Dissertations / Theses: 'Boundary conditions and coupling'

1

Castro, Nicholas D. "Numerical Modeling of Synthetic Jets in Quiescent Air with Moving Boundary Conditions." VCU Scholars Compass, 2005. http://scholarscompass.vcu.edu/etd/1466.

Full text

Abstract:

Flow control is a key factor in optimizing the performance of any vehicle moving through fluids. Particularly, in aerodynamics there are many potential benefits for implementing synthetic jets to achieve aircraft designs with less moving parts, uper- maneuverability, and separation control for fuel economy. Piezoelectric synthetic jets are of special interest because of their lightweight and low power consumption. Numerous publications on such jets are available. Actuator properties and boundary conditions relevant to this particular application however are often overlooked. The focus of this project is to numerically model synthetic jets in quiescent air to study the influence of cavity geometry and boundary conditions of the piezoelectric diaphragm on jet velocity. Numerical simulation is performed for two synthetic jet cavities of different height and orifice diameter. The numerical modeling utilizes a turbulent RNG κ ε model and a moving boundary condition with two oscillating deflection profiles, parabolic and logarithmic, applied to the diaphragm. The actuators modeled are typical Bimorph and Thunder piezoelectric actuators. The initial conditions for the actuators are obtained experimentally resulting in 0.396mm and 0.07mm respectively when driven with a sinusoidal wave input at 1524 V/m and 4064 V/m. Although the velocity boundary numerical model gave overall better results than the current moving-boundary numerical model, the moving-boundary model is more accurate since it better approximates the movement of the diaphragm. From an optimizing viewpoint the moving boundary is more suitable to attempt to optimize the design because displacement magnitude of the diaphragm can be measured directly from experiments. For the higher displacement Bimorph actuator, a logarithmic profile matches the experimental results, whereas the parabolic profile provided better results for the relatively small displacement Thunder actuator. It is thus hypothesized that both tested actuators, Bimorph and Thunder, oscillate according to the specified logarithmic and parabolic profile respectively. Cavity height was briefly investigated for the Bimorph actuator. Results show that cavity height did not make a difference in the centerline velocity for the numerical model. The model fails to consider the important effect of the dynamic coupling of the actuator displacement and the pressure that develops inside the cavity. The pressure values obtained are comparable to the theoretical blocking pressure for the Bimorph in the cavity. The results of this study show that jet formation and development has unique characteristics for each actuator and cavity configuration. The smaller orifice cavity configuration produced a faster, longer, thinner jet with larger vortices than the bigger orifice. During max expulsion, t = 0.25T, and max ingestion, t = 0.75T, a low-pressure area localized at the corners of the orifice, inlet and exit respectively, were observed. All cavity configurations passed all three known jet formation criterions that include, Lo/Do>1, Re > 50, and Re/S2 > 0.16.

APA, Harvard, Vancouver, ISO, and other styles

2

Ablikim, Medina. "Boundary sinh-Gordon model and its supersymmetric extension." Thesis, Durham University, 1999. http://etheses.dur.ac.uk/4853/.

Full text

Abstract:

Three different aspects of the sinh-Gordon model are explored in this thesis. We begin, in chapter one, with a summary of the model and the necessary background. Chapter two studies the model with two boundary conditions. Two approaches are presented to investigate the reflection factors off the boundaries and the energy of the theory. In chapter three, perturbation theory is developed to study the theory with one general boundary condition. A contribution to the quantum reflection factor is obtained and compared with the result obtained for the special boundary condition. Chapters four and five investigate the supersymmetric extension of the model in the presence of a single boundary. Firstly, the classical limits of the supersymmetric reflection matrices are checked. The exact reflection factors are studied perturbatively up to the second order of the coupling constant. Secondly, the perturbation theory and the path integral formalism are employed in the supersymmetric model to study the quantum reflection factors. We conclude with a brief sixth chapter describing the outlook for further investigations.

APA, Harvard, Vancouver, ISO, and other styles

3

Trovant, Michael. "A boundary condition coupling strategy for the modeling of metal casting processes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0011/NQ35346.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

4

Chenaghlou, Alireza. "Quantum corrections to the classical reflection factor of the sinh-Gordon model." Thesis, Durham University, 2000. http://etheses.dur.ac.uk/4347/.

Full text

Abstract:

This thesis studies the quantum reflection factor of the sinh-Gordon model under boundary conditions consistent with integrability. First, we review the affine Toda field theory in Chapter One. In particular, the classical and quantum integrability of the theory are reviewed on the whole line and on the half-line as well, that is, in the presence of a boundary. We next consider the sinh-Gordon model which is restricted to a half-line by boundary conditions maintaining integrability in Chapter Two. A perturbative calculation of the reflection factor is given to one loop order in the bulk coupling and to first order in the difference of the two parameters introduced at the boundary. The result provides a further verification of Ghoshal's formula. The calculation is consistent with a conjecture for the general dependence of the reflection factor on the boundary parameters and the bulk coupling. In Chapter Three, quantum corrections to the classical reflection factor of the sinh-Gordon model are studied up to second order in the difference of boundary data and to one loop order in the bulk coupling. Chapter Four deals with the quantum reflection factor for the sinh-Gordon model with general boundary conditions. The model is studied under boundary conditions which are compatible with integrability and in the framework of the conventional perturbation theory generalised to the affine Toda field theory. It is found that the general form of a subset of the related quantum corrections are hypergeometric functions. Finally, we sum up this thesis in Chapter Five along with some conclusions and suggestions for further future studies.

APA, Harvard, Vancouver, ISO, and other styles

5

Jolley, Kenny. "Multiscale methods for nanoengineering." Thesis, University of Leicester, 2009. http://hdl.handle.net/2381/7809.

Full text

Abstract:

This thesis is presented in two sections. Two different multiscale models are developed in order to increase the computational speed of two well known atomistic algorithms, Molecular Dynamics (MD) and Kinetic Monte Carlo (KMC). In Section I, the MD method is introduced. Following this, a multiscale method of linking an MD simulation of heat conduction to a finite element (FE) simulation is presented. The method is simple to implement into a conventional MD code and is independent of the atomistic model employed. This bridge between the FE and MD simulations works by ensuring that energy is conserved across the FE/MD boundary. The multiscale simulation allows for the investigation of large systems which are beyond the range of MD. The method is tested extensively in the steady state and transient regimes, and is shown to agree with well with large scale MD and FE simulations. Furthermore, the method removes the artificial boundary effects due to the thermostats and hence allows exact temperatures and temperature gradients to be imposed on to an MD simulation. This allows for better study of temperature gradients on crystal defects etc. In Section II, the KMC method is introduced. A continuum model for the KMC method is presented and compared to the standard KMC model of surface diffusion. This method replaces the many discrete back and forth atom jumps performed by a standard KMC algorithm with a single flux that can evolve in time. Elastic strain is then incorporated into both algorithms and used to simulate atom deposition upon a substrate by Molecular Beam Epitaxy. Quantum dot formation due to a mismatch in the lattice spacing between a substrate and a deposited film is readily observed in both models. Furthermore, by depositing alternating layers of substrate and deposit, self-organised quantum dot super-lattices are observed in both models.

APA, Harvard, Vancouver, ISO, and other styles

6

Moretti, Rocco. "Étude et amélioration des méthodologies de couplage aérothermique fluide-structure." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEM076.

