Dissertations / Theses on the topic 'Spatial stability analysis'

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
Freqüentemente as análises de estabilidade de taludes são feitas por métodos determinísticos, nos quais é obtido um Fator de Segurança (FS). Estes métodos não quantificam as incertezas existentes nas variáveis de entrada (parâmetros de resistência) na análise. Tampouco mostram detalhes sobre qual variável afeta mais o resultado. Os métodos probabilísticos permitem superar estes problemas. A presente pesquisa tem como objetivo comparar dois métodos probabilísticos geralmente utilizados (Estimativas Pontuais e Segundo Momento). Três projetos utilizados comumente na geotecnia são analisados: barragem de rejeitos, talude de solo e aterro sobre argila mole.Um aspecto importante na análise probabilística é a quantificação adequada do desvio padrão. É sabido que as propriedades dos solos mostram uma correlação no espaço, pelo que o desvio padrão calculado por métodos clássicos da estatística é superestimado em comparação ao valor real no campo. El-Ramly (2001) desenvolveu uma metodologia para o cálculo da probabilidade de ruptura considerando a correlação espacial das propriedades do solo. Esta metodologia é de difícil aplicação prática. Serão, portanto, avaliadas na presente pesquisa as técnicas geralmente usadas de probabilidade e estabilidade de taludes, juntamente com um fator de correção proposto por Vanmarcke (1977a). Estas técnicas de probabilidade de estabilidade de taludes são factíveis de serem utilizadas junto com este fator de correção. Verificou-se que o Método de Segundo Momento é de mais fácil utilização, e portanto adequado para emprego em projetos geotécnicos.
The analysis of slope stability is often determined by deterministic methods, in which a Factor of Security (FS) is obtained. These methods do not quantify the uncertainty in the input variables (strength parameters). Neither these methods show details which variable affects mostly the outcome results. The probabilistic methods allow overcoming these restrictions. This study aims at comparing two probabilistic methods of general use (Point Estimates and First Order Second Moment). Three projects commonly used in geotechnical engineering are analyzed: tailings dam, slope soil and embankment on soft clay. An important aspect of the probabilistic analysis is the proper quantification of the standard deviation. It is known that the soil properties show a correlation in space, so the standard deviation, calculated by traditional methods of statistics, is overestimated when compared to the real field value. El-Ramly (2001) developed a methodology for calculating the probability of failure considering the spatial correlation of the soil properties. This methodology is difficult to apply in practice. The present study will therefore evaluate the techniques generally used in probability of failure of slopes. These probability techniques applied to slope stability can be used together with a correction factor proposed by Vanmarcke (1977b). The Second Moment Method was found to be easier to use, and therefore more suitable for geotechnical projects.

FUNDAÇÃO DE APOIO À PESQUISA DO ESTADO DO RIO DE JANEIRO
Em geral, a análise da estabilidade e a análise das respostas de poços de petróleo são realizadas de forma determinística em relação às propriedades mecânicas e hidráulicas do meio rochoso. No entanto, sabe- se que os meios rochosos e em particular rochas sedimentares, podem mostrar um considerável grau de heterogeneidades, em micro, meso e macro-escala. Essas heterogeneidades produzem variabilidade espacial nas propriedades mecânicas e hidráulicas dos meios rochosos. Essa variabilidade mostra em geral um caráter espacial pronunciado. O presente estudo propõe o desenvolvimento de procedimentos de análise numérica, utilizando elementos finitos, de processos fluido mecânicos acoplados, monofásicos e bifásicos, que levem em conta a variabilidade espacial de propriedades hidráulicas e mecânicas e a variabilidade das condições iniciais de tensões e poro pressões. Nesse estudo, empregam-se os procedimentos numéricos desenvolvidos em duas fases distintas. Na análise probabilística da estabilidade de poços e na análise probabilística das respostas dos poços durante a produção, considerando o acoplamento fluido mecânico com fluxo bifásico.
In general, borehole stability analysis and borehole response analysis are carried out considering that both hydraulic and mechanical parameters of the rock mass are deterministic. However, it is a well known fact, that rock masses and in particular sedimentary rock masses may show a considerable degree of heterogeneity, in micro, meso and macro scale. These heterogeneities produce spatial variability in mechanical and hydraulic properties of the rock medium. This variability can be very pronounced. The present work proposes the development of numerical analysis procedures, using finite elements, in order to analyze single and two phases flow, coupled fluid mechanical processes that take into account the spatial variability of hydraulic and mechanical properties and the variability of the initial stresses and pore pressures. In this study, the developed numerical procedures are used in two distinct phases. In the borehole stochastic stability analysis and in the borehole stochastic response analysis during the production, considering fluid mechanical coupling and two phase flow.

