The atomic physics relevant to gaseous nebulae is critically examined using modelling software with particular emphasis on radio recombination lines (RRLs). The theoretical spectral line intensities can be deduced if we know the population structure of the bound electrons in the gas under non-thermal equilibrium conditions. The population structure of hydrogen is solved for various environments using a capture-collision-cascade model that incorporates an ambient radiation eld. The validity of assuming Case B (Baker & Menzel, 1938) for nebulae is investigated. It is known that Case B is appropriate for levels with small principal quantum numbers (n < 40), but this assumption is re-examined for high levels which are relevant to RRLs. E ects of an ambient radiation eld on the population structure is examined and processes that are stimulated by a radiation eld are included in the model. This is done as a preliminary investigation to extend the model to a photoionization code. / Physics / (M. Sc. (Astronomy)

Thesis (D.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban Institute of Technology, 2005 xxi, 265 leaves / The aim of this study is to develop, using neural networks, a model to aid the performance monitoring of operational diesel engines in industrial settings. Feed-forward and modular neural network-based models are created for the prediction of the specific fuel consumption on any normally aspirated direct injection four-stroke diesel engine. The predictive capability of each model is compared to that of a published quadratic method. Since engine performance maps are difficult and time consuming to develop, there is a general scarcity of these maps, thereby limiting the effectiveness of any engine monitoring program that aims to manage the fuel consumption of an operational engine. Current methods applied for engine consumption prediction are either too complex or fail to account for specific engine characteristics that could make engine fuel consumption monitoring simple and general in application. This study addresses these issues by providing a neural network-based predictive model that requires two measured operational parameters: the engine speed and torque, and five known engine parameters. The five parameters are: rated power, rated and minimum specific fuel consumption bore and stroke. The neural networks are trained using the performance maps of eight commercially available diesel engines, with one entire map being held out of sample for assessment of model generalisation performance and application validation. The model inputs are defined using the domain expertise approach to neural network input specification. This approach requires a thorough review of the operational and design parameters affecting engine fuel consumption performance and the development of specific parameters that both scale and normalize engine performance for comparative purposes. Network architecture and learning rate parameters are optimized using a genetic algorithm-based global search method together with a locally adaptive learning algorithm for weight optimization. Network training errors are statistically verified and the neural network test responses are validation tested using both white and black box validation principles. The validation tests are constructed to enable assessment of the confidence that can be associated with the model for its intended purpose. Comparison of the modular network with the feed-forward network indicates that they learn the underlying function differently, with the modular network displaying improved generalisation on the test data set. Both networks demonstrate improved predictive performance over the published quadratic method. The modular network is the only model accepted as verified and validated for application implementation. The significance of this work is that fuel consumption monitoring can be effectively applied to operational diesel engines using a neural network-based model, the consequence of which is improved long term energy efficiency. Further, a methodology is demonstrated for the development and validation testing of modular neural networks for diesel engine performance prediction.