Full text

Abstract:

Ces travaux s’inscrivent dans la résolution de problèmes couplés aérothermiques. Il s’agit notamment d’améliorer les méthodes de couplage en termes de précision et de robustesse. La stabilité du couplage aérothermique en régime permanent a été d’abord étudiée (couplage faible). Un nombre de Biot numérique a été défini ce qui permet d’évaluer l’intensité de l’interaction thermique fluide-structure. Plusieurs méthodes (Dirichlet-Robin, Neumann-Robin et Robin-Robin) ont été étudiées et leur domaine de validité a été défini. La méthode Dirichlet-Robin avec un coefficient de sécurité s’est avérée la plus robuste et simple à mettre en œuvre. La prise en compte du rayonnement implique une déstabilisation majeure du problème aérothermique. La méthode de stabilisation a donc été modifiée afin de pouvoir stabiliser ce type de calcul. Dans un deuxième temps, la résolution de problèmes couplés aérothermiques en régime transitoire a été étudiée via l’algorithme partitionné quasi-instationnaire. Il s’agit d’une procédure itérative (couplage fort) entre le solide en régime transitoire et des états fluides en régime stationnaire, assurant ainsi l’égalité des flux de chaleur et de la température à chaque instant de couplage. La précision de cet algorithme a été analysée et améliorée. Enfin, l’algorithme quasi-instationnaire a été analysé sur des problèmes aérothermiques quasi-industriels de disques de turbine et compresseur aéronautiques
This work deals with the solution of coupled aerothermal problems. The aim of the work is to improve the accuracy and robustness of the coupling techniques. The stability of the aerothermal coupling for steady state problems is first studied (weak coupling). A numerical Biot number is defined, which allows to evaluate the intensity of the fluid-structure thermal interaction. Several methods (Dirichlet-Robin, Neumann-Robin and Robin-Robin) are studied and their ranges of validity are defined. The Dirichlet-Robin method with a safety coefficient, which presents the easiest implementation, proves to be the most robust. Taking radiation into account implies a major destabilization of the aerothermal problem. Thus, the stabilization method is modified in order to be able to stabilize this type of calculation. In the second part, the solution of coupled aerothermal problems in the transient regime is studied using the quasidynamic partitioned algorithm. This is an iterative procedure (strong coupling) between the thermal problem in the solid solved in the transient regime and the flow field at the steady state, thus ensuring equal heat flux and temperature at each coupling time. The accuracy of this algorithm is analysed and improved. Finally, the quasi-dynamic algorithm is analysed on quasi-industrial aerothermal problems of aeronautical compressor and turbine discs

APA, Harvard, Vancouver, ISO, and other styles

7

Mikhaylenko, Maxim A. "Development and Application of the Boundary Singularity Method to the Problems of Hydrodynamic and Viscous Interaction." University of Akron / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=akron1442423671.

Full text

APA, Harvard, Vancouver, ISO, and other styles

8

Maati, Amel. "Mise en œuvre de formalismes pour la modélisation de grands réseaux périodiques d'antennes." Thesis, Limoges, 2018. http://www.theses.fr/2018LIMO0009/document.

Full text

Abstract:

Cette thèse se place dans le contexte général de la modélisation de réseaux d’antennes de grande taille, avec pour objectif d’atteindre un niveau de précision suffisamment élevé pour permettre une optimisation complète des performances et en particulier une amélioration de l’efficacité énergétique. Partant du constat que l’optimisation électromagnétique de grands réseaux représente un verrou si les couplages doivent être modélisés efficacement, cette thèse propose la mise en œuvre d’une méthode permettant la modélisation fine de grands réseaux d'antennes tout en réduisant les temps de calcul et en conservant un haut degré de précision. L'objectif est de montrer qu'une approche dérivée des formalismes périodiques infinis connus de la littérature permet d'obtenir une matrice [S] complète d'un réseau depuis l'étude d'une cellule unitaire. Après avoir présenté un état de l'art sur les réseaux d'antennes et leurs méthodes d'analyse, l'approche de modélisation proposée est détaillée. Des véhicules de test numériques et expérimentaux, permettant de valider cette méthode de modélisation, sont ensuite réalisés. L'approche est enfin utilisée avec succès pour deux types d'application
This thesis is organized in the general context of modeling a large antenna arrays with the aim of achieving a high level of precision. This modeling allows a complete optimization of the performances and an enhancement of the energy efficiency. Given that the electromagnetic optimization of large arrays still represents a challenge if the mutual coupling is not efficiently modeled. This work offers the implementation of a method allowing the precise modeling of large antenna arrays while reducing the computation time and maintaining a high degree of precision. The aim is to demonstrate that the derived approach from the infinite periodic formalisms makes it possible to obtain a full [S] matrix of an array based on the study of a unit cell. After presenting a state of the art of antenna arrays and their methods of analysis, the proposed modeling approach is explained. Numerical and Experimental demonstrators are then made for the validation. Finally, this method is successfully used for two types of applications

APA, Harvard, Vancouver, ISO, and other styles

9

Warman, Craig S. "Understanding the spatial and temporal variation in anthropogenically induced channel response in the Irwin River catchment." University of Western Australia. School of Earth and Geographical Sciences, 2008. http://theses.library.uwa.edu.au/adt-WU2008.0214.

Full text

Abstract:

The Irwin River catchment, located in the central western region of Western Australia, has been the scene of significant geomorphological change over both historical and geological timescales. This thesis focuses on the most recent of these changes, the anthropogenic imprint, through the development of a catchment-scale understanding of system behaviour. Analysis and modelling of changes in the hydrological behaviour of the system indicates that while the Irwin River has displayed a natural susceptibility to large flood events, these have been exacerbated by the widespread clearing of native vegetation throughout the catchment. As a result, when such events do occur, the catchment response is now larger, more direct and has a greater ability to cause erosion. However, the nature and detail of sediment yield processes and stream channel response varies markedly throughout the system. A series of representative channel reaches, as defined by their planform characteristics, geometry and architecture, are presented to illustrate spatial changes in stream channel behaviour. A distinct variation in river morphotypes is seen both downstream throughout the system as well as across the tributary sub-catchments of the Irwin River, Lockier River and Green Brook. This inter and intra sub-catchment variation in stream channel response can be attributed to changes in the boundary conditions and coupling mechanisms in operation throughout the Irwin River system. The pronounced spatial variability in response to human disturbance and the changing nature of catchment-scale connectivity seen in the Irwin River system differs markedly to that reported elsewhere in the literature. Appreciation of the variability in form, behaviour and evolutionary history throughout the Irwin River catchment not only provides the foundation for effective management but also contributes to a wider understanding of fluvial system behaviour. Unlike the majority of existing literature, which tends to identify and measure channel changes in a single catchment where historical variation to the sediment and discharge regime is well known, this study demonstrates the role of boundary conditions in determining the response of the fluvial system to changing environmental controls.

APA, Harvard, Vancouver, ISO, and other styles

10

Fajardo, Peña Pablo. "Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow." Doctoral thesis, Universitat Politècnica de València, 2012. http://hdl.handle.net/10251/16878.

Full text

Abstract:

The increasing use of turbochargers is leading to an outstanding research to understand the internal flow in turbomachines. In this frame, computational fluid dynamics (CFD) is one of the tools that can be applied to contribute to the analysis of the fluid-dynamic processes occurring in a turbine. The objective of this thesis is the development of a methodology for performing simulations of radial turbomachinery optimizing the available computational resources. This methodology is used for the characterization of a vaned-nozzle turbine under steady and pulsating flow conditions. An important effort has been devoted in adjusting the case configuration to maximize the accuracy achievable with a certain computational cost. Concerning the cell size, a local mesh independence analysis is proposed as a procedure to optimize the distribution of cells in the domain, thus allowing to use a finer mesh in the most suitable places. Particularly important in turbomachinery simulations is the influence of the approach for simulating rotor motion. In this thesis two models have been compared: multiple reference frame and sliding mesh. The differences obtained using both methods were found to be significant in off-design regions. Steady flow CFD results have been validated against global measurements taken on a gas-stand. The modeling of a turbine, installed either on a turbocharger test rig or an engine, requires the calculation of the flow in the ducts composing the system. Those ducts could be simulated assuming a one-dimensional (1D) approximation, and thus reducing the computational cost. In this frame of ideas, two CFD boundary conditions have been developed. The first one allows performing coupled 1D-3D simulations, communicating the flow variables from each domain through the boundary. The second boundary condition is based in a new formulation for a stand-alone anechoic end, which intends to represent the flow behavior of an infinite duct. Finally, the turbine was simulat
Fajardo Peña, P. (2012). Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16878
Palancia

APA, Harvard, Vancouver, ISO, and other styles

11

Müller-Bierl, Bernd Michael. "Rayonnement et couplage entre les interconnexions dans les circuits hyperfréquences : étude au moyen de la méthode tlm." Grenoble INPG, 1997. http://www.theses.fr/1997INPG0026.