Given the large volume of data that is regularly accumulated, the need to properly manage, efficiently display and correctly interpret, becomes more important. Complex analysis of data is best performed using statistical models and in particular those with a geographical element are best analysed using Spatial Statistical Methods, including local regression. Spatial Statistical Methods are employed in a wide range of disciplines to analyse and interpret data where it is necessary to detect significant spatial patterns or relationships. The topic of the research presented in this thesis is an exploration of the most effective methods of visualising results. A human being is capable of processing a vast amount of data as long as it is effectively displayed. However, the perceptual load will at some point exceed the cognitive processing ability and therefore the ability to comprehend data. Although increases in data scale did increase the cognitive load and reduce processing, prior knowledge of geographical information systems did not result in an overall processing advantage. The empirical work in the thesis is divided into two parts. The first part aims to gain insight into visualisations which would be effective for interpretation and analysis of Geographically Weighted Regression (GWR), a popular Spatial Statistical Method. Three different visualisation techniques; two dimensional, three dimensional and interactive, are evaluated through an experiment comprising two data set sizes. Interactive visualisations perform best overall, despite the apparent lack of researcher familiarity. The increase in data volume can present additional complexity for researchers. Although the evaluation of the first experiment augments understanding of effective visualisation display, the scale at which data can be adequately presented within these visualisations is unclear. Therefore, the second empirical investigation seeks to provide insight into data scalability, and human cognitive limitations associated with data comprehension. The general discussion concludes that there is a need to better inform researchers of the potential of interactive visualisations. People do need to be properly trained to use these systems, but the limits of human perceptual processing also need to be considered in order to permit more efficient and insightful analysis.

Parametric instability in a system is caused by periodically varying coefficients in its governing differential equations. Parametric instability regions of a second-order non-dispersive distributed structural system in this work are obtained using the wave solution and the fixed point theory without spatially discretizing the governing partial differential equation. The parametric instability regions are classified as period-1 and period- i (i > 1) instability regions, where the former is analytically obtained, and the latter can be numerically calculated using bifurcation diagrams. The parametric instability phenomenon is characterized by a bounded displacement and an unbounded vibratory energy, due to formation of infinitely compressed shock-like waves. Parametric instability in a taut string with a periodically moving boundary is then investigated. The free linear vibration of the taut string is studied first, and three corresponding nonlinear models are introduced next. It is shown that the responses and vibratory energies of the nonlinear models are close to those of the linear model, which indicates that the parametric instability in the linear model can also exist in the nonlinear models.

A new global spatial discretization method for one- and two-dimensional continuous systems is investigated. General formulations for one- and two-dimensional systems that can achieve uniform convergence are established, whose displacements are divided into internal terms and boundary-induced terms. For one-dimensional systems, natural frequencies, mode shapes, harmonic steady-state responses, and transient responses of a rod and a tensioned Euler-Bernoulli beam are calculated using the new method and the assumed modes method, and results are compared with those from exact analyses. The new method gives better results than the assumed modes method in calculating eigensolutions and responses of a system, and it can use sinusoidal functions as trial functions for the internal term rather than possibly complicated eigenfunctions in exact analyses. For two-dimensional systems, natural frequencies and dynamic responses of a rectangular Kirchhoff plate that has three simply-supported boundaries and one free boundary with an attached Euler-Bernoulli beam are calculated using both the new method and the assumed modes method, and compared with results from the finite element method and the finite difference method, respectively. Advantages of the new method over local spatial discretization methods are fewer degrees of freedom and less computational effort, and those over the assumed modes method are better numerical property, a faster calculation speed, and much higher accuracy in calculation of high-order spatial derivatives of the displacement.

