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Rudholmer, Elin. "Inlärningseffekten på Limits of Stability på Transtibialt Amputerade Protesbrukare." Thesis, Hälsohögskolan, Högskolan i Jönköping, HHJ, Avd. för rehabilitering, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-30427.
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Syftet med studien är att undersöka om det finns någon inlärningseffekt på testet Limits of Stability (LoS) för transtibialt amputerade protesbrukare och en kontrollgrupp. Sju transtibialt amputerade protesbrukare och en kontrollgrupp bestående av sju friska vuxna män upprepade testet LoS fem gånger under fyra testtillfällen. Två kraftplattor och 69 reflexmarkörer användes för att samla in data. Testpersonerna placerades med en fot på varje kraftplatta och blev instruerade att förflytta sin center of pressure genom att luta kroppen från anklarna mot åtta mål som visades på en skärm tillsammans med deras center of pressure. Ordningen på målen var slumpvist utvalda. Datan analyserades med Friedmans test, eftersom den inte var normalfördelad, för att se om det fanns någon skillnad i resultatet mellan upprepningarna av testet och resultatet mellan testtillfällena. Det fanns några signifikanta skillnader mellan upprepningarna och mellan testtillfällena som tyder på att det finns en inlärningseffekt efter första upprepningen och första testtillfället, men resultatet var inte tillräckligt entydigt för att kunna dra några konkreta slutsatser. Vidare studier rekommenderas.<br>The aim of the study is to examine the learning effect of Limits of Stability (LoS) test concerning transtibially amputated prosthetic users and a healthy control group. Seven transtibially amputated prosthetic users and a control group of seven healthy males repeated the LoS test five times during four sessions, one in the morning and one in the afternoon in two consecutive days. Two forceplates and 69 reflex markers were used to collect the data. The test subjects were placed with on the foot on each forceplate and were instructed to move their center of pressure by leaning the body from the ankles towards eight targets presented on a screen along with their center of pressure. The targets were selected in random order. The data was analyzed with Friedmans test, since the data wasn’t normally distributed, to see if there was any discrepancy between the repetitions and between the sessions. There were some significant discrepancies between repetitions and between sessions mainly involving the first repetition and the first session. However the results weren’t distinct enough to draw any concrete conclusions. Further studies are recommended.
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Reinert, Senia Smoot. "Enhancing Posturography Stabilization Analysis and Limits of Stability Assessment." University of Dayton / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1470227622.
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Dunton, Elizabeth Marie. "Kinematic and stability motion limits for a hexapod walking machine." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA295654.
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Thesis (M.S. in Computer Science) Naval Postgraduate School, March 1995.<br>"March 1995." Thesis advisor(s): Robert B. McGhee, Michael J. Zyda. Includes bibliographical references. Also available online.
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Huo, Feng. "Limits of stability and postural sway in young and older people." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0005/MQ42635.pdf.
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Sauba, Rooktabir Nandan. "Stability limits and combustion measurements in low calorific value gas flames." Thesis, Middlesex University, 1987. http://eprints.mdx.ac.uk/13380/.
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A Hilton combustor was substantially modified to a suitable symmetrical configuration for research purposes. Provisions for swirl, preheat and injection of LCV gases were incorporated with appropriate burner management systems for safe operation. Instrumentation for temperature, velocity and concentration measurements was developed and fully automated by interfacing to a microprocessor for rapid data acquisition. Flame stability limits were determined over a wide range of operating conditions by varying swirl, secondary air temperature and excess air levels while maintaining the burner momentum constant. Addition of swirl up to a limit of O~ 69 generally improved stability. Preheating secondary air alone was beneficial only if the temperature was raised to at least 250 oC. A combination of intermediate swirl and moderate preheat of the secondary air resulted in satisfactory flame stability over a wide range of calorific values of the fuel. Thus, existing concentric pipe burner systems may be easily modified at low cost to burn LCV gases of variable compositions. With LCV gas flames the excess air factor (EAF) had a major influence on values of temperature, species concentration and velocity. Unburnt hydrocarbon (UHC) and CO not surprisingly increased in concentration close to the blow-off limits under the majority of operating conditions. This indicated incomplete combustion probably resulting from the lowered flame temperatures and partial flame lift-off. On the other hand, burnout efficiencies at the exhaust were reasonably high for most operating conditions involving LCV gases. The combustion data were analysed to extract the characteristic mixing and chemical reaction times the ratio of which gave the parameter epsilon, originally proposed for unconfined flames. Close to the blow-off limit epsilon took the value 4.9 compared with 6.2 for fully stable flames. This finding showed that the criterion was also valid for confined flames, supporting the extinction mechanism proposed by Peters and Williams, and providing an important basis for predicting stability limits and burner design parameters.
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Tamimi, Ala. "Voltage stability limits for weak power systems with high wind penetration." Diss., Kansas State University, 2011. http://hdl.handle.net/2097/13100.
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Doctor of Philosophy<br>Department of Electrical and Computer Engineering<br>Anil Pahwa<br>Shelli K. Starrett<br>Analysis of power system voltage stability has practical value in increasing wind penetration levels. As wind penetration levels increase in power systems, voltage stability challenges arise due to locating wind resources far away from load centers. This dissertation presents several different voltage stability methods for sizing new wind farms. Power system wind penetration levels depend on the available voltage stability margins (VSMs) of the existing power system and system load characteristics. Three new iterative methods have been developed to maximize wind penetration level in weak power systems based on systems’ VSMs. The first two methods use an iterative approach for increasing the size of each wind farm until reaching the collapse point. Wind farms with less negative impact on system VSMs are sized larger than others. A third wind farm sizing method has been developed using modal analysis in conjunction with the traditional voltage stability method (Q-V method). Wind farms are placed at buses in the power system which have the lowest negative impact on voltage instability modes (strong wind injection buses). By placing the wind farms at the strongest wind injection buses, higher amounts of wind power can be injected into the power system. To further increase wind penetration in weak power systems, two additional techniques are introduced and applied to the western Kansas power system. The first technique uses modes of voltage instability to place voltage support equipment like static var compensators at locations in the power system where they provide the needed reactive power support for increasing levels of wind penetration. The second technique uses the fact that wind patterns at a wind farm site may rarely allow the wind farm to produce its maximum capacity during the peak loading hours. Wind farm maximum sizes can be increased above their maximum voltage stable size limit without driving the power system into becoming voltage unstable. Preventing voltage collapse for the additional increases in wind farm sizes is accomplished by disconnecting some wind turbines inside the wind farm during critical times to reduce its power output to a voltage stable level.
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Hill, Martin John. "The limits and control of mixture leanness for stable engine operation." Thesis, University of Nottingham, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367163.
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Rodríguez, Tinoco Cristian. "Calorimetric study of vapour deposited glasses: beyond conventional stability and temperature limits." Doctoral thesis, Universitat Autònoma de Barcelona, 2016. http://hdl.handle.net/10803/386488.
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La deposición de capas a partir de la fase vapor se ha convertido recientemente en una herramienta muy potente de cara a la fabricación de vidrios de una estabilidad sin precedentes. Los vidrios depositados a partir de la fase vapor pueden tener propiedades seleccionadas a medida, simplemente ajustando las condiciones de deposición. En particular, los vidrios de máxima estabilidad se obtienen cuando la temperatura del substrato se encuentra alrededor de 0.85Tg. Por otra parte, la nanocalorimetría a altas velocidades de escaneo ha demostrado ser una herramienta muy útil para la caracterización térmica de la transición vítrea, a temperaturas muy superiores a las exploradas habitualmente. Es esta tesis presentamos un estudio detallado de diferentes aspectos de la transición vítrea en vidrios orgánicos de Indometacina y Celecoxib depositados a partir de la fase vapor. El estudio se ha realizado mediante el uso de diferentes técnicas calorimétricas, desde calorimetría convencional, a nanocalorimetría a alta velocidad de escaneo e incluyendo calorimetría a alta presión.
Los vidrios estables producidos a partir de la fase vapor exhiben un mecanismo de transformación diferente a los vidrios obtenidos de forma convencional por enfriamiento rápido del líquido. Mientras que estos últimos se transforman de forma homogénea en todo el volumen, la transformación de los vidrios estables se produce a partir de un frente paralelo que comienza en la y se propaga hacia el interior de la muestra. A partir normalizar los datos considerando la superficie de la muestra ha sido posible identificar este mecanismo de transformación así como medir la velocidad del frente de propagación. Tanto la orientación molecular como la estabilidad termodinámica del vidrio parecen influir en la velocidad de transformación. Vidrios con destacada birefringencia y baja estabilidad presentan velocidades de transformación más altas.
Una vez el frente de transformación ha avanzado una cierta distancia el mecanismo de transformación homogéneo se activa y pasa a dominar el ritmo al que se transforma el vidrio. El análisis de muestras más gruesas que esta distancia de cruce permite estudiar la dinámica de la transformación homogénea en volumen. Hemos visto que la evolución del tiempo de relajación en función de la temperatura sigue una dependencia de tipo super-Arrhenius. Esta nueva expresión describe de forma satisfactoria la dinámica del líquido sobreenfriado y de vidrios de diferente estabilidad de forma simultánea. Esta visión unificada de vidrios y líquido puede ayudar a entender la conexión entre las propiedades dinámicas y termodinámicas del vidrio.
Hemos medido también la variación del inicio de la transformación en vidrios de diferente estabilidad en función de la presión hidrostática. Hemos visto que la temperatura de transformación tiende a un valor común, independientemente de la estabilidad del vidrio, mostrando un escenario en el que tanto el vidrio ultra estable como los vidrios convencionales se transformarían a la misma temperatura.
