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1
Crooks, Elaine Craig Mackay. « Travelling-wave solutions for parabolic systems ». Thesis, University of Bath, 1996. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319218.
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Yolcu, Türkay. « Parabolic systems and an underlying Lagrangian ». Diss., Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29760.
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In this thesis, we extend De Giorgi's interpolation method to a class of parabolic equations which are not gradient flows but possess an entropy functional and an underlying Lagrangian. The new fact in the study is that not only the Lagrangian may depend on spatial variables, but also it does not induce a metric. Assuming the initial condition is a density function, not necessarily smooth, but solely of bounded first moments and finite "entropy", we use a variational scheme to discretize the equation in time and construct approximate solutions. Moreover, De Giorgi's interpolation method is revealed to be a powerful tool for proving convergence of our algorithm. Finally, we analyze uniqueness and stability of our solution in L¹.
3
Yolcu, Türkay. « Parabolic systems and an underlying Lagrangian ». Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29760.
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Thesis (Ph.D)--Mathematics, Georgia Institute of Technology, 2010.Committee Chair: Gangbo, Wilfrid; Committee Member: Chow, Shui-Nee; Committee Member: Harrell, Evans; Committee Member: Swiech, Andrzej; Committee Member: Yezzi, Anthony Joseph. Part of the SMARTech Electronic Thesis and Dissertation Collection.
4
Reichelt, Sina. « Two-scale homogenization of systems of nonlinear parabolic equations ». Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://dx.doi.org/10.18452/17385.
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Ziel dieser Arbeit ist es zwei verschiedene Klassen von Systemen nichtlinearer parabolischer Gleichungen zu homogenisieren, und zwar Reaktions-Diffusions-Systeme mit verschiedenen Diffusionslängenskalen und Gleichungen vom Typ Cahn-Hilliard. Wir betrachten parabolische Gleichungen mit periodischen Koeffizienten, wobei die Periode dem Verhältnis der charakteristischen mikroskopischen zu der makroskopische Längenskala entspricht. Unser Ziel ist es, effektive Gleichungen rigoros herzuleiten, um die betrachteten Systeme besser zu verstehen und den Simulationsaufwand zu minimieren. Wir suchen also einen Konvergenzbegriff, mit dem die Lösung des Ausgangsmodells im Limes der Periode gegen Null gegen die Lösung des effektiven Modells konvergiert. Um die periodische Mikrostruktur und die verschiedenen Diffusivitäten zu erfassen, verwenden wir die Zwei-Skalen Konvergenz mittels periodischer Auffaltung. Der erste Teil der Arbeit handelt von Reaktions-Diffusions-Systemen, in denen einige Spezies mit der charakteristischen Diffusionslänge der makroskopischen Skala und andere mit der mikroskopischen diffundieren. Die verschiedenen Diffusivitäten führen zu einem Verlust der Kompaktheit, sodass wir nicht direkt den Grenzwert der nichtlinearen Terme bestimmen können. Wir beweisen mittels starker Zwei-Skalen Konvergenz, dass das effektive Modell ein zwei-skaliges Modell ist, welches von der makroskopischen und der mikroskopischen Skale abhängt. Unsere Methode erlaubt es uns, explizite Raten für die Konvergenz der Lösungen zu bestimmen. Im zweiten Teil betrachten wir Gleichungen vom Typ Cahn-Hilliard, welche ortsabhängige Mobilitätskoeffizienten und allgemeine Potentiale beinhalten. Wir beweisen evolutionäre Gamma-Konvergenz der zugehörigen Gradientensysteme basierend auf der Gamma-Konvergenz der Energien und der Dissipationspotentiale.The aim of this thesis is to derive homogenization results for two different types of systems of nonlinear parabolic equations, namely reaction-diffusion systems involving different diffusion length scales and Cahn-Hilliard-type equations. The coefficient functions of the considered parabolic equations are periodically oscillating with a period which is proportional to the ratio between the charactersitic microscopic and macroscopic length scales. In view of greater structural insight and less computational effort, it is our aim to rigorously derive effective equations as the period tends to zero such that solutions of the original model converge to solutions of the effective model. To account for the periodic microstructure as well as for the different diffusion length scales, we employ the method of two-scale convergence via periodic unfolding. In the first part of the thesis, we consider reaction-diffusion systems, where for some species the diffusion length scale is of order of the macroscopic length scale and for other species it is of order of the microscopic one. Based on the notion of strong two-scale convergence, we prove that the effective model is a two-scale reaction-diffusion system depending on the macroscopic and the microscopic scale. Our approach supplies explicit rates for the convergence of the solution. In the second part, we consider Cahn-Hilliard-type equations with position-dependent mobilities and general potentials. It is well-known that the classical Cahn-Hilliard equation admits a gradient structure. Based on the Gamma-convergence of the energies and the dissipation potentials, we prove evolutionary Gamma-convergence, for the associated gradient system such that we obtain in the limit of vanishing periods a Cahn-Hilliard equation with homogenized coefficients.
5
Liu, Weian, Yin Yang et Gang Lu. « Viscosity solutions of fully nonlinear parabolic systems ». Universität Potsdam, 2002. http://opus.kobv.de/ubp/volltexte/2008/2621/.
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In this paper, we discuss the viscosity solutions of the weakly coupled systems of fully nonlinear second order degenerate parabolic equations and their Cauchy-Dirichlet problem. We prove the existence, uniqueness and continuity of viscosity solution by combining Perron's method with the technique of coupled solutions. The results here generalize those in [2] and [3].