Abstract In this thesis we study the spatial throughput of interference-limited wireless networks from different perspectives, considering that the spatial distribution of nodes follows a 2-dimensional homogeneous Poisson point process and transmitters employ Gaussian point-to-point codes. To carry out this analysis, we model the interrelations between network elements using concepts from stochastic geometry, communication theory and information theory. We derive closed-form equations to compute/approximate the performance metric that is chosen to evaluate the system for each given specific scenario. Our first contribution is an investigation about whether it is preferable to have a large number of short single-hop links or a small number of long hops in multi-hop wireless networks, using a newly proposed metric denominated aggregate multi-hop information efficiency. For single-hop systems, we revisit the transmission capacity framework to study medium access protocols that use asynchronous transmissions and allow for packet retransmissions, showing when a carrier sensing capability is more suitable than synchronous transmissions, and vice-versa. We also cast the effective link throughput and the network spatial throughput optimization problems to find the combination of medium access probability, coding rate and maximum number of retransmissions that maximize each metric under packet loss and queue stability constraints, evincing when they do (and do not) have the same solution. Furthermore we analyze the expected maximum achievable sum rates over a given area – or spatial capacity – based on the capacity regions of Gaussian point-to-point codes for two decoding rules, namely (i) treating interference as noise (IAN) and (ii) jointly detecting the strongest interfering signals treating the others as noise (OPT), proving the advantages of the second. We additionally demonstrate that, when the same decoding rule and network density are considered, the spatial-capacity-achieving scheme always outperforms the spatial throughput obtained with the best predetermined fixed rate strategy. With those results in hand, we discuss general guidelines on the construction of ad hoc adaptive algorithms that would improve the information flow throughout the interference network, respecting the nodes’ internal and external constraints
Tiivistelmä Tässä työssä tutkitaan häiriörajoitteisten langattomien verkkojen tila-alueen suorituskykyä, olettaen verkkosolmujen sijoittuvan 2-ulotteisen Poissonin pisteprosessin mukaisesti, sekä olettaen lähettimien hyödyntävän Gaussisia pisteestä-pisteeseen -koodeja. Suorituskykyanalyysi pohjautuu stokastiseen geometriaan, tietoliikenneteoriaan sekä informaatioteoriaan. Suljetun muodon suorituskyky-yhtälöitä hyödyntäen arvioidaan suorityskykymetriikoita eri skenaarioissa. Työn aluksi esitetään uusi monihyppyverkkojen informaatiotehokkuuteen perustuva metriikka. Sen avulla voidaan tutkia onko tehokkaampaa käyttää useita lyhyen hypyn linkkejä vai pienempää määrää pidempien hyppyjen linkkejä. Yhden hypyn verkoissa tutkitaan mediaanpääsyprotokollia asynkronisissa verkoissa pakettien uudelleenlähetykseen perustuen ja verrataan tätä synkroniseen lähetykseen ilman vapaan kanavan tunnistusmekanismia. Työssä tutkitaan myös linkin efektiivisen suorituskyvyn ja verkon tila-alueen suorituskyvyn optimointia, jotta sopiva yhdistelmä mediaan pääsyn todennäköisyydelle, koodausnopeudelle ja uudelleenlähetysten maksimilukumäärälle löytyisi ja samalla maksimoisi jokaisen käytetyn metriikan ehdollistettuna paketin menetyksille ja jonon stabiilisuudelle. Lisäksi arvioidaan maksimaalista odotettavaa nettosiirtonopeutta tietyllä alueella, eli tila-alueen kapasiteettia, Gaussimaisen pisteestä-pisteeseen koodien kapasiteettialueisiin perustuen kahta eri dekoodaussääntöä hyödyntäen: (i) olettaen häiriön olevan kohinaa tai (ii) ilmaisemalla voimakkaimmat häiriösignaalit ja olettaen muiden olevan kohinaa. Jälkimmäinen osoittautui tehokkaammaksi menetelmäksi. Työssä osoitetaan myös, että samalla dekoodaussäännöllä ja verkon tiheydellä tila-alueen kapasiteetin saavuttava menetelmä on aina tehokkaampi kuin tavanomainen tila-alueen suorituskykyyn perustuva kiinteän siirtonopeuden menetelmä. Saavutettujen tulosten valossa työssä esitetään yleisiä suunnittelumenetelmiä mukautuville ad hoc -algoritmeille, joiden avulla voidaan parantaa tiedonsiirtoa häiriörajoitteisissa verkoissa, ehdollistettuna verkon solmujen sisäisille ja ulkoisille rajoitteille