Full text

Abstract:

La these est constitue de 3 parties principales. Apres une introduction a l'analyse des lignes micro-ruban sous approximation de propagation des signaux en mode tem, la premiere partie traite d'une methode d'anaulyse dynamique valable pour des distributions de champs qui peuvent meme etre eloignees du mode tem, la methode tlm (transmission line matrix method), introduite 1971 par johns et al. La methode tlm est presentee comme un processus de repartition base sur la decomposition des equations de maxwell sous forme matricielle. Dans une deuxieme partie nous traitons de la possibilite d'utiliser la methode tlm dans les problemes dits de l'espace ouvert, c'est a dire des espaces de calcul limites par des parois absorbant toute l'energie quittant le domaine de calcul. Cela est necessaire afin de reduire l'espace memoire necessaire pour calculer par example des effets de rayonnement et de diffraction. Dans la troisieme partie nous appliquons la methode tlm pour calculer les parametres des differents discontinuites micro-ruban, en particulier les effets de couplage et de rayonnement. Au moyen d'une transformation des champs proches en champs lointain nous determinons egalement les diagrammes de rayonnement correspondants. L'ensemble du travail se situe dans le cadre des problemes de compatibilite electromagnetique.

APA, Harvard, Vancouver, ISO, and other styles

12

Ehrhardt, Matthias. "Discrete artificial boundary conditions." [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=965232921.

Full text

APA, Harvard, Vancouver, ISO, and other styles

13

le, coupanec erwan. "Boundary conditions for the lattice Boltzmann method : Mass conserving boundary conditions for moving walls." Thesis, Norwegian University of Science and Technology, Department of Energy and Process Engineering, 2010. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-10154.

Full text

Abstract:

Based on the no-slip boundary condition for walls at rest for the lattice Boltzmann Bathnagar-Gross-Krook method by J.C.G. Verschaeve [Phys. Rev. 80,036703 (2009)], a no-slip boundary condition for walls with a tangential movement is derived. Numerical tests verify that the present boundary condition is second order accurate and stable for relaxation frequencies close to two.

APA, Harvard, Vancouver, ISO, and other styles

14

Boström, Erik. "Boundary Conditions for Spectral Simulations of Atmospheric Boundary Layers." Licentiate thesis, KTH, Stabilitet, Transition, Kontroll, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-218054.

Full text

Abstract:

An atmospheric boundary layer (ABL) is generally a very high Reynolds number boundary layer over a fully rough surface that is influenced by different external forces. Numerical simulations of ABLs are typically demanding, particularly due to the high Reynolds numbers. Large eddy simulation (LES) where the grid filtered Navier--Stokes equations are solved together with a turbulence model for the subgrid-scale motions is the most accurate and widely used technique to date for ABLs. However, high Reynolds numbers, filtered equations and rough surfaces do not support the simple no-slip boundary conditions together with a feasible grid resolution. A paramount part for the performance of an ABL LES simulation therefore lies in the quality of approximate wall boundary conditions, so called wall models. The vast majority of LES codes used for ABL simulations rely on spatial discretization methods with low order finite difference approximations for the derivatives in the inhomogeneous wall normal direction. Furthermore, the wall boundary conditions are typically chosen in a mesh-dependent, non-local way, relying on the finite differences formulation. In this thesis we focus on solving the ABL LES equations with a fully (pseudo) spectral Fourier--Chebyshev code. We present how wall boundary conditions can be formulated through Robin boundary conditions and how to implement these in the normal-velocity normal-vorticity formulation that we solve. A new idea of specifying boundary conditions directly in Fourier space where also the turbulence intensity statistics can be controlled is presented and verified. The present results show that the Robin-type formulation is effective at least in near-equilibrium boundary layers. The code and boundary conditions were tested in both low and high Reynolds number (open and full) channel flows of neutral and stable stratification. Results were validated with both low to moderate Reynolds number DNS statistics as well as with the logarithmic law. Our results indicate great potential for both the the new boundary condition formulation and the specific code implementation. Further analysis of more complex flow situations will show whether the Robin-type formulation will give similarly good results.
Ett atmosfäriskt gränsskikt (ABL) är generellt sett ett gränsskikt med väldigt högt Reynolds-tal över en rand med ojämn yta och som är påverkad av yttre krafter. Numeriska simuleringar av ABLs är typiskt sett väldigt krävande, speciellt på grund av de höga Reynolds-talen. Large eddy simulation (LES) där de filtrerade Navier--Stokes ekvationerna är lösta tillsammans med en turbulensmodell för the oupplösta skalorna är den mest noggranna och mest använda tekniken för ABLs. Men, för höga Reynolds-tal, filtrerade ekvationer och ytojämnheter är inte ``no-slip'' randvillkor giltiga för en genomförbart hög nätupplösning. En viktig del för kvalitén hos en ABL LES simulering ligger därför i prestandan i approximativa randvillkor, så kallade väggmodeller. Majoriteten av alla LES koder som används för ABL simuleringar är baserade på en lågordnings finita-differens diskretisering för derivatorna i den inhomogena väggnormalriktningen. Dessutom så är vägg-randvillkoren typiskt valda nätberoende och icke-lokala och direkt beroende av finita-differens diskretiseringen. I den här avhandlingen så fokuserar vi i att lösa ABL LES ekvationerna med en fullt (pseudo) spektral Fourier--Chebyshev kod. Vi förklarar vidare hur väggrandvillkor kan formuleras genom Robin-randvillkor och hur man implementerar dessa i normalhastighet normalvorticitet formuleringen som vi löser. En ny idé i att specifiera randvillkor direkt i Fourier-rummet där statistiken för den turbulenta intensiteten kan kontrolleras är också presenterad och verifierad. Resultaten vi härmed presenterar visar att Robin-randvillkor formuleringen är effektiv åtminstone for gränsskikt i nära jämvikt. Den numeriska koden och randvillkoren var testade för kanalströmning (öppen och stängd) av både neutral och stabil stratifikation och för både låga och höga Reynolds-tal. Våra resultat visar på en god potential hos både den nya randvillkorsformuleringen och den nya kodimplementationen. Vidare analys i mer komplexa flödessituationer kommer att visa om Robin-randvillkor formuleringen ger lika goda resultat.

QC 20171122

APA, Harvard, Vancouver, ISO, and other styles

15

Gamlin, Samuel. "Boundary conditions in Abelian sandpiles." Thesis, University of Bath, 2016. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.687371.

Full text

Abstract:

The focus of this thesis is to investigate the impact of the boundary conditions on configurations in the Abelian sandpile model. We have two main results to present in this thesis. Firstly we give a family of continuous, measure preserving, almost one-to-one mappings from the wired spanning forest to recurrent sandpiles. In the special case of $Z^d$, $d \geq 2$, we show how these bijections yield a power law upper bound on the rate of convergence to the sandpile measure along any exhaustion of $Z^d$. Secondly we consider the Abelian sandpile on ladder graphs. For the ladder sandpile measure, $\nu$, a recurrent configuration on the boundary, I, and a cylinder event, E, we provide an upper bound for $\nu(E|I) − \nu(E)$.

APA, Harvard, Vancouver, ISO, and other styles

16

Fouchet-Incaux, Justine. "Modélisation, analyse numérique et simulations autour de la respiration." Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112043.