6
Floater, Michael S. « Blow-up of solutions to nonlinear parabolic equations and systems ». Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235037.
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Chen, Mingxiang. « Structural stability of periodic systems ». Diss., Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/29341.
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Floridia, Giuseppe. « Approximate multiplicative controllability for degenerate parabolic problems and regularity properties of elliptic and parabolic systems ». Doctoral thesis, Università di Catania, 2012. http://hdl.handle.net/10761/1051.
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This thesis consists of two parts, both related to the theory of parabolic equations and systems.
The first part is devoted to control theory which studies the possibility of influencing
the evolution of a given system by an external action called control. Here we address approximate controllability problems via multiplicative controls, motivated by our interest in some differential models for the study of climatology.
In the second part of the thesis we address regularity issues on the local differentiability and H\"older regularity for weak solutions of nonlinear systems in divergence form.
In order to improve readability, the two parts have been organized as completely independent chapters, with
two separate introductions and bibliographies.
All the new results of this thesis have been presented at conferences and workshops, and
most of them appeared or are to appear as research articles in international journals.
Related directions for future research are also outlined in body of the work.
9
Al, Refai Mohammed. « Sequential eigenfunction expansion for certain non-linear parabolic systems and wave type equations ». Thesis, McGill University, 2000. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=36747.
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In trying to solve nonlinear partial differential equations with time dependence using the Galerkin method, one ends up with solving nonlinear systems of ordinary differential equations, which are not easily solved. In this thesis we introduce a new iterative method based on eigenfunction expansion to deal with the finite non-linear systems sequentially.We apply the new method to integrate the semi-linear parabolic equation ut=12u+fu ,x∈D with homogeneous Dirichlet or Robin boundary conditions. We prove the convergence of the new iterative method, and use it to find the multiple solutions of the system, which are difficult to obtain using the Galerkin method.
We next apply the new method to solve a parabolic system of two semi-linear equations
ut=12u+f u,q qt=12 q+gu,q ,x∈D with homogeneous boundary conditions Au = 0 and Bv = 0. We prove the convergence of the new method for the case when A = B. If A ≠ B no analytical statements are obtained. However, the proof of convergence is a sufficient, but not a necessary condition, and numerical calculations indicate that the solution obtained by the new method still converges to that obtained by the Galerkin method for the case when A ≠ B. We apply the new method to integrate a system in combustion theory, and we are able to find critical (as defined in Chapter 4) solutions for the system, which are not easily found using the Galerkin method.
To see that the new method can be applied to more general systems, we use it to integrate the Kuramoto-Sivashinsky equation
6tu+41+e 264xu+e6 2xu+12 6xu2=0 ,x∈0,ℓ We prove the convergence of the iterative method and use it to find the first term of the eigenfunction expansion analytically, and from that we notice that the equation has two solutions, one stable and the other unstable. This kind of observation can not be obtained using the Galerkin method.
Finally, we apply the new method to solve a wave type equation governing the motion of a fluid in a conveying pipe,
EI64w 6x4+&parl0;MU2 t+M6U6t L-x&parr0;6 2w6x2+2MU6 2w6x6t+M +m62w 6t2=0. In all of the above systems, numerical calculations indicate that the solutions obtained by the new method and the Galerkin method coincide.
10
Zhao, Yaxi. « Numerical solutions of nonlinear parabolic problems using combined-block iterative methods / ». Electronic version (PDF), 2003. http://dl.uncw.edu/etd/2003/zhaoy/yaxizhao.pdf.
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11
Cao, Yanzhao. « Analysis and numerical approximations of exact controllability problems for systems governed by parabolic differential equations ». Diss., Virginia Tech, 1996. http://hdl.handle.net/10919/37771.
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The exact controllability problems for systems modeled by linear parabolic differential equations and the Burger's equations are considered. A condition on the exact controllability of linear parabolic equations is obtained using the optimal control approach. We also prove that the exact control is the limit of appropriate optimal controls. A numerical scheme of computing exact controls for linear parabolic equations is constructed based on this result. To obtain numerical approximation of the exact control for the Burger's equation, we first construct another numerical scheme of computing exact controls for linear parabolic equations by reducing the problem to a hypoelliptic equation problem. A numerical scheme for the exact zero control of the Burger's equation is then constructed, based on the simple iteration of the corresponding linearized problem. The efficiency of the computational methods are illustrated by a variety of numerical experiments.Ph. D.
12
ADDONA, DAVIDE. « Parabolic operators with unbounded coefficients with applications to stochastic optimal control games ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2015. http://hdl.handle.net/10281/76535.
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The aim of this thesis is to improve some results on parabolic Cauchy problems with unbounded coefficients and their connection with stochastic optimal games.
In the first part we summarize the recent results on parabolic operators with unbounded coefficients and on stochastic optimal control problem. In particular, in the matter of analyitic results, we recall the main exstence and uniqueness theorems for parabolic Cauchy problems with unbounded coefficients, the gradient estimates for the associated evolution operator, and its continuity and compactness properties. About the stochastic part, we briefly show the strong and weak formulation, which are the settings where the stochastic control problems are located, and we introduce the backward stochastic differential equations, which allow to connect a semilinear Cauchy problem with a class of stochastic control problem.
In the second part we prove the existence and uniqueness of a mild solution to a semilinear parabolic Cauchy problem of Hamilton-Jacobi-Bellman (HJB) type. Moreover, we show that the solution to a Forward Backward Stochastic Differential Equation (FBSDE) can be expressed in terms of the solution to the HJB equation. Combining HJB equation and FBSDE, we show that, for a class of stochastic control problemin weak formulation, there exists an optimal control, and by means of the regularity of the solution to the HJB equation, we can identify the feedback law.