Avalanche hazard evaluation relies on snowpack stability observations. Because snowpack properties can vary extensively over time and space, estimating slope-scale stability is difficult. This study addressed these challenges by implementing a methodology that 1. quantified spatial and temporal patterns of snowpack stability, 2. identified spatial associations between the strength and stability of a weak layer and slab load, and radiation properties, 3. identified internal associations between weak layer thickness, shear strength, microstructural properties, and slab load. An instability associated with a buried surface hoar weak layer was examined on an inclined forest opening at Lionhead, southwest Montana, during February and March, 2005. During five sampling days, 824 snow depth and SnowMicroPen resistance profiles and 352 shear frame tests were performed. An objective texture-based stratigraphic sampling approach was developed to obtain microstructural estimates of a stratigraphic weakness and instability from SnowMicroPen profiles, utilizing the coefficient of variation of rupture force. Spatial models of hemispheric sky visibility, and incoming long- and shortwave radiation were generated for the surface hoar formation period using a Geographic Information System and independent optical observations. Despite relative topographic uniformity, in a distance of 30 m, the buried surface hoar weak layer thicknesses varied between 3 - 21 mm. Before burial, the surface hoar persisted despite moderate winds and above freezing air temperatures. Spatial patterns of modeled incoming longwave and shortwave radiation explained the large variation in weak layer thickness and strength properties. Areas exposed to large amounts of radiation contained a strong, thin buried surface hoar layer, while in areas with limited incoming longwave (due to high sky visibility) and shortwave radiation (due to shading), the layer was thicker and possessed low shear and microstructural strengths. Over time, the shear frame stability index and SnowMicroPen-derived microstrength of the surface hoar layer increased and values became spatially more variable (divergence): it became harder to predict stability as the snowpack became more stable. A loading event then decreased stability and micro-strength and caused spatial uniformity (convergence), thereby increasing predictive strength of observations. The findings illustrate the usefulness of the SnowMicroPen for evaluating spatial patterns and load-related changes in snowpack stability.

Soil moisture represents a crucial factor in the mass and energy exchanges between land and atmosphere. Thus, the knowledge of its spatial and temporal dynamic is fundamental for several hydrological, agricultural and climatological applications. The purpose of this study is to shed light on soil moisture patterns at catchment scale in alpine environment with focus on the role of climate, soil and topography. For this purpose, a dataset of 13 measurement points at two different depths for two years have been assessed. The analyses highlight significant differences in soil moisture at 5 and 20 cm, showing diverse influence of climate, topography and soil type on the two. We found that climate factors in the shallow layer contribute to 75% of the soil moisture variance and only 25% is related to topography and soil. On the other hand, at 20 cm the climatic and the time-invariant components (topography, soil, etc…) have almost equal weights in determining soil moisture spatial variance. The analysis on seasonal basis showed the presence of frozen-related phenomena during the winter season, which prevented a clear understanding of the diverse factors on the soil moisture spatial distribution and opened new potential researches on this topic. By contrast, during the summer season well-defined soil moisture patterns mainly controlled by climate and land use were observed. The presence of weak correlations with land data and high covariances during dry periods confirms that soil moisture is the result of an interaction of multiple components rather than the result of single controlling factor. The study extends the knowledge of soil moisture dynamic in alpine environment, confirming some outcomes found in previous studies and providing new evidences on site dependency. Moreover, a database for further hydrological researches, as well as practices for future surveys, have been settled.