Dado que la cristalización del sistema amorfo sigue siendo el mayor impedimento para la implementación completa de los vidrios en la industria farmacéutica, hemos analizado si la elevada estabilidad de los vidrios depositados en fase vapor podía ser relevante en este sentido. Hemos visto que los cristales crecen un 30% más lento en la superficie de los vidrios ultra estables. También hemos analizado la influencia de la estabilidad del vidrio en la absorción del agua, la cual puede inducir la cristalización del sistema, entre otros efectos no deseados. En este trabajo mostramos que la cantidad de agua absorbida por el vidrio depende de la estabilidad del mismo, alcanzando valores insignificantes en el caso de los vidrios ultra estables. El empaquetamiento compacto y la reducción de enlaces de hidrógeno disponibles en este tipo de vidrios podrían ser las causas de este comportamiento.<br>Vapour deposition has emerged as a very powerful tool to produce glasses of unprecedented stability. By tuning the deposition conditions, one can tailor the properties of the deposited glass. In particular, at substrate temperatures around 0.85Tg, the maximum stability is reached. On the other hand, fast-scanning nanocalorimetry has proven to be an extremely useful technique to evaluate different characteristics of the mechanisms behind the glass transition at temperatures much above the typically explored low temperature range. In this thesis, we present a detailed study of different aspects of the glass transition on vapour deposited organic glasses of Indomethacin and Celecoxib, by combining a variety of calorimetric techniques, from conventional to fast-scanning calorimetry, including high pressure-calorimetry.
Stable glasses produced from the vapour phase exhibit a different transformation mechanism with respect to glasses obtained by cooling the liquid. While the latter transform homogeneously throughout the volume of the sample, stable glasses initiate the transformation via a parallel transformation front that starts at the surface or interfaces and propagates into the bulk. By means of an ad-hoc surface normalization procedure we are able to identify transformation fronts and measure the front velocity. This velocity can be expressed as a function of the relaxation time of the liquid, v_gr=Cτ^(-0.79), where C depends on the deposition conditions of the sample and is not temperature dependent. This expression holds at least from Tg+10 K up to Tg+75 K. Both molecular orientation and thermodynamic stability, which are determined by deposition conditions, seem to play a role in the transformation rate. In particular, glasses with strong birefringence and low stability present faster front velocities.
Eventually, once the front has advance a particular distance, called cross-over length, the homogeneous transformation is triggered and dominates the transformation. By analysing glassy films thicker than the crossover length, we can study the bulk transformation dynamics. From the onset of devitrication and the width of the transformation peaks we infer the relaxation time of the glass. We have seen that the evolution of relaxation time as a function of temperature follows a super-Arrhenius dependence, similar to the VFT expression for the equilibrium alpha relaxation time of the supercooled liquid. This expression satisfactorily describes the dynamics of supercooled liquid and glasses of different stability simultaneously. This unified view of glasses and liquids may help understating the connection between the dynamic and thermodynamic characteristics of a glass. Indeed, we have shown that glasses of different stability follow density scaling relationships that were conceived to describe uniquely equilibrium liquid states. In that sense, the dynamics of liquid and glasses are influenced, in the same manner, by temperature and density uniquely.
We have also measured the variation of the onset temperature of devitrification of glasses with very different stability as a function of hydrostatic pressure, finding that the devitrification temperature tends to a unique Ton value, regardless of the stability of the glass, depicting a scenario in which both ultrastable and conventional glasses transform into the liquid at very similar temperatures.
Being crystallization a major drawback for the use of glasses in the pharmaceutical industry, we consider if ultrastability would somehow have a significant impact. We see that crystals grow a 30% slower on ultrastable glass surfaces. We have also analysed the influence of stability on water absorption, which may induce glass crystallization, among other undesired effects. We show that the amount of absorbed water depends on the stability of the glass, reaching an almost negligible value in the case of ultrastable glasses. The improved packing and reduction of accessible hydrogen bonds in this type of glasses could explain this behaviour.
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Nechipurenko, Inna. "FoxO limits microtubule stability and is itself negatively regulated by microtubule disruption." Case Western Reserve University School of Graduate Studies / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=case1333500439.
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Hauptmann, Marek, Sebastian Kaulfürst, and Jens-Peter Majschak. "Advances on Geometrical Limits in the Deep Drawing Process of Paperboard." NC State University, 2016. https://tud.qucosa.de/id/qucosa%3A30229.
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The geometrical limits of the deep drawing process of paper to advanced shapes are not known. This report examines the technological limits of convex elements of the base shape in relation to the drawing height and shows the material behavior in the bottom radius of 3D shapes with regard to special material properties. In the bottom radius, non-compressed wrinkles occurred due to the in-plane compression, but wrinkles were reduced by an increased blank holder force or tool temperatures and improved extensibility or in-plane compressive strain. The forming ratio during deep drawing (drawing height related to base diameter) was increased to a value of more than 1 by a blank holder force, which increased with the drawing height such that the initial blank holder force was reduced concurrently. Straight sections in the base shape reduced the risk for ruptures in the edge radii of rectangular shapes, producing a forming ratio in these radii of 2.5. The forming ratio was further supported by a pattern of creasing lines at the blanks with a radial orientation and a number near the expected maximum number of wrinkles. The spring-back at rectangular shapes mainly depended on the drawing height and edge radius.
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Kasuya, Yuko. "The presidential connection : party system instability and executive term limits in the Philippines /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC IP addresses, 2005. http://wwwlib.umi.com/cr/ucsd/fullcit?p3159873.
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Quinonez-Varela, Gustavo. "Modelling determination of the stability and penetration limits of wind turbines in autonomous power systems." Thesis, University of Strathclyde, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.436841.
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Brown, Kaitlyn. "Understanding the role of size limits in optimal fisheries management." Thesis, Queensland University of Technology, 2022. https://eprints.qut.edu.au/229264/1/Kaitlyn_Brown_Thesis.pdf.
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This thesis examines the optimality of size limit regulations in fisheries management using dynamical systems theory and mathematical optimisation. Analytical analysis of the proposed model reveals that the solution is confined to the boundary of the solution space. The findings show that a common approach to fisheries management (maximum- and minimum-size limits) are an optimal solution to an age structured model of a fishery.
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Karbassi, Massoumeh. "Analytical and experimental studies of the stability limits of nonpremixed flames in a co-flowing stream." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq24541.pdf.
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Papanikolaou, Nancy. "An experimental investigation of the flow structure and stability limits of jet diffusion flames in a co-flowing oxidizing stream." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0013/NQ31062.pdf.
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Yakouchyk, Katsiaryna [Verfasser], and Daniel [Akademischer Betreuer] Göler. "Post-Soviet Region, Authoritarian Stability, and Autocracy Promotion: Limits to the European Union’s External Governance / Katsiaryna Yakouchyk ; Betreuer: Daniel Göler." Passau : Universität Passau, 2019. http://d-nb.info/1180025814/34.
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Yakouchyk, Katsiaryna Verfasser], and Daniel [Akademischer Betreuer] [Göler. "Post-Soviet Region, Authoritarian Stability, and Autocracy Promotion: Limits to the European Union’s External Governance / Katsiaryna Yakouchyk ; Betreuer: Daniel Göler." Passau : Universität Passau, 2019. http://nbn-resolving.de/urn:nbn:de:bvb:739-opus4-6146.
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Zhou, Shuai. "Improvement of the prediction of the ductility limits of polycrystalline materials by using relevant multiscale schemes." Electronic Thesis or Diss., Paris, ENSAM, 2025. http://www.theses.fr/2025ENAME008.
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L’objectif principal de cette thèse de doctorat est d’améliorer les capacités des méthodes et outils numériques développés dans la prédiction des limites de ductilité des tôles métalliques polycristallines à l’aide de la méthode des éléments finis en plasticité cristalline (CPFEM). Cette méthode utilise un Volume Elémentaire Représentatif (VER) polycristallin pour capter avec précision les caractéristiques mécaniques des tôles étudiées. La technique d’homogénéisation périodique est adoptée pour assurer la transition entre l’échelle du VER et celle du monocristal. À l’échelle du monocristal, le comportement mécanique suit une formulation élastoplastique en grandes déformations, avec un écoulement plastique régi par la loi de Schmid. Pour améliorer les capacités prédictives de l’approche numérique développée, les effets de l’écrouissage cinématique, l’écrouissage isotrope basé sur la densité des dislocations, la taille de grains et l’endommagement microscopique sont intégrés dans le modèle constitutif du monocristal. De plus, les effets des paramètres microstructuraux (e.g., textures morphologiques et cristallographiques) sont étudiés. À l’échelle macroscopique, les limites de ductilité sont prédites sous l’hypothèse de contraintes planes en utilisant le critère de bifurcation de Rice. Les résultats de cette thèse démontrent que ces approches multi-échelles avancées fournissent un outil robuste et fiable pour prédire avec précision les limites de ductilité des tôles métalliques polycristallines<br>The main objective of this PhD thesis is to improve the capabilities of the developed numerical methods and tools in the prediction of the ductility limits of polycrystalline metal sheets using the Crystal Plasticity Finite Element Method (CPFEM). This method uses a polycrystalline Representative Volume Element (RVE) to accurately capture the mechanical characteristics of metal sheets. The periodic homogenization multiscale scheme is adopted to ensure the transition between the RVE and single crystal scales. At the single crystal scale, the constitutive framework follows a finite strain rate-independent formulation, with the plastic flow governed by the Schmid law. To improve the predictive capabilities of the developed computational approach, the effects of kinematic hardening, dislocation density-based hardening, grain size, and damage within the framework of Continuum Damage Mechanics (CDM) are incorporated into the single crystal constitutive modeling. Furthermore, the effects of microstructural parameters (e.g., morphological and crystallographic textures) are investigated. At the macroscopic scale, the ductility limits are predicted under the plane-stress assumption using the Rice bifurcation theory. The findings of this thesis demonstrate that these advanced multiscale approaches provide a robust and reliable tool for accurately predicting the ductility limits of polycrystalline metal sheets
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Venter, Marike. "Computerized dynamic posturography in ballet dancers." Diss., University of Pretoria, 2016. http://hdl.handle.net/2263/59162.