The third part of the thesis is devoted to the study of a class of system of nonautonomous linear parabolic equations with unbounded coefficients, coupled both at first and zero order. We provide sufficient conditions which guarantee the existence and uniqueness of a classical solution to the Cauchy problem, and throughout this classical solution we define an evolution operator on the space of bounded and continuous functions. Further, we prove continuity properties of the evolution operator and that, under additional hypotheses, it is compact on the space of bounded and continuous functions.
In the last chapter, we deal with a semilinear system of parabolic equations and its application to differential games. At first, we prove the existence of a mild solution to the system by an approximation argument. Throughout this mild solution, we show the existence of an adapted solution to a system of FBSDE which allows us to prove the existence of a Nash equilibrium for a class of differential games.
13
Moreno, Claudia. « Control of partial differential equations systems of dispersive type ». Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASV031.
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Il existe peu de résultats dans la littérature sur la contrôlabilité du système d'équations aux dérivées partielles. Dans cette thèse, nous considérons l'étude des propriétés de contrôle pour trois systèmes couplés d'équations aux dérivées partielles de type dispersif et un problème inverse de récupération d’un coefficient. Le premier système est formé par N équations de Korteweg-de Vries sur un réseau en forme d'étoile. Pour ce système, nous étudierons la contrôlabilité exacte avec N contrôles placés aux extrémités du réseau. Le deuxième système couple trois équations de Korteweg-de Vries. Ce système est appelé dans la littérature le système Hirota-Satsuma généralisé. Nous étudions la contrôlabilité exacte avec trois contrôles frontières.Après, nous étudierons un système parabolique du quatrième ordre formé par deux équations de Kuramoto-Sivashinsky. Nous prouvons l’existence et l’unicité de la solution du système. Ensuite, nous étudions la nulle contrôlabilité du système avec deux contrôles, pour supprimer un contrôle, nous avons besoin d’une inégalité de Carleman qui n’est pas encore prouvée. Finalement, nous présentons pour le système parabolique du quatrième ordre le problème inverse de récupérer le coefficient anti-diffusion à partir des mesures de la solutionThere are few results in the literature about the controllability of partial differential equations system. In this thesis, we consider the study of control properties for three coupled systems of partial differential equations of dispersive type and an inverse problem of recovering a coefficient. The first system is formed by N Korteweg-de Vries equations on a star-shaped network. For this system we will study the exact controllability using N controls placed in the external nodes of the network. The second system couples three Korteweg-de Vries equations. This system is called in the literature the generalized Hirota-Satsuma system. We study the exact controllability with three boundary controls.On the other hand, we will study a fourth-order parabolic system formed by two Kuramoto-Sivashinsky equations. We prove the well-posedness of the system with some regularity results. Then we study the null controllability of the system with two controls, to remove a control, we need a Carleman inequality which is not proven yet. Finally, we present for the fourth-order parabolic system the inverse problem of retrieving the anti-diffusion coefficient from the measurements of the solution
14
Zelley, Christopher Andrew. « Radiowave propagation over irregular terrain using the parabolic equation method ». Thesis, University of Birmingham, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390682.
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Reichelt, Sina [Verfasser], Alexander [Akademischer Betreuer] Mielke, Dorothee [Akademischer Betreuer] Knees et Adrian [Akademischer Betreuer] Muntean. « Two-scale homogenization of systems of nonlinear parabolic equations / Sina Reichelt. Gutachter : Alexander Mielke ; Dorothee Knees ; Adrian Muntean ». Berlin : Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://d-nb.info/1080558411/34.
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Song, Yongcun. « An ADMM approach to the numerical solution of state constrained optimal control problems for systems modeled by linear parabolic equations ». HKBU Institutional Repository, 2018. https://repository.hkbu.edu.hk/etd_oa/551.
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We address in this thesis the numerical solution of state constrained optimal control problems for systems modeled by linear parabolic equations. For the unconstrained or control-constrained optimal control problem, the first order optimality condition can be obtained in a general way and the associated Lagrange multiplier has low regularity, such as in the L²(Ω). However, for state-constrained optimal control problems, additional assumptions are required in general to guarantee the existence and regularity of Lagrange multipliers. The resulting optimality system leads to difficulties for both the numerical solution and the theoretical analysis. The approach discussed here combines the alternating direction of multipliers (ADMM) with a conjugate gradient (CG) algorithm, both operating in well-chosen Hilbert spaces. The ADMM approach allows the decoupling of the state constraints and the parabolic equation, in which we need solve an unconstrained parabolic optimal control problem and a projection onto the admissible set in each iteration. It has been shown that the CG method applied to the unconstrained optimal control problem modeled by linear parabolic equation is very efficient in the literature. To tackle the issue about the associated Lagrange multiplier, we prove the convergence of our proposed algorithm without assuming the existence and regularity of Lagrange multipliers. Furthermore, a worst case O(1/k) convergence rate in the ergodic sense is established. For numerical purposes, we employ the finite difference method combined with finite element method to implement the time-space discretization. After full discretization, the numerical results we obtain validate the methodology discussed in this thesis.
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Post, Katharina. « A System of Non-linear Partial Differential Equations Modeling Chemotaxis with Sensitivity Functions ». Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät II, 1999. http://dx.doi.org/10.18452/14365.