The aim of this research was to assess three hierarchical aspects of alga-grazer interactions in intertidal communities on a small scale: spatial heterogeneity, grazing effects and spatial stability in grazing effects. First, using semivariograms and cross-semivariograms I observed hierarchical spatial patterns in most algal groups and in grazers. However, these patterns varied with the level on the shore and between shores, suggesting that either human exploitation or wave exposure can be a source of variability. Second, grazing effects were studied using manipulative experiments at different levels on the shore. These revealed significant effects of grazing on the low shore and in tidal pools. Additionally, using a transect of grazer exclusions across the shore, I observed unexpected hierarchical patchiness in the strength of grazing, rather than zonation in its effects. This patchiness varied in time due to different biotic and abiotic factors. In a separate experiment, the effect of mesograzers effects were studied in the upper eulittoral zone under four conditions: burnt open rock (BOR), burnt pools (Bpool), non-burnt open rock (NBOR) and non-burnt pools (NBpool). Additionally, I tested spatial stability in the effects of grazing in consecutive years, using the same plots. I observed great spatial variability in the effects of grazing, but this variability was spatially stable in Bpools and NBOR, meaning deterministic and significant grazing effects in consecutive years on the same plots. Both the significance in grazing effects and spatial stability depended on the level of resolution (species, functional, biomass) at which the algal assemblage was evaluated, suggesting hierarchical variability. In order to be able to predict spatial variability in the effects of grazers in the upper eulittoral zone using biotic and abiotic micro- and macrofactors, a conceptual model was proposed, based on data from several multiple-regressions. This linked the interactions among three elements: idiosyncratic heterogeneity, micro and macrofactors. This suggests that spatial variability can be a product of these factors, while spatial stability can be caused by the same or different combinations of factors. In conclusion, grazing and other ecological phenomena must be studied hierarchically, not only through spatiotemporal scales, but also at different levels of resolution, as these also influence our perception of patterns.

Analyses of nuclear reactor safety have increasingly required coupling of full three dimensional neutron kinetics (NK) core models with system transient thermal–hydraulics (TH) codes.  In order to produce results within a reasonable computing time, the coupled codes use two different spatial description of the reactor core.  The TH code uses few, typically 5 to 20 TH channels, which represent the core.  The NK code uses explicit one node for each fuel assembly.  Therefore, a spatial mapping of a coarse grid TH and a fine grid NK domain is necessary.  However, improper mappings may result in loss of valuable information, thus causing inaccurate prediction of safety parameters. The purpose of this thesis is to study the effectiveness of spatial coupling (channel refinement and spatial mapping) and develop recommendations for NK/TH mapping in simulation of safety transients.  Additionally, sensitivity of stability (measured by Decay Ratio and Frequency) to the different types of mapping schemes, is analyzed against OECD/NEA Ringhals–1 Stability Benchmark data. The research methodology consists of spatial coupling convergence study, by increasing the number of TH channels and varying mapping approaches, up to and including the reference case.  The reference case consists of one-to-one mapping: one TH channel per one fuel assembly.  The comparisons of the results are done for steady–state and transient results.  In this thesis mapping (spatial coupling) definition is formed and all the existing mapping approaches were gathered, analyzed and presented.  Additionally, to increase the efficiency and applicability of spatial mapping convergence, a new mapping methodology has been proposed.  The new mapping approach is based on hierarchical clustering method; the method of unsupervised learning that is adopted by many researchers in many different scientific fields, thanks to its flexibility and robustness.  The proposed new mapping method turns out to be very successful for spatial coupling problem and can be fully automatized allowing for significant time reduction in mapping convergence study. The steady–state results obtained from three different plant models for all the investigated cases are presented.  All models achieved well converged steady–state and local parameters were compared and it was concluded that solid basis for further transient analysis was found.  Analyzing the mapping performance, the best predictions for steady–state conditions are the mappings that include the power peaking factor feature alone or with any combination of other features.  Additionally it is of value to keep the core symmetry (symmetry feature).  The big part of this research is devoted to transient analysis.  The selection of transients was done such that it covers a wide range of transients and gathered knowledge may be used for other types of transients.  As a representative of a local perturbation, Control Rod Drop Accident was chosen.  A specially prepared Feedwater Transient was investigated as a regional perturbation and a Turbine Trip is an example of a global one.  In the case of local perturbation, it has been found that a number of TH channels is less important than the type of mapping, so a high number of TH channels does not guarantee improved results.  To avoid unnecessary averaging and to obtain the best prediction, hot channel and core zone where accident happens should be always separated from the rest.  The best performance is achieved with mapping according power peaking factors, and therefore this one is recommended for such type of perturbation. The regional perturbation has been found to be more challenging than the others.  This kind of perturbation is strongly dependent on mapping type that affects the power increase rate, SCRAM time, onset of instability, development of limit cycle, etc.  It has been also concluded that a special effort is needed for input model preparation.   In contrast to the regional perturbation, the global perturbation is found to be the least demanding transient.  Here, the number of TH channels and type of mapping do not have significant impact on average plant behaviour – general plant response is always well recreated.  A special effort has also been paid to investigate the core stability performance, in both global and regional mode.  It has been found that in case of unstable cores, a low number of TH channels significantly suppresses the instability.  For these cases number of TH channels is very important and therefore at least half of the core has to be modeled to have a confidence in predicted DR and FR.  In case of regional instability in order to get correct performance of out-of-phase oscillations, it is recommended to use full-scale model.  If this is not possible, the mapping which is a mixture of 1st power mode and power peaking factors, should be used. The general conclusions and recommendations are summarized at the end of this thesis.  Development of these recommendations was one of the purposes of this investigation and they should be taken into consideration while designing new coupled TH/NK models and choosing mapping strategy for a new transient analysis.