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Balance is the ability to maintain an individual’s centre of mass over his or her base of support while standing. Ballet dancers have better balance than non-dancer control groups as they presented with better sensory-motor integrative skills when required to maintain a given posture for a longer period. To evaluate functional balance abilities, assessing individuals with above average balance abilities may provide valuable information about function. The balance of trained ballet dancers (N1) was investigated by comparing Sensory Organisaiton Test (SOT), Motor Control Test (MCT) and Limits of Stability (LOS) test results to matched non-trained individuals (N2). A quasi-experimental and two group design was used. Ten matched trained ballet dancers (N1) and 10 untrained individuals were included in the study. Trained ballet dancers (N1) had an average of 16.6 (± 4.24) years of experience, while non-trained individuals (N2) had none. No significant dufferences in demographic data was present between groups. Both groups reflected good overall balance and similar sensory organization. No differences in reflex latencies and weight symmetry of the left or right leg was present. Trained ballet dancers (N1) reflected higher response strength for medium forward translations (p.05) with the left leg, indicating poorer amplitude scaling in response to platform translations than those of non-trained individuals (N2). In non-trained individuals (N2), there was less variation in the response strength between legs. Research indicated that stretch reflex amplitude was attenuated as load stability was reduced. Co-contraction was also heightened as stability was reduced, but not enough to oppose the induced instability, probably due to feed-forward strategies instead of rapid involuntary feedback. Trained ballet dancers (N1) were able to extend further out of their Centre of Pressure (COP) than non-trained individual (N2) during forward (p<.05) and right forward (p<.05) movements. It was concluded that that for forward and right forward movements, ballet dancers used the feedback they received during the movements better than non-trained individuals (N2), resulting in a better awareness of where to go in space and how to reach that position after a subsequent attempt. This difference may be as a result of continuous training. Ballet training exercises can be used to rehabilitate individuals with impaired limits of stability. Further studies should be done on exactly which ballet training exercise results in increased limits of stability.<br>Mini Dissertation (MCommunication Pathology)--University of Pretoria, 2016.<br>Speech-Language Pathology and Audiology<br>MCommunication Pathology<br>Unrestricted
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Assefa, Hana Yohannes. "Stability Investigation of an Advanced Electrical Rail Vehicle : Investigation of the Effect of Nonlinearity Introduced by a Switching Model of an Advanced Electrical Rail Vehicle on the General Performance and The Stability Limits." Thesis, Norwegian University of Science and Technology, Department of Electrical Power Engineering, 2009. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-9094.
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<p>Reducing grid harmonics and increasing grid stability are both major issues for the operation of rail vehicles. For stability investigation of complex power systems, simplified system models are in need in order to reduce the model complexity and the simulation time. In this thesis work the effect of modelling a voltage source converter (VSC) for traction power system with and without the detailed pulse width modulated (PWM)-switch model is modeled. Effect of different operating conditions for the switching model on the harmonic content of the system is also analyzed. The same disturbance is imposed for the two models and the low frequency oscillation of the DC- link voltage response is compared and analyzed. The effect of semiconductor switching on the stability limit of the system is also investigated. Furthermore, the performance of a PWM time delay compensation technique during transient is analyzed. The result shows that in the model including the switching the DC- link voltage oscillation is damped and has a better stability margin compared to the average model. In the detailed switching model a converter loss is included while in the average model a no loss ideal case scenario is considered. As far as the switching harmonic is considered, the switching model with an operational condition of a high switching frequency and a switching frequency with an integer multiple of the fundamental frequency has a low harmonic content on the system compared to the operating condition of a low switching frequency which is not integer multiple of the fundamental frequency. A unipolar voltage switching technique has also a tremendous advantage over the bipolar voltage switching technique as far as this harmonic content in the system is concerned. Using a unipolar voltage switching technique reduces the harmonic content in the overall railway system. For triangular carrier modulators, an average time delay from the reference voltage to the actuated PWM terminal voltage of half the switching frequency is assumed .The delay in DC- link voltage control loop caused by the switching dead-time effect was improved by compensation of dead-time in the inverse-park transformation block of the control loop. The comparison of the compensated and non-compensated model proves that the compensated model is better in terms of the overshoot of amplitude of transient.</p>
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Chan, Teck-Wai. "Proximity-to-separation based energy function control strategy for power system stability." Thesis, Queensland University of Technology, 2003. https://eprints.qut.edu.au/15840/1/Teck-Wai_Chan_Thesis.pdf.
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The issue of angle instability has been widely discussed in the power engineering literature. Many control techniques have been proposed to provide the complementary synchronizing and damping torques through generators and/or network connected power apparatus such as FACTs, braking resistors and DC links. The synchronizing torque component keeps all generators in synchronism while damping torque reduces oscillations and returns the power system to its pre-fault operating condition. One of the main factors limiting the transfer capacity of the electrical transmission network is the separation of the power system at weak links which can be understood by analogy with a large spring-mass system. However, this weak-links related problem is not dealt with in existing control designs because it is non-trivial during transient period to determine credible weak links in a large power system which may consist of hundreds of strong and weak links. The difficulty of identifying weak links has limited the performance of existing controls when it comes to the synchronization of generators and damping of oscillations. Such circumstances also restrict the operation of power systems close to its transient stability limits. These considerations have led to the primary research question in this thesis, "To what extent can the synchronization of generators and damping of oscillations be maximized to fully extend the transient stability limits of power systems and to improve the transfer capacity of the network?" With the recent advances in power electronics technology, the extension of transfer capacity is becoming more readily achievable. Complementary to the use of power electronics technology to improve transfer capacity, this research develops an improved control strategy by examining the dynamics of the modes of separation associated with the strong and weak links of the reduced transmission network. The theoretical framework of the control strategy is based on Energy Decomposition and Unstable Equilibrium Points. This thesis recognizes that under extreme loadings of the transmission network containing strong and weak links, weak-links are most likely to dictate the transient stability limits of the power system. We conclude that in order to fully extend the transient stability limits of power system while maximizing the value of control resources, it is crucial for the control strategy to aim its control effort at the energy component that is most likely to cause a separation. The improvement in the synchronization amongst generators remains the most important step in the improvement of the transfer capacity of the power system network.
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Chan, Teck-Wai. "Proximity-to-Separation Based Energy Function Control Strategy for Power System Stability." Queensland University of Technology, 2003. http://eprints.qut.edu.au/15840/.
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The issue of angle instability has been widely discussed in the power engineering literature. Many control techniques have been proposed to provide the complementary synchronizing and damping torques through generators and/or network connected power apparatus such as FACTs, braking resistors and DC links. The synchronizing torque component keeps all generators in synchronism while damping torque reduces oscillations and returns the power system to its pre-fault operating condition. One of the main factors limiting the transfer capacity of the electrical transmission network is the separation of the power system at weak links which can be understood by analogy with a large spring-mass system. However, this weak-links related problem is not dealt with in existing control designs because it is non-trivial during transient period to determine credible weak links in a large power system which may consist of hundreds of strong and weak links. The difficulty of identifying weak links has limited the performance of existing controls when it comes to the synchronization of generators and damping of oscillations. Such circumstances also restrict the operation of power systems close to its transient stability limits. These considerations have led to the primary research question in this thesis, "To what extent can the synchronization of generators and damping of oscillations be maximized to fully extend the transient stability limits of power systems and to improve the transfer capacity of the network?" With the recent advances in power electronics technology, the extension of transfer capacity is becoming more readily achievable. Complementary to the use of power electronics technology to improve transfer capacity, this research develops an improved control strategy by examining the dynamics of the modes of separation associated with the strong and weak links of the reduced transmission network. The theoretical framework of the control strategy is based on Energy Decomposition and Unstable Equilibrium Points. This thesis recognizes that under extreme loadings of the transmission network containing strong and weak links, weak-links are most likely to dictate the transient stability limits of the power system. We conclude that in order to fully extend the transient stability limits of power system while maximizing the value of control resources, it is crucial for the control strategy to aim its control effort at the energy component that is most likely to cause a separation. The improvement in the synchronization amongst generators remains the most important step in the improvement of the transfer capacity of the power system network.
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Abrantes, Adriano Lima. "Bifurcações induzidas por limites no contexto de estabilidade de tensão." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/18/18154/tde-31082016-113911/.
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A interligação dos sistemas elétricos de potência (SEPs) resultou em um aumento na complexidade dos mesmos. Além disso, devido a pressões econômicas e ambientais, os sistemas têm operado mais próximos aos seus limites de transmissão, o que aumenta a relevância de análises de segurança no contexto de estabilidade de tensão. Neste cenário, problemas associados à capacidade de transmissão do SEP, como a bifurcação induzida por limite (BIL), tornam-se mais importantes, criando a necessidade de ferramentas de análise apropriadas. Um dos objetivos deste projeto de mestrado é estudar mais profundamente as BILs para que se possa tratar melhor o fenômeno no contexto de estabilidade de tensão. Outro objetivo é o desenvolvimento de técnicas para avaliação da margem de estabilidade de tensão (MET) considerando a possibilidade de ocorrência de BIL. Finalmente, dada a grande importância e ampla utilização da análise de sensibilidade da MET devido à bifurcação sela-nó (BSN) em estudos de estabilidade de tensão, o equacionamento do problema de análise de sensibilidade da MET devido à BIL é o terceiro objetivo deste trabalho. A análise de sensibilidade é importante pois, não só fornece mais informações sobre o fenômeno de instabilidade e seus mecanismos, mas também auxilia na análise de segurança de SEPs, fornecendo informações sobre quais ações de controle serão mais eficazes na manutenção da MET e quais contingências serão mais severas. Isto é, quais mudanças no sistema mais afetam a MET. No entanto, este tipo de análise só foi realizado para o caso em que a MET é determinada por um ponto de BSN, não para o caso da BIL. Com o intuito de possibilitar que ferramentas de seleção de controles preventivos tratem o fenômeno da BIL, realizou-se uma análise de sensibilidade do ponto de BIL similar à análise usual baseada na BSN. Outra contribuição deste trabalho é uma formulação suave de limites complementares que foi aplicada ao problema de limites de injeção de potência reativa de unidades geradoras. A formulação proposta transforma, pelo menos numericamente, a BIL em uma BSN que pode ser detectada através de métodos já consolidados na literatura.<br>The interconnection of electric power systems (EPSs) led to an increase in security assessment complexity. Besides, due to economical and environmental inuences, EPSs have been operating closer to their transmission limits, which raises the relevance of security assessment in the context of voltage stability. In this scenario, problems related to EPS power transmission capacity, such as the limit induced bifurcation (LIB), become more important and bring the need of appropriate analysis tools. One of the goals of this project is to study the LIB problem more deeply, so it can be better understood in the context of voltage stability. Another objective is the development of methods for evaluating the load margin (LM) considering the possible occurrence of LIBs. Finally, since the LM sensitivity analysis due to saddle-node bifurcation (SNB) plays a highly important role in voltage stability studies, developing a method for LM sensitivity analysis due to LIB is our third objective. The sensitivity analysis is important not only because it provides information on the instability phenomenon and its mechanisms, but it is also useful for EPS security assessment, since it may provide knowledge on which control actions will be more eective in increasing the LM and which contingencies may be more severe. However, this analysis has been performed only for the case in which the LM is determined by a SNB point, not for the LIB case. With the intention of enabling pre-existing preventive control selection tools to treat the LIB phenomenon, a sensitivity analysis was performed at the LIB point similarly to what was developed for the SNB. Another contribution of this work is a smoothing formulation for complementary limits that was applied to the problem of limited reactive power injection of generating units. The proposed formulation transforms, at least numerically, the LIB in a SNB, which may be detected through methods already established in literature.