Full text

Abstract:

Cette thèse est consacrée à la modélisation de la ventilation mécanique chez l'humain et à l'analyse numérique des systèmes en découlant. Des simulations directes d'écoulement d'air dans l'ensemble des voies aériennes étant impossibles (maillages indisponibles et géométrie trop complexe), il est nécessaire de considérer un domaine d'intérêt réduit, qui implique de travailler dans une géométrie tronquée, comportant des frontières artificielles ou encore de considérer des modèles réduits simples mais représentatifs. Si on cherche à effectuer des simulations numériques 3D où l'écoulement du fluide est décrit par les équations de Navier-Stokes, différentes problématiques sont soulevées :- Si on considère que la ventilation est la conséquence de différences de pression, les conditions aux limites associées sont des conditions de type Neumann. Cela aboutit à des questions théoriques en terme d'existence et d'unicité de solution et à des questions numériques en terme de choix de schémas et de méthodes adaptées.- Lorsque l'on travaille dans un domaine tronqué, il peut être nécessaire de prendre en compte les phénomènes non décrits grâce à des modèles réduits appropriés. Ici nous considérons des modèles 0D. Ces couplages 3D/0D sont à l'origine d'instabilités numériques qu'on étudie mathématiquement et numériquement dans ce manuscrit. Par ailleurs, lorsqu'on s'intéresse à des régimes de respiration forcée, les modèles usuels linéaires sont invalidés par les expériences. Afin d'observer les différences entre les résultats expérimentaux et numériques, il est nécessaire de prendre en compte plusieurs types de non linéarités, comme la déformation du domaine ou les phénomènes de type Bernoulli. Une approche par modèles réduits est adoptée dans ce travail.Pour finir, on a cherché à valider les modèles obtenus en comparant des résultats numériques et des résultats expérimentaux dans le cadre d'un travail interdisciplinaire.Parvenir à modéliser et simuler ces écoulements permet de mieux comprendre les phénomènes et paramètres qui entrent en jeu lors de pathologies (asthme, emphysème...). Un des objectifs à moyen terme est d'étudier l'influence du mélange hélium-oxygène sur le dépôt d'aérosol, toujours dans le cadre du travail interdisciplinaire. A plus long terme, l'application de ces modèles à des situations pathologiques pourrait permettre de construire des outils d'aide à la décision dans le domaine médical (compréhension de la pathologie, optimisation de thérapie...)
In this thesis, we study the modelling of the human mecanical ventilation and the numerical analysis of linked systems. Direct simulations of air flow in the whole airways are impossible (complex geometry, unavailable meshes). Then a reduced area of interest can be considered, working with reduced geometries and artificial boundaries. One can also use reduced models, simple but realistic. If one try to make 3D numerical simulations where the fluid flow is described by the Navier-Stokes equations, various issues are raised:- If we consider that ventilation is the result of pressure drops, the associated boundary conditions are Neumann conditions. It leads to theoretical questions in terms of existence and uniqueness of solution and numerical issues in terms of scheme choice and appropriate numerical methods.- When working in a truncated domain, it may be necessary to take into account non-described phenomena with appropriate models. Here we consider 0D models. These 3D/0D couplings imply numerical instabilities that we mathematically and numerically study in this thesis.Furthermore, when we focus on forced breathing, linear usual models are invalidated by experiments. In order to observe the differences between the experimental and numerical results, it is necessary to take into account several types of non-linearities, such as deformation of the domain or the Bernoulli phenomenon. A reduced model approach is adopted in this work. Finally, we sought to validate the obtained models by comparing numerical and experimental results in the context of interdisciplinary work.Achieving model and simulate these flows allow to better understand phenomena and parameters that come into play in diseases (asthma, emphysema ...). A medium-term objective is to study the influence of helium-oxygen mixture in the aerosol deposition. In the longer term, the application of these models to pathological situations could afford to build decision support tools in the medical field (understanding of pathology, therapy optimization ...)

APA, Harvard, Vancouver, ISO, and other styles

17

Roman, Svetlana. "Green's functions for boundary-value problems with nonlocal boundary conditions." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2011. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2011~D_20111227_092148-01085.

Full text

Abstract:

In the dissertation, second-order and higher-order differential and discrete equations with additional conditions which are described by linearly independent linear functionals are investigated. The solutions to these problems, formulae and the existence conditions of Green's functions are presented, if the general solution of a homogeneous equation is known. The relation between two Green's functions of two nonhomogeneous problems for the same equation but with different additional conditions is obtained. These results are applied to problems with nonlocal boundary conditions. In the introduction the topicality of the problem is defined, the goals and tasks of the research are formulated, the scientific novelty of the dissertation, the methodology of research, the practical value and the significance of the results are presented. m-order differential problem and its Green's function are investigated in the first chapter. The relation between two Green's functions and the existence condition of Green's function are obtained. In the second chapter, the main definitions and results of the first chapter are formulated for the second-order differential equation with additional conditions. In the examples the application of the received results is analyzed for problems with nonlocal boundary conditions in detail. In the third chapter, the second-order difference equation with two additional conditions is considered. The expression of Green's function and its existence... [to full text]
Disertacijoje tiriami antros ir aukštesnės eilės diferencialinis ir diskretusis uždaviniai su įvairiomis, tame tarpe ir nelokaliosiomis, sąlygomis, kurios yra aprašytos tiesiškai nepriklausomais tiesiniais funkcionalais. Pateikiamos šių uždavinių Gryno funkcijų išraiškos ir jų egzistavimo sąlygos, jei žinoma homogeninės lygties fundamentalioji sistema. Gautas dviejų Gryno funkcijų sąryšis uždaviniams su ta pačia lygtimi, bet su papildomomis sąlygomis. Rezultatai pritaikomi uždaviniams su nelokaliosiomis kraštinėmis sąlygomis. Įvadiniame skyriuje aprašyta tiriamoji problema ir temos objektas, išanalizuotas temos aktualumas, išdėstyti darbo tikslai, uždaviniai, naudojama tyrimų metodika, mokslinis darbo naujumas ir gautų rezultatų reikšmė, pateikti ginamieji teiginiai ir darbo rezultatų aprobavimas. m-tosios eilės diferencialinis uždavinys ir jo Gryno funkcija nagrinėjami pirmajame skyriuje. Surastas uždavinio sprendinys, išreikštas per Gryno funkciją. Pateikta Gryno funkcijos egzistavimo sąlyga. Antrajame skyriuje pateikti pirmojo skyriaus pagrindiniai apibrėžimai ir rezultatai antros eilės diferencialinei lygčiai. Pavyzdžiuose išsamiai išanalizuotas gautų rezultatų pritaikymas uždaviniams su nelokaliosiomis kraštinėmis sąlygomis. Trečiajame skyriuje nagrinėjama antros eilės diskrečioji lygtis su dviem sąlygomis. Surastos diskrečiosios Gryno funkcijos išraiška ir jos egzistavimo sąlyga. Taip pat pateiktas dviejų Gryno funkcijų sąryšis, kuris leidžia surasti diskrečiosios... [toliau žr. visą tekstą]

APA, Harvard, Vancouver, ISO, and other styles

18

Onyango, Thomas Tonny Mboya. "Boundary element methods for solving inverse boundary conditions identification problems." Thesis, University of Leeds, 2008. http://etheses.whiterose.ac.uk/11283/.

Full text

Abstract:

This thesis explores various features of the boundary element method (BEM) used in solving heat transfer boundary conditions identification problems. In particular, we present boundary integral equation (BIE) formulations and procedures of the numerical computation for the approximation of the boundary temperatures, heat fluxes and space, time or temperature dependent heat transfer coefficients. There are many practical heat transfer situations where such problems occur, for example in high temperature regions or hostile environments, such as in combustion chambers, steel cooling processes, etc., in which the actual method of heat transfer on the surface is unknown. In such situations the boundary condition relating the heat flux to the difference between the boundary temperature and that of the surrounding fluid is represented by an unknown function which may depend on space, time, or temperature. In these inverse heat conduction problems (IHCP), the BEM is formulated as a minimization of some functional that measures the discrepancy between the measured data, say the average temperature on a portion of the boundary or at an instant over the whole domain. The minimization provides solutions that are consistent with the data. This indicates that the BEM algorithms for the IRCP are robust, stable and predict reliable results. When the input data is noisy, we have used the truncated singular value decomposition and the Tikhonov regularisation methods to stabilise the solution of the IRCI' boundary conditions identification. Numerical approximations have been obtained and, where possible, the results obtained are compared to the analytical solutions.