QC 20130516

Methods in hydrodynamic stability, systems and control theory are applied to spatially developing flows, where the flow is not required to vary slowly in the streamwise direction. A substantial part of the thesis presents a theoretical framework for the stability analysis, input-output behavior, model reduction and control design for fluid dynamical systems using examples on the linear complex Ginzburg-Landau equation. The framework is then applied to high dimensional systems arising from the discretized Navier–Stokes equations. In particular, global stability analysis of the three-dimensional jet in cross flow and control design of two-dimensional disturbances in the flat-plate boundary layer are performed. Finally, a parametric study of the passive control of two-dimensional disturbances in a flat-plate boundary layer using streamwise streaks is presented.

Le contexte général des travaux de recherche de cette thèse concerne les problématiques liées à l’optimisation glob-ale liée à la conception des futurs satellites d’observation terrestre et de missions scientifiques, nécessitantune très haute stabilité en pointage (capacité du satellite à garder son point de visée). Plus particulièrement,les travaux concernent le contrôle actif des modes micro-vibratoires.Dans une mission satellitaire d’observation terrestre, la qualité des images dépend bien évidemmentdes instruments de mesure optique (diamètre du miroir, aberrations optiques et qualité du polissage)mais également des performances de la stabilité de la ligne de visée du satellite qui peut s’avérer dégradéepour cause de micro-vibrations. La présence de ces micro-vibrations est liée aux divers éléments tournantdu satellite tels que les mécanismes de rotation des panneaux solaires ou de contrôle d’orientation dusatellite (on parle de contrôle d’attitude réalisé au moyen de roues inertielles).Le contrôle des micro-vibrations représentent ainsi un défit technologique, conduisant l’ESA et les ac-teurs industriels du monde spatial, a considéré cette problématique comme hautement prioritaire pour ledéveloppement des satellites d’observation terrestre nouvelle génération.Il existe à l’heure actuelle deux principes fondamentaux de contrôle des micro-vibrations :• le contrôle dit passif: la stratégie consiste à introduire des dispositions constructives et des matériauxparticuliers permettant de minimiser la transmission des vibrations à l’environnement.• le contrôle dit actif : le concept de contrôle actif des vibrations est tout autre : l’idée est cette fois-ci,de bloquer la micro-vibration en exerçant une vibration antagoniste créée artificiellement avec despropriétés en opposition, à tout instant, relativement à la vibration indésirable, pour rendre nulleleur somme.L’industrie spatiale aborde cette problématique en plaçant des isolateurs en élastomère au voisinage dechaque source de micro-vibrations. Cette solution, qui a fait ses preuves puisqu’elle équipe actuelle-ment nombre de satellites en orbite, permet de rejeter nombre de micro-vibrations. Malheureusement,la demande de plus en plus importante de grande stabilité de la ligne de visée pour les futures missionsd’observation terrestres telles que les missions GAIA rend l’approche passive insuffisante.L’ESA et Airbus Defence and Space, ont donc collaborer conjointement avec l’équipe ARIA au travers decette thèse, dans des travaux de recherche dans le domaine du contrôle actif pour palier ces problèmes.L’objectif visé est de coupler les approches passives et actives afin de rejeter à la fois les micro-vibrations enhautes fréquences (approche passive existant) et en basses fréquences (approche active objet des travauxde la thèse)
Next generation satellite missions will have to meet extremely challenging pointing stability requirements. Even low levels of vibration can introduce enough jitter in the optical elements to cause a significant reduction in image quality. The success of these projects is therefore constrained by the ability of on-board vibration isolation and optical control techniques to keep stable the structural elements of the spacecraft in the presence of external and internal disturbances.In this context, the research work presented in this thesis combines the expertise of the European Space Agency (ESA), the industry (Airbus Defence and Space) and the IMS laboratory (laboratoire de l’Intégration du Matériau au Système) with the aim of developing new generation of robust microvibration isolation systems for future space observation missions. More precisely, the thesis presents the development of an Integrated Modeling, Control and Analysis framework in which to conduct advanced studies related to reaction wheel microvibration mitigation.The thesis builds upon the previous research conducted by Airbus Defence and Space and ESA on the use of mixed active/passive microvibration mitigation techniques and provides a complete methodology for the uncertainty modeling, robust control system design and worst-case analysis of such systems for a typical satellite observation mission. It is shown how disturbances produced by mechanical spinning devices such as reaction wheels can be significantly attenuated in order to improve the pointing stability of the spacecraft even in the presence of model uncertainty and other nonlinear phenomenon.Finally, the work introduces a new disturbance model for the multi harmonic perturbation spectrum produced by spinning reaction wheels that is suitable for both controller synthesis and worst-case analysis using modern robust control tools. This model is exploited to provide new ways of simulating the image distortions induced by such disturbances