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Wir betrachten ein System nichtlinearer parabolischer partieller Differentialgleichungen zur Modellierung des biologischen Phänomens Chemotaxis, das unter anderem in Aggregationsprozessen in Lebenszyklen bestimmter Einzeller eine wichtige Rolle spielt. Unser Chemotaxismodell benutzt Sensitivitäts funktionen, die die vorkommenden biologischen Prozesse genauer spezifizieren. Trotz der durch die Sensitivitätsfunktionen eingebrachten, zusätzlichen Nichtlinearitäten in den Gleichungen erhalten wir zeitlich globale Existenz von Lösungen für verschiedene biologisch realistische Klassen von Sensitivitätsfunktionen und können unter unterschiedlichen Bedingungen an die Systemdaten Konvergenz der Lösungen zu trivialen und nicht-trivialen stationären Punkten beweisen.We consider a system of non-linear parabolic partial differential equations modeling chemotaxis, a biological phenomenon which plays a crucial role in aggregation processes in the life cycle of certain unicellular organisms. Our chemotaxis model introduces sensitivity functions which help describe the biological processes more accurately. In spite of the additional non-linearities introduced by the sensitivity functions into the equations, we obtain global existence of solutions for different classes of biologically realistic sensitivity functions and can prove convergence of the solutions to trivial and non-trivial steady states.
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Rolland, Guillaume. « Global existence and fast-reaction limit in reaction-diffusion systems with cross effects ». Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2012. http://tel.archives-ouvertes.fr/tel-00785757.
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This thesis is devoted to the study of parabolic systems of partial differential equations arising in mass action kinetics chemistry, population dynamics and electromigration theory. We are interested in the existence of global solutions, uniqueness of weak solutions, and in the fast-reaction limit in a reaction-diffusion system. In the first chapter, we study two cross-diffusion systems. We are first interested in a population dynamics model, where cross effects in the interactions between the different species are modeled by non-local operators. We prove the well-posedness of the corresponding system for any space dimension. We are then interested in a cross-diffusion system which arises as the fast-reaction limit system in a classical system for the chemical reaction C1+C2=C3. We prove the convergence when k goes to infinity of the solution of the system with finite reaction speed k to a global solution of the limit system. The second chapter contains new global existence results for some reaction-diffusion systems. For networks of elementary chemical reactions of the type Ci+Cj=Ck and under Mass Action Kinetics assumption, we prove the existence and uniqueness of global strong solutions, for space dimensions N<6 in the semi-linear case, and N<4 in the quasi-linear case. We also prove the existence of global weak solutions for a class of parabolic quasi-linear systems with at most quadratic non-linearities and with initial data that are only assumed to be nonnegative and integrable. In the last chapter, we generalize a global well-posedness result for reaction-diffusion systems whose nonlinearities have a "triangular" structure, for which we now take into account advection terms and time and space dependent diffusion coefficients. The latter result is then used in a Leray-Schauder fixed point argument to prove the existence of global solutions in a diffusion-electromigration system.
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Krainer, Thomas, et Bert-Wolfgang Schulze. « On the inverse of parabolic systems of partial differential equations of general form in an infinite space-time cylinder [Part 1 : Chapter 1+2] ». Universität Potsdam, 2001. http://opus.kobv.de/ubp/volltexte/2008/2598/.
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We consider general parabolic systems of equations on the infinite time interval in case of the underlying spatial configuration is a closed manifold. The solvability of equations is studied both with respect to time and spatial variables in exponentially weighted anisotropic Sobolev spaces, and existence and maximal regularity statements for parabolic equations are proved. Moreover, we analyze the long-time behaiour of solutions in terms of complete asymptotic expansions.
These results are deduced from a pseudodifferential calculus that we construct explicitly. This algebra of operators is specifically designed to contain both the classical systems of parabolic equations of general form and their inverses, parabolicity being reflected purely on symbolic level. To this end, we assign t = ∞ the meaning of an anisotropic conical point, and prove that this interprtation is consistent with the natural setting in the analysis of parabolic PDE. Hence, major parts of this work consist of the construction of an appropriate anisotropiccone calculus of so-called Volterra operators.
In particular, which is the most important aspect, we obtain the complete characterization of the microlocal and the global kernel structure of the inverse of parabolicsystems in an infinite space-time cylinder. Moreover, we obtain perturbation results for parabolic equations from the investigation of the ideal structure of the calculus.
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Krainer, Thomas, et Bert-Wolfgang Schulze. « On the inverse of parabolic systems of partial differential equations of general form in an infinite space-time cylinder [Part 2 : Chapter 3-5] ». Universität Potsdam, 2001. http://opus.kobv.de/ubp/volltexte/2008/2599/.
Texte intégralRésumé :
We consider general parabolic systems of equations on the infinite time interval in case of the underlying spatial configuration is a closed manifold. The solvability of equations is studied both with respect to time and spatial variables in exponentially weighted anisotropic Sobolev spaces, and existence and maximal regularity statements for parabolic equations are proved. Moreover, we analyze the long-time behaiour of solutions in terms of complete asymptotic expansions.
These results are deduced from a pseudodifferential calculus that we construct explicitly. This algebra of operators is specifically designed to contain both the classical systems of parabolic equations of general form and their inverses, parabolicity being reflected purely on symbolic level. To this end, we assign t = ∞ the meaning of an anisotropic conical point, and prove that this interprtation is consistent with the natural setting in the analysis of parabolic PDE. Hence, major parts of this work consist of the construction of an appropriate anisotropiccone calculus of so-called Volterra operators.
In particular, which is the most important aspect, we obtain the complete characterization of the microlocal and the global kernel structure of the inverse of parabolicsystems in an infinite space-time cylinder. Moreover, we obtain perturbation results for parabolic equations from the investigation of the ideal structure of the calculus.
21
Krainer, Thomas, et Bert-Wolfgang Schulze. « On the inverse of parabolic systems of partial differential equations of general form in an infinite space-time cylinder [Part 3 : Chapter 6+7] ». Universität Potsdam, 2001. http://opus.kobv.de/ubp/volltexte/2008/2600/.
Texte intégralRésumé :
We consider general parabolic systems of equations on the infinite time interval in case of the underlying spatial configuration is a closed manifold. The solvability of equations is studied both with respect to time and spatial variables in exponentially weighted anisotropic Sobolev spaces, and existence and maximal regularity statements for parabolic equations are proved. Moreover, we analyze the long-time behaiour of solutions in terms of complete asymptotic expansions.
These results are deduced from a pseudodifferential calculus that we construct explicitly. This algebra of operators is specifically designed to contain both the classical systems of parabolic equations of general form and their inverses, parabolicity being reflected purely on symbolic level. To this end, we assign t = ∞ the meaning of an anisotropic conical point, and prove that this interprtation is consistent with the natural setting in the analysis of parabolic PDE. Hence, major parts of this work consist of the construction of an appropriate anisotropiccone calculus of so-called Volterra operators.
In particular, which is the most important aspect, we obtain the complete characterization of the microlocal and the global kernel structure of the inverse of parabolicsystems in an infinite space-time cylinder. Moreover, we obtain perturbation results for parabolic equations from the investigation of the ideal structure of the calculus.
22
Hill, Thomas Ian. « Complete Blow Up for Parabolic System Arising in a Theory of Thermal Explosion of Porous Energetic Materials ». University of Akron / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=akron1428939894.
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Le, Balc'h Kévin. « Contrôlabilité de systèmes de réaction-diffusion non linéaires ». Thesis, Rennes, École normale supérieure, 2019. http://www.theses.fr/2019ENSR0016/document.
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Cette thèse est consacrée au contrôle de quelques équations aux dérivées partielles non linéaires. On s’intéresse notamment à des systèmes paraboliques de réaction-diffusion non linéaires issus de la cinétique chimique. L’objectif principal est de démontrer des résultats de contrôlabilité locale ou globale, en temps petit, ou en temps grand.Dans une première partie, on démontre un résultat de contrôlabilité locale à des états stationnaires positifs en temps petit, pour un système de réaction-diffusion non linéaire.Dans une deuxième partie, on résout une question de contrôlabilité globale à zéro en temps petit pour un système 2 × 2 de réaction-diffusion non linéaire avec un couplage impair.La troisième partie est consacrée au célèbre problème ouvert d’Enrique Fernández-Cara et d’Enrique Zuazua des années 2000 concernant la contrôlabilité globale à zéro de l’équation de la chaleur faiblement non linéaire. On démontre un résultat de contrôlabilité globale à états positifs en temps petit et un résultat de contrôlabilité globale à zéro en temps long.La dernière partie, rédigée en collaboration avec Karine Beauchard et Armand Koenig, est une incursion vers l’hyperbolique. On étudie des systèmes linéaires à coefficients constants, couplant une dynamique transport avec une dynamique parabolique. On identifie leur temps minimal de contrôle et l’influence de leur structure algébrique sur leurs propriétés de contrôleThis thesis is devoted to the control of nonlinear partial differential equations. We are mostly interested in nonlinear parabolic reaction-diffusion systems in reaction kinetics. Our main goal is to prove local or global controllability results in small time or in large time.In a first part, we prove a local controllability result to nonnegative stationary states in small time, for a nonlinear reaction-diffusion system.In a second part, we solve a question concerning the global null-controllability in small time for a 2 × 2 nonlinear reaction-diffusion system with an odd coupling term.The third part focuses on the famous open problem due to Enrique Fernndez-Cara and Enrique Zuazua in 2000, concerning the global null-controllability of the weak semi-linear heat equation. We show that the equation is globally nonnegative controllable in small time and globally null-controllable in large time.The last part, which is a joint work with Karine Beauchard and Armand Koenig, enters the hyperbolic world. We study linear parabolic-transport systems with constant coeffcients. We identify their minimal time of control and the influence of their algebraic structure on the controllability properties
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Rieger, Marc Oliver. « Nonconvex Dynamical Problems ». Doctoral thesis, Universitätsbibliothek Leipzig, 2004. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-37269.
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Many problems in continuum mechanics, especially in the theory of elastic materials, lead to nonlinear partial differential equations. The nonconvexity of their underlying energy potential is a challenge for mathematical analysis, since convexity plays an important role in the classical theories of existence and regularity. In the last years one main point of interest was to develop techniques to circumvent these difficulties. One approach was to use different notions of convexity like quasi-- or polyconvexity, but most of the work was done only for static (time independent) equations. In this thesis we want to make some contributions concerning existence, regularity and numerical approximation of nonconvex dynamical problems.
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Pires, Leonardo. « Rate of convergence of attractors for abstract semilinear problems ». Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/55/55135/tde-27102016-090449/.
Texte intégralRésumé :
In this work we study rate of convergence of attractors for parabolic equations. We consider various types of problems where the diffusion coefficient has varied profiles: large diffusion, localized large diffusion and large diffusion except in the neighborhood of a point where it becomes small. In all cases we obtain a singular perturbation where a rate of convergence of attractors is obtained.Neste trabalho estudamos taxa de convergência de atratores para equações parabólicas. Consideramos vários tipos de problemas onde o coeficiente de difusão apresenta perfís variados: difusão grande, difusão grande localizada e difusão grande exceto na vizinhança de um ponto onde ela torna-se pequena. Em todos os casos consideramos perturbações singulares e uma taxa de convergência para os atratores é obtida.