Thesis (PhD)--Stellenbosch University, 1995. / ENGLISH ABSTRACT: New advanced control techniques for attitude determination and control of small (micro) satellites are presented. The attitude sensors and actuators on small satellites are limited in accuracy and performance due to physical limitations, e.g. volume, mass and power. To enhance the application of sophisticated payloads such as high resolution imagers within these confinements, a multi-mode control approach is proposed, whereby various optimized controller functions are utilized during the orbital life of the satellite. To keep the satellite's imager and antennas earth pointing with the minimum amount of control effort, a passive gravity gradient boom, active magnetic torquers and a magnetometer are used. A "cross-product" detumbling controller and a robust Kalman filter angular rate estimator are presented for the preboom deployment phase. A fuzzy controller and magnetometer full state extended Kalman filter are presented for libration damping and Z-spin rate control during inactive imager periods. During imaging, when high performance is required, additional fine resolution earth horizon, sun and star sensors plus 3-axis reaction wheels are employed. Full state attitude, rate and disturbance estimation is obtained from a horizon/sun extended Kalman filter. A quaternion feedback reaction wheel controller is presented to point or track a reference attitude during imaging. A near-minimum time, eigenaxis rotational reaction wheel controller for large angular maneuvers. Optimal linear quadratic and minimum energy algorithms to do momentum dumping using magnetic torquers, are presented. A new recursive magnetometer calibration method is designed to enhance the magnetic in-flight measurements. Finally, a software structure is proposed for the future onboard implementation of the multi-mode attitude control system. / AFRIKAANSE OPSOMMING: Nuwe gevorderde beheertegnieke vir die oriëntasiebepaling en -beheer van klein (mikro-) satelliete word behandel. Die oriëntasiesensors en -aktueerders op klein satelliete het 'n beperkte akkuraatheid en werkverrigting as gevolg van fisiese volume, massa en kragleweringbeperkings. Om gesofistikeerde loonvragte soos hoë resolusie kameras binne hierdie tekortkominge te kan hanteer, word 'n multimode beheerbenadering voorgestel. Hiermee kan 'n verskeidenheid van optimale beheerfunksies gedurende die wentelleeftyd van die satelliet gebruik word. Om die satellietkamera en -antennas aardwysend te rig met 'n minimale beheerpoging, word 'n passiewe graviteitsgradiëntstang, aktiewe magneetspoele en 'n magnetometer gebruik. 'n "Kruisproduk" onttuimellings beheerder en 'n robuuste hoektempo Kalmanfilter afskatter is ontwikkel vir die periode voordat die graviteitsgradiëntstang ontplooi word. 'n Wasige beheerder en 'n volledige toestand, uitgebreide Kalmanfilter afskatter is ontwikkel om librasiedemping en Z-rotasietempo beheer te doen gedurende tydperke wanneer die kamera onaktief is. Gedurende kamera-opnames word hoë werkverrigting verlang. Fyn resolusie aardhorison, son en stersensors met 3-as reaksiewiele kan dan gebruik word. 'n Volledige oriëntasie, hoektempo en steurdraaimoment Kalmanfilter afskatter wat inligting van bogenoemde sensors gebruik, is ontwikkel. 'n “Quaternion” reaksiewiel terugvoerbeheerder waarmee die satelliet na verwysings oriëntasiehoeke gerig kan word of waarmee oriëntasiehoektempos gevolg kan word, word behandel. 'n Naby minimumtyd, "eigen"-as reaksiewielbeheerder vir groothoek rotasies is ontwikkel. Optimale algoritmes om momentumontlading van reaksiewiele met lineêre kwadratiese en minimumenergie metodes te doen, word afgelei en aangebied. 'n Nuwe rekursiewe kalibrasietegniek waarmee 'n magnetometer outomaties gedurende vlug ingestel kan word, is ontwikkel. Ten slotte, word 'n programstruktuur voorgestel vir aanboord implementering van die nuwe multimode beheerstelsel.

Thesis (PhD (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 1995. / Computer simulation programs were developed for the analysis of dry-cooling systems for power plant applications. Both forced draft direct condensing air"cooled condensers and hyperbolic natural draft indirect dry-cooling towers are considered. The results of a considerable amount of theoretical and experimental work are taken into account to model all the physical phenomena ofthese systems, to formu1ate the problems in formal mathematical terms and to design and apply suitable computational algorithms to solve these problems effectively and reliably. The dry-cooling systems are characterized by equation-based models. These equations are simultaneously solved by a specially designed constrained nonlinear least squares algorithm to determine the performance characteristics of the dry-cooling systems under fixed prescnoed operating conditions, or under varying operating conditions when coupled to a turbo-generator set. The solution procedure is very fast and effective. A capital and operating cost estimation procedure, based on information obtained from dry-cooling system component manufacturers and the literature, is proposed. Analytical functions express the annual cost in terms ofthe various geometrical and operating parameters ofthe dry-cooling systems. The simu1ation and the cost estimation procedures were coupled to a constrained nonlinear programming code which enable the design of minimum cost dry-cooling systems at fixed prescribed operating conditions, or dry-cooling systems which minimize the ratio of total annual cost to the annual net power output of the corresponding turbo-generator set. Since prevailing atmospheric conditions, especially the ambient temperature, influence the performance of dry-cooling systems, wide fluctuations in turbine back pressure occur. Therefore, in the latter case the optimal design is based on the annual mean hourly frequency ofambient temperatures, rather than a fixed value. The equation-based models and the optimization problems are simultaneously solved along an infeasible path (infeasible path integrated approach). The optimization model takes into consideration all the parameters that may affect the capital and operating cost of the dry-cooling systems and does not prescribe any limits, other than those absolutely essential due to practical limitations and to simulate the systems effectively. The influence that changes ofthe constraint limits and some problem parameters have on the optinmm solution, are evaluated (sensitivity analysis). The Sequential Quadratic Programming (SQP) method is used as the basis in implementing nonlinear optimization techniques to solve the cost minimirnti~n problems. A stable dual active set algorithm for convex quadratic programming (QP) problems is implemented that makes use of the special features ofthe QP subproblems associated with the SQP methods. TIrls QP algorithm is also used as part of the algorithm that solves the constrained nonlinear least squares problem This particular implementation of the SQP method proved to be very reliable and efficient when applied to the optimization problems based on the infeasible path integrated approach. However, as the nonlinear optimization problems become large, storage requirements for the Hessian matrix and computational expense of solving large quadratic programming (QP) subproblems become prohibitive. To overcome these difficulties, a reduced Hessian SQP decomposition strategy with coordinate bases was implemented. This method exploits the low dimensionality of the subspace of independent decision variables. The performance of this SQP decomposition is further improved by exploiting the mathematical structure of the engineering model, for example the block diagonal structure ofthe Jacobian matrix. Reductions ofbetween 50-90% in the total CPU time are obtained compared to conventional SQP optimization methods. However, more function and gradient evaluations are used by this decomposition strategy. The computer programs were extensively tested on various optimization problems and provide fast and effective means to determine practical trends in the manufacturing and construction of costoptimal dry-cooling systems, as well as their optimal performance and operating conditions in power plant applications. The dissertation shows that, through the proper application of powerful optimization strategies and careful tailoring of the well constructed optimization model, direct optimization of complex models does not need to be time consuming and difficult. Reconnnendations for further research are made. / Imported from http://etd.sun.ac.za April 2010.

Thesis (PhD)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: As the title suggests, the major challenge that this study faces is to set out and design a model for analysing and enabling the management of political risk as investment risk - a model that is both sensitive to and reflective of the comprehensive business and investment climate in a country, not only credit or country risk, or only pure political risk in its narrowest definition. In reading about past and more recent research in the field of political risk analysis, it becomes clear that many authors begin by noting the diversity and the discrepancies of the existing definitions of political risk, but evidence in political risk insurance shows that the major perceived political risks that investors insure their interests against seem to be confiscation, expropriation and nationalisation. In the light of this study's findings though, a case can be put forward for urging that the conceptualisation of political risk be