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Pan, Qiujing. "Deterministic and Probabilistic Assessment of Tunnel Face Stability." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAI044.
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The main work for Qiujing PAN’s PhD thesis is to develop the stability analysis for underground structures, which contains two parts, deterministic model and probabilistic analysis. During his 1st year of PhD research, he has mainly finished the deterministic model study. In the 2nd year, I developed a probabilistic model for high dimensional problems<br>In the contemporary society, the utilization and exploitation of underground space has become an inevitable and necessary measure to solve the current urban congestion. One of the most important requirements for successful design and construction in tunnels and underground engineering is to maintain the stability of the surrounding soils of the engineering. But the stability analysis requires engineers to have a clear ideal of the earth pressure, the pore water pressure, the seismic effects and the soil variability. Therefore, the research aimed at employing an available theory to design tunnels and underground structures which would be a hot issue with high engineering significance. Among these approaches employed to address the above problem, limit analysis is a powerful tool to perform the stability analysis and has been widely used for real geotechnical works. This research subject will undertake further research on the application of upper bound theorem to the stability analysis of tunnels and underground engineering. Then this approach will be compared with three dimensional analysis and experimental available data. The final goal is to validate new simplified mechanisms using limit analysis to design the collapse and blow-out pressure at the tunnel face. These deterministic models will then be used in a probabilistic framework. The Collocation-based Stochastic Response Surface Methodology will be used, and generalized in order to make possible at a limited computational cost a complete parametric study on the probabilistic properties of the input variables. The uncertainty propagation through the models of stability and ground movements will be evaluated, and some methods of reliability-based design will be proposed. The spatial variability of the soil will be taken into account using the random field theory, and applied to the tunnel face collapse. This model will be developed in order to take into account this variability for much smaller computation times than numerical models, will be validated numerically and submitted to extensive random samplings. The effect of the spatial variability will be evaluated
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Velleret, Aurélien. "Mesures quasi-stationnaires et applications à la modélisation de l'évolution biologique." Thesis, Aix-Marseille, 2020. http://www.theses.fr/2020AIXM0226.
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Je décris le comportement en temps long de plusieurs processus qui illustrent les mécanismes de sélection naturelle. Il arrive que ces effets de sélection s’interprètent comme un conditionnement qui biaise la dynamique d’un processus aléatoire "neutre". Ce processus évolue sur un espace potentiellement très général, notamment continu et non borné. On peut ainsi caractériser aussi bien la dynamique du profil complet de la population que celle du profil d’un individu choisi uniformément dans la population. On voit naturellement apparaître dans ces modèles des transitions brutales de ces lois qui rendent l’analyse plus délicate que pour les modèles sans conditionnement. La première partie de ce travail consiste à mettre en place des conditions permettant d’obtenir des résultats analogues à la stationnarité lorsqu’on prend en compte un tel conditionnement. La seconde porte sur l’application de ces critères à différents modèles de population sous sélection et sur leur interprétation. Les exemples d'applications concernent le modèle de l'optimum mobile pour l'adaptation à un changement environnemental, un modèle de sélection de groupes et celui du cliquet de Müller sur le maintien d'une sous-population préservée des mutations délétères<br>I describe the long term behavior of several processes that model the mechanisms of natural selection. In the cases under consideration, one can interpret those selective effects as a conditioning which introduces a bias on the dynamics of some « neutral » stochastic process. This process evolves in a potentially very general space, notably continuous and unbounded. By these means, one can characterize the dynamics of the whole profile of individuals in the population of study as well as the profile of an individual uniformly chosen in the population. One can naturally see in these descriptions some brutal transitions of the distribution laws as time evolves, which makes the analysis much trickier than for the models without conditioning. The first part of this thesis consists in setting the conditions under which one can prove results analogous to the notion of stationarity while taking the conditioning into account. The second part is dedicated to the application of these criteria for a diversity of models of populations under selection and their interpretation. Examples of application include the mobile optimum model for adaptation to environmental change, a group selection model and Müller's ratchet model that describes how the sub-population unaffected by deleterious mutations can maintain itself
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EL-Kahky, Ayman Mohamed. "Video posturography near the limit of stability." [Maastricht : Maastricht : Universiteit Maastricht] ; University Library, Maastricht University [Host], 1998. http://arno.unimaas.nl/show.cgi?fid=8542.
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Liu, Ying, and 劉影. "Limit equilibrium methods for slope stability analysis." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B42576684.
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Liu, Ying. "Limit equilibrium methods for slope stability analysis." Click to view the E-thesis via HKUTO, 2002. http://sunzi.lib.hku.hk/hkuto/record/B42576684.
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Bonne, Nicolas. "Stabilité de l'intéraction onde de choc/ couche limite laminaire." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLX025/document.
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Le phénomène d’interaction onde de choc/couche limite (CL) est omniprésent en aérodynamique. De manière générale, il génère des oscillations basses fréquences qui peuvent être néfastes pour les machines. L’exemple typique est le tremblement de l’onde de choc sur profil d’aile en régime transsonique, dangereux car il peut exciter les modes de structure de l’aile et potentiellement la rompre. Ce type de phénoménologies a été largement étudié en condition de CL amont turbulente, ce qui a donné lieu à des scénarios physiques crédibles et des méthodologies d’étude efficaces, notamment les analyses de stabilité sur champs turbulents moyennés (RANS). Toutefois la technologie laminaire, c’est-à-dire l’utilisation de CL laminaires en vue de réduire la consommation des aéronefs représente un nouveau challenge scientifique sur cette problématique. La physique est en effet fortement impactée par la nature laminaire de la CL, notamment du fait de la faible résilience de celle-ci aux gradients de pression adverses et à la transition turbulente. Cette thèse a ainsi porté les méthodes d’analyse de stabilité sur champ RANSpour les situations de CL laminaire. L’originalité et l'apport de l’étude résident dans la prise en compte des modèles de transition dans l’approchelinéarisée sur champ RANS. Les modèles utilisés (RANS et transition) ont donc été linéarisés afin de réaliser des études de stabilité en perturbant toutes les variables aérodynamiques. La validation de la méthode a été réalisée par comparaison avec des résultats expérimentaux et de simulation (LES) sur deux conifurations d'application. La première configuration est le cas de la réflexion d’un choc oblique sur une plaque plane. La deuxième est celle du choc droit à l’extrados d’un profil en condition transsonique. Ces deux cas sont en condition de CL laminaire à l'amont du choc.Des analyses de stabilité et de résolvent ont été réalisées.Ces approches ont permis de caractériser le comportement d’oscillateur/amplificateur des écoulements en question et d'analyser la physique des instationnarités observées dans les expériences.Le cas de la réflexion de choc est caractérisé par trois fréquences. L'analyse de stabilité montre que celles-ci ne correspondent pas à des modes globaux instables mais à une dynamique d'amplificateur de l'écoulement. L'analyse de résolvent identifie bien ces trois fréquences. L’analyse des réponses optimales, couplée à une analyse de stabilité locale, a ensuite permis de proposer des scénarios physiques de ces dynamiques.Dans le cas du choc droit sur profil en régime transsonique, l'écoulement apparaît globalement instable. Deux modes d'instabilité sont identifiés. Le premier à basse fréquence correspond au phénomène de tremblement observé en conditions turbulentes. Le deuxième apparaît à plus haute fréquence, et correspond à un mode d'oscillation de la bulle de séparation présente sous le pied de choc.Plus largement, la thèse permet de suggérer que certaines dynamiques dans ce type d’interaction procèdent de mécanismes similaires liés à la respiration de la bulle de séparation laminaire<br>The shock wave boundary layer (BL) interaction phenomenon is ubiquitous in aerodynamic. In general this interaction generates some low frequency oscillations which can be disastrous for the machines. The typical example is the buffet phenomenon on an airfoil in transonic conditions. Buffet is dangerous since its low frequency can excite the structural modes of the airfoil and break it. The phenomenology has been wildly studied when the incoming BL is turbulent. These studies have derived several credible scenarii and efficient methodologies to capture its dynamic, especially the stability analysis tools on an averaged turbulent flow (RANS). However laminar technologies, the use of laminar BL to reduce the fuel consumption of planes, represent a new scientific challenge on this problematic. In fact, the physic of the interaction is importantly impacted by the laminar nature of the BL especially because of its weak resilience to an adversed pressure gradient and of the transition to turbulence.The thesis deals with the methodologies for the stability analysis on a RANS base flow in the case of a laminar BL. The originality and the contribution of this work have been to take into account a transition criteria in the linearised dynamic on a RANS base flow. The model used (RANS and transition) have then been linearized in order to make a stability analysis which take into account all the aerodynamic varaibles. The validation of this methodology has been made by comparison to expermient and simulation (LES) on two configurations of application. The first one is a weak reflected shock wave on a flat plate. The second one is the strong shock around an airfoil in a transonic regime. In both cases the incoming BL is laminar.Stability and resolvent analysis have been made. These approches have been able to caratirized the ocillator/noise amplifier behavior of the flow and to enabled a physical analysis of the unsteadinesses observed in the experiments.The case of the reflected shock wave is caracterized by three frequencies. The stability analysis shows that they don't correspond to globally unstable modes but to a noise amplifier behavior of the flow. The resolvent analysis identifies the three frequencies. The analysis of the optimal response, coupled with a local stability analysis, enables to proposed physical scenarii of these dynamics.In the case of the strong shock on an airfoil in transonic regime, the flow is globally unstable. Two unstable modes have been identified. The first one, at low frequency, correspond to the buffet phenomenon also observed in the turbulent case. The second one appears at higher frequency and correspond to the oscillation of the separation bubble formed at the feet of the shock.More generally, this thesis suggests that some dynamics of these two interactions result from the same mecanism linked to the breathing motion of the laminar separation bubble
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Berrouane, Salah. "Les lois limites des k-iemes valeurs de record et leurs concomitants." Paris 6, 1986. http://www.theses.fr/1986PA066388.