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Li, Ji. « Analyse mathématique de modèles d'intrusion marine dans les aquifères côtiers ». Thesis, Littoral, 2015. http://www.theses.fr/2015DUNK0378/document.
Texte intégralRésumé :
Le thème de cette thèse est l'analyse mathématique de modèles décrivant l'intrusion saline dans les aquifères côtiers. On a choisi d'adopter la simplicité de l'approche avec interface nette : il n'y a pas de transfert de masse entre l'eau douce et l'eau salée (resp. entre la zone saturée et la zone sèche). On compense la difficulté mathématique liée à l'analyse des interfaces libres par un processus de moyennisation verticale nous permettant de réduire le problème initialement 3D à un système d'edps définies sur un domaine, Ω, 2D. Un second modèle est obtenu en combinant l'approche 'interface nette' à celle avec interface diffuse ; cette approche est déduite de la théorie introduite par Allen-Cahn, utilisant des fonctions de phase pour décrire les phénomènes de transition entre les milieux d'eau douce et d'eau salée (respectivement les milieux saturé et insaturé). Le problème d'origine 3D est alors réduit à un système fortement couplé d'edps quasi-linéaires de type parabolique dans le cas des aquifères libres décrivant l'évolution des profondeurs des 2 surfaces libres et de type elliptique-parabolique dans le cas des aquifères confinés, les inconnues étant alors la profondeur de l'interface eau salée par rapport à eau douce et la charge hydraulique de l'eau douce. Dans la première partie de la thèse, des résultats d'existence globale en temps sont démontrés montrant que l'approche couplée interface nette-interface diffuse est plus pertinente puisqu'elle permet d'établir un principe du maximum plus physique (plus précisèment une hiérarchie entre les 2 surfaces libres). En revanche, dans le cas de l'aquifère confiné, nous montrons que les deux approches conduisent à des résultats similaires. Dans la seconde partie de la thèse, nous prouvons l'unicité de la solution dans le cas non dégénéré, la preuve reposant sur un résultat de régularité du gradient de la solution dans l'espace Lr (ΩT), r > 2, (ΩT = (0,T) x Ω). Puis nous nous intéressons à un problème d'identification des conductivités hydrauliques dans le cas instationnaire. Ce problème est formulé par un problème d'optimisation dont la fonction coût mesure l'écart quadratique entre les charges hydrauliques expérimentales et celles données par le modèleThe theme of this thesis is the analysis of mathematical models describing saltwater intrusion in coastal aquifers. The simplicity of sharp interface approach is chosen : there is no mass transfer between fresh water and salt water (respectively between the saturated zone and the area dry). We compensate the mathematical difficulty of the analysis of free interfaces by a vertical averaging process allowing us to reduce the 3D problem to system of pde's defined on a 2D domain Ω. A second model is obtained by combining the approach of 'sharp interface' in that with 'diffuse interface' ; this approach is derived from the theory introduced by Allen-Cahn, using phase functions to describe the phenomena of transition between fresh water and salt water (respectively the saturated and unsaturated areas). The 3D problem is then reduced to a strongly coupled system of quasi-linear parabolic equations in the unconfined case describing the evolution of the DEPTHS of two free surfaces and elliptical-parabolic equations in the case of confined aquifer, the unknowns being the depth of salt water/fresh water interface and the fresh water hydraulic head. In the first part of the thesis, the results of global in time existence are demonstrated showing that the sharp-diffuse interface approach is more relevant since it allows to establish a mor physical maximum principle (more precisely a hierarchy between the two free surfaces). In contrast, in the case of confined aquifer, we show that both approach leads to similar results. In the second part of the thesis, we prove the uniqueness of the solution in the non-degenerate case. The proof is based on a regularity result of the gradient of the solution in the space Lr (ΩT), r > 2, (ΩT = (0,T) x Ω). Then we are interest in a problem of identification of hydraulic conductivities in the unsteady case. This problem is formulated by an optimization problem whose cost function measures the squared difference between experimental hydraulic heads and those given by the model
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Baysal, Arzu. « Inverse Problems For Parabolic Equations ». Master's thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/12605623/index.pdf.
Texte intégralRésumé :
In this thesis, we study inverse problems of restoration of the unknown function in a boundary condition, where on the boundary of the domain there is a convective heat exchange with the environment. Besides the temperature of the domain, we seek either the temperature of the environment in Problem I and II, or the coefficient of external boundary heat emission in Problem III and IV. An additional information is given, which is the overdetermination condition, either on the boundary of the domain (in Problem III and IV) or on a time interval (in
Problem I and II). If solution of inverse problem exists, then the temperature can be defined everywhere on the domain at all instants.
The thesis consists of six chapters. In the first chapter, there is the introduction where the definition and applications of inverse problems are given and definition of the four inverse problems, that we will analyze in this thesis, are stated. In the second chapter, some definitions and theorems which we will use to obtain some conclusions about the corresponding direct problem of our four inverse problems are stated, and the conclusions about direct problem are obtained.
In the third, fourth, fifth and sixth chapters we have the analysis of inverse problems I, II, III and IV, respectively.
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Sa, Ngiamsunthorn Parinya. « Domain perturbation for parabolic equations ». Thesis, The University of Sydney, 2011. http://hdl.handle.net/2123/7775.
Texte intégralRésumé :
We study the effect of domain perturbation on the behaviour of parabolic equations. The first aspect considered in this thesis is the behaviour of solutions under changes of the domain. We show how solutions of linear and semilinear parabolic equations behave as a sequence of domains $\Omega_n$ converges to an open set $\Omega$ in a certain sense. In particular, we are interested in singular domain perturbations so that a change of variables is not possible on these domains. For autonomous linear equations, it is known that convergence of solutions under domain perturbation is closely related to the corresponding elliptic equations via a standard semigroup theory. We show that there is also a relation between domain perturbation for non-autonomous linear parabolic equations and domain perturbation for elliptic equations. The key result for this is the equivalence of Mosco convergences between various closed and convex subsets of Banach spaces. An important consequence is that the same conditions for a sequence of domains imply convergence of solutions under domain perturbation for both parabolic and elliptic equations. By applying variational methods, we obtain the convergence of solutions of initial value problems under Dirichlet or Neumann boundary conditions. A similar technique can be applied to obtain the convergence of weak solutions of parabolic variational inequalities when the underlying convex set is perturbed. Using the linear theory, we then study domain perturbation for initial boundary value problems of semilinear type. We are also interested in the behaviour of bounded entire solutions of parabolic equations defined on the whole real line. We establish a convergence result for bounded entire solutions of linear parabolic equations under $L^2$ and $L^p$-norms. For the $L^p$-theory, we also prove H\"{o}lder regularity of bounded entire solutions with respect to time. In addition, the persistence of some classes of bounded entire solutions is given for semilinear equations using the Leray-Schauder degree theory. The second aspect is to study the dynamics of parabolic equations under domain perturbation. In this part, we consider parabolic equation as a dynamical system in an $L^2$ space and study the stability of invariant manifolds near a stationary solution. In particular, we prove the continuity (upper and lower semicontinuity) of both, the local stable invariant manifolds and the local unstable invariant manifolds under domain perturbation.
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Yung, Tamara. « Traffic Modelling Using Parabolic Differential Equations ». Thesis, Linköpings universitet, Kommunikations- och transportsystem, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-102745.
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The need of a working infrastructure in a city also requires an understanding of how the traffic flows. It is known that increasing number of drivers prolong the travel time and has an environmental effect in larger cities. It also makes it more difficult for commuters and delivery firms to estimate their travel time. To estimate the traffic flow the traffic department can arrange cameras along popular roads and redirect the traffic, but this is a costly method and difficult to implement. Another approach is to apply theories from physics wave theory and mathematics to model the traffic flow; in this way it is less costly and possible to predict the traffic flow as well. This report studies the application of wave theory and expresses the traffic flow as a modified linear differential equation. First is an analytical solution derived to find a feasible solution. Then a numerical approach is done with Taylor expansions and Crank-Nicolson’s method. All is performed in Matlab and compared against measured values of speed and flow retrieved from Swedish traffic department over a 24 hours traffic day. The analysis is performed on a highway stretch outside Stockholm with no entries, exits or curves. By dividing the interval of the highway into shorter equal distances the modified linear traffic model is expressed in a system of equations. The comparison between actual values and calculated values of the traffic density is done with a nominal average difference. The results reveal that the numbers of intervals don’t improve the average difference. As for the small constant that is applied to make the linear model stable is higher than initially considered.
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Hofmanová, Martina. « Degenerate parabolic stochastic partial differential equations ». Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2013. http://tel.archives-ouvertes.fr/tel-00916580.
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In this thesis, we address several problems arising in the study of nondegenerate and degenerate parabolic SPDEs, stochastic hyperbolic conservation laws and SDEs with continues coefficients. In the first part, we are interested in degenerate parabolic SPDEs, adapt the notion of kinetic formulation and kinetic solution and establish existence, uniqueness as well as continuous dependence on initial data. As a preliminary result we obtain regularity of solutions in the nondegenerate case under the hypothesis that all the coefficients are sufficiently smooth and have bounded derivatives. In the second part, we consider hyperbolic conservation laws with stochastic forcing and study their approximations in the sense of Bhatnagar-Gross-Krook. In particular, we describe the conservation laws as a hydrodynamic limit of the stochastic BGK model as the microscopic scale vanishes. In the last part, we provide a new and fairly elementary proof of Skorkhod's classical theorem on existence of weak solutions to SDEs with continuous coefficients satisfying a suitable Lyapunov condition.
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Dekkers, Sophia Antonia Janna. « Degenerate parabolic equations on Riemannanian manifolds ». Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.405755.
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Parvin, S. « Diffusion-convection problems in parabolic equations ». Thesis, University of Manchester, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382761.
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Ribeiro, Saraiva L. M. « Removable singularities and quasilinear parabolic equations ». Thesis, University of Sussex, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356520.
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Elbirki, Asma. « On parabolic equations with gradient terms ». Thesis, University of Sussex, 2016. http://sro.sussex.ac.uk/id/eprint/66012/.
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This thesis is concerned with the study of the important effect of the gradient term in parabolic problems. More precisely, we study the global existence or nonexistence of solutions, and their asymptotic behaviour in finite or infinite time. Particularly when the power of the gradient term can increase to the power function of the solution. This thesis consists of five parts. (i) Steady-State Solutions, (ii) The Blow-up Behaviour of the Positive Solutions, (iii) Parabolic Liouville-Type Theorems and the Universal Estimates, (iv) The Global Existence of the Positive Solutions, (v) Viscous Hamilton-Jacobi Equations (VHJ). Under certain conditions on the exponents of both the function of the solution and the gradient term, the nonexistence of positive stationary solution of parabolic problems with gradient terms are proved in (i). In (ii), we extend some known blow-up results of parabolic problems with perturbation terms, which is not too strong, to problems with stronger perturbation terms. In (iii), the nonexistence of nonnegative, nontrivial bounded solutions for all negative and positive times on the whole space are showed for parabolic problems with a strong perturbation term. Moreover, we study the connections between parabolic Liouville-type theorems and local and global properties of nonnegative classical solutions to parabolic problems with gradient terms. Namely, we use a general method for derivation of universal, pointwise a priori estimates of solutions from Liouville type theorems, which unifies the results of a priori bounds, decay estimates and initial and final blow up rates. Global existence and stability, and unbounded global solutions are shown in (iv) when the perturbation term is stronger. In (v) we show that the speed of divergence of gradient blow up (GBU) of solutions of Dirichlet problem for VHJ, especially the upper GBU rate estimate in n space dimensions is the same as in one space dimension.
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Pang, Huadong. « Parabolic equations without a minimum principle ». Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/38958.
Texte intégralRésumé :
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2007.Includes bibliographical references (p. 63-64).
In this thesis, we consider several parabolic equations for which the minimum principle fails. We first consider a two-point boundary value problem for a one dimensional diffusion equation. We show the uniqueness and existence of the solution for initial data, which may not be continuous at two boundary points. We also examine the circumstances when these solutions admit a probabilistic interpretation. Some partial results are given for analogous problems in more than one dimension.
by Huadong Pang.
Ph.D.
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Fontana, Eleonora. « Maximum Principle for Elliptic and Parabolic Equations ». Bachelor's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/12061/.
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Nel primo capitolo si riporta il principio del massimo per operatori ellittici. Sarà considerato, in un primo momento, l'operatore di Laplace e, successivamente, gli operatori ellittici del secondo ordine, per i quali si dimostrerà anche il principio del massimo di Hopf. Nel secondo capitolo si affronta il principio del massimo per operatori parabolici e lo si utilizza per dimostrare l'unicità delle soluzioni di problemi ai valori al contorno.
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Takayama, Yuuya. « Nahm’s equations, quiver varieties and parabolic sheaves ». 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/204570.
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Keras, Sigitas. « Numerical methods for parabolic partial differential equations ». Thesis, University of Cambridge, 1997. https://www.repository.cam.ac.uk/handle/1810/251611.
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Sande, Olow. « Boundary Estimates for Solutions to Parabolic Equations ». Doctoral thesis, Uppsala universitet, Matematiska institutionen, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-281451.
Texte intégralRésumé :
This thesis concerns the boundary behavior of solutions to parabolic equations. It consists of a comprehensive summary and four scientific papers. The equations concerned are different generalizations of the heat equation. Paper I concerns the solutions to non-linear parabolic equations with linear growth. For non-negative solutions that vanish continuously on the lateral boundary of an NTA cylinder the following main results are established: a backward Harnack inequality, the doubling property for the Riesz measure associated with such solutions, and the Hölder continuityof the quotient of two such solutions up to the boundary. Paper 2 concerns the solutions to linear degenerate parabolic equations, where the degeneracy is controlled by a Muckenhoupt weight of class 1+2/n. For non-negative solutions that vanish continuously on the lateral boundary of an NTA cylinder the following main results are established: a backward Harnack inequality, the doubling property for the parabolic measure, and the Hölder continuity of the quotient of two such solutions up to the boundary. Paper 3 concerns a fractional heat equation. The first main result is that a solution to the fractional heat equation in Euclidean space of dimension n can be extended as a solution to a certain linear degenerate parabolic equation in the upper half space of dimension n+1. The second main result is the Hölder continuity of quotients of two non-negative solutions that vanish continuously on the latteral boundary of a Lipschitz domain. Paper 4 concerns the solutions to uniformly parabolic linear equations with complex coefficients. The first main result is that under certain assumptions on the opperator the bounds for the single layer potentials associated to the opperator are bounded. The second main result is that these bounds always hold if the opperator is realvalued and symmetric.
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Sockell, Michael Elliot. « Similarity solutions of stochastic nonlinear parabolic equations ». Diss., Virginia Polytechnic Institute and State University, 1987. http://hdl.handle.net/10919/49898.
Texte intégralRésumé :
A novel statistical technique introduced by Besieris is used to study solutions of the nonlinear stochastic complex parabolic equation in the presence of two profiles. Specifically, the randomly modulated linear potential and the randomly perturbed quadratic focusing medium. In the former, a class of solutions is shown to admit an exact statistical description in terms of the moments of the wave function. In the latter, all even-order moments are computed exactly, whereas the odd-order moments are solved asymptotically. Lastly, it is shown that this statistical technique is isomorphic to mappings of nonconstant coefficient partial differential equations to constant coefficient equations. A generalization of this mapping and its inherent restrictions are discussed.Ph. D.
incomplete_metadata
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Petitta, Francesco. « Nonlinear parabolic equations with general measure data ». Doctoral thesis, La Sapienza, 2006. http://hdl.handle.net/11573/917105.
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Tsegmid, Munkhgerel. « Modélisation d’aquifères peu profonds en interaction avec les eaux de surfaces ». Thesis, Littoral, 2019. http://www.theses.fr/2019DUNK0526/document.
Texte intégralRésumé :
Nous présentons une classe de nouveaux modèles pour décrire les écoulements d’eau dans des aquifères peu profonds non confinés. Cette classe de modèles offre une alternative au modèle Richards 3d plus classique mais moins maniable. Leur