APA, Harvard, Vancouver, ISO, and other styles

19

Schulze, Bert-Wolfgang, and Nikolai Tarkhanov. "Boundary value problems with Toeplitz conditions." Universität Potsdam, 2005. http://opus.kobv.de/ubp/volltexte/2009/2983/.

Full text

Abstract:

We describe a new algebra of boundary value problems which contains Lopatinskii elliptic as well as Toeplitz type conditions. These latter are necessary, if an analogue of the Atiyah-Bott obstruction does not vanish. Every elliptic operator is proved to admit up to a stabilisation elliptic conditions of such a kind. Corresponding boundary value problems are then Fredholm in adequate scales of spaces. The crucial novelty consists of the new type of weighted Sobolev spaces which serve as domains of pseudodifferential operators and which fit well to the nature of operators.

APA, Harvard, Vancouver, ISO, and other styles

20

Stephenson, P. W. "Glueball spectra with twisted boundary conditions." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.276836.

Full text

APA, Harvard, Vancouver, ISO, and other styles

21

Park, Kyeong M. "Boundary conditions of font size effects." Thesis, The University of Alabama in Huntsville, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10130786.

Full text

Abstract:

Prior research has shown that people perceive items in a larger font size as being more memorable than items in a smaller font size. This perception leads to higher judgments of learning (JOLs; i.e., confidence ratings regarding the likelihood of recalling an item) for larger font size items than smaller font size items. Yet other research has shown that people recalled more when the information was presented in a smaller font than when it was presented in a larger font size. The present study examined if there are boundary conditions of font sizes affecting JOLs and actual recall performance. As we expected, the results show that JOLs increased as a function of the size category. The results also show that font size impacted recall performance such that items in the Smallest size category were recalled at a higher rate than items in the other three font size categories.

APA, Harvard, Vancouver, ISO, and other styles

22

Deadman, Edvin. "Outer boundary conditions in numerical relativity." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608394.

Full text

APA, Harvard, Vancouver, ISO, and other styles

23

Schulze, Bert-Wolfgang, and Y. Wei. "Edge-boundary problems with singular trace conditions." Universität Potsdam, 2008. http://opus.kobv.de/ubp/volltexte/2009/3031/.

Full text

Abstract:

The ellipticity of boundary value problems on a smooth manifold with boundary relies on a two-component principal symbolic structure (σψ; σ∂), consisting of interior and boundary symbols. In the case of a smooth edge on manifolds with boundary we have a third symbolic component, namely the edge symbol σ∧, referring to extra conditions on the edge, analogously as boundary conditions. Apart from such conditions in integral form' there may exist singular trace conditions, investigated in [6] on closed' manifolds with edge. Here we concentrate on the phenomena in combination with boundary conditions and edge problem.

APA, Harvard, Vancouver, ISO, and other styles

24

Cousins, William Bryan. "Boundary Conditions and Uncertainty Quantification for Hemodynamics." Thesis, North Carolina State University, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=3575896.

Full text

Abstract:

We address outflow boundary conditions for blood flow modeling. In particular, we consider a variety of fundamental issues in the structured tree boundary condition. We provide a theoretical analysis of the numerical implementation of the structured tree, showing that it is sensible but must be performed with great care. We also perform analytical and numerical studies on the sensitivity of model output on the structured tree's defining geometrical parameters. The most important component of this dissertation is the derivation of the new, generalized structured tree boundary condition. Unlike the original structured tree condition, the generalized structured tree does not contain a temporal periodicity assumption and is thus applicable to a much broader class of blood flow simulations. We describe a numerical implementation of this new boundary condition and show that the original structured tree is in fact a rough approximation of the new, generalized condition.

We also investigate parameter selection for outflow boundary conditions, and attempt to determine a set of structured tree parameters that gives reasonable simulation results without requiring any calibration. We are successful in doing so for a simulation of the systemic arterial tree, but the same parameter set yields physiologically unreasonable results in simulations of the Circle of Willis. Finally, we investigate the extension of recently introduced PDF methods to smooth solutions of systems of hyperbolic balance laws subject to uncertain inputs. These methods, currently available only for scalar equations, would provide a powerful tool for quantifying uncertainty in predictions of blood flow and other phenomena governed by first order hyperbolic systems.

APA, Harvard, Vancouver, ISO, and other styles

25

Zacher, Rico. "Quasilinear parabolic problems with nonlinear boundary conditions." [S.l. : s.n.], 2003. http://deposit.ddb.de/cgi-bin/dokserv?idn=969321899.

Full text

APA, Harvard, Vancouver, ISO, and other styles

26

Stigner, Carl. "A classifying algebra for CFT boundary conditions." Licentiate thesis, Karlstad University, Faculty of Technology and Science, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-4890.

Full text

Abstract:

Conformal field theories (CFT) constitute an interesting class of twodimensionalquantum field theories, with applications in string theoryas well as condensed matter physics. The symmetries of a CFT can beencoded in the mathematical structure of a conformal vertex algebra.The rational CFT’s are distinguished by the property that the categoryof representations of the vertex algebra is a modular tensor category.The solution of a rational CFT can be split off into two separate tasks, apurely complex analytic and a purely algebraic part.

The TFT-construction gives a solution to the second part of the problem.This construction gets its name from one of the crucial ingredients,a three-dimensional topological field theory (TFT). The correlators obtainedby the TFT-construction satisfy all consistency conditions of thetheory. Among them are the factorization constraints, whose implicationsfor boundary conditions are the main topic of this thesis.

The main result reviewed in this thesis is that the factorization constraintsgive rise to a semisimple commutative associative complex algebrawhose irreducible representations are the so-called reflection coefficients.The reflection coefficients capture essential information aboutboundary conditions, such as ground-state degeneracies and Ramond-Ramond charges of string compactifications. We also show that the annuluspartition function can be derived fromthis classifying algebra andits representation theory.

APA, Harvard, Vancouver, ISO, and other styles

27

Olsen, Robert. "Time-dependent boundary conditions for multiphase flow." Doctoral thesis, [Trondheim : Norges teknisk-naturvitenskapelige universitet, 2004. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-237.

Full text

APA, Harvard, Vancouver, ISO, and other styles

28

Moore, Stuart Alex. "Lateral boundary conditions in numerical ocean models." Thesis, University of Reading, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.402199.

Full text

APA, Harvard, Vancouver, ISO, and other styles

29

Gunson, James Reginald. "Estimating open-ocean boundary conditions : sensitivity studies." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/53024.

Full text

Abstract:

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995.
Includes bibliographical references (leaves 145-149).
by James Reginald Gunson.
Ph.D.

APA, Harvard, Vancouver, ISO, and other styles

30

Savi, L. L. [UNESP]. "A geometric approach to cosmological boundary conditions." Universidade Estadual Paulista (UNESP), 2015. http://hdl.handle.net/11449/132136.

Full text

Abstract:

Made available in DSpace on 2015-12-10T14:24:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-06-09. Added 1 bitstream(s) on 2015-12-10T14:30:15Z : No. of bitstreams: 1 000852869.pdf: 540584 bytes, checksum: 876f75004ad5a8e7b7dbf74986332aea (MD5)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
A assimetria temporal observada na física macroscópica se deve à conﬁguração de entropia extremamente baixa do universo primordial. Apesar de a matéria estar muito quente e com uma temperatura uniforme naquele estágio, os graus de liberdade gravitacionais estavam em grande medida suprimidos, fato este que contribui para o baixo valor da entropia e está codiﬁcado no alto grau de simetria espacial (caráter aproximadamente Friedman-Lemaître-Robertson-Walker) da superfície de último espalhamento. Analisamos diferentes tentativas de explicar a origem de tal conﬁguração especial. O paradigma inﬂacionário é testado com respeito a esse problema, e é concluído que a baixa entropia inicial não pode ser explicada dentro dele. Conclusões similares são obtidas com respeito a formulações estatísticas (i.e. antrópicas). Por outro lado, o paradigma conhecido como cosmologia cíclica conforme (CCC) se apresenta como uma nova alternativa que ultrapassa muitas das diﬁculdades enfrentadas pelos seus rivais, apesar de levantar suas próprias questões em aberto. Introduzimos o modelo juntamente com a estrutura matemática das geometrias de Cartan como um meio possível de atingir um melhor entendimento das condições de contorno cosmológicas. Um elemento que é crucial nessa análise é a modelagem de uma estrutura geométrica de Cartan sobre o espaço de de Sitter SOe(4,1)/SOe(3,1) com um parâmetro de comprimento variável. A introdução de um parâmetro de comprimento na cinemática é favorecida pela observação de uma constante cosmológica positiva e também desejável por motivos oriundos da gravitação quântica, devido à escala natural determinada pelo comprimento de Planck
The observed T-asymmetry of macroscopic physics is traced back to the extremely low entropy conﬁguration of the early universe. Although matter was very hot and with a uniform temperature at that stage, the gravitational degrees of freedom were largely suppressed, which fact contributes to the lowness of the entropy and is encoded in the high level of spatial symmetry (nearly Friedman-Lemître-Robertson-Walker character) of the last scattering surface. I analyze diﬀerent attempts to explain the origin of such special conﬁguration. The inﬂation paradigm is probed with respect to this problem, and it is concluded that the initial low entropy cannot be accounted for within it. Similar conclusions are reached with respect to statistical (i.e. anthropic) reasonings. On the other hand, the paradigm known as conformal cyclic cosmology (CCC) presents itself as a new alternative which surpasses many of the diﬃculties faced by its rivals, although raising its own open questions. I introduce the model together with the mathematical structure of Cartan geometries as a possible means of achieving a better understanding of cosmological boundary conditions. One element which is crucial in this analysis is the modeling of the Cartan geometric structure over a de Sitter space SOe(4,1)/SOe(3,1) with varying length parameter. The introduction of a length parameter in the kinematics is favored by the observation of a positive cosmological constant and also desirable for quantum gravity reasons, due to the natural scale set by Planck's constant

APA, Harvard, Vancouver, ISO, and other styles

31

Savi, L. L. (Lucas Lolli). "A geometric approach to cosmological boundary conditions /." São Paulo, 2015. http://hdl.handle.net/11449/132136.

Full text

Abstract:

Orientador: José Geraldo Pereira
Banca: Bruto Max Pimentel Escobar
Banca: Roldão da Rocha Junior
Banca: Davi Cabral Rodrigues
Banca: José abdalla Helayel-Neto
Resumo: A assimetria temporal observada na física macroscópica se deve à conﬁguração de entropia extremamente baixa do universo primordial. Apesar de a matéria estar muito quente e com uma temperatura uniforme naquele estágio, os graus de liberdade gravitacionais estavam em grande medida suprimidos, fato este que contribui para o baixo valor da entropia e está codiﬁcado no alto grau de simetria espacial (caráter aproximadamente Friedman-Lemaître-Robertson-Walker) da superfície de último espalhamento. Analisamos diferentes tentativas de explicar a origem de tal conﬁguração especial. O paradigma inﬂacionário é testado com respeito a esse problema, e é concluído que a baixa entropia inicial não pode ser explicada dentro dele. Conclusões similares são obtidas com respeito a formulações estatísticas (i.e. antrópicas). Por outro lado, o paradigma conhecido como cosmologia cíclica conforme (CCC) se apresenta como uma nova alternativa que ultrapassa muitas das diﬁculdades enfrentadas pelos seus rivais, apesar de levantar suas próprias questões em aberto. Introduzimos o modelo juntamente com a estrutura matemática das geometrias de Cartan como um meio possível de atingir um melhor entendimento das condições de contorno cosmológicas. Um elemento que é crucial nessa análise é a modelagem de uma estrutura geométrica de Cartan sobre o espaço de de Sitter SOe(4,1)/SOe(3,1) com um parâmetro de comprimento variável. A introdução de um parâmetro de comprimento na cinemática é favorecida pela observação de uma constante cosmológica positiva e também desejável por motivos oriundos da gravitação quântica, devido à escala natural determinada pelo comprimento de Planck
Abstract:The observed T-asymmetry of macroscopic physics is traced back to the extremely low entropy conﬁguration of the early universe. Although matter was very hot and with a uniform temperature at that stage, the gravitational degrees of freedom were largely suppressed, which fact contributes to the lowness of the entropy and is encoded in the high level of spatial symmetry (nearly Friedman-Lemître-Robertson-Walker character) of the last scattering surface. I analyze diﬀerent attempts to explain the origin of such special conﬁguration. The inﬂation paradigm is probed with respect to this problem, and it is concluded that the initial low entropy cannot be accounted for within it. Similar conclusions are reached with respect to statistical (i.e. anthropic) reasonings. On the other hand, the paradigm known as conformal cyclic cosmology (CCC) presents itself as a new alternative which surpasses many of the diﬃculties faced by its rivals, although raising its own open questions. I introduce the model together with the mathematical structure of Cartan geometries as a possible means of achieving a better understanding of cosmological boundary conditions. One element which is crucial in this analysis is the modeling of the Cartan geometric structure over a de Sitter space SOe(4,1)/SOe(3,1) with varying length parameter. The introduction of a length parameter in the kinematics is favored by the observation of a positive cosmological constant and also desirable for quantum gravity reasons, due to the natural scale set by Planck's constant
Doutor

APA, Harvard, Vancouver, ISO, and other styles

32

Li, Hongwei. "Local absorbing boundary conditions for wave propagations." HKBU Institutional Repository, 2012. https://repository.hkbu.edu.hk/etd_ra/1434.

Full text

APA, Harvard, Vancouver, ISO, and other styles

33

Wintz, Nick. "Eigenvalue comparisons for an impulsive boundary value problem with Sturm-Liouville boundary conditions." Huntington, WV : [Marshall University Libraries], 2004. http://www.marshall.edu/etd/descript.asp?ref=414.

Full text

APA, Harvard, Vancouver, ISO, and other styles

34

Bertolaccini, Jonathan. "Modélisation et simulation des écoulements de contre-courant de l'hélium superfluide par la méthode Boltzmann sur réseau." Thesis, Lyon, École normale supérieure, 2015. http://www.theses.fr/2015ENSL1063/document.

Full text

Abstract:

Les propriétés thermiques exceptionnelles de l’hélium superfluide, ou He-II, sont mises à profit pour la réfrigération cryogénique d'installations de forte puissance, bien que les mécanismes physiques sous-jacents restent mal compris. L’He-II peut être décrit à l’échelle macroscopique comme la superposition de deux fluides en interaction : un fluide normal se comportant comme un liquide ordinaire, et un superfluide sans viscosité. En présence d’une source de chaleur, un contre-courant s’établit naturellement entre ces deux composantes. L’évacuation de la chaleur par ce contre-courant est limitée par l’apparition d’instabilités dans des conditions mal comprises ; la grande dispersion des données expérimentales ne permettant pas de discriminer les différents modèles théoriques. Cette thèse examine à l’aide de simulations numériques le rôle des conditions aux bords et du couplage mutuel entre les deux composantes de l’He-II dans le déclenchement des instabilités de contre-courant.Une approche originale de type Boltzmann sur réseau a été développée pour modéliser à l’échelle mésoscopique l'interaction entre les deux composantes de l’He-II. Un code reproduisant les écoulements de contre-courant en conduite 2d et 3d a été développé et validé. Les résultats obtenus indiquent des effets d’entrée de conduite amplifiés pour la composante superfluide, qui engendrent des pertes de charge anormalement élevées. Le mécanisme responsable de ces effets d’entrée a été étudié et il est montré qu'il peut fausser la détection du seuil de transition dans des conduites trop courtes ; ceci peut expliquer en partie la dispersion des données expérimentales.Pour illustrer la puissance de l'approche dans une géométrie complexe, le sillage d'un obstacle dans un écoulement de contre-courant a été simulé. La présence de zones de recirculation des deux côtés de l’obstacle, déjà observée expérimentalement, est retrouvée et expliquée par un mécanisme original de parois virtuelles
The exceptional thermal properties of superfluid helium, or He-II, are exploited to the cryogenic refrigeration of high power installations, although the underlying physical mechanisms remain poorly understood. The He-II can be described macroscopically as the superposition of two fluids in interaction: a normal fluid behaves as an ordinary liquid, and a superfluid without viscosity. In the presence of a heat source, a counterflow established between these two components. The heat dissipation by this counterflow is limited by the occurrence of instabilities in misunderstood condition; the wide dispersion of experimental data does not allow to discriminate between the different theoretical models. This thesis examines using numerical simulations the role of boundary conditions and the mutual coupling between the two components of the He-II in triggering instabilities in counterflow.An innovative lattice Boltzmann type approach was developed to model the mesoscopic scale interaction between the two components of the He-II. A code reproducing counterflow in 2D and 3D conducts has been developed and validated. The results obtained indicate amplified entrance effects for superfluid component, which generate abnormally high pressure drops. The head of these entrance effects mechanism has been studied and it is shown that it can distort the detection of the transition threshold in too short pipes; This may partly explain the dispersion of experimental data.To illustrate the power of the approach in a complex geometry, the wake of an obstacle in a counterflow was simulated. The presence of recirculation areas on both sides of the obstacle, already observed experimentally, is found and explained by a new mechanism using "virtual walls"