extended to further include any or all of the micro political risk factors and their indicators that have been identified to ensure that political events do not impact negatively on a foreign company's profitability. Foreign investors put assets at risk to achieve their objectives and the assessment of these risks, including political risks, is the key to successful operations. Opportunities and risks are often two sides of the same coin and political risk comprises a large part of the environmental forces in terms of the management challenges a Multinational Company (MNC) faces in any investment climate. A firm's foreign investment strategy deals with the positioning of the organisation in an uncertain host country environment and investment climate. This study attempts to explain how a firm's political risk exposure, which refers to the sensitivity of a firm's projected profitability and operationability in a host country to changes in the investment climate, could be managed and reduced. It is hoped that political risk analysis and management can assist foreign operations in managing the risks that might have otherwise proven to be destructive to profitability and operationability. It is irresponsible to present a potential investor with a risk assessment that does not incorporate political risk factors and their indicators, let alone environmental, societal and socio-economic risk factor indicators. Ultimately any business climate, regardless of the country being studied, is underwritten by a political system, political climate, political culture and business culture of the system in which foreign business wishes to operate profitably. What is often labelled as unnecessary and irrelevant detail in risk analysis often results in a lack of using micro risk factors and their indicators and an underestimation of the importance of such micro risk indicators. Hopefully this study takes up the challenge of showing that political risk can be managed and political risk analysis can be made more precise - that it is possible to measure and manage political risk. / AFRIKAANSE OPSOMMING: Soos die titel van hierdie studie voorstel is een van die grootste uitdagings die ontwerp van 'n model vir die analise van politieke risiko as beleggingsrisiko - 'n model wat ter selfde tyd sensitief is vir en weerspieëlend van 'n land se algemeen omvattende besigheids- en beleggingsklimaat, en nie slegs suiwer politieke risiko in die nouste sin van die woord nie. 'n Literatuurstudie van meer onlangse navorsing, asook navorsing wat in die verlede gedoen is oor politieke risiko en die analise daarvan, dui daarop dat baie outeurs melding maak van die diversiteit en teenstrydighede in die bestaande definisies van politieke risiko. Die teenwoordigheid van versekering teen politieke risiko wys egter daarop dat die primêre politieke risiko's waarteen beleggers hulle belange verseker meesal nasionalisering en onteiening is, asook die beslaglegging op beleggings. Teen die agtergrond van hierdie studie se bevindinge, kan daar egter 'n saak uitgemaak word vir die verbreeding van die konseptualisering van politieke risiko om enige of alle van die mikro-politieke risiko faktorindikatore wat in hierdie studie identifiseer word in te sluit, om sodoende te verseker dat die negatiewe gevolge wat politieke gebeure moontlik mag inhou vir 'n buitelandse maatskappy se belange, sover moontlik beperk word. Buitelandse beleggers stel bates bloot aan risiko's ten einde voorafgestelde doelwitte te bereik en die assessering van hierdie risiko's, insluitende politieke risiko's, is 'n groot bydraende' faktor tot die suksesvolle bedryf van buitelandse beleggings. Geleenthede en risiko's is dikwels twee kante van diesIefde muntstuk en politieke risiko maak 'n groot deel uit van die uitdagende beleggingsomgewing waarin die bestuur van 'n multinasionale korporasie (MNK) daagliks moet funksioneer. 'n Maatskappy se buitelandse beleggingstrategie handel met die posisionering van die organisasie in die onvoorspelbare beleggingsklimaat van 'n vreemde land. Hierdie studie poog ook om te verduidelik hoe die mate waarin 'n firma blootgestel word aan politieke risiko, met ander woorde die sensitiwiteit van 'n firma se voorgenome winsgewendheid en bedryf teenoor veranderinge in die beleggingsklimaat van 'n vreemde land, bestuur en verminder kan word. Daar word gehoop dat politieke risiko analise en die bestuur daarvan 'n bydra kan lewer tot buitelandse besighede se bestuur van hierdie risiko's, wat andersins 'n vemietgende impak kan hê op die winsgewendheid van buitelandse bedrywighede. Dit is onverantwoordelik om aan 'n buitelandse belegger 'n risiko analise voor te lê wat nie politieke risiko faktore en die daarmee gepaardgaande indikatore insluit nie. Die studie argumenteer verder dat faktorindikatore wat die fisiese omgewing, sosiale asook sosio-ekonomiese faktore aanspreek ook in 'n risiko analise ingesluit moet word. Oplaas is enige besigheidsklimaat, nieteenstaande die land wat bestudeer word, onderskryf deur 'n politieke stelsel, politieke klimaat, politieke kultuur en besigheidskultuur van die stelsel waarin die buitelandse besigheid winsgewende resultate as doelwit het. Wat dikwels beskou word as onnodige en irrelevante detail in risiko analise lei dikwels tot 'n gebrek aan die insluiting van mikro-risiko faktore en hulle indikatore weens 'n onderskatting van die noodsaaklikheid daarvan om juis sulke mikro-risiko faktorindikatore in 'n risiko analise in te bou. Hierdie studie aanvaar hopelik die uitdaging om te wys dat politieke risiko tog bestuur kan word en dat politieke risiko analise tog meer eksak gemaak kan word - dat dit wel moontlik is om politieke risiko te meet en bestuur.

Thesis (PhD)--Stellenbosch University, 2005. / ENGLISH ABSTRACT: Since their inception, biology and biochemistry have been spectacularly successful in characterising the living cell and its components. As the volume of information about cellular components continues to increase, we need to ask how we should use this information to understand the functioning of the living cell? Computational systems biology uses an integrative approach that combines theoretical exploration, computer modelling and experimental research to answer this question. Central to this approach is the development of computational models, new modelling strategies and computational tools. Against this background, this study aims to: (i) develop a new modelling package: PySCeS, (ii) use PySCeS to study discontinuous behaviour in a metabolic pathway in a way that was very difficult, if not impossible, with existing software, (iii) develop an interactive, web-based repository (JWS Online) of cellular system models. Three principles that, in our opinion, should form the basis of any new modelling software were laid down: accessibility (there should be as few barriers as possible to PySCeS use and distribution), flexibility (pySCeS should be extendable by the user, not only the developers) and usability (PySCeS should provide the tools we needed for our research). After evaluating various alternatives we decided to base PySCeS on the freely available programming language, Python, which, in combination with the large collection of science and engineering algorithms in the SciPy libraries, would give us a powerful modern, interactive development environment. / AFRIKAANSE OPSOMMING: Sedert hul totstandkoming was biologie en, meer spesifiek, biochemie uiters suksesvol in die karakterisering van die lewende sel se komponente. Steeds groei die hoeveelheid informasie oor die molekulêre bestanddele van die sel daagliks; ons moet onself dus afvra hoe ons hierdie informasie kan integreer tot 'n verstaanbare beskrywing van die lewende sel se werking. Om dié vraag te beantwoord gebruik rekenaarmatige sisteembiologie 'n geïntegreerde benadering wat teorie, rekenaarmatige modellering en eksperimenteeIe navorsing kombineer. Sentraal tot die benadering is die ontwikkeling van nuwe modelle, strategieë vir modellering, en sagteware. Teen hierdie agtergrond is die hoofdoelstelling van hierdie projek: (i) die ontwikkeling van 'n nuwe modelleringspakket, PySCeS (ii) die benutting van PySCeS om diskontinue gedrag in n metaboliese sisteem te bestudeer (iets wat met die huidiglik beskikbare sagteware redelik moeilik is), (en iii) die ontwikkeling vann interaktiewe, internet-gebaseerde databasis van sellulêre sisteem modelle, JWS Online. Ons is van mening dat nuwe sagteware op drie belangrike beginsels gebaseer behoort te wees: toeganklikheid (die sagteware moet maklik bekombaar en bruikbaar wees), buigsaamheid (die gebruiker moet self PySCeS kan verander en ontwikkel) en bruikbaarheid (al die funksionalitiet wat ons vir ons navorsing nodig moet in PySCeS ingebou wees). Ons het verskeie opsies oorweeg en besluit om die vrylik verkrygbare programmeringstaal, Python, in samehang die groot kolleksie wetenskaplike algoritmes, SciPy, te gebruik. Hierdie kombinasie verskaf n kragtige, interaktiewe ontwikkelings- en gebruikersomgewing. PySCeS is ontwikkel om onder beide die Windows en Linux bedryfstelsels te werk en, meer spesifiek, om gebruik te maak van 'n 'command line interface'. Dit beteken dat PySCeS op enige interaktiewe rekenaar-terminaal Python ondersteun sal werk. Hierdie eienskap maak ook moontlik die gebruik van PySCeS as 'n modelleringskomponent in 'n groter sagteware pakket onder enige bedryfstelsel wat Python ondersteun. PySCeS is op 'n modulere ontwerp gebaseer, wat dit moontlik vir die eindgebruiker maak om die sagteware se bronkode verder te ontwikkel. As 'n toepassing is PySCeS gebruik om die oorsaak van histeretiese gedrag van 'n lineêre, eindproduk-geïnhibeerde metaboliese pad te ondersoek. Ons het hierdie interessante gedrag in 'n vorige studie ontdek, maar kon nie, met die sagteware wat op daardie tydstip tot ons beskikking was, hierdie studie voortsit nie. Met PySCeS se ingeboude vermoë om parameter kontinuering te doen, kon ons die oorsake van hierdie diskontinuë gedrag volledig karakteriseer. Verder het ons 'n nuwe metode ontwikkel om hierdie gedrag te visualiseer as 'n interaksie tussen die volledige sisteem se subkomponente. Tydens PySCeS se ontwikkeling het ons opgemerk dat dit baie moeilik was om metaboliese modelle wat in die literature gepubliseer is te herbou en te bestudeer. Hierdie situasie is grotendeels die gevolg van die feit dat nêrens 'n sentrale databasis vir metaboliese modelle bestaan nie (soos dit wel bestaan vir genomiese data of proteïen strukture). Die JWS Online databasis is spesifiek ontwikkel om hierdie leemte te vul. JWS Online maak dit vir die gebruiker moontlik om, via die internet en sonder die installasie van enige gespesialiseerde modellerings sagteware, gepubliseerde modelle te bestudeer en ook af te laai vir gebruik met ander modelleringspakkette soos bv. PySCeS. JWS Online het alreeds 'n onmisbare hulpbron vir sisteembiologiese navorsing en onderwys geword.

The Fire Dynamics Simulator (FDS) code recently developed by the National Institute of Standards and Technology (NIST) is particularly well-suited for use by fire protection engineers for studying fire behavior. It makes use of Large Eddy Simulation (LES) techniques to directly calculate the large-scale fluid motions characteristic of buoyant turbulent diffusion flames. However, the underlying model needs further development and validation against experiment in the areas of soot formation/oxidation and radiation before it can be used to calculate flame heat transfer and predict the burning of solid or liquid fuels. WPI, Factory Mutual Research, and NIST have undertaken a project to make FDS capable of calculating the flame heat transfer taking place in fires of hazardous scale. The temperatures predicted by the FDS code were generally too high on the fuel side and too low on the oxidant side when compared to experimental data from small-scale laminar diffusion flames. For this reason, FDS was reformulated to explicitly solve the conservation of energy equation in terms of total (chemical plus sensible) enthalpy. This allowed a temperature correction to be applied by removing enthalpy from the fuel side and adding it to the oxidant side. This reformulation also has advantages when using probability density function (PDF) techniques in larger turbulent flames because the radiatively-induced nonadiabaticity is tracked locally with each fluid parcel. The divergence of the velocity field, required to obtain the flow-induced perturbation pressure, is calculated from an expression derived from the continuity equation. A new approach to soot modeling in diffusion flames was developed and added to the FDS code. The soot model postulated as part of this work differs from others because it is intended for engineering calculations of soot formation and oxidation in an arbitrary hydrocarbon fuel. Previous models contain several fuel-specific constants that generally can only be determined by calibration experiments in laminar flames. The laminar smoke point height, an empirical measure of a fuel?s sooting propensity, is used in the present model to characterize fuel-specific soot chemistry. Two separate mechanisms of soot growth are considered. The first is attributed to surface growth reactions and is dependent on the available surface area of the soot aerosol. The second is attributed to homogeneous gas-phase reactions and is independent of the available soot surface area. Soot oxidation is treated empirically in a global (fuel-independent) manner. The local soot concentration calculated by the model drives the rate of radiant emission. Calibration against detailed soot volume fraction and temperature profiles in laminar axisymmetric flames was performed. This calibration showed that the general approach postulated here is viable, yet additional work is required to enhance and simplify the model. The essential mathematics for modeling larger turbulent flames have also been developed and incorporated into the FDS code. An assumed-beta PDF is used to approximate the effect of unresolved subgrid-scale fluctuations on the grid-scale soot formation/oxidation rate. The intensity of subgrid-scale fluctuations is quantified using the principle of scale similarity. The modified FDS code was used to calculate the evolution of soot in buoyant turbulent diffusion flames. This exercise indicated that the subgrid-scale fluctuations are quantitatively important in LES of turbulent buoyant diffusion flames, although no comparison of prediction and experiment was performed for the turbulent case.

Bibliography: leaves 155-169. Studies the effect of nitrogen fertilizer on dry matter production under differing watering regimes. Investigates the accuracy of different crop and soil organic matter models for predicting crop yield, nitrogen uptake and changes in soil organic carbon and nitrogen. Compares the models with data from long-term field experiments on wheat in Australia and sorghum in Ethiopia. Finds that a higher crop yield and better nitrogen and water utilisation can be achieved if addition of nitrogen fertilizer is balanced with soil water.

Bibliography: leaves 155-169. / xiv, 169 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Studies the effect of nitrogen fertilizer on dry matter production under differing watering regimes. Investigates the accuracy of different crop and soil organic matter models for predicting crop yield, nitrogen uptake and changes in soil organic carbon and nitrogen. Compares the models with data from long-term field experiments on wheat in Australia and sorghum in Ethiopia. Finds that a higher crop yield and better nitrogen and water utilisation can be achieved if addition of nitrogen fertilizer is balanced with soil water. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agronomy & Farming Systems, 1997?

As the second largest river in the U.S., the entrance to the Columbia River is home to some of the most extreme wave conditions on the Pacific Coast. Winter storms commonly generate waves 6-8 m in height, which in combination with strong tidal currents, can produce dangerous navigation conditions. To improve understanding of the wave dynamics in this complex setting, the SWAN model is applied; 2 hindcasts are conducted and an operations forecast is developed. The model is forced with offshore wave heights obtained from a buoy located in 134 m water depth (for the hindcasts) and a specialized WaveWatchIII forecast (for the forecast). In both cases tidal currents are obtained from SELFE, a circulation model of the Columbia River. The hindcasts are validated through measurements obtained from an inshore buoy located in 25 m water depth, a 4-week field experiment and remote sensing methods. The model performs best at the location of the buoy, with a normalized root-mean-squared error (NRMSE) of 11%, primarily because it is outside the area of strong tidal currents. Within the river mouth, the model is able to predict the changes in the wave field due to currents, but its performance is limited by errors in velocity estimates and strong shears in the tidal current profile. From the modeling work, it is evident that wave transformations at the mouth of the river are dominated by the tidal currents. The forecast has been operational since August 2011 and provides 45-hours of predictive wave information. In comparison with measured wave heights at the buoy, the forecast performs well, with a NRMSE of 16%. The majority of errors are caused by errors in the input conditions, since they themselves are forecasted. Additional errors arise from phase-resolved properties in the wave field that the model is unable to produce; these errors are also present in the hindcasts. Despite the limitations, this forecast provides valuable information to bar pilots since it includes the effects of the tidal currents. / Graduation date: 2013