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Nous généraliserons tous les résultats des lois limites des k-iemes valeurs de record (k >ou= 1). Nous énoncerons deux résultats concernant la structure poissonienne des k-iemes valeurs de record. Nous étudierons en détail la stabilité en probabilité ; et nous énoncerons un résultat concernant les records issus d'une fonction de répartition normale. Nous utiliserons le processus de winner w(t) pour l'approximation forte des k-iemes valeurs de record. Une application du théorème de Komlos, Major et Tusnady donne un résultat concernant la stabilité forte. En second lieu, nous définissons les concomitants des k-iemes valeurs de record. Nous déterminerons la distribution générale des concomitants des k-iemes valeurs de record, dans le cas général, et dans le cas de la régression linéaire simple. Nous étudierons les lois limites des concomitants des k-iemes valeurs de record dans le modèle de la régression linéaire simple, et dans le cas général.
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Butler, Bradley D. "AXIAL COMPRESSOR FLOW BEHAVIOR NEAR THE AERODYNAMIC STABILITY LIMIT." UKnowledge, 2014. http://uknowledge.uky.edu/me_etds/34.
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In this investigation, casing mounted high frequency response pressure transducers are used to characterize the flow behavior near the aerodynamic stability limit of a low speed single stage axial flow compressor. Time variant pressure measurements are acquired at discrete operating points up to the stall inception point and during the transition to rotating stall, for a length of time no shorter than 900 rotor revolutions. The experimental data is analyzed using multiple techniques in the time and frequency domains.
Experimental results have shown an increase in the breakdown of flow periodicity as the flow coefficient is reduced. Below a flow coefficient of 0.40 a two node rotating disturbance develops with a propagation velocity of approximately 23% rotor speed in the direction of rotation. During rotating stall, a single stall cell is present with a propagation velocity of approximately 35% rotor speed. The stall inception events present are indicative of a modal stall inception.
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GILLIBERT, ALAIN. "Mesures de masse aux limites de la stabilite." Paris 7, 1987. http://www.theses.fr/1987PA077290.
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Dhingra, Manuj. "Compressor stability management." Diss., Available online, Georgia Institute of Technology, 2006, 2006. http://etd.gatech.edu/theses/available/etd-01102006-174943/.
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Thesis (Ph. D.)--Aerospace Engineering, Georgia Institute of Technology, 2006.<br>Prasad, J.V.R., Committee Chair ; Neumeier, Yedidia, Committee Member ; Seitzman, Jerry, Committee Member ; Sankar, Lakshmi, Committee Member ; Wadia, Aspi, Committee Member.
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Aryal, Krishna Prasad. "Slope Stability Evaluations by Limit Equilibrium and Finite Element Methods." Doctoral thesis, Norwegian University of Science and Technology, Department of Civil and Transport Engineering, 2006. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-1868.
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<p>This thesis deals with slope stability evolutions carried out by commonly used limit equilibrium (LE) and finite element (FE) methods. The study utilizes two LE based software (SLOPE/W and SLIDE) and one FE based software (PLAXIS). The principal difference between these two analyses approaches is that the LE methods are based on the static of equilibrium whereas FE methods utilise the stress‐strain relationship or constitutive law. </p><p>To fulfil one of the aims of the study, the LE based methods are compared based on the factor of safety (FOS) obtained for various load combinations. The comparison is mainly based on simplified slope geometry and assumed input parameters. Among the LE methods, the Bishop simplified (BS), Janbu simplified (JS) and Janbu GPS methods are compared with the Morgenstern‐Price method (M-PM). The two latter methods satisfy both force and moment equilibriums. In addition, M-PM allows a variable interslice force function. These LE methods are well established for many years, and thus some of them are still commonly used in practice for stability analysis. Simplicity and relatively good results are the advantages of these methods. </p><p>The comparative study among the LE methods shows that BSM is as good as M-PM for normal condition in circular shear surface (SS) analysis. Similarly, JSM is as good as M‐PM for plane SS analysis. Moreover, the study shows that BSM is insensitive to interslice forces in circular SS analysis, whereas JSM is insensitive to interslice forces in plane SS analysis.This is true for the analyses without any external forces acting in slopes. With some external forces, both methods may be sensitive to the interslice force function, and thus the analyses may result in either higher or lower FOS compared with M-PM. Even if the M-PM assumes an interslice force function in analysis, the method is found suitable, because the method satisfies both force and moment equilibrium conditions. The comparison between Janbu’s GPS method and the M‐PM shows a very good agreement with nominal difference (<2%) in the FOS.</p><p>Moreover, the M-PM has been compared with results from the FE analyses. Compared with the FE (PLAXIS) analyses, the LE (M-PM) analyses may estimate 5 – 14% higher FOS, depending on the conditions of a dry slope and a fully saturated slope with hydrostatic pore pressure distributions. For fully saturated conditions in the slope, inaccurate computation of stresses in LE methods may have resulted in larger difference in the computed FOS. </p><p>Since, the FE software is based on stress-strain relationship, stress redistributions are surely better computed even for a complicated problem. This has been found one of the advantages in FE simulations. A parameter study shows that the application of a positive dilatancy angle in FE analysis can significantly improve the FOS (4 - 10%). On contrast, the shear surface optimization in LE (M-PM in SLOPE/W) analysis results in lower FOS, and thus minimizing the difference in FOS compared with FE analysis. Thus, the optimisation of the circular SS in LE analyses and application of dilatancy angle in FE analyses may provide very good agreement in FOS keeping the variations less than 5%. </p><p>Instability of natural and built-in slopes has been a serious geotechnical challenge, particularly in mountainous countries like Nepal, where a large number of water-induced disasters are experienced every year. Since the country lies in a relatively active earthquake zone, earthquake events may adversely affect the long-term stability of slopes along the highways, around hydropower installations and elsewhere. The selected case study from the Middle Marsyangdi Hydroelectric Project (MMHEP) in Nepal was carried out, focusing on both the groundwater and earthquake effects on long-term stability of slopes. In addition, the analyses were performed for pore pressure distributions by seepage analyses and pseudo-static and dynamic earthquake events imposed in the case slopes. </p><p>Another important contribution of this study has been the investigations of the shear strength parameters for the case study slopes. The investigation ranges from the basic index tests, which were carried out to characterize material properties and establish a basis for building-in the test specimens, to the most advanced triaxial tests. Additionally, both consolidated drained and undrained triaxial tests were conducted, aiming to find any differences in the strength parameters. A total of twenty-four triaxial tests from four different soil samples, taken from the case study slopes, were conducted to define the Mohr-Coulomb failure envelope, and thus obtained the effective shear strength parameters (c’, φ’) for stability evaluations. In addition, the coefficients of permeability were investigated as an important input for seepage analyses. </p><p>Two slopes, one located at the dam site and the other at the powerhouse of the MMHEP, were evaluated for long-term stability conditions. The study, carried out using the monitored groundwater table, reveals that the groundwater is the potential destabilizing factor in the slopes. Both hydrostatic pore pressure distributions with phreatic surface correction and seepage analysis were carried out in the stability analysis. The pore pressure distributions from the seepage analyses indicated critically stable slopes particularly at the toe areas. However, a significant improvement in the FOS was found with lowered groundwater table. Moreover, the study, carried out for the pseudo-static analysis with the selected earthquake acceleration coefficient, shows that otherwise stable slopes become critical in earthquake events. However, the case slopes were found to have higher FOS than the recommended minimum value for such conditions. The dynamic earthquake simulations indicate both positive excess pore pressure and suction developed during the earthquakes. The applied earthquake acceleration of 0.15g (35% of PGA, where PGA = 4.2 m/s2)) shows critical conditions in the slopes. Moreover, larger accumulated displacements were found close to the slope surface compared to the inner side of the slopes. In addition, the dynamic analyses further indicate plastic points all over the slopes and increased shear strains in a particular location. </p><p>Nevertheless, the difference in FOS found from the LE and FE analyses may have a little interest, if there is a large uncertainty in the input parameters. Therefore, priority should be given to investigate the shear strength parameters and precise mapping of the slope geometry before selecting an appropriate analysis method. Most preferably, a slope should be analyzed by FE methods, else otherwise by LE based M‐PM or GPS method with investigated input parameters. </p>
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Potgieter, Jean-Timothy. "Finite element versus limit equilibrium stability analyses for surface excavations." Diss., University of Pretoria, 2017. http://hdl.handle.net/2263/63001.
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Limit equilibrium methods are widely and routinely used in practice. In several codes, limit
equilibrium methods are recommended to evaluate the stability of a lateral support systems,
such as soil-nails and anchors, to an acceptable defined factor of safety.
For decades, limit equilibrium methods have been used successfully in providing an acceptable
margin of safety against failure (movements, which can be significantly more complex, is not
considered). However, due to the advances in computational power offered by personal
computers, finite element modelling has become increasingly accessible.
Since the idea immerged of using a strength reduction factor in finite element displacement
analyses, an increase in the use thereof to calculate the factor of safety has been observed.
However, the use of finite elements has often led to misinterpretation of the results. Several
authors have cautioned engineers to the complexities involved in using finite element analyses
to model geotechnical problems. Studies have been conducted comparing the use of finite
elements to other methods. However, most of these studies consider only slope problems. Few
studies have been conducted for lateral support systems.
Several codes of practice use the numerical quantity of ‘factor of safety’ to define the suitability
of geotechnical design. Whether finite element- or limit equilibrium methods are used, the accurate calculation of the factor of safety remains paramount to quantifying the stability of a
geotechnical structure.
The aim of this research is to compare limit equilibrium and finite element methods in
evaluating the stability, in terms of factor of safety, of soil-nailed and anchored lateral support
systems in surface excavations.
This was done by using four methods of analysis to calculate the factor of safety. Two
traditional limit equilibrium methods were used (trail wedge and method of slices). The newer,
finite element strength reduction technique was used. Finally, a hybrid method which combines
a finite element analysis with limit equilibrium slip surface analysis was used.
These methods of analysis were applied to three different geometries. A uniform slope without
any reinforcing was analysed. This was followed by the analysis of an 8.5m soil-nail supported
face and a 17m face supported by anchors.
A parametric study was conducted for the soil-nailed and anchored excavations. Material
properties (friction angle, cohesion etc.), modelling parameters (boundary distances, mesh
resolution etc.) and engineering design variables (reinforcement capacity etc.) were varied in
order to observe the influence on the factor of safety.
It is concluded that limit equilibrium methods, such as a trial wedge method and the method of
slices, compare well with each other throughout the analyses. Using a combination of finite
elements with a slip surface analysis compares poorly with the other methods. By using the
finite element strength reduction technique, an optimised failure mechanism is found. The finite
element strength reduction technique compares well with limit equilibrium methods if the
following two conditions are met:
• The same failure mechanism is evaluated for both methods; and
• the capacity of reinforcement is consistently specified in both methods.<br>Dissertation (MEng)--University of Pretoria, 2017.<br>Civil Engineering<br>MEng<br>Unrestricted
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DELCOIGNE, FRANCK. "Stabilite et limite thermodynamique dans certains reseaux a polling." Paris 6, 1999. http://www.theses.fr/1999PA066143.
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On aborde dans cette these les problemes du calcul des conditions necessaires et suffisantes d'ergodicite et du comportement en limite thermodynamique de certains systemes a polling decrivant l'evolution de reseaux de transport. Le modele propose dans le premier chapitre se presente comme une variante a routage markovien, ou les serveurs arrivant a une file vide attendent la prochaine arrivee de client a cette file (e. G. Probleme du taxi). Sous des hypotheses assez larges de stationnarite pour les processus d'arrivee et de service, on montre que le reseau ne peut etre que transient ou recurrent-nul. On obtient en outre une classification complete de chaque file ainsi que des theoremes limites apres renormalisation temporelle ad-hoc. L'abondante litterature consacree a l'etude des systemes a polling montre qu'il est tres difficile voire impossible meme dans les cas les plus simples, de calculer la mesure invariante de tels systemes. Dans cette perspective, on demontre, dans le deuxieme chapitre, la propagation du chaos pour une suite de modeles a polling symetriques a n files et v serveurs lorsque n et v tendent simultanement vers l'infini. On peut alors caracteriser completement la dynamique et le regime stationnaire d'un p-uplet arbitraire de files. Dans la derniere partie, on traite de l'extension de ces resultats au cas ou chaque reseau est constitue d'un nombre fini de sous-systemes symetriques.
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Sartor, Fulvio. "Unsteadiness in transonic shock-wave/boundary layer interactions : experimental investigation and global stability analysis." Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4705.
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Dans cette étude nous considérons l'interaction entre une onde de choc et une couche limite turbulente sur un écoulement transsonique sur une bosse d'un point de vue expérimentale et théorique.Des mesures expérimentales ont permis de montrer que l'interaction est caractérisée par la coexistence de deux fréquences caractéristiques distinctes, mais l'origine des oscillations est controversée. Des simulations numériques permettent une description de l'écoulement moyen, mais ne sont pas capables de reproduire le comportement instable de l'interaction. Nous proposons d'abord une étude de stabilité globale: une décomposition en valeurs propres de l'opérateur de Navier-Stokes linéarisé indique que l'interaction est un phénomène stable, et la dynamique de l'écoulement ne peut pas être décrite par un mode global instable.Nous considérons ensuite une approche linéarisée, où la réceptivité de l'écoulement à un forçage externe est analysée à travers une décomposition en valeurs singulières du Résolvant global. Cette nouvelle approche est proposée afin d'expliquer le processus de sélection de fréquence dans cet écoulement, et montre que l'interaction filtre et amplifie le bruit résiduel existant.La même approche est enfin appliquée sur un cas d'écoulement transsonique autour d'un profil d'aile, qui peut présenter des oscillations périodiques de l'onde de choc. La décomposition en valeurs propres de opérateur de Navier-Stokes linéarisé est capable de décrire la dynamique du choc, tandis que la décomposition en valeurs singulières du Résolvant global peut indiquer la présence des instabilité convectives<br>A transonic interaction between a shock wave and a turbulent boundary layer is experimentally and theoretically investigated. The configuration is a channel flow over a bump, where a shock wave causes the separation of the boundary layer and a recirculating bubble is observed downstream of the shock foot.The mean flow is experimentally investigated by means of PIV, then different techniques allows to identify the main unsteadiness of this shock-wave/boundary-layer interaction. As recognised in similar configurations, the flow presents two distinguished characteristic frequencies, whose origins are still unknown.Numerical simulations are performed solving RANS equations. Results are in good agreement with the experimental mean flow, but the approach fails to predict the unsteady. The solution of RANS equations is then considered as a base flow, and a global stability analysis is performed. Eigenvalue decomposition of the Navier-Stokes operator indicates that the interaction is stable, and the dynamics cannot be described by unstable global modes.A linearised approach based on a singular-value decomposition of the Resolvent is then proposed: the noise-amplifier behaviour of the flow is highlighted by the linearised approach. Medium-frequency perturbations are shown to be the most amplified in the mixing layer, whilst the shock wave behaves as a low-pass filter.The same approach is then applied to a transonic flow over a profile, where the flow can present high-amplitude shock oscillations. The stability analysis can describe both the buffet phenomenon when an unstable mode is present, and the convective instabilities responsible of medium-frequency unsteadiness
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Poruba, Michal. "Návrh sanace sesuvu." Master's thesis, Vysoké učení technické v Brně. Fakulta stavební, 2015. http://www.nusl.cz/ntk/nusl-227466.
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Main goal of thesis entitled "Landslide remediation design" is - based on acquired knowledge and submitted documents about the problematic of landslides - to determine the causes of the landslides on II. class road between villages Koryčany and Jestřabice and to propose optimal redevelopment.
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ARAUJO, LUIZ GONZAGA DE. "NUMERICAL STUDIES OF THE STABILITY OF GEOTECHNICAL MATERIALS THROUGH LIMIT ANALYSIS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1997. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=2084@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR<br>O presente trabalho apresenta um estudo de problemas de
estabilidade, freqüentemente,encontrados, em Engenharia
Geotécnica, através da técnica da Análise Limite associada
ao Método dos Elementos Finitos (MEF).Inicialmente, faz-se
uma revisão das formulações da Análise Limite, via
MEF,encontradas, com maior freqüência, na literatura
técnica especializada.Uma formulação mista que é descrita
em detalhe na tese foi escolhida para implementação.
Extensões das formulações da Análise Limite de meios
contínuos são propostas para contemplar características de
maciços rochosos fraturados. É proposto,também, um
procedimento numérico para tratar de problemas de
estabilidade de meios que exibem fluxo plástico não
associado.As implementações realizadas foram validadas
através de problemas cujas soluções
podem ser obtidas por via analítica.Finalmente, um número
considerável de problemas de interesse em Engenharia
Geotécnica é estudado utilizando a implementação realizada.
Os resultados destes estudos sugerem a viabilidade da
utilização da técnica estudada na solução de problemas
práticos de Engenharia Civil.<br>This work presents a study of stability problems often
encountered in Geotechnical Engineering, through the use of
Limit Analysis in conjunction with the Finite Element Method
(FEM).Initially, a literature survey of the most often
found formulations in Limit Analysis through the FEM is
carried out.A mixed formulation of Limit Analysis was
chosen for implementation and its details are fully
described. Extensions of the formulation to deal with
stability problems in fractured rock media are also
proposed and described. A numerical procedure to take into
account the effect of non associative plastic flow is
proposed.The implementations carried out were validated
through problems to which analytical solutions could be
found.Finally, a considerable number of problems of
interest to Geotechnical Engineering is studied with the
implemented formulation. The results of these studies
suggest that Limit Analysis can be considered as a viable
tool in the solution of practical problems in Geotechnical
Engineering.
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Ukritchon, Boonchai 1970. "Application of numerical limit analyses for undrained stability problems in clay." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/9591.
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Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1998.<br>Includes bibliographical references.<br>This thesis describes the development and application of numerical limit analyses for planar, undrained stability problems in geotechnical engineering. The proposed analyses formulate upper and lower bound theorems for perfectly plastic, cohesivefrictional soils as linear programming problems, with spatial variations of the unknown velocities and stresses approximated by standard finite element interpolation functions. The thesis includes three substantial modifications of the pre-existing software: l) inclusion of structural beam and joint elements, that enable the program to model failure in combined bending, shear and axial loading; 2) implementation of functions to represent the undrained strength anisotropy of soft clays; and 3) representation of pore water pressures and effective stress strength parameters for stability analyses that include free draining soil layers. In all of the examples presented in this thesis, the proposed numerical limit analyses are able to bracket the true collapse load within ±5%. The stability of surface footings under combined effects of vertical, horizontal and moment (V, H, M) loading is solved in the form of three-dimensional failure envelopes, that include effects of underbase suction and non-homogeneous undrained strength profiles. Existing empirical bearing capacity factors for inclined, eccentric loading are shown to be conservative, often underestimating the exact collapse solutions for footings on homogeneous clay by more than 25%. However, the same correction factors can become completely unreliable when there is a significant undrained strength gradient. The numerically-derived failure envelope for footings on homogeneous clay is described approximately by curve fitting techniques using relatively simple functions that can be used to update the existing bearing capacity factors. The undrained (short-term) pullout capacity of a single caisson cell in clay is initially investigated as a planar problem. Limit analyses show that the capacity can be presented as a normalized function of the undrained strength gradient in the clay, and interior surface area of the caisson. The results show that there are significant errors in limit equilibrium calculations which assume mechanisms of reverse end-bearing. Pullout predictions are compared with measured data from 1) small scale laboratory element tests on miniature caissons, 2) centrifuge models and 3) field tests on a four-cell caisson unit. These comparisons introduce approximations in the scaling of the planar solutions to axisymmetric cell geometries, and inclined loading cases. Good agreement is obtained with all three sets of experiments, and confirms that the proposed limit analyses are at least comparable in predictive capability to the limit equilibrium calculations currently used in practice. The limit analyses represent the undrained strength anisotropy of soft clays using a yield function originally proposed by Davis and Christian ( 1970). Undrained strength anisotropy is found to have a major influence on the stability and failure mechanisms for unbraced vertical cuts in clay. In contrast, there is minimal effect on the failure mechanisms for kinematically constrained problems such as the lateral load capacity of deeply embedded pile or pipe sections. Applications of limit analyses for braced excavations show important errors in existing calculations of basal stability. The analyses also provide the first theoretical solutions that quantify effects of wall embedment and bending strength on the stability of well-braced cuts. The results have also been compared with previous displacement-based finite element calculations based on the anisotropic MIT-E3 soil model. Good matching is obtained using the limit analyses together with anisotropic strength parameters mobilized at shear strains, y = 0.6 - 1.0%, corresponding to failure conditions in the finite element solutions. Limit analyses have been used to re-assess the stability of tied-back walls at Bird Island Flats in East Boston. Detailed analyses were carried out for two cases l) estimated excavation and support geometry at an unstable section; and 2) at final excavated grade after placement of a remedial buttress constructed by deep soil-mixing and jet grouting. Reliability analyses show that uncertainties in the undrained strength profile represent the major factor affecting predictions of stability. Successful performance of the improved cross-section is not consistently explained by strength parameters derived from the unstable cross-section. Additional calculations compare the stabilizing effects of different designs for the wall, tiebacks and remedial buttress. The failure of an offshore breakwater in Brazil was analyzed extensively using conventional slope stability methods in a recent Ph.D thesis by Lee ( 1995). Limit analyses, using the same soil parameters, shows similar mechanisms of failure controlled by the undrained shear strength of the underlying Sergipe clay. However, the factors of safety computed by Spencer's method are shown to be unrealistically low due to the assumed directions of interslice forces in the overlying sand and rockfill layers.<br>by Boonchai Ukritchon.<br>Sc.D.