APA, Harvard, Vancouver, ISO, and other styles

35

Schulze, Bert-Wolfgang, and Jörg Seiler. "Pseudodifferential boundary value problems with global projection conditions." Universität Potsdam, 2002. http://opus.kobv.de/ubp/volltexte/2008/2623/.

Full text

Abstract:

Contents: Introduction 1 Operators with the transmission property 1.1 Operators on a manifold with boundary 1.2 Conditions with pseudodifferential projections 1.3 Projections and Fredholm families 2 Boundary value problems not requiring the transmission property 2.1 Interior operators 2.2 Edge amplitude functions 2.3 Boundary value problems 3 Operators with global projection conditions 3.1 Construction for boundary symbols 3.2 Ellipticity of boundary value problems with projection data 3.3 Operators of order zero

APA, Harvard, Vancouver, ISO, and other styles

36

Mossige, Endre Joachim. "Curved Boundary Conditions for the Lattice Boltzmann Method." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for energi- og prosessteknikk, 2011. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-14667.

Full text

Abstract:

The lattice Boltzmann method is a modern method in computational fluid dynamics. Its primary use is the simulation of incompressible flows. It has computational advantages over conventional methods like the finite volume method. However, the implementation of boundary conditions is still an unsolved topic for this method. The method is defined on a Cartesian grid such that curved walls need special treatment as they are generally not aligned with the grid lines. We investigated a number of straight and curved boundary conditions and performed four different benchmark tests to verify these. Based on a formulation for curved walls with no-slip from the literature, we showed that this method could be extended to simulate flows with arbitrary velocity boundary conditions. Our scheme conserved the second order accuracy of the lattice Boltzmann method in time and space.

APA, Harvard, Vancouver, ISO, and other styles

37

Jung, Florian [Verfasser]. "Canonical group quantization and boundary conditions / Florian Jung." Mainz : Universitätsbibliothek Mainz, 2012. http://d-nb.info/1025490088/34.

Full text

APA, Harvard, Vancouver, ISO, and other styles

38

Appelö, Daniel. "Non-reflecting Boundary Conditions for Wave Propagation Problems." Licentiate thesis, KTH, Numerical Analysis and Computer Science, NADA, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1664.

Full text

Abstract:

We consider two aspects of non-reflecting boundaryconditions for wave propagation problems. First we evaluate aproposed Perfectly Matched Layer (PML) method for thesimulation of advective acoustics. It is shown that theproposed PML becomes unstable for a certain combination ofparameters. A stabilizing procedure is proposed andimplemented. By numerical experiments the performance of thePML for a problem with nonuniform flow is investigated. Furtherthe performance for different types of waves, vorticity andsound waves, are investigated.

The second aspect concerns spurious waves, which areintroduced by any discretization procedure. We constructdiscrete boundary conditions, that are nonreflecting for bothphysical and spurious waves, when combined with a fourth orderaccurate explicit discretization of one-way wave equations. Theboundary condition is shown to be GKS-stable. The boundaryconditions are extended to hyperbolic systems in two spacedimensions, by combining exact continuous non-reflectingboundary conditions and the one dimensional discretelynon-reflecting boundary condition. The resulting boundarycondition is localized by the standard Pad´eapproximation.

Numerical experiments reveal that the resulting methodsuffers from boundary instabilities. Analysis of a relatedcontinuous problem suggests that the discrete boundarycondition can be stabilized by adding tangential viscosity atthe boundary. For the lowest order Pad´e approximation weare able to stabilize the discrete boundary condition.

APA, Harvard, Vancouver, ISO, and other styles

39

Burgess, N. A. "Stable boundary conditions for the shallow water equations." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380000.

Full text

APA, Harvard, Vancouver, ISO, and other styles

40

Ilie, Alexandra. "Unethical Pro-Organizational Behaviors: Antecedents and Boundary Conditions." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4085.

Full text

Abstract:

The goals of the current study were to examine the antecedents and boundary conditions of a new construct called unethical pro-organizational behavior (UPB) defined as behaviors that are unethical but at the same time helping the organizations (e.g., giving a low performing employee a letter of recommendation to help him/her find a job in another organization). Drawing from social exchange theory, antecedents such as leader-member exchange, perceived organizational support, idiosyncratic deals, and leader-member exchange were hypothesized to be positively related to UPB Three moderators of the impact of the social exchange variables on UPB: were also investigated: moral identity, psychological entitlement, and supervisor's embodiment of the organization. Data was collected in a cross-sectional survey from 269 employees and 144 supervisors. The hypotheses were tested using correlations and moderated multiple regressions. The results indicate that none of the hypotheses were supported. However, there were some interesting unexpected findings as some social exchange variables were found to correlate negatively with UPB. Implications for future research and practice are discussed.

APA, Harvard, Vancouver, ISO, and other styles

41

Arbegard, Fredrik, and Håkan Jakobsson. "Evaluation of Boundary Conditions in Chassis Test Rig." Thesis, KTH, Hållfasthetslära (Inst.), 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-176497.

Full text

Abstract:

Trucks are constantly developed in order to achieve new goals in terms of demands from the customers and regulation. In order to develop the truck chassis to withstand the load, different tests are conducted and among those life time testing. At Scania, service life of frames and suspensions is tested using a purpose built test rig known as Ramriggen. The test rig subjects the frame to a quasi-static load created of measurements from a test track. The current rig for frame testing has boundary conditions that distribute the loads unevenly within the chassis, especially for trucks with added frame twisting. In the current rig, the front axle load is absorbed solely by the engine and the frame twisting is inserted by a cradle. With increased axle load the error in vertical loading is also increased and with the current configuration for frame twisting, unexpected cracks in the frame has been observed. In order to improve the emulation of a real truck the boundary conditions requires development. The purpose of this study is to compare different boundary conditions at the front of the truck. An investigation of how new boundary conditions at the front of truck can redistribute the loads has been conducted. This investigation utilized FEM as a tool and method to reach the end results. In order to compare different boundary conditions two synthetic load cases were defined from measurements conducted on a truck at a test track. The two load cases consisted of a vertical load and two cases of frame twisting. This investigation resulted in new boundary conditions for a concept test rig. The new boundary conditions consist of a cab mock-up that is able to absorb vertical load and induce frame twisting. The result of the investigation of the two load cases shows that the engine beam is overloaded at most 40% and the base mount with 150% with the current boundary conditions compared with the new suggested concept.
Lastbilar utvecklas ständigt för att uppnå nya krav och regelverk. När lastbilschassin optimeras för hållfasthet måste olika prover utföras bland annat livslängdsprov. Livslängdsprov på ramar och hjulupphängningar utförs på Scania i en speciell kvasistatisk rigg, Ramriggen. Riggen utsätter ramarna för en lasthistorik skapad utifrån provbanemätningar. Dagens rigg för provning av ramar har randvillkor som snedfördelar lasterna inom chassiet, speciellt för dragbilar med pålagd ramvridning. Dragbilens framaxeltyck tags enbart upp utav motorn och vridningen av ramen införs i basfästena via en krängningsvagga. Vid större axeltryck blir felet ifrån vertikallasten allt större och vid ramvridning så har oförutsedda utfall skett i framvagnen. För att förbättra Ramriggens förmåga att efterlikna en verklig lastbil behöver randvillkoren utvecklas. Syftet med undersökningen är att jämföra olika randvillkor i dragbilens framvagn. En undersökning om nya randvillkor för dragbilens framvagn i riggen kan omfördela lasterna har genomförts. I undersökningen användes FEM. För att jämföra olika randvillkor har två lastfall syntetiserats utifrån olika mätningar inmätta på dragbil. Ett lastfall för hög vertikallast och två lastfall med ramvridning. Genom undersökningen har nya randvillkorskoncept för riggen tagits fram. De nya randvillkoren innefattar en hyttattrapp som kan bidra med mothåll för vertikallaster samt inducera ramvridning. Resultatet av undersökningen av de två lastfallen visar att motorbalken överlastas med som mest 40% och basfästet 150% med de nuvarande randvillkoren jämfört med det föreslagna konceptet.

APA, Harvard, Vancouver, ISO, and other styles

42

Shepherd, Roger. "Coupled non-linear diffusion under periodic boundary conditions." Thesis, University of South Wales, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239985.

Full text

APA, Harvard, Vancouver, ISO, and other styles

43

Padbidri, Jagan. "Minimal boundary conditions for simulations of disordered materials." WSU all campuses online access, 2003. http://www.dissertations.wsu.edu/Thesis/Fall2003/j%5Fpadbidri%5F121503.pdf.

Full text

APA, Harvard, Vancouver, ISO, and other styles

44

Dade, William Brian. "Studies on boundary conditions for fine-sediment transport /." Thesis, Connect to this title online; UW restricted, 1992. http://hdl.handle.net/1773/11052.

Full text

APA, Harvard, Vancouver, ISO, and other styles

45

Bélanger-Rioux, Rosalie. "Compressed absorbing boundary conditions for the Helmholtz equation." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/90179.

Full text

Abstract:

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mathematics, 2014.
56
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 101-105).
Absorbing layers are sometimes required to be impractically thick in order to offer an accurate approximation of an absorbing boundary condition for the Helmholtz equation in a heterogeneous medium. It is always possible to reduce an absorbing layer to an operator at the boundary by layer-stripping elimination of the exterior unknowns, but the linear algebra involved is costly. We propose to bypass the elimination procedure, and directly fit the surface-to-surface operator in compressed form from a few exterior Helmholtz solves with random Dirichlet data. We obtain a concise description of the absorbing boundary condition, with a complexity that grows slowly (often, logarithmically) in the frequency parameter. We then obtain a fast (nearly linear in the dimension of the matrix) algorithm for the application of the absorbing boundary condition using partitioned low rank matrices. The result, modulo a precomputation, is a fast and memory-efficient compression scheme of an absorbing boundary condition for the Helmholtz equation.
by Rosalie Bélanger-Rioux.
Ph. D.

APA, Harvard, Vancouver, ISO, and other styles

46

LaGrone, John. "Application and optimization of complete radiation boundary conditions." Thesis, Southern Methodist University, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10144064.

Full text

Abstract:

We describe the implementation of optimal local radiation boundary condition sequences for second order finite difference approximations to Maxwell's equations and the scalar wave equation using the double absorbing boundary formulation. Numerical experiments are presented which demonstrate that the design accuracy of the boundary conditions is achieved and exceeds that of perfectly matched layers for comparable effort.

We also describe the application of CRBC type boundary conditions for elastic waves in (an)isotropic media. The results show that we can optimize the CRBC problems for a subset of elastic media. Additionally, we propose a generalized CRBC type boundary conditions that may be more applicable to elastic wave equations and present some preliminary results.

APA, Harvard, Vancouver, ISO, and other styles

47

Mezzadri, Francesco. "Boundary conditions for torus maps and spectral statistics." Thesis, University of Bristol, 1999. http://hdl.handle.net/1983/dcba3276-00be-4bae-a89f-f864750e77ff.

Full text

APA, Harvard, Vancouver, ISO, and other styles

48

Massa, Kenneth L. "Control of Burgers' Equation With Mixed Boundary Conditions." Thesis, Virginia Tech, 1998. http://hdl.handle.net/10919/36681.

Full text

Abstract:

We consider the problems of simulation and control for Burgers' equation with mixed boundary conditions. We first conduct numerical experiments to test the convergence and stability of two standard finite element schemes for various Robin boundary conditions and a variety of Reynolds numbers. These schemes are used to compute LQR feedback controllers for Burgers' equation with boundary control. Numerical studies of these feedback control laws are used to evaluate the performance and practicality of this approach to boundary control of non-linear systems.
Master of Science

APA, Harvard, Vancouver, ISO, and other styles

49

Berrabah, Nassif. "On High Order On-Surface Radiation Boundary Conditions." University of Akron / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1415620836.

Full text

APA, Harvard, Vancouver, ISO, and other styles

50

Ahmed, Ashfaq. "Investigation of boundary conditions for hydrological design analysis." Thesis, University of Birmingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.396238.

Full text

Abstract:

This study deals with the fundamental problems of hydrological design. Specifically, it explores the boundary conditions for design flood analysis. The problem of extrapolation of design relationships has been investigated by the systematic analysis of important design parameters namely unit hydrograph time to peak (tp), catchment antecedent conditions (CWI), runoff losses (PR) and the relationship between rainfall and flood return periods. In particular, this thesis investigates the combination of these variables representative of design analysis. A review of the hydrological design tools of response identification along with the physical evidence of catchment response is presented. The results of the investigation regarding time to peak (tp) show that it varies significantly between events, and the relationships developed between tp and flood magnitude (Qp) show the non-linear catchment behaviour which conform with most of the physical and field investigations. The relationships suggest that the unit hydrograph (UH) parameters derived from moderate events should be adjusted for extreme events and therefore a correction in UH tp has been developed which depends on the flood return period (Ashfaq and Webster, 2000a). The analysis of catchment wetness index (CWI) from a large number of observed events showed that antecedent conditions observed in the flood season are reasonably representative of the major events. This contrasts with the existing design recommendations which suggest consistently lower values. An alternative relationship of CWI therefore has been developed for design purposes (Webster and Ashfaq, submitted manuscript). The investigation showed that the percentage runoff (PR) characteristics of large events are consistently different than available from the existing design PR-method. The design method underestimates for the standardised conditions especially for large events because of its limited range of estimates for a wide range of return periods (e.g. 11% range in PR for 2 to 1000 year return periods). This problem is related with the PR-method itself for catchments having higher mean annual rainfall (SAAR> 800 mm) whereas for lower SAAR areas « 800 mm), it is related to the selection of design CWI values. The analysis of observed large events also revealed that a flood is generally associated with the storm of less return period than that of the flood. This contrasts with the suggested design rainfall-flood return periods relationship in the FSR (NERC, 1975; [H, 1999), but conforms with the curves presented by Webster (1998, 1999). This observation is further established by a detailed investigation of the characteristics of extreme events through a continuous model as well as the hydrological analysis of observed extreme floods of Easter-1998 (Ashfaq and Webster,2000b). The study demonstrates that the characteristics of extreme floods are different from those of small and moderate events. Relationships based on moderate events should be adjusted for the design of major events. The aggregated and integrated findings based on a comprehensive series of analysis led to the proposal of an alternative combination of design parameters. The performance of this combination showed improved flood estimates without any prior calibration in comparison to the FSR as well as FEH. A revised design methodology has therefore been proposed on the philosophy of 'independent treatment of input variables'. The application of this methodology on new catchments also provides encouraging estimates of flood quantiles. It is suggested that the methodology is equally applicable for both gauged and ungauged catchments especially where the observed data are limited or no data are available.

APA, Harvard, Vancouver, ISO, and other styles

Dissertations / Theses on the topic 'Boundary conditions and coupling'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles