydraulic parameters is the only way to guarantee qualitative results from CFD modeling.

Abstract In ladle metallurgy, gas stirring and the behaviour of the slag layer are very important for alloying and the homogenization of the steel. When gas is injected through a nozzle located at the bottom of the ladle into the metal bath, the gas jet exiting the nozzle breaks up into gas bubbles. The rising bubbles break the slag layer and create an open-eye. The size of the open-eye is very important as the efficiency of the metal-slag reactions depend on the interaction between the slag and steel created during the stirring process, and information about the position and size of the open-eye is important for effective alloying practice. Moreover, the open-eye has an effect on the energy balance since it increases heat losses. In this study, experimental measurements and numerical simulations were performed to study the effect of different operating parameters on the formation of the open-eye and mixing time in a water model and industrial ladle. Experimental measurements were performed to study the effect of the gas flow rate, slag layer thickness, slag layer densities and number of porous plugs in a 1/5 scale water model and in a 150-ton steelmaking ladle. For numerical modelling, a multi-phase volume of fluid (VOF) model was used to simulate the system including the behaviour of the slag layer. The numerical simulation of the open-eye size and mixing time was found to be in good agreement with the experimental data obtained from the water model and data obtained from the industrial measurements
Tiivistelmä Senkkametallurgiassa kaasuhuuhtelu ja kuonakerroksen käyttäytyminen ovat tärkeitä teräksen seostamisen ja homogenisoinnin näkökulmasta. Senkan pohjalla sijaitsevasta suuttimesta puhallettava kaasu hajoaa kupliksi, jotka rikkovat kuonakerroksen ja muodostavat avoimen silmäkkeen. Avoimen silmäkkeen koko on yhteydessä voimakkaampaan kuonan emulgoitumiseen, joka tehostaa metallisulan ja kuonan välisiä reaktioita. Tietoa avoimen silmäkkeen paikasta ja koosta tarvitaan myös tehokkaaseen seostuspraktiikkaan. Avoin silmäke vaikuttaa lisäksi prosessin energiataseeseen lisäten sen lämpöhäviöitä. Tässä tutkimuksessa tutkittiin kokeellisesti ja laskennallisesti erilaisten operointiparametrien vaikutusta avoimen silmäkkeen muodostumiseen vesimallissa ja terässenkassateollisessa senkassa. Kokeellisia mittauksia tehtiin kaasuhuuhtelun, kuonakerroksen paksuuden, ja suuttimien määrän vaikutuksen tutkimiseksi 1/5-mittakaavan vesimallissa ja 150 tonnin terässenkassa. Numeerisessa mallinnuksessa systeemin ja siihen lukeutuvan kuonakerroksen käyttäytymisen simuloimiseen käytettiin volume of fluid (VOF) –monifaasimenetelmää. Avoimen silmäkkeen kokoon ja sekoittumisaikaan liittyvien numeeristen simulointien havaittiin vastaavan hyvin vesimallista ja teollisista mittauksista saatua kokeellista aineistoa

At the Torpshammar dam two rectangular beams are situated upstream of the spillway gates to stabilize the sidewalls holding the embankment of the dam. A computational fluid dynamics (CFD) simulation of the dam with the bottom outlets open was made to investigate how the flow and discharge capacity is affected by the beams. The results can be used to avoid unexpected consequences due to turbulence caused by the beams, make the beams strong enough to hold the pressure from the flow and get an estimation of the discharge capacity with the beams. Turbulence is one of the hardest things to simulate so the results were compared with previous simulation work made without the beams and physical model tests to validate the results. Also, a sensitivity analysis was made to investigate the method used. The beams lowered the velocity (to 17 m/s) and the discharge capacity (to 255 m3/s) compared to the previous work. The force on the beams was directed upward and downstream. The beams increased the turbulence and the vortex shedding frequency was higher for the beam closest to the outlet. The velocity and discharge capacity differed with 6 % compared to model test results. The results can therefore only be used as an estimation, a more detailed computational model and more computational cells are needed to get a better result. The sensitivity analysis showed that the velocity and turbulence depend on the method and further studies need to be made to decide which method gives the closest similarity with reality.

2008-2009
The mechanics of wave breaking in shallow water has been a major research field for many years, and a very large number of published results are available. No attempt is made here to review the whole literature. Some interesting – if somewhat outdated - descriptions of waves breaking on beaches are presented by Peregrine (1983), Battjes (1988) or Liberatore-Petti (1992). In fact, the most important process in the near coast zone of the shoreline motion is wave breaking. Some waves break in shallow water, some of them break at the water’s edge and in other circumstances waves do not break at all (with steep beach slopes, incident waves with low steepness - or long waves). In general, breaking in deep water is rarer than breaking in shallow water. The latter is triggered by the bottom and is more predictable, although the simple question ‘where breaking starts’ is far from having a unique answer, even in controlled physical experiments. The breaker types are, generally, classified as spilling (where the water spills down the front face), plunging (with a jet emanating from the front crest), surging (characterized by a rise in water surface before the breaking) and collapsing (between plunging and surging). The fluid dynamics of non-breaking waves can be described using potential theory in most of the flow field except near the bottom and near the free surface, where vorticity develops and is confined to a boundary layer. As long as the details near the free surface (e.g. necessary for wind–wave interaction) and/or near the bottom (e.g. necessary for sediment transport analysis) are not of interest, the potential theory approach is sufficient. After breaking, ‘waves’ and ‘eddies’, essentially a potential component and a rotational component of the flow field, are intimately mixed. The surf and swash zones are characterized by the complete transformation of the organized motion of the incident, sea-swell, waves into motions of different types and scales, including small-scale (less than a wave period) turbulence, and large-scale (much greater than the wave period) mean flows [Battjes, 1988]. It is obvious that [Stive and Wind, 1982; Lin and Liu, 1998a; Svendsen et al., 2000; Svendsen, 2005] contributions from terms which have traditionally been neglected in the traditional assumptions of hydrostatic pressure, depth uniform velocity profile, and negligible turbulence, are important and must be taken into full account in surf zone hydrodynamics. Non Linear Shallow Water equations (‘800) and Boussinesq models [Peregrine, 1967] have intrinsic limitations and can only simulate wave breaking and its evolution by assuming on semi-empirical ad hoc assumptions and threshold values to represent wave dissipation. Moreover, these models lack the capability to determine spatial distribution of the turbulent kinetic energy, which is of great importance for sediment transport studies [Lin and Liu, 1998b]. Given all this, it was only natural that the Navier-Stokes solvers now widely tested and developed in other fields of fluid mechanics, with less restricted assumptions involved, no wave theory assumed beforehand, and the capability to simulate complex turbulent processes, should soon become one of the main approaches to describe nearshore processes. Numerical modeling of three-dimensional breaking waves is extremely difficult. Several challenging tasks must be overcome. First of all, one must be able to track accurately the free surface location during the wave breaking process so that the near surface dynamics is captured. Secondly, one must properly model the physics of turbulence production, transport and dissipation throughout the entire wave breaking process. Thirdly, one needs to overcome the huge demand in computational resources. There have been some successful two-dimensional results. For instance, more recent is the treatment of the free surface within such an Eulerian framework with the marker and cell (MAC) method [e.g., Johnson et al.1994] and the volume of fluid method (VOF) [e.g., Ng and Kot 1992, Lin and Liu, 1998a]. The most common approach for simulating breaking waves is presently the application of 2D-Reynolds Averaged Navier-Stokes (RANS) equations with a Volume of Fluid (VOF) surface computation and a turbulence closure model. Such an approach, while being often tested for many years by many various Authors (see for instance Bovolin et al, 2004) only reached full maturity with a fundamental paper by Lin and Liu (1998a). This line of research has been going on successfully for many years to the point that reliable procedures now exist to simulate wave breaking, run up and interaction with structures. The next obvious step. i.e. the application of Large Eddy Simulation (LES) models has so far not been equally successful [Watanabe and Saeki, 1999; Christensen and Deigaard, 2001; Lubin et al, 2006; Christensen, 2006]. LES models necessarily require a fully three-dimensional solution and three-dimensional turbulence effects might be indeed important in the prediction of velocity within the surf zone, especially in the case of plunging breaker [Watanabe and Saeki, 1999]. Such models certainly are a promising tool in the study of surf zone hydrodynamics; however, the LES approach requires much finer grid resolution and a lager computational domain than the RANS approach, resulting in the very high demand on computational resource, at least for the time being. They are however a definite perspective for the future. Smoothed Particle Hydrodynamics (SPH) method, adapted from astrophysics into a number of fields, is a relatively new method for examining the propagation of highly nonlinear and breaking waves [Monaghan et al, 1977; Dalrymple et al, 2005; Viccione et al, 2007-2008]. SPH offers a variety of advantages for fluid modeling, particularly those with a free surface. The Lagrangian method is meshfree; the equivalents of mesh points are the fluid particles moving with the flow. The free surface requires no special approaches, such as the volume-of-fluid method or a Lagrangian surface tracking. Furthermore, the method can treat rotational flows with vorticity and turbulence. SPH is a technique based on computing the trajectories of particles of fluid, which interact according to the Navier–Stokes equations. Each of such particles carries scalar information, density, pressure, velocity components, etc. The work presented here is therefore mainly based on the application of the Eulerian 2-dimensional RANS/VOF equations to the study of surf zone processes on a beach. In particular the work is aimed at demonstrating the capability of RANS/VOF to improve the current modeling of surf zone hydrodynamics on sloping natural beach and in front of shallow water coastal structures , comparing its performance with laboratory observations and other theoretical and numerical results. [edited by author]
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Refrigerant charge minimization is one of the most important targets for heating and air conditioning applications when using natural refrigerants like hydrocarbons or ammonia to cope with the new environmental challenges. Some applications of minichannels for charge minimization in heat pumps are presented and discussed in this thesis. The design of an innovative condenser, an evaporator and an internal heat exchanger is presented. These devices are shell-and-tube heat exchangers using 2 mm i.d. minichannels and realized for the use with propane. Computational procedures based on empirical correlations available in the literature and a simplified model of the heat transfer and pressure drop processes have been developed and used for the design. Experimental performance data of the heat exchangers when using R22 and propane is reported and compared against the predictions given by the computational procedures. The shell-and-tube minichannel heat exchangers have been installed in a 100 kW heat pump using propane as the refrigerant. The unit has been designed for laboratory tests and the minichannel shell-and-tube heat exchangers have been installed in the facility together with a conventional plate condenser and an evaporator. Different configurations have been tested in order to quantify the advantages of operating the heat pump using the low charge heat exchangers, with regard to both energy performance and propane charge. In particular, the experimental performance when using the minichannel condenser is compared to the one obtained when using the plate condenser, and the influence of the internal heat exchanger on the performance of the equipment is measured and discussed. Experimental data about the efficiency with propane of the semihermetic compressor installed in the heat pump is also reported. Besides empirical correlations to predict the global thermal performance, a more complete understanding of the two-phase flow and heat transfer in minichannels is needed for the design and optimization of heat exchangers. Some CFD simulations are presented in this thesis using the innovative Volume Of Fluid (VOF) method, which is able to directly compute multiphase flows without using any empirical closure law to model the interaction between the phases. In order to assess the capability of the method to compute the motion of the gas-liquid interface, which is crucial for two-phase flow and heat transfer, simulations of the adiabatic churn flow regime of air-water mixture at different pipe diameters and liquid and gas superficial velocities have been initially performed. A comparison of the numerical results with experimental visualizations is reported and a simplified theoretical model of the wave levitation process has been developed and used to explain the numerical results. The VOF simulations were then extended to the study of condensation of R134a inside a minichannel with 1 mm internal diameter. Computational results about the evolution of the vapour-liquid interface and the heat transfer coefficient along the channel are reported.
La riduzione della carica di refrigerante nelle applicazioni di condizionamento e riscaldamento è uno dei vincoli di progetto principali quando vengono utilizzati, per motivi di carattere ambientale, refrigeranti naturali come idrocarburi ed ammoniaca. Alcune applicazioni dei minicanali per la minimizzazione della carica nelle pompe di calore vengono presentate e discusse nella presente tesi. Viene presentato il progetto di un condensatore, un evaporatore ed uno scambiatore di calore rigenerativo innovativi. Questi componenti sono degli scambiatori di calore a fascio tubiero utilizzanti minicali del diametro di 2 mm e progettati per l’uso con propano. Delle procedure di calcolo basate su di correlazioni disponibili in letteratura ed un modello semplificato del processo di scambio termico sono state utilizzate per il progetto. Le prestazioni sperimentali degli scambiatori con R22 e propano vengono riportate e confrontate con le stime fornite dalle procedure di calcolo. Gli scambiatori di calore sono stati installati in una pompa di calore della capacità termica di 100 kW utilizzante propano come fluido frigorigeno. Nell’impianto della pompa di calore, destinata a test di laboratorio, sono stati installati anche un condensatore ed un evaporatore a piastre convenzionali. In questo modo è stato possibile confrontare diverse configurazioni al fine di quantificare in via sperimentale i vantaggi apportati dall’utilizzo degli scambiatori a minicanali, in termini sia di prestazioni energetiche, sia di carica di propano richiesta. In particolare, le prestazioni delle configurazioni utilizzanti il condensatore a minicanali vengono confrontate con quelle delle configurazioni utilizzanti lo scambiatore a piastre, e l’influenza sulle prestazioni energetiche dello scambiatore rigenerativo viene misurata e discussa. Vengono inoltre riportati dati sperimentali relativi all’efficienza con propano del compressore semiermetico installato nella pompa di calore. Oltre a correlazioni empiriche in grado di stimare le prestazioni termiche globali, il progetto e l’ottimizzazione di scambiatori di calore richiede una più approfondita conoscenza del deflusso e dello scambio termico all’interno di minicanali. Vengono presentate in questa tesi delle simulazioni di termofluidodinamica computazionale tramite l’innovativo metodo VOF (Volume Of Fluid) in grado di simulare direttamente deflussi multifase senza la necessità di utilizzare correlazioni empiriche per la modellazione dell’interazione tra le fasi. Al fine di validare l’efficacia di questo metodo nel calcolare il moto dell’interfaccia gas-liquido, il quale è un aspetto cruciale nello scambio termico bifase, sono state in un primo momento eseguite delle simulazioni del regime di deflusso ”churn flow” per una miscela aria-acqua nel caso di un tubo liscio verticale adiabatico, a differenti valori di diametro del tubo e di velocità superficiale delle due fasi. I risultati sono stati confrontati con visualizzazioni sperimentali ed un modello teorico semplificato del processo di levitazione delle onde è stato sviluppato ed utilizzato per commentare i risultati numerici. Le simulazioni con il metodo VOF sono state in un secondo momento estese allo studio della condensazione di R134a all’interno di un minicanale del diametro di 1 mm. Vengono riportati risultati computazionali relativi all’evoluzione dell’interfaccia vapore-liquido e dei coefficienti di scambio termico lungo il minicanale.

Cette thèse porte sur la modélisation et l’analyse de la propagation électromagnétique dans un milieu forestier dans les bandes VHF et UHF. L’objectif principal est le développement d’un modèle numérique "full-wave" tridimensionnel de diffusion par la forêt permettant de caractériser l’interaction d’une onde électromagnétique avec un milieu forestier. Un tel modèle s’avère, actuellement, un outil indispensable à l’analyse des mesures radar pour l’étude des paramètres caractéristiques de la forêt tels que la biomasse forestière, la hauteur des arbres et leur densité. La complexité numérique de ce modèle a limité son domaine d’application à de petites parcelles de forêt et aux basses fréquences. Pour pouvoir traiter de larges zones forestières tout en montant en fréquence, et s’approcher ainsi des besoins et exigences des utilisateurs potentiels de notre modèle, nous avons intégré à ce modèle une méthode numérique efficace dédiée à l’analyse de larges problèmes électromagnétiques. La méthode en question,connue sous le nom de Characteristic Basis Function Method (CBFM) était récemment développée dans le laboratoire de Communication et Electromagnétisme de l’université PennState dirigé par le Professeur Mittra. Après une optimisation et une adaptation au problème d’intérêt, la CBFM réalise d’excellentes performances et nous permet une diminution considérable du temps de calcul et des besoins en espace mémoire sans pour autant dégrader la qualité des résultats obtenus ou altérer la fidélité du modèle à la réalité du problème électromagnétique traité
A 3-D full-wave model, based on the integral representation of the electric field and dedicatedto the analysis of bi-static scattering mechanisms by a forest in the VHF and UHF bands wasefficiently enhanced. In order to overcome the limitation of a previous 3D model to small simulationsscenes and low frequencies, we have developed, during this research work, a new model using basis functionsadapted to the problem of interest, in the context of the Characteristic Basis Function Method(CBFM) and we investigated the suitability of this direct method for computing the electromagneticfields inside and outside three-dimensional dielectric objects representing the tree trunks and branches.The CBFM has shown great performances, when applied to the forest scattering modeling, both interms of CPU time and memory resources needed. Once properly set, the CBFM-E is so efficient thatit is able to treat in few minutes electromagnetic problems totally intractable with the classical MoM.Consequently, we have developed a powerful 3D forest electromagnetic scattering tool which allows ustoday to compute large forest electromagnetic problems in few minutes without worrying about theaccuracy of the solution. On the other hand, we have demonstrated the efficiency and accuracy of theCBFM-E when applied to 3D dielectric objects in the context of the electric volumetric integral equation,and have consolidated thus its leading position in the computational electromagnetics, especiallyagainst the iterative solvers based numerical methods

The scope of the present thesis is the development of a Computational Fluid Dynamics model to describe the multiphase flow inside a structured packing absorber for postcombustion carbon capture. The work focuses mainly on two flow characteristics: the interface tracking and the reactive mass transfer between the gas and the liquid. The interface tracking brings the possibility of studying the liquid maldistribution phenomenon, which strongly affects the mass transfer performance. The development of a user-defined function to account for the reactive mass transfer between phases constitutes the second major concept considered in this thesis. Numerical models found in the literature are divided into three scales due to the current computational capacity: small-, meso- and large-scale. Small-scale has usually dealt with interface tracking in 2D computational domains. Meso-scale has usually been considered to assess the dry pressure drop performance of the packing (considering only the gas phase). Large-scale studies the liquid distribution over the whole column assuming that the structured packing behaves as a porous medium. This thesis focuses on small- and meso-scale. The novelty of this work lies in expanding the capabilities of the aforementioned scales. At small-scale, the interfacial tracking is implemented in a 3D domain, instead of 2D. The user-defined function that describes the reactive mass transfer of CO2 into the aqueous MEA solution is also included to assess the influence of the liquid maldistribution on the mass transfer performance. At the meso-scale, the Volume of Fluid method for interface tracking is included (instead of only the gas phase) to describe flow characteristics such as the liquid hold-up, the interfacial area and the mass transfer. At the theoretical level, this model presents the particularity of including both a mass and a momentum source term in the conservation equations. A comprehensive mathematical development shows the influence of the mass source terms on the momentum equation.

Defrosting in the refrigeration industry is used to remove the frost layer accumulated on the evaporators after a period of running time. It is one way to improve the energy efficiency of refrigeration systems. There are many studies about the defrosting process but none of them use computational fluid dynamics (CFD) simulation. The purpose of this thesis is (1) to develop a defrost model using the commercial CFD solver FLUENT to simulate numerically the melting of frost coupled with the heat and mass transfer taking place during defrosting, and (2) to investigate the thermal response of the evaporator and the defrost time for different hot gas temperatures and frost densities. A 3D geometry of a finned tube evaporator is developed and meshed using Gambit 2.4.6, while numerical computations were conducted using FLUENT 12.1. The solidification and melting model is used to simulate the melting of frost and the Volume of Fluid (VOF) model is used to render the surface between the frost and melted frost during defrosting. A user-defined-function in C programming language was written to model the frost evaporation and sublimation taking place on the free surface between frost and air. The model was run under different hot gas temperatures and frost densities and the results were analyzed to show the effects of these parameters on defrosting time, input energy and stored energy in the metal mass of the evaporator. The analyses demonstrate that an optimal hot gas temperature can be identified so that the defrosting process takes place at the shortest possible melting time and with the lowest possible input energy.

During intense rain events a stormwater system can fill rapidly and undergo a transition from open channel flow to pressurized flow. This transition can create large discrete pockets of trapped air in the system. These pockets are pressurized in the horizontal reaches of the system and then are released through vertical vents. In extreme cases, the transition and release of air pockets can create a geyser feature. The current models are inadequate for simulating mixed flows with complicated air-water interactions, such as geysers. Additionally, the simulation of air escaping in the vertical dropshaft is greatly simplified, or completely ignored, in the existing models. In this work a two-phase numerical model solving the Navier-Stokes equations is developed to investigate the key factors that form geysers. A projection method is used to solve the Navier-Stokes Equation. An advanced two-phase flow model, Volume of Fluid (VOF), is implemented in the Navier-Stokes solver to capture and advance the interface. This model has been validated with standard two-phase flow test problems that involve significant interface topology changes, air entrainment and violent free surface motion. The results demonstrate the capability of handling complicated two-phase interactions. The numerical results are compared with experimental data and theoretical solutions. The comparisons consistently show satisfactory performance of the model. The model is applied to a real stormwater system and accurately simulates the pressurization process in a horizontal channel. The two-phase model is applied to simulate air pockets rising and release motion in a vertical riser. The numerical model demonstrates the dominant factors that contribute to geyser formation, including air pocket size, pressurization of main pipe and surcharged state in the vertical riser. It captures the key dynamics of two-phase flow in the vertical riser, consistent with experimental results, suggesting that the code has an excellent potential of extending its use to practical applications.

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Este trabalho tem como objetivo otimizar o custo operacional dependente dos parâmetros de vazão de ar do condensador, número de aletas e de circuitos do condensador, frequência do compressor e parâmetros do dispositivo de expansão de um sistema de um condicionador de ar do tipo split hiwall unitário com vazão variável de refrigerante (VRV). Para a otimização foram consideradas as freqüências de ocorrência de quatro condições de temperatura do ambiente externo na cidade de Porto Alegre. O condicionador de ar abordado neste trabalho troca calor utilizando um ciclo de compressão de vapor de simples estágio, tendo como fluido de trabalho o refrigerante R-410A. O sistema completo, composto de um compressor, um condensador, um evaporador e tubo capilar, foi modelado analiticamente utilizando o software EES (Engineering Equation Solver) e ajustado de acordo com testes realizados em calorímetro. Para a otimização, um algoritmo genético foi programado no próprio EES. Após a otimização do custo operacional em função dos parâmetros físicos de projeto, o COP aumentou em até 12 % na condição de ar externo a 35 °C. A otimização em função dos parâmetros de controle mostrou aumento ainda mais significativo do COP do sistema. Também é feita uma comparação entre os valores de COP de um sistema de vazão constante de ar e refrigerante e um sistema com vazão variável de ar e refrigerante.
This work aims to optimize the operational cost of an air conditioning system with variable refrigerant flow (VRF). The cost is dependent of the following parameters: condenser air flow, number of condenser fins and circuits, compressor frequency and the expansion device. The optimization considered the frequencies of occurrence of four distinct ambient temperatures in the city of Porto Alegre. The air conditioner presented in this work exchange heat using a single stage vapor compression cycle, with the refrigerant R-410A as the working fluid. The complete system comprises a compressor, a condenser, an evaporator and a capillary tube and was analytically modeled using the EES (Engineering Equation Solver) software. The parameters were fitted according to results obtained in a calorimeter and for optimization, a genetic algorithm was programmed using the same software. After the optimization of the operational cost as a function of the project parameters, the COP was raised up to 12% in external ambient air at 35 °C. The optimization of the operational cost as a function of the control parameters showed even a more significant system COP raise. A comparison between the COP of a constant air and refrigerant flow system and a variable air and refrigerant flow system was also performed.

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Fundação de Amparo a Pesquisa do Estado de São Paulo
We study the behavior of the Earth-ionosphere waveguide through the modeling of the propagation of very low frequency radio waves (VLF). We use the computational model LWPC (Long Wave Propagation Capability) to estimate changes in amplitude and phase of the VLF signals detected by the SAVNET network (South America VLF NETwork), and thus try to understand the behavior of the lower ionosphere under different ionization conditions. The research was divided into two parts. The first part investigates the behavior of the VLF signals in quiescent regimes of ionization. Amplitude and phase simulations for the were carried out, modifying adapting polynomials for the β and h parameters (or Wait s parameters) as a function of the zenithal angle. The second part of this research, uses these polynomials in the study of the lower ionosphere under transient ionization regimes in two distinct conditions: first during of solar flares and second during solar eclipse. For the simulations under solar flare conditions, we calculate the changes in β and ℎ′ parameters during the 25/03/2008 solar explosion. With these values, we calculate the electronic density profile through an exponential model and we find that the electronic density at 75 km is ∼ 104 cm−3, that is twenty times higher than during quiescent conditions. To evaluate our parameter estimates, we calculate the variation of the Wait s parameters for the case of twelve solar events of different classes. We note that the variations Δℎ′ found in this work are larger than that in Muraoka, Murata e Sato (1977) because they consider the variations in the conductivity gradient. For the solar eclipse simulations on 11/07/2011, we investigate its effect on the VLF phase. For this, we use the obscuration coefficient to estimate the guide height variation along the whole path during the eclipse. The simulations reproduce the phase behavior during the eclipse. However, a delay of about twenty four minutes was observed between the simulated and observed measurements. The observed delay is a direct consequence of own estimates of the perturbed ionospheric height and it causal relation with the obscuration during the eclipse. lower ionosphere, VLF, modeling, ionospheric disturbances, solar flares, solar eclipse.
Neste trabalho realizamos o estudo do comportamento do guia de ondas terra-ionosfera através da modelagem da propagação ondas de rádio de frequência muito baixa (VLF). Para isto, utilizamos o modelo computacional LWPC (Long Wave Propagation Capability) para estimar as variações de amplitude e fase de sinais de VLF detectados nos trajetos da rede SAVNET (South America VLF NETwork) e assim compreender o comportamento da baixa ionosfera em diferentes regimes de ionização. A pesquisa foi dividida em duas partes. A primeira parte, investigou o comportamento do sinal VLF em regimes quiescente de ionização, assim realizou-se simulações de amplitude e fase adaptando polinômios que definem os parâmetros β e ℎ′ (ou parâmetros de Wait) em função do ângulo zenital solar. Na segunda parte desta pesquisa, aplicou-se os polinômios no estudo da baixa ionosfera sob regimes transientes de ionização em duas condições distintas. A primeira para o caso de explosões solares e a segunda um para eclipse solar. Nas simulações relativas a explosões solares, calculamos as variações dos parâmetros β e ℎ′ durante o evento do dia 25/03/2008. Com esses valores, calculamos o perfil de densidade eletrônica, através de um modelo exponencial e observamos que a densidade eletrônica em 75 km é ∼ 104 cm−3, ou seja, vinte vezes maiores que antes da explosão. Para avaliar nossas estimativas, calculamos a variação dos parâmetros de Wait para doze eventos de diferentes classes. Observamos que as variações Δℎ′ neste trabalho são sempre maiores do que as descritas em Muraoka, Murata e Sato (1977), devido elas considerarem as variações no gradiente de condutividade. Nas simulações relativa ao eclipse solar do dia 11/07/2011, investigamos seu efeito na fase observada. Para esse estudo, utilizou-se o coeficiente de obscurecimento para realizar as simulações, desta forma foi possível estimar a variação da altura do guia ao longo de todo o trajeto durante o eclipse. As simulações reproduziram o comportamento da fase durante o eclipse. Entretanto, foi observado um atraso entre as medidas calculadas e observadas de aproximadamente ∼ vinte e quatro minutos. O atraso observado é diretamente decorrente da estimativa da altura de referência da ionosfera pertubada e de sua relação causal com o obscurecimento durante o eclipse.

Les télécommunications très basses fréquences (Very Low Frequency/Low Frequency) sont utilisées pour les communications militaires à couverture mondiale avec des sous-marins en plongée. La simulation de ces antennes nécessite l'utilisation de méthodes numériques bien souvent en limite de leur domaine de validité. L'objectif de la thèse est de développer un outil de modélisation globale des structures antennaires VLF/LF [3-300 kHz] prenant en compte l'environnement proche tels que les isolateurs, les structures de soutien, le plan de sol, les bâtiments annexes, ... Une première partie est consacrée à la caractérisation des isolateurs. Le comportement de ces dispositifs soumis à de très hautes tensions est en effet peu connu dans la bande de fréquences VLF/LF. Une seconde partie est consacrée à l'élaboration et au développement d'un modèle de "Fil Mince" arbitrairement orienté intégrant des composants localisés. Trois méthodes sont présentées puis validées par comparaison à des résultats théoriques. Enfin l'étude de l'influence de la structure de soutien sur le fonctionnement des antennes VLF/LF est présentée.

Le travail présenté dans ce manuscrit a pour objet l’étude et le développement d’un banc de caractérisation généraliste appliqué à l’extraction du modèle comportemental d’amplificateur TPM-NIM (Two-Path Memory Nonlinear Integral). Ce modèle qui dispose d’une des architectures les plus abouties au laboratoire XLIM requiert une instrumentation microonde haut de gamme, très onéreuse, hors de portée de la majorité des concepteurs pour sa mise en œuvre expérimentale. L’objectif est donc de proposer des principes de mesure originaux permettant d’identifier le modèle TPM-NIM avec une instrumentation standard. Dans ces travaux, deux bancs sont présentés : tout d’abord, un banc de caractérisation développé autour d’une instrumentation de pointe disposant des meilleures propriétés pour extraire le modèle. Puis, un banc construit autour d’une instrumentation standard mais incluant des méthodes de traitement et de mesure novatrices. Ces deux bancs ont été utilisés avec plusieurs véhicules de tests et il ressort que le second permet de diminuer le bruit des mesures de phase tout en réduisant le coût total des équipements. Enfin, une dernière partie est consacrée à la comparaison du modèle TPM-NIM avec deux modèles comportementaux classiques mettant en avant sa polyvalence
The work presented in this manuscript is devoted to the study and development of a general characterization bench applied to the extraction of the TPM-NIM (Two-Path Memory Nonlinear Integral) amplifier behavioral model. This model, has one of the most advanced architectures at the XLIM laboratory. It requires a high-end microwave instrumentation, overpriced and beyond reach for most of the designers for its experimental implementation. The aim is to propose some original measurements principles allowing the TPM-NIM model’s identification with a standard instrumentation. Two benches are presented in these works : firstly, a characterization bench, developed using a high performance instrumentation with the best properties to extract the model. Then, a bench, built with a standard instrumentation but through innovative processing and measurement methods. These two benches have been used with several test vehicles and it appears that the second one decreases the noise of phase measurements while reducing the equipment’s total cost. Finally, a last part is dedicated to the comparison of the TPM-NIM model with two classic behavioral models by emphasizing its versatility

The transportation of gas-liquid mixtures in horizontal pipes as two-phase stratified flow is examined using computational fluid dynamics (CFD) method. The design of these pipelines requires accurate prediction of flow parameters such as pressure drop and liquid holdup. Many empirical correlations have been developed in the last 70 years and are well documented in the literature to obtain these parameters using experimental, analytical and numerical methods. In this investigation, the numerical method based on CFD code-FLUENT is used as an alternative to the experimental method to obtained numerical data such as gas wall shear stress, liquid holdup and pressure drop in order to calculate interfacial shear stress using semi-mechanistic flow model for stratified-smooth and stratified-wavy flow based on the 3D CFD models developed in FLUENT DesignModeler. The Volume of Fluid (VOF) model and k-ω SST turbulence model were used to obtain numerical data from the CFD models for validations. In the 3D CFD model for gas flow over stationary liquid surface, the average gas velocities and corresponding liquid heights from the experimental data were validated in the two-phase flow domain. The interfacial friction factor correlation proposed was in good agreement against the existing two-phase friction factors using conventional two-phase flow calculation method, while the mathematical formulations involving hydrostatic force for the interfacial and gas wall shear stresses were poorly correlated against existing correlations. In the co-current gas-liquid flow 3D CFD model, the pressure drop, gas wall shear stress, interfacial shear stress and liquid holdups were in excellent agreement and the interfacial friction factor correlations proposed were in good agreement with the published correlations. The flow patterns were correctly predicted as stratified-smooth and wavy flow on the flow map. A design procedure involving both 3D CFD models was proposed and presented in Appendix D.

Les méthodes de lubrification par injection d'air sont considérées par la communauté scientifique comme la principale percée technologique à venir pour la réduction de la trainée des navires de commerce. A cette fin, il est impératif de modéliser finement les phénomènes physiques en jeu dans la lubrification par injection d'air, qui intègrent la représentation précise de la tension superficielle, des instabilités d'interface et des écoulements avec entraînement d'air. Au cours de ce travail, nous nous sommes attachés au développement d'outils de programmation, de schémas de reconstruction d'interface et de modélisations de tension superficielle dans le code de calcul ISIS-CFD. Deux nouvelles méthodes de calcul de la tension superficielle sont présentées. Elles utilisent un schéma de reconstruction globale de l'interface et sont couplées avec les schémas de discrétisation compressifs de la fraction volumique utilisés dans la formulation volumes finis non structurés sur laquelle est basé le code ISIS-CFD. Les résultats démontrent que des interactions dynamiques complexes, d'une ou plusieurs interfaces, peuvent être modélisées de façon précise. Ce travail permet d'envisager la réalisation ultérieure de calcul d'écoulements sur des navires lubrifiés par injection d'air, d’améliorer la compréhension physique et de contribuer à la modélisation macroscopique de modèles d'entraînement
Air Lubrication methods are regarded by the scientific community as the next major technological breakthrough in Naval Engineering to achieve the reduction of drag in commercial vessels. The accurate modeling of the physical phenomena governing the drag reduction mechanisms of Air Lubrication methods, namely, the dynamics of surface tension, the instabilities of the air-water interfaces and air entrainment, are imperative for the design of air-lubricated hulls. To that end, we have implemented to ISIS-CFD several programming tools, interface reconstruction schemes and surface tension modeling. Two new surface tension methods were developed. Both use a global interface reconstruction scheme and are coupled with the compressive discretization volume fraction schemes for the unstructured finite volume formulation that the flow solver ISIS-CFD is based on. The results demonstrate that complicated dynamic interactions of either a single or multiple interfaces can be accurately captured. In the context of a future research study, the proposed approaches could lead to the further enhancement of the modeling capabilities of ISISCFD by introducing a macroscopic air entrainment model and eventually the assessment of different physical effects encountered in lubricated naval vessels using ISIS-CFD

Work in this dissertation is motivated by reducing the design cost at the early design stage while maintaining high design accuracy throughout all design stages. It presents four key design methods to improve the performance of Efficient Global Optimization for multidisciplinary problems. First, a fidelity-calibration method is developed and applied to lower-fidelity samples. Function values analyzed by lower fidelity analysis methods are updated to have equivalent accuracy to that of the highest fidelity samples, and these calibrated data sets are used to construct a variable-fidelity Kriging model. For the design of experiment (DOE), a dynamic sampling method is developed and includes filtering and infilling data based on mathematical criteria on the model accuracy. In the sample infilling process, multi-objective optimization for exploitation and exploration of design space is carried out. To indicate the fidelity of function analysis for additional samples in the variable-fidelity Kriging model, a dynamic fidelity indicator with the overlapping coefficient is proposed. For the multidisciplinary design problems, where multiple physics are tightly coupled with different coupling strengths, multi-response Kriging model is introduced and utilizes the method of iterative Maximum Likelihood Estimation (iMLE). Through the iMLE process, a large number of hyper-parameters in multi-response Kriging can be calculated with great accuracy and improved numerical stability. The optimization methods developed in the study are validated with analytic functions and showed considerable performance improvement. Consequentially, three practical design optimization problems of NACA0012 airfoil, Multi-element NLR 7301 airfoil, and all-moving-wingtip control surface of tailless aircraft are performed, respectively. The results are compared with those of existing methods, and it is concluded that these methods guarantee the equivalent design accuracy at computational cost reduced significantly.
Doctor of Philosophy
In recent years, as the cost of aircraft design is growing rapidly, and aviation industry is interested in saving time and cost for the design, an accurate design result during the early design stages is particularly important to reduce overall life cycle cost. The purpose of the work to reducing the design cost at the early design stage with design accuracy as high as that of the detailed design. The method of an efficient global optimization (EGO) with variable-fidelity analysis and multidisciplinary design is proposed. Using the variable-fidelity analysis for the function evaluation, high fidelity function evaluations can be replaced by low-fidelity analyses of equivalent accuracy, which leads to considerable cost reduction. As the aircraft system has sub-disciplines coupled by multiple physics, including aerodynamics, structures, and thermodynamics, the accuracy of an individual discipline affects that of all others, and thus the design accuracy during in the early design states. Four distinctive design methods are developed and implemented into the standard Efficient Global Optimization (EGO) framework: 1) the variable-fidelity analysis based on error approximation and calibration of low-fidelity samples, 2) dynamic sampling criteria for both filtering and infilling samples, 3) a dynamic fidelity indicator (DFI) for the selection of analysis fidelity for infilled samples, and 4) Multi-response Kriging model with an iterative Maximum Likelihood estimation (iMLE). The methods are validated with analytic functions, and the improvement in cost efficiency through the overall design process is observed, while maintaining the design accuracy, by a comparison with existing design methods. For the practical applications, the methods are applied to the design optimization of airfoil and complete aircraft configuration, respectively. The design results are compared with those by existing methods, and it is found the method results design results of accuracies equivalent to or higher than high-fidelity analysis-alone design at cost reduced by orders of magnitude.

Within the military there is great need for reliable communication between vehicles. During the planning and construction of a military RIB, Rigid-hulled Inflatable Boat, it is important to design an efficient antenna system that does not deteriorate out of disorder. It must also be ensured that the antennas transmitted power does not stay in the RIB boat. We have been given assignment to model and simulate a proposed antenna system and assess which tool is best suitable for the task. To analyze the antenna system we will look at the radiated electrical field together with the reflectionand EMC properties. The tool that we choose to use is a software called EMPro produced by Keysight Technologies. In this program we will create 3Dstructures for each individual object, the boat, the three antennas and the seawater. It’s also important to include each objects properties concerning material, so that they correctly reflect the reality. We are covering a broad spectrum with our antennas reaching from 1.6-30MHz, 30-88MHz and 100512MHz. The resulting simulation verifies that electromagnetic field would be powerful enough and that the antennas would not affect each other with the proposed placement. We could also confirm that our antennas reflected an inordinate amount of power but with cause that our models were not an exact replica of the antenna. The software EMPro is a suitable tool for this kind of projects concerning modeling and simulating antenna systems.Within the military there is great need for reliable communication between vehicles. During the planning and construction of a military RIB, Rigid-hulled Inflatable Boat, it is important to design an efficient antenna system that does not deteriorate out of disorder. It must also be ensured that the antennas transmitted power does not stay in the RIB boat. We have been given assignment to model and simulate a proposed antenna system and assess which tool is best suitable for the task. To analyze the antenna system we will look at the radiated electrical field together with the reflectionand EMC properties. The tool that we choose to use is a software called EMPro produced by Keysight Technologies. In this program we will create 3Dstructures for each individual object, the boat, the three antennas and the seawater. It’s also important to include each objects properties concerning material, so that they correctly reflect the reality. We are covering a broad spectrum with our antennas reaching from 1.6-30MHz, 30-88MHz and 100512MHz. The resulting simulation verifies that electromagnetic field would be powerful enough and that the antennas would not affect each other with the proposed placement. We could also confirm that our antennas reflected an inordinate amount of power but with cause that our models were not an exact replica of the antenna. The software EMPro is a suitable tool for this kind of projects concerning modeling and simulating antenna systems.
Inom militären finns stort behov av pålitlig kommunikation mellan fordon. Vid konstruktion av ett småfartyg i militärtoch bevakningssyfte är det viktigt att designa ett välfungerande antennsystem som inte försämras utav störningar, det ska även ses till att antennernas utsända effekt inte fastnar i småfartyget. Vi har fått en ritning av hur antennplaceringen är planerad, denna rapport går ut på att verifiera dess funktionalitet samt hitta en mjukvara som kan användas för att verifiera olika antennsystem. För att bedöma antennsystemet kommer denna rapport att undersöka att dess elektriska fält samt reflektionsoch EMC egenskaper, en uppgift som kan lösas med programvaran EMPro (Keysight). I programvaran skapas en 3Dstruktur som innehåller småfartyget, dess 3 stycken antenner samt omfattande havsvatten. Här tas hänsyn till objektens materialegenskaper, antennernas jordning samt de frekvenser antennerna arbetar på, 1.6-30MHz, 30-88MHz samt 100-512MHz. Simulering av systemet gav positiva resultat kring antennsystemets elektromagnetiskafält, antennerna kommer inte heller att störa varandra. Antennerna som vi har modellerat reflekterar orimligt mycket effekt, detta bortser vi från då vi inte haft tillgång till exakt avbildning av antennerna. EMPro är ett verktyg som är lämpligt att använda i detta samt liknande projekt. Dock krävs det att en kraftig dator finns tillgänglig då simuleringar av stora antennsystem baseras på stora uträkningar, som generellt tar lång tid.

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, turbulence, velocity seen by the bubbles, momentum and mass exchange term over the cells or bubble expansion, among others. But also new implementations as an algorithm to seed the bubbles in the system have been incorporated. As a result, this new solver gives more accurate results as the provided up to date. Following the decrease on resolution level, and therefore the required computational resources, a 3D TFM have been developed with a population balance equation solved with an implementation of the Quadrature Method Of Moments (QMOM). The solver is implemented with the same closure models as the CFD-DEM to analyze the effects involved with the lost of information due to the averaging of the instantaneous Navier-Stokes equation. The analysis of the results with CFD-DEM reveals the discrepancies found by considering averaged values and homogeneous flow in the models of the classical TFM formulation. Finally, for the lowest resolution level approach, the system code RELAP5/MOD3 is used for modelling the bubbly flow regime. The code has been modified to reproduce properly the two-phase flow characteristics in vertical pipes, comparing the performance of the calculation of the drag term based on drift-velocity and drag coefficient approaches.
El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación de más de unas pocas decenas de burbujas. Como alternativa, se propone el uso de técnicas CFD-DEM (Discrete Element Methods) en la que se representa a las burbujas como partículas discretas. En esta tesis se ha desarrollado un nuevo solver para bubbly flow en el que se han añadido un gran número de nuevos modelos, como los necesarios para contemplar los choques entre burbujas o con las paredes, la turbulencia, la velocidad vista por las burbujas, la distribución del intercambio de momento y masas con el fluido en las diferentes celdas por cada una de las burbujas o la expansión de la fase gaseosa entre otros. Pero también se han tenido que incluir nuevos algoritmos como el necesario para inyectar de forma adecuada la fase gaseosa en el sistema. Este nuevo solver ofrece resultados con un nivel de resolución superior a los desarrollados hasta la fecha. Siguiendo con la reducción del nivel de resolución, y por tanto los recursos computacionales necesarios, se efectúa el desarrollo de un solver tridimensional de TFM en el que se ha implementado el método QMOM (Quadrature Method Of Moments) para resolver la ecuación de balance poblacional. El solver se desarrolla con los mismos modelos de cierre que el CFD-DEM para analizar los efectos relacionados con la pérdida de información debido al promediado de las ecuaciones instantáneas de Navier-Stokes. El análisis de resultados de CFD-DEM permite determinar las discrepancias encontradas por considerar los valores promediados y el flujo homogéneo de los modelos clásicos de TFM. Por último, como aproximación de nivel de resolución más bajo, se investiga el uso uso de códigos de sistema, utilizando el código RELAP5/MOD3 para analizar el modelado del flujo en condiciones de bubbly flow. El código es modificado para reproducir correctamente el flujo bifásico en tuberías verticales, comparando el comportamiento de aproximaciones para el cálculo del término d
L'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa en aqueix cas es proposa l'ús de tècniques CFD-DEM (Discrete Element Methods) en la qual es representa a les bambolles com a partícules discretes. En aquesta tesi s'ha desenvolupat un nou solver per a bubbly flow en el qual s'han afegit un gran nombre de nous models, com els necessaris per a contemplar els xocs entre bambolles o amb les parets, la turbulència, la velocitat vista per les bambolles, la distribució de l'intercanvi de moment i masses amb el fluid en les diferents cel·les per cadascuna de les bambolles o els models d'expansió de la fase gasosa entre uns altres. Però també s'ha hagut d'incloure nous algoritmes com el necessari per a injectar de forma adequada la fase gasosa en el sistema. Aquest nou solver ofereix resultats amb un nivell de resolució superior als desenvolupat fins la data. Seguint amb la reducció del nivell de resolució, i per tant els recursos computacionals necessaris, s'efectua el desenvolupament d'un solver tridimensional de TFM en el qual s'ha implementat el mètode QMOM (Quadrature Method Of Moments) per a resoldre l'equació de balanç poblacional. El solver es desenvolupa amb els mateixos models de tancament que el CFD-DEM per a analitzar els efectes relacionats amb la pèrdua d'informació a causa del promitjat de les equacions instantànies de Navier-Stokes. L'anàlisi de resultats de CFD-DEM permet determinar les discrepàncies ocasionades per considerar els valors promitjats i el flux homogeni dels models clàssics de TFM. Finalment, com a aproximació de nivell de resolució més baix, s'analitza l'ús de codis de sistema, utilitzant el codi RELAP5/MOD3 per a analitzar el modelatge del fluxos en règim de bubbly flow. El codi és modificat per a reproduir correctament les característiques del flux bifàsic en canonades verticals, comparant el comportament d'aproximacions per al càlcul del terme de drag basades en velocitat de drift flux model i de les basades en coe
Peña Monferrer, C. (2017). Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90493
TESIS

Les systèmes de 3G offrent de nouveaux services support (bearer services) à plus hauts débits pour les modes de transmission "paquet". Ces services vont coexister avec la voix (ou d'autres services temps réels), des scénarios de trafic mixte, voix et données, doivent être envisagés. La norme UMTS permet effectivement aux utilisateurs d'avoir plus d'un service activé simultanément. Les différentes classes de trafic augmentent la complexité de la gestion des ressources radios. Dans ce contexte, deux types de fonctions sont étudiés: l'allocation de TFCI et l'ordonnancement de paquets. Leur impact sur la qualité de service (QoS) ainsi que sur la capacité du système est évalué. On propose des améliorations de ces mécanismes dans le but d'augmenter la capacité du système et par conséquent d'améliorer la QoS des utilisateurs. Les études se restreignent au sens montant, c'est à dire aux transmissions du mobile (User Equipment ou UE) vers le réseau. Un premier mécanisme pour lequel un effort d'amélioration est fait, est l'adaptation du lien radio par variation du débit instantané transmis. On simule le cas d'une transmission multiservice (voix et données). L'UE doit partager un débit global qui lui est alloué entre les différents services activés. Ces derniers sont véhiculés dans des radio bearers (tuyaux supports). À chaque intervalle élémentaire de transmission (Transmission Time Interval, TTI), l'UE sélectionne un sous-débit pour chaque bearer; ceci se fait par la sélection d'un "format de transport" à appliquer pendant la durée TTI. Cette procédure s'effectue dans la couche MAC (Medium Access Control); le résultat de la sélection est une combinaison de formats de transport (Transport Format Combination, TFC) que la couche physique doit utiliser. La procédure, nommée sélection de TFC, permet d'adapter la transmission des différents services aux conditions variables de la propagation radio: elle détermine notablement la performance de transmission. L'algorithme de sélection de TFC est tracé dans ses grandes lignes dans la norme. Un de ses principes est de favoriser le trafic temps réel au détriment des services de données par paquet. Cependant, le trafic temps réel peut être perturbé par le trafic de données sous certaines conditions, en particulier pour les mobiles éloignés de la station de base (Node B). On propose un algorithme de sélection de TFC qui limite ces perturbations et qui offre une plus large zone de couverture aux services temps réels. En plus, il améliore la QoS du service de données et le débit effectif de l'UE sans augmenter sa puissance de transmission. Un autre type d'études concerne l'ordonnancement de paquets entre les différents utilisateurs ou UEs. C'est une procédure qui est contrôlée par la partie fixe du réseau. Nous l'étudions principalement par simulation en considérant divers mécanismes ou variations. Un premier mécanisme est nommé fast Variable Spreading Factor (fast VSF): les UEs distants changent rapidement leur facteur d'étalement (SF) afin de conserver une puissance de transmission constante, ce qui vise à stabiliser l'interférence inter-cellulaire. Un deuxième mécanisme étudié est un accès paquet décentralisé (decentralized mode) utilisant une information en retour sur le niveau global d'interférence dans la cellule. Un troisième mécanisme nommé "fast scheduling" (ordonnancement rapide) raccourcit le cycle d'ordonnancement. Les résultats ont montré que dans le cas de faible ou moyenne charge dans la cellule, le mode décentralisé réduit le délai par paquet jusqu'à 25 %. L'ordonnancement rapide augmente la capacité du système jusqu' à 10%. En plus, il améliore la QoS perçue par les utilisateurs en terme de débit par utilisateur et délai par paquet transmis.

Στόχος της παρούσας διπλωματικής εργασίας είναι η ηλεκτρομαγνητική μοντελοποίηση υπό βρoχόπτωση στις VHF και UHF περιοχές συχνοτήτων. Προκειμένου να εξαχθούν συμπεράσματα για την ποιότητα των παρεχόμενων υπηρεσιών (κυτταρική και δορυφορική τηλεφωνία, μετάδοση τηλεοπτικών σημάτων) προσδιορίζονται η ισχύς λήψης και μία σειρά από παραμέτρους.
The aim of this master thesis is the RF modeling during rainfall for the VHF and UHF ranges. In order to infer conclusions for the quality of the supplied services (cellular and satellite telephony, transmission of television signals) the received power and a series of parameters are defined.

碩士
中原大學
化學工程研究所
99
In this study, the plasma diagnostics tools (including actinometrics OES and QMS) were used to investigate the influences of operating parameters on intrinsic layer deposition process of μc-Si:H thin film solar cell in parallel-plate PECVD system with an excitation frequency of 40.68 MHz (VHF). The diagnostics results were correlated to deposition rate and crystalline fraction of deposited silicon films. A combination of experimental diagnostics and computational modeling was utilized to understand the main reaction mechanism in the plasma. The plasma diagnostic results show that when plasma power is increased, both the silane dissociation efficiency and deposition rate are increased. Meanwhile, it also benefits to film’s crystallinity. Higher silane flow rate leads to a slightly decreased in silane dissociation efficiency, however there are much more dissociated radicals contributed to film growth. When operating pressure is increased, both the silane dissociation efficiency and deposition rate are decreased. Since higher pressure corresponds to higher species collision frequency, the H atoms get higher probability to etch silicon surface, resulting in better film’s crystallinity. By comparing OES and QMS results, quantitative analysis of the main species in the plasma can be achieved. On the basis of theoretical deposition model, we have proposed a flux ratio of H atom to SiHx radical as a new indicator of crystallinity during film growth of μc-Si:H. Finally, a global model of SiH4/H2 plasma is developed. Both gas phase plasma chemistry and silicon film deposition process are included in the model. The model is found to capture the trend in experiment results on the effect of operating parameters on film deposition rate and active species concentrations in the plasma.

Trabalho de Projeto do Mestrado Integrado em Engenharia Biomédica apresentado à Faculdade de Ciências e Tecnologia
Nos últimos 20 anos, os polímeros electroactivos (EAPs) têm recebido mais atenção no mundo dos mecanismos de actuação. O termo "electroactivo" significa que o polímero é electricamente activo ou reactivo, ou seja, que terá uma resposta mecânica a um estímulo eléctrico. Como qualquer polímero, este é conhecido por ser leve e fácil de processar. Para além disso, os EAPs são resistentes e capazes de produzir grandes tensões de actuação, quando comparados com actuadores convencionais. Portanto, a sua importância está a aumentar em várias aplicações, nomeadamente biomédicas, tais como instrumentos médicos, implantes biónicos, próteses e "músculos artificiais". Os EAPs dividem-se em iónicos e electrónicos, sendo a principal diferença o transporte físico: os EAPs iónicos utilizam cargas iónicas, enquanto os EAPs electrónicos utilizam cargas eléctricas. O P(VDF-TrFE-CTFE) é um polímero "ferrorelaxor" pertencente aos EAPs electrónicos e é considerado um material promissor pela sua elevada constante dieléctrica (~50), bem como pela elevada resposta electromecânica e densidade de energia elástica.Neste trabalho, três actuadores (cinco camadas) foram caracterizados electromecanicamente para estudar a influência de dois parâmetros geométricos: a espessura da camada do P(VDF-TrFE-CTFE), e a largura total do actuador. A defleção, a força de bloqueio e a rigidez à flexão de cada actuador foram obtidas através de testes mecânicos sob três valores de campo eléctrico. Subsequentemente, foi executada uma análise por MEF em COMSOL Multiphysics®. Realizou-se um estudo estacionário de cada actuador para avaliar o comportamento electromecânico sob três valores de campo eléctrico. A defleção em cada simulação foi comparada com os testes experimentais.A partir dos resultados dos testes, concluiu-se que a largura não influencia o desempenho do actuador. Além disso, um aumento da espessura do P(VDF-TrFE-CTFE) provocou um aumento da defleção, força de bloqueio e rigidez de flexão. Mais ainda, a simulação foi validada, pois o comportamento observado revelou-se semelhante nos testes experimentais, ou seja, a defleção aumentava linearmente com o aumento do campo eléctrico aplicado.Finalmente, comparando os três actuadores com outras tecnologias electroactivas, é possível afirmar que as defleções observadas são mais elevadas, mas a força de bloqueio exercida por estes actuadores é significativamente inferior a outras tecnologias de actuação já estabelecidas.
In the last 20 years, electroactive polymers (EAPs) have gained attention in the world of actuation mechanisms. "Electroactive" means that the polymer is electrically active or responsive, i.e. that it will have a mechanical response to an electric stimulus. As any polymer, they are known for being lightweight and easy to process. Adding to this, EAPs are resilient and able to produce large actuation strains, when compared to conventional actuators. Therefore, their importance is increasing in several applications, namely biomedical applications such as surgery-assisting tools, bionic implants, prosthetics and "artificial muscles". EAPs are divided into ionic and electronic, with the major different being the physic transport: ionic EAPs use ionic charges, while electronic EAPs use electric charges. P(VDF-TrFE-CTFE) is a ferrorelaxor polymer that belongs to electronic EAPs and it is considered one of the most promising materials for its high dielectric constant (~50), as well as its high electromechanical response and elastic energy density.In this work, three cantilever-like multilayered actuators (five layers) were electromechanically characterized to study the influence of two geometry parameters: the thickness of the active layer, P(VDF-TrFE-CTFE), and the total width of the actuator. The tip displacement, blocking force and bending stiffness of each actuator were obtained through mechanical testing under three values of electric field. Subsequently, a FEM analysis was executed on COMSOL Multiphysics®. A stationary study was performed for each actuator, to evaluate the electromechanical behavior under the three values of electric field. The tip displacement in each simulation was also obtained and compared with the experimental tests.From the tests' results, it was concluded that the width of the actuator does not play a major influence in the performance of the actuator. Moreover, an increase in the thickness of the active layer translated in an increase of the tip displacement, blocking force and bending stiffness. On the other hand, the simulation was validated as the tip displacement increased linearly with the electric field, which was a behavior already observed in the experimental tests.Finally, comparing these three actuators with other electroactive technologies, it is possible to state the displacements observed are higher, but the blocking force exerted by these actuators is significantly lower than other existing actuation technologies.

The Very Low Frequency Electromagnetic (VLF-EM) method, which enables surveying without contact with the ground, is suitable for ground surveys in a wide area and has been used in mapping geology for decades. The technique makes use of signal radiation from military navigation radio transmitters operating in the frequency range of 15-30 kHz. When the electromagnetic wave impinges on the surface it is both reflected back into the air and refracted into the earth. By measuring the shifted reflected magnetic field relative to the primary field, subsurface structures can be constrained. Although the VLF method has been widely used to map geology in the last several decades, few modeling studies have been published. Particularly the effect of topography on VLF measurements is poorly characterized. The objective of my research is to study and simulate the VLF topographic responses, and therefore to distinguish between such responses and actual subsurface resistivity anomalies in VLF-EM data. A few basic models (homogenous half-space, horizontal contact and vertical contact) were first created using the finite-elements modeling software Comsol Multiphysics and verified with theoretical solutions. Subsequently, features such as hills and lakes were incorporated into these basic models and further analyzed. When modeling topographic effects, two relationships between max inphase / max slope and max Quad / max slope versus skin depth / hill width are found, which can be used to predict topographic effects when the slope of a hill and resistivity of the ground are known. Two different sets of field data acquired at Saskatoon’s Diefenbaker Hill and at Cameco’s Cree Extension are compared with the modeling results.

碩士
國立交通大學
電信研究所
83
In this report, field strength measurements of TV-Band (UHF/VHF) propagation in the middle region of Taiwan have been conducted. The measurement procedure follows FCC's regulations. Meanwhile, a theoretical model, based on geometric optics and knife-edge diffraction theory has been developed. By comparing the measured field strength with that precdicted by FCC's, CCIR's or the theorectical model, it shows that FCC and CCIR have better predicting accuracy in the VHF band and the theoretical model is more accurate in the UHF band.

碩士
國立海洋大學
導航與通訊系碩士班
91
Automatic Identification System (AIS), which integrates the mobile communication and satellite positioning technologies, has great potential in contributing to navigation safety and maritime security. All vessels subject to the Safety of Life at Sea Convention（SOLAS）are required to be fitted with AIS within a very tight schedule. It is therefore of urgency needs of AIS researches to support the planning and implementations of shore-based AIS facilities and services. However, as compared with the situation on land, field measurements with sea trials are really costly, subject to weather/sea conditions and vessel motions, even restricted in route by the concern about navigation safety. Available database of the coastal land area such as the digital terrain model is also rather incomplete due to either the remoteness or confidential factors. This study focuses on making the most use of limited sea trial measurements and environmental database for the planning of AIS shore-based facilities. Matlab numerical methods and the Mapinfo geographical information system (GIS) are used for the simulation and analysis of AIS signal propagation in the maritime VHF band around 162MHz. Lee Model, which takes field measurement and terrain height data as important parameters, is adopted for the simulation. Research area mainly covers coastal waters of northern Taiwan and the Kaohsiung Harbor, where sea trials are performed by using an AIS base station and a class A AIS mobile station. Comparison between field measurements and simulations by linear regression analysis shows that the difference in path loss slope is about 1.3dB/dec, corresponding to a coverage range difference about 3.7km in the case of northern coastal waters. While the difference in path loss slope is only about 0.35dB/dec, corresponding to a coverage range difference about 0.2km in the case of Kaohsiung Harbor. Considering the fact that measurements were taken at a much worse sea state for the case of northern Taiwan than the case of Kaohsiung Harbor, the simulation fits well with the measurement, especially when in calm sea. Consequently, the mention presented should be applicable to large-scale predictions along the coast of Taiwan with reasonable results.

This thesis is concerned with the construction and application of MONASH-VN (hereafter MVN), a large-scale dynamic computable general equilibrium (CGE) model of Vietnam. The model is designed to provide the capacity for not only detailed analysis of economic policies but also forecasts and analyses of economic history. After constructing the model, I use it to conduct an analysis of the structural changes in the Vietnamese economy over the period 1996-2003. This was a period of rapid growth of around seven percent per annum, despite adverse shocks such as the Asian economic crisis in the late 1990s. MVN’s theoretical structure is based on that of the MONASH model of the Australian economy (Dixon and Rimmer 2002). It has a detailed disaggregation of industries and factors of production, a detailed representation of technologies and preferences, taxes and margins, and a detailed representation of government accounts, balance of payment accounts, and net foreign liabilities. The dynamic features include the inter-temporal links between flow and stock variables for physical capital accumulation, for debt and savings, and a wage-employment adjustment mechanism. The MVN model captures the structure and characteristics of the Vietnamese economy via the calibration of the model using the Vietnamese input-output tables, the adoption of appropriate substitution elasticities and the representation of features which are pertinent to the Vietnamese economy, such as particular types of taxes and transfers, and types of transactions with the rest of the world specific to Vietnam. In particular, it includes many equations which allow the use of statistical data reported by Vietnamese statistical agency at different aggregation and reliability levels. I carry out two major simulations with MVN to analyse in detail the structural changes in the Vietnamese economy over the period 1996-2003. The first is the historical simulation, in which observed changes in economic variables (such as changes in GDP, industries outputs, labour and capital stocks, household consumption, imports and exports) are imposed on the model, and changes in structural variables (such as technologies and preferences) are calculated endogenously. The second simulation is the decomposition simulation, in which the contributions of those structural changes to the economic performance of the economy over the period under study are evaluated. Simulation results show that there have been significant structural changes in the Vietnamese economy over the period. The most important change is technical improvement in using primary factors and other inputs in most sectors. Overall, technical improvement accounted for 34 percent of the growth in real GDP over the period. The second important structural change is the favourable shifts in foreign preferences for Vietnamese exports and the improvement of the terms of trade, which contributed about 23 percent to the growth in real GDP. Employment growth also contributed strongly to the growth in real GDP (17 percent). Other sources of growth in real GDP include changes in household preferences toward manufactured goods and services and away from primary products; the increase in land area for primary industries, the decrease in barriers to international trade; the shift in preferences of all users toward domestically-produced goods and away from imported goods; the increase in the marginal propensity to save; and the increase in the investment-capital ratios. All of those factors help explain the strong performance of the Vietnamese economy during the period under study. This type of analysis provides much richer insights into structural changes in the economy than the standard growth accounting framework. The results of the simulations can be used to evaluate the effectiveness of economic policies or to inform forecasts into the future.

Στις σύγχρονες τηλεπικοινωνίες, χρησιμοποιείται μια μεγάλη γκάμα από RF μοντέλα, για τον υπολογισμό την μέσης τιμής ισχύος του σήματος εκπομπής για δεδομένη απόσταση από τον πομπό και για δεδομένη συχνότητα. Στις διεργασίες αυτές , επιδρούν στο δικό τους βαθμό διάφορες παράμετροι τοπολογικού (πληθυσμός , τύπος εμποδίων, πυκνότητα εμποδίων) και γεωγραφικού (μορφολογία εδάφους, υγρασία, διαμόρφωση χώρου) χαρακτήρα, αλλά και χαρακτηριστικά των πομποδεκτών ( συνήθως το ύψος και το κέρδος των κεραιών). Στις περισσότερες περιπτώσεις οι απώλειες όδευσης (Path Loss όπως είναι γνωστές στη διεθνή βιβλιογραφία) , εκφράζονται σε dB , σα συνάρτηση της συχνότητας λειτουργίας του υπό μελέτη συστήματος και της απόστασης ανάμεσα στον πομπό και το δέκτη ( πάντα για δεδομένα χαρακτηριστικά κεραιών και τύπο περιβάλλοντος). Έτσι μια ντετερμινιστική γνώση του μέσου path loss, που σε συνδυασμό με άλλες πιθανές απώλειες μας δίνει το σύνολο των απωλειών είναι εφικτή. O περιορισμός όσον αφορά στη συχνότητα και στις αποστάσεις , έχουν οδηγήσει τη σύγχρονη έρευνα στην επέκταση των υπαρχόντων μοντέλων , τόσο εξωτερικού όσο και εσωτερικού χώρου. Μια βασική παράμετρος της έρευνας στηρίζεται στην παραδοχή πως ο νόμος του αντιστρόφου τετραγώνου , ο οποίος περιγράφεται από την εξίσωση του Friis δε βρίσκει εφαρμογή, παρά μόνο σε περιβάλλοντα όπου επιτυγχάνεται LOS( Line of Sight) όδευση. Η τροποποίηση της παραπάνω εξίσωσης με δυναμικό ως προς το εκάστοτε περιβάλλον τρόπο , επιτρέπει πλέον τον υπολογισμό της μέσης ισχύος του σήματος σε σχετικά ρεαλιστικό βαθμό. Για παράδειγμα έχει προταθεί τροποποίηση με την τρίτη δύναμη της απόστασης για ένα σύνολο εφαρμογών , που αφορούν συστήματα ασυρμάτων τηλεπικοινωνιών εξωτερικού περιβάλλοντος.Ένα εσωτερικό περιβάλλον απαιτεί μια πολύ πιο ντετερμινιστική φόρμουλα υπολογισμού των απωλειών ώστε να μπορέσει ο μελετητής να υπολογίσει με αξιοπιστία την ισχύ του σήματος σε μια δεδομένη θέση. Η ακρίβεια λοιπόν των μοντέλων εξαρτάται άμεσα από την ικανότητά τους να απεικονίσουν και να αποδώσουν με τη σειρά τους , μέσω των υπολογισμών, όλα αυτά τα σύνθετα φαινόμενα με τον καλύτερο δυνατό τρόπο. Η πλειοψηφία των RF μοντέλων που αναπτύσσονται στα πλαίσια ακόλουθων παραγράφων, έχει αναπτυχθεί και αξιολογηθεί για συστήματα κυτταρικής κινητής τηλεφωνίας (GSM, PCS-1800, GPRS, UMTS). Προκειμένου να ξεπεραστούν διάφοροι περιορισμοί των αρχικών μοντέλων ως προς την συχνότητα λειτουργίας του συστήματος και την απόσταση πομπού-δέκτη (ουσιαστικά την εμβέλεια του μοντέλου), ορισμένες προεκτάσεις έχουν παρουσιαστεί σε διάφορες ερευνητικές εργασίες και χρησιμοποιούνται ευρέως . Ορισμένα νέα μοντέλα έχουν επίσης προστεθεί στα ήδη υπάρχοντα για τις συχνότητες συστημάτων κινητής τηλεφωνίας . Για άλλες περιοχές συχνοτήτων με έντονο ενδιαφέρον, πχ στα 5,2 GHz, διάφορες προτάσεις έχουν πραγματοποιηθεί, συχνά υποστηριζόμενες από μετρήσεις. Λαμβάνοντας υπόψη την συχνότητα των 2,4 GHz, δεν μπορούμε παρά να συμπεράνουμε ότι η όσο το δυνατόν πιο ακριβής και επιστημονικά αξιόπιστη μοντελοποίηση του ασύρματου διαύλου, ιδιαίτερα – άλλα όχι αποκλειστικά – για τοπολογίες εσωτερικών χώρων, είναι πολύ σημαντική για τον σχεδιασμό και την εύρυθμη λειτουργία των Wi-Fi συστημάτων και των WLAN δικτύων. Παρόλα αυτά, αν και στην υπάρχουσα βιβλιογραφία και στην διεθνή επιστημονική και ερευνητική κοινότητα υπάρχουν αρκετά σημαντικές εργασίες για την μοντελοποίηση του ασύρματου διαύλου στα 2,4 GHz , εντούτοις απουσίαζε, για πολύ καιρό, μία ολοκληρωμένη συγκριτική αντιπαραβολή και αξιολόγηση των βασικών θεωρητικών μοντέλων για τον υπολογισμό της μέσης απώλειας οδεύσεως στη συγκεκριμένη συχνότητα. Στα πλαίσια της εργασίας αυτής , θα προσπαθήσουμε να αξιοποιήσουμε το σύνολο των μετρήσεων και των δεδομένων που συλλέξαμε , σε ρεαλιστικές συνθήκες και για πραγματικά συστήματα μετάδοσης , τόσο να αξιολογήσουμε τα ήδη υπάρχοντα RF μοντέλα , όσο και να προβούμε στις απαραίτητες προτάσεις και τροποποιήσεις όπου αυτό κριθεί σκόπιμο. Επίσης για τη σκίαση επιχειρούμε εδώ μια προσέγγιση υπολογισμού μέσω της χρήσης των RF μοντέλων και την ενσωμάτωση όλων των χαρακτηριστικών διάδοσης , που αφορούν και στα δύο στοιχεία των μεγάλης κλίμακας διαλείψεων. Οι μετρήσεις μέσης λαμβανόμενης ισχύος πραγματοποιήθηκαν σε πέντε διαφορετικές τοπολογίες. Σε κάθε μία από αυτές τις τοπολογίες ελήφθησαν μετρήσεις της μέσης λαμβανόμενης ισχύος σε διάφορες δειγματοληπτικές αποστάσεις πομπού-δέκτη ούτως ώστε να είναι εφικτή, με έναν υψηλό βαθμό αξιοπιστίας, η αξιολόγηση των βασικών μοντέλων απωλειών οδεύσεως. Ταυτόχρονα, μέσα από αυτή την διαδικασία, αξιολογήθηκε η αξιοπιστία των μοντέλων αναφορικά με τις ιδιαιτερότητες της κάθε τοπολογίας, που αποτυπώνονται ποιοτικά και ποσοτικά στους μηχανισμούς διάδοσης του ραδιοσήματος σε κάθε περίπτωση.
In modern wireless communications, a wide range of RF models are used to provide the median (average) value of the signal strength at a given distance from the transmitter and for a given frequency spectrum. In this procedure, certain geographical (ground, humidity, terrain irregularities), topological (heavy or scattered population, type of obstacles, density of the buildings) characteristics of the area, as well as certain specifications of the transmitter and receiver antennas (most notably antenna height and antenna gain) have to be taken into consideration. In most cases, the mean value of the path loss is expressed in dB in dependence with the frequency of the operating system and the distance between the transmitter and the receiver (for given antenna characteristics and a certain type of environment where the system operates). Thus, a deterministic knowledge of the average path loss (which along with the average rain loss and diffraction loss provides the overall propagation loss in dB) is available. However, distance and frequency limitations have led research to a further study and expanding of the existing empirical and semi-empirical models , for both outdoor and indoor scenarios. A fundamental parameter-based study of the path loss models is based on the concept that the second power law that is predicted by the Friis equation does not apply in real-life scenarios except for standard LOS paths. The modified power law research allows engineers and scholars to calculate the mean received power of a signal transmitted over a wireless link in a more realistic approach. It has been suggested that the third-power law is more suitable for a plethora of applications based on wireless communications for an outdoor environment.The indoor propagation channel, in particular, demands a lot more than a deterministic formula of calculating the average signal strength as a function of distance and frequency. The increased impact of multipath and other propagation phenomena such as reflection and scattering, as well as the existence of many objects whose proportions are comparatively close to the wavelength of the operating wireless systems, render the propagation of a signal and its arrival at a receiver (mobile or fixed) a rather complex procedure. The precision of the path loss models depends heavily on their ability to demonstrate and reflect, in the calculations, these complex phenomena to the best possible way .The majority of these models have been developed and validated for mobile telephony systems in both outdoor and indoor schemes (GSM, PCS-1800, GPRS, UMTS). Certain extensions of many of these models were conducted in order to expand the frequency and distance limitation of the original formulas. New empirical models have also been suggested for these certain frequencies .Taking into account the very sensitive and subject to many different factors nature of the indoor propagation channel, it is easily concluded that both researchers in academia and engineers in industry need to have reliable models that will predict precisely the average path loss over the indoor 2.4 GHz channel which is of utter importance as the de facto frequency of Wi-Fi and WLAN systems. A gap in aforementioned research, however, is that it either concentrates on multipath parameters or does not feature a full comparative validation of most well-known indoor path loss models.The purpose of this work is to present, all the data collected through experiments in realistic conditions and real telecom systems, in order to validate and modify (where necessary) the existing RF models. Furthermore an empirical formula to measure attenuation due to shadowing is derived through these RF models.Measurements took part in five different topologies. In each of them the mean received power was recorded, for various distances between the transmitter and the receiver, in order for our endeavor to validate the RF models in question to be reliable. Through this procedure, RF models where examined towards each topology’s distinctive characteristics that reflect in quality and quantity all the attenuation mechanisms that affect the propagated signal.