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Warren, Sean N. "Limit equilibrium stability analysis utilizing geotechnical data at Mount Baker, Washington /." abstract and full text PDF (UNR users only), 2008. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1461537.
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Thesis (M.S.)--University of Nevada, Reno, 2008.<br>"December 2008." Includes bibliographical references (leaves 109-112). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2009]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web. Library also has electronic version on CD-ROM
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Desrayaud, Gilles. "Analyse de stabilite lineaire dans un milieu semitransparent : determination experimentale des limites de stabilite dans un milieu transparent." Paris 6, 1987. http://www.theses.fr/1987PA066089.
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Etude de la stabilite des regimes d'ecoulement de convection naturelle dans un fluide semi-transparent confine entre deux parois verticales chauffees differentiellement. Resolution des equations de l'ecoulement et du probleme aux valeurs propres par une methode spectrale tau utilisant les polynomes de chebyshev. Etude experimentale des limites de stabilite dans des cavites verticales remplies d'air par interferometrie holographique et injection de fumee
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Dantzer, Jean-François. "Stabilité des réseaux de files d'attente et limites fluides stochastiques." Versailles-St Quentin en Yvelines, 2000. http://www.theses.fr/2000VERS0010.
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Le domaine étudié est celui de la stabilité du trafic et du stockage de messages dans les réseaux informatiques. On rappelle les principaux résultats sur les fonctions de Liapunov. La suite du premier chapitre est consacrée aux limites fluides, méthode développée depuis les années 90. Elles ont l'avantage de décrire un modèle sous une forme macroscopique et permettent ainsi d'écrire de façon simplifiée les équations régies par la dynamique établie sur le réseau. Une condition de stabilité est établie à partir du modèle fluide. On étudie deux modèles de réseaux dont les serveurs tombent en panne de façon aléatoire. Les conditions de stabilité ainsi qu'une convergence de mesures invariantes sont obtenues à l'aide des fonctions de Liapunov. L'anneau de Cambridge, un modèle de réseau local, est étudié sous des hypothèses presque générales. Les serveurs sont repartis à l'intérieur d'un anneau tournant toujours dans le même sens. Les files d'attente sont disposées régulièrement autour de l'anneau. On décrit le modèle fluide et un système d'équations régi par celui-ci. Il permet de trouver les conditions de stabilité de l'anneau. Le modèle suivant est dérive de celui de l'anneau de cambridge. Son but est d'analyser les différents comportements d'un anneau de Cambridge stable, d'un état initial jusqu'a son retour a l'équilibre. On sature certaines files d'attente de façon permanente. Le modèle fluide permet de donner une condition suffisante de stabilité. La dernière application est un modèle simplifie de partage de bande passante. L'étude du modèle fluide apparaît beaucoup plus délicate que pour les modèles précédents. Dans un cas particulier, on établit les conditions de stabilité. La description détaillée du comportement du modèle transient, permet d'obtenir des limites fluides non déterministes.
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Guiho, Florian. "Analyse de stabilité linéaire globale d'écoulements compressibles : application aux interactions onde de choc / couche limite." Thesis, Paris, ENSAM, 2015. http://www.theses.fr/2015ENAM0003/document.
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Cette thèse a pour objectif d'améliorer la compréhension de la dynamique d'une interaction entre une onde de choc droite ou oblique et une couche limite laminaire ou turbulente. En particulier, nous nous sommes intéressés aux mécanismes responsables de l'apparition d'oscillations auto-entretenues basses fréquences. Ce phénomène survient dans de nombreux cas applicatifs comme dans des entrées d'air d'avions supersoniques, autour d'un profil d'aile en régime transsonique et au sein de tuyère en régime de sur-détente. La première partie de ce mémoire traite des différentes études réalisées pour déterminer la phénoménologie de ce type de dynamique. Dans un deuxième temps, nous expliquons la stratégie retenue pour effectuer notre étude qui consiste à développer un outil d'étude des instabilités, adapté à des écoulements turbulents présentant une interaction entre une onde de choc et une couche limite. Le développement d'un outil CFD linéarisé couplé à une méthode de résolution d'un problème aux valeurs propres par une approche dite sans matrice ou de « time-stepping », a permis la réalisation d'une telle étude. Après une étape de validation de notre outil, nous avons étudié des cas d'écoulements présentant une interaction entre une onde de choc et une couche limite. Trois cas en particulier ont été traités. Le premier cas correspond à une interaction entre une onde de choc oblique impactant une couche limite laminaire se développant sur une plaque plane. Ce cas est généralement qualifié dans la littérature de cas de « réflexion de choc ». Nous montrons qu'un tel écoulement est globalement stable et que sa dynamique peut être caractérisée par des mécanismes de réceptivité et par la réponse de l'écoulement vis-à-vis de perturbations extérieures. Les deux autres cas abordés dans ce travail ont été le cas d'un écoulement transsonique autour d'un profil d'aile de type NACA0012 en régime d'entrée en tremblement aérodynamique et un cas de tuyère transsonique plane de type Sajben en régime de sur-détente. Dans le premier cas, l'analyse de stabilité nous permet de mettre en évidence le phénomène de « buffet » sur le profil NACA0012, ce qui montre que le phénomène est lié à une instabilité globale linéaire . Dans le second cas, l'analyse de stabilité ne permet pas d'expliquer le phénomène auto-entretenues basses fréquences, et montre que l'écoulement est linéairement globalement stable. Dans ce cas, la dynamique est convective, transitoire et pilotée par des mécanismes de réceptivité<br>The general purpose of this study is to provide a better understanding of the dynamics of an interaction between a shock wave and a laminar or turbulent boundary layer. In particular, we were interested in mechanisms responsible for the emergence of low-frequency self-sustained oscillations. This phenomenon arises in numerous industrial cases as in air inlets of supersonic aircrafts, around a profile of wing in transonic regime and within over-extended nozzle. The first part of this report handles various studies carried out to determine the phenomenology of this kind of dynamics. Secondly, we explain the strategy adopted to make our study which consists in developing a tool of study of the instabilities adapted to turbulent flows including an interaction between a shock wave and a boundary layer. The development of a linearized CFD tool coupled with a method of resolution of a eigenvalue problem by a free-matrix approach ( " time-stepping " approach), allowed the realization of such a study. After a stage of validation of our tool, we studied cases of flows including an interaction between a shock wave and a boundary layer. Three cases in particular were handled. The first case corresponds to an interaction enter an oblique shock wave impacting on a laminar boundary layer developing on a flat plate. This case is generally qualified in the literature of case as " reflected shock wave". We show that such a flow is globally stable and that the dynamics of such a flow behaves as a selective noise amplifier, the dynamic is mainly driven by receptivity mechanisms and by the response of upstream white nose disturbance. Two other cases have been studied on this work, the case of a transonic flow around a profile wing of NACA0012 type around the onset of buffet phenomenon and the case of transonic nozzle of Sajben type on over-extended regime. In the first case, the global stability analysis allows us to highlight the buffet phenomenon of on the profile NACA0012, what shows that the phenomenon is linked to a linear global instability. In the second case, the analysis of stability does not allow to explain the self-sustained low frequencies phenomenon, and shows that the flow is linearly globally stable. In this case, the dynamics is convective, passing and piloted by receptivity mechanisms
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Hammachi, Riwan. "Simulation haute fidélité du contrôle de la transition de la couche limite au moyen de métasurfaces acoustiques." Electronic Thesis or Diss., Toulouse, ISAE, 2025. http://www.theses.fr/2025ESAE0003.
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L'écoulement de la couche limite visqueuse proche d'une paroi peut évoluer d'un régime laminaire à turbulent, ce qui entraîne une augmentation significative du frottement et des échanges de chaleur à la surface. En conséquence, les véhicules à haute vitesse peuvent subir une traînée accrue et un échauffement pariétal excessif. Ce phénomène de transition, d'un écoulement ordonné à chaotique, débute lorsque de faibles perturbations atmosphériques pénètrent l'écoulement de la couche limite près de la paroi, excitant ses instabilités naturelles. Ces instabilités se développent ensuite par amplification linéaire jusqu'à atteindre des amplitudes critiques. Au-delà de ces amplitudes, des interactions non-linéaires et des phénomènes tridimensionnels émergent, entraînant la formation de régions turbulentes.Cette thèse se concentre sur la croissance linéaire des instabilités au sein des couches limites bidimensionnelles subsoniques et hypersoniques, et explore l'utilisation de dispositifs de contrôle passifs, tels que les métasurfaces acoustiques, pour retarder ou prévenir le déclenchement de la turbulence. La première année a été consacrée à l'adaptation et à l'implémentation de conditions aux limites dans un code de simulation haute fidélité, afin d'étendre ses capacités pour intégrer les effets de réponse acoustique de métasurfaces complexes et simuler de manière précise les interactions entre les instabilités et les dispositifs de contrôle. En utilisant ces outils, des simulations numériques directes (DNS) de configurations canoniques ont ensuite été effectuées, où la réponse acoustique des surfaces acoustiquement réactives a été modélisée par le biais d'une condition aux limites d'impédance temporelle (TDIBC). Ces simulations instationnaires, couvrant une large gamme d’échelles spatio-temporelles, visaient à fournir une description fine de la dynamique du développement des ondes de Tollmien-Schlichting en régime subsonique et du second mode de Mack en régime hypersonique, ainsi que de leur interaction avec une surface acoustiquement réactive.Ces investigations numériques ont été complétées par des analyses de stabilité linéaire. Un premier cas de couche limite sur une plaque plane à Mach 0,12 a été simulé en utilisant une impédance acoustique mesurée expérimentalement comme condition aux limites, afin de reproduire l'effet passif de la réponse acoustique d'un système de contrôle de la laminarité de l'écoulement (LFC). De façon analogue, l'écoulement autour d'un cône émoussé à Mach 7,4 a été simulé numériquement en tenant compte de la réponse acoustique d'un matériau composite à matrice céramique (CMC) utilisé dans les systèmes de protection thermique (TPS) des véhicules hypersoniques. Un processus d'optimisation des paramètres géométriques d'échelle macroscopique d'un matériau CMC a également été mis en œuvre pour améliorer ses performances en absorption acoustique.Un dernier volet de la thèse a porté sur l'étude de la dynamique du second mode de Mack, sur paroi froide et sur paroi adiabatique, sur un cas académique de plaque plane à Mach 6. Cette dynamique est décrite via une approche originale en termes d'interaction des structures cohérentes associées aux composantes thermodynamiques, vorticales, acoustiques et thermiques du champ de perturbation<br>The wall-bounded boundary layer flow can transition from a laminar to a turbulent regime, leading to a significant increase in friction and heat transfer at the surface. Consequently, high-speed vehicles may experience increased drag and excessive wall heating. This transition phenomenon, from an ordered flow to chaotic motion, is initiated when relatively small atmospheric perturbations penetrate the boundary layer flow, triggering the flow's natural instabilities. These instabilities then undergo linear amplification until critical amplitudes. Beyond these amplitudes, nonlinear interactions and three-dimensional effects arise, progressively filling the wall-bounded flow until it becomes fully turbulent.This thesis focuses on the linear growth of instabilities in both subsonic and hypersonic two-dimensional boundary layers and explores the use of acoustic metasurfaces passive control systems to delay or prevent the onset of turbulence. The first year was dedicated to adapting and implementing boundary conditions in a high-fidelity simulation code to extend its capabilities for incorporating the acoustic response effects of complex metasurfaces and accurately simulating the interactions between instabilities and control devices. Leveraging these tools, direct numerical simulations (DNS) of canonical configurations were subsequently performed, with the acoustic response of reactive acoustic surfaces modeled by a time-domain impedance boundary condition (TDIBC). These unsteady simulations, covering a wide range of spatiotemporal scales, aimed to provide a detailed description of the dynamics of Tollmien-Schlichting wave development in subsonic flows and the second Mack mode in hypersonic flows, as well as their interaction with an acoustically reactive surface.These numerical investigations were complemented by linear stability analyses. A first case involving the boundary layer on a flat plate at Mach 0.12 was simulated with an experimentally measured acoustic impedance as the boundary condition, aiming to mimic the passive effect of the acoustic response of a laminar flow control (LFC) system. In a similar fashion, the Mach 7.4 flow around a blunt cone was numerically simulated, accounting for the acoustic response of a ceramic matrix composite (CMC) material used in thermal protection systems (TPS) for hypersonic vehicles. An optimization process for the macroscopic geometric parameters of a CMC material was also carried out in order to enhance its acoustic absorption performance.Another aspect of the thesis concerned the analysis of the dynamics of the second mode of Mack, both on a cold wall and on an adiabatic wall, in the context of an academic case of a flat plate at Mach 6. These dynamics are described using an original approach in terms of the interaction of coherent structures associated with the fluid-thermodynamic, vortical, acoustic, and thermal components of the perturbation field
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Bugeat, Benjamin. "Stabilité et perturbations optimales globales d'écoulements compressibles pariétaux." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066504/document.
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Une méthode de calcul de forçage optimal a été employée afin d'analyser le caractère amplificateur sélectif de bruit d'écoulements compressibles pariétaux. Une telle approche inclut la prise en compte de croissances non-modales induites par la non-normalité des équations de Navier-Stokes linéarisées. La méthode numérique repose sur le calcul de la matrice résolvante globale et la résolution d'un problème aux valeurs propres associé à un problème d'optimisation. Les densités d'énergie des forçages et réponses optimaux calculés pour une couche limite supersonique ont pu être reliés à la courbe neutre expérimentale obtenue par Laufer et Vrebalovich, à condition de contraindre la localisation du forçage en amont de la branche inférieure. Par la suite, une étude paramétrique en nombre de Mach de la réceptivité 2D d'une interaction choc/couche limite laminaire a permis de caractériser le développement d'instabilités convectives de Kelvin-Helmholtz et Tollmien-Schlichting (TS) à haute fréquence. La réceptivité basse fréquence de ce système a été mise en relation avec la résonance d'un mode global stable. Par ailleurs, une extension de la méthode numérique 2D a été proposée pour le calcul de perturbations 3D. Son application au calcul du forçage optimal d'une couche limite à M=4.5 a permis de mettre en évidence la croissance non-modale 3D de streaks ainsi que le développement d'ondes TS obliques dont la croissance, en régime compressible, est favorisée par rapport à celle des ondes 2D. Cette étude a également permis d'observer la croissance du mode de Mack à plus haute fréquence<br>Parietal compressible flows have been studied by means of optimal forcing computations in order to characterize the noise amplifier nature of these flows. This approach is able to take into account the non-modal growth of linear perturbations induced by the non-normality of the linearized Navier-Stokes equations. The numerical strategy is based on the computation of the global resolvent matrix and an eigenvalue problem stemming from an optimization problem. Optimal forcing and response energy densities of a supersonic boundary layer have been linked to the experimental neutral curve obtained by Laufer et Vrebalovich, provided that the forcing localization is constrained upstream from the lower branch. Afterwards, a parametric study with respect to the Mach number of the 2D receptivity of the laminar shock wave/boundary layer interaction flow has allowed to analyze the growth of Kelvin-Helmholtz and Tollmien-Schlichting instabilities (TS) occurring at high frequencies. At low frequencies, the receptivity of the system has been linked to the resonance of a stable global mode. Furthermore, the 2D numerical method has been extended to allow the computation of 3D perturbations. This approach has been applied to a supersonic boundary layer flow at M=4.5 in which the 3D non-modal growth of streaks has been identified, as well as the development of oblique TS waves, whose growth is larger than the one associated to 2D waves in compressible regime. This study has also allowed to detect the growth of Mack mode at higher frequencies
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Firat, Seyhan. "Critical assessment of existing slope stability formulae and application to slope stabilisation." Thesis, University of South Wales, 1998. https://pure.southwales.ac.uk/en/studentthesis/critical-assessment-of-existing-slope-stability-formulae-and-application-to-slope-stabilisation(339ddd93-e0c1-466d-8278-143beca4fb20).html.
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In this research, extensive use has been made of limit equilibrium methods of analysis for studying the stability of slopes. For the determination of the factor of safety (FOS) of slopes, the usual two-step process has been adopted; (a) assuming a slip surface for the soil mass, and (b) using the appropriate limit equilibrium equation(s). Eight wellknown limit equilibrium methods have been programmed to calculate different FOS values. The comparative performance of the various analyses has been carried out successfully using case studies. The innovative use of Gauss quadrature to calculate the FOS values has been shown to reducet he iterative sequencesd ramatically with no loss of accuracy. A visco-plastic flow model has been proposed to estimate lateral forces on piles used for slope stabilisation. The present research data occupies an "in-between" position to the previously reported values, with the variation trend being confirmed satisfactorily in all cases. Slope stabilisation due to the presence of a row of piles has been investigated using two distinct lateral load estimations. These include theories of plastic deformation and the proposed visco-plastic flow which are modelled and implemented in a computer program. Eight well-known methods of slope stability analyses have been adopted and computer coded to re-calculate FOS values for a slope reinforced by a row of piles. A Finite Element computer program has been developed to evaluate the displacement, bending moment and shear force along the pile axis. The pile is analysed at two levels above and below the slip failure surface.
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Salinas, Hervé. "Stabilité linéaire et faiblement non linéaire d'une couche limite laminaire compressible tridimensionnelle par l'approche PSE." Toulouse, ENSAE, 1998. http://www.theses.fr/1998ESAE0024.
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Prasad, Anamika 1979. "Development of user interface for numerical limit analysis of geotechnical stability problems." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/85751.
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Souchet, Michel. "Etude de stabilite d'une couche limite thermocapillaire avec surface libre deformable." Paris 6, 1991. http://www.theses.fr/1991PA066639.
Full textAbstract:
La transition vers la turbulence des ecoulements thermocapillaires est encore mal connue aujourd'hui. Nous nous sommes propose d'etudier ici la stabilite d'un tel ecoulement, avec surface libre deformable. Un logiciel de calcul de stabilite lineaire a ete developpe permettant de determiner mes courbes de stabilite neutre. Cet outil nous a permis de mettre en evidence deux grandes classes d'instabilite: les ondes convectives thermocapillaires, les ondes de surface. Aux faibles longueurs d'onde le mode privilegie sera une onde convective oblique a 80; la surface reste plane dans ce cas. Alors qu'aux grandes longueurs d'ondes le mode le plus instable est une onde de surface transverse. Les mecanismes physiques de ces differents types d'instabilite ont ete dans un second temps analyses a l'aide d'une approche energetique. Nous avons ainsi montre que, dans le cas des ondes convectives transverses, l'energie des perturbations provenait du gradient vertical de vitesse axiale et que dans le cas des ondes convectives longitudinales, l'energie provenait des forces thermocapillaires. Les ondes obliques ont un mecanisme intermediaire. Pour les ondes de surface, l'energie provient du travail des forces thermocapillaires, des forces de cisaillements et des forces tangentielles; l'ecoulement de base est dans ce cas stabilisant par les contraintes de reynolds