Les réseaux spatiaux sont des réseaux dans lesquels les sommets occupent une position dans l'espace Euclidien. Les interactions dans ces réseaux sont déterminées par cette géometrie sous-jacente des sommets. Les réseaux de communications offrent un vaste champ d'application et une source de nouveaux modèles autour de ce thème. La thèse aborde trois sujets dans des domaines differents. Le premier concerne l'étude de certains arbres couvrant géométriques de processus ponctuels de Poisson. Ces travaux portent notamment sur le phenomene "petit monde", les arbres couvrants radiaux et l'arbre couvrant minimal. Un autre sujet de recherche porte sur la stabilité stochastique de réseaux de files d'attente pour lesquelles les files ont des interactions spatiales. La dernière partie de la thèse aborde des thèmes reliés à la géometrie stochastique: une étude du modèle de feuilles mortes et un travail sur la sensibilité de fonctionnelles de processus ponctuels de Poisson.

碩士
國立成功大學
航空太空工程學系碩博士班
95
The objective of this work is to study the asymmetric vortices structure and the induced side force on a cone-cylinder body at high incidence. Both control-free and micro-balloon actuated flows were simulated. In the control-free case the effect of small geometric irregularity on the apex was studied first to see the sensitivity of the natural vortical flow development over a cone-cylinder body. Flow control enforced with micro-balloon actuation was then investigated. A previously developed incompressible flow solver which employs the artificial compressibility and the Osher-Chakravarthy upwind MUSCL type total variation diminishing scheme was used. Time-accuracy is achieved by implicit ALU(Approximate Lower/Upper factorization) time-marching augmented by Newton sub-iterations. Geometric alteration was modeled using blowing/suction boundary conditions. Results show that the asymmetry of the natural vortical structure changes with the circumferential position of the imposed disturbance on the apex. Moreover, at sensitive forcing locations, the strength of the disturbances may cause sudden structural change of the separated vortical flow when the volume flux, which simulates the disturbance on the apex, exceeds certain threshold value. The original weak separated vortices situated remotely from the body surface migrate closer to the body surface, while the original strong vortices get pushed far away from the body. The flip of the influential vortical structure results in the side force reversal of the cone-cylinder body. For the controlled flow study, blowing and suction strength was adjusted to yield similar micro-balloon height generated in the experiment. It was observed that, in the same effective forcing region, the enforcement of balloon control over the weak vortex side causes the basic pattern change of the vortex structure. A newly generated organized vortex structure in the vicinity of the original weak vortex side takes the role of the influential vortex, making the side force change significantly in the reversed trend.

Geotechnical engineering design has relied upon deterministic methods of analysis whereby values for analysis parameters and conditions are selected subjectively based on judgment with the intent of providing acceptable margins of safety. The objective of this research was to improve the use of probabilistic slope stability analysis in practice so that the design of slopes can be made on a consistent and probabilistic basis. The current research involved the development of a methodology for the measurement and modeling of the anisotropic autocorrelation distance of cohesive soils, which was demonstrated at Dyke 17 West of the McArthur Falls Generating Station. In-situ testing using the piezocone and laboratory testing was conducted to characterize the spatial variability of the effective-shear strength envelope. Vertical (down-hole) and horizontal (cross-hole) geostatistical analysis was conducted to assess the anisotropy of the semivariogram. The investigation identified that heterogeneous inclusions had significant impacts on the results, but that simplistic (visual) identification and filtering procedures were adequate. The effective-stress shear strength envelope was statistically characterized as a random field, which was simulated as a first-order Markov process using customized add-in functions in a limit-equilibrium slope stability analysis. The analysis accounts for the spatial variability of shear strength and is capable of simulating both isotropic and anisotropic autocorrelation functions. The study showed that the critical slip surface geometry and the probability of failure can be significantly different when the anisotropy of spatial correlation is accounted for. The study also showed that neglecting spatial correlation may over-estimate the probability of failure, however this finding was noted to be likely case-specific. The primary conclusion of the study was that appropriate representation of spatial correlation is essential to calculating the probability of failure. Finally, convergence of the probabilistic simulation was evaluated using bootstrapping of the simulated factor of safety distribution to assess the standard error in the mean factor of safety, standard deviation of factor of safety and the probability of failure. A convergence criterion based on the percentage standard error in the probability of failure was proposed and used to define the number of Monte-Carlo iterations required.

Riverbank collapse is a natural and expected phenomenon associated with the evolution of rivers worldwide and has been studied extensively over the last two decades and remains an active research topic. The evolution of riverbank stability analysis has followed closely the developments in analytical methods, investigation tools, stabilisation methods and data acquisition technology. Furthermore, the stability of riverbanks is a multifaceted issue which involves the study of geology, topography, stratigraphy, hydrology, climate, spatial variation and geotechnical engineering. The River Murray is one of the only river systems in the world that can fall below sea level due to the barrages preventing the inflow of sea water during periods of low river flows. Over the last few years, an unprecedented period of dry conditions and low flows between 2005 – 2010 led to more than 162 reported riverbank collapse-related incidents along the Lower River Murray, in South Australia (downstream of Lock 1 at Blanchetown to Wellington). Those collapse-related incidents threatened public infrastructure, private property and the safety of river users, and also provide significant challenges for environmental and river management. From the inventory of the South Australian Department of Environment, Water and Natural Resources (DEWNR), riverbank collapse, erosion, cracking, tree leaning and collapse and levee problems are the main forms of the recorded incidents. Geographical information systems (GIS) is well known for its efficient and cost-effective spatial data processing capabilities, which include spatial data collection, manipulation and analysis, and has been widely used in riverbank instability research. As a significant feature of this thesis, GIS, incorporating high-resolution spatial data, such as aerial photographs and LIDAR (light detecting and ranging) images, facilitates the assessment of riverbank instability in several ways. Firstly, the actual location of the historical collapse can be determined and verified by the use of high-resolution aerial image comparison and interpretation to facilitate accurate back-analyses. Secondly, the 2D and 3D geometry of the riverbank is able to be readily extracted from the LIDAR digital elevation models (DEMs). Thirdly, the dimensions of the predicted collapsed regions can be validated against high-resolution aerial images, and finally, the influencing factors are able to be manipulated and mapped with GIS to predict regions susceptible to riverbank collapse. This thesis aims to: (1) examine the failure mechanisms affecting riverbank collapse along the Lower River Murray and identify the most relevant mechanism; (2) identify potential triggers for riverbank collapse events that should be monitored and managed in the future; (3) develop a framework, incorporating spatial information, GIS and geotechnical data, to facilitate the prediction of riverbank collapse along the Lower River Murray (between Blanchetown and Wellington, South Australia); and (4) develop a framework, based on GIS and geotechnical data, to identify regions susceptible to high risk of riverbank collapse along the Lower River Murray. In order to realise these aims, numerical analyses have been performed using two commercially available software programs, ArcGIS and SVOffice, which integrate the limit equilibrium method, back-analysis of collapse incidents, transient unsaturated flow modelling, steady state modelling, and DEMs and high-resolution aerial images within a GIS framework. The modelling has been informed by a series of geotechnical investigations undertaken at various sites along the River Murray.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Civil, Environmental & Mining Engineering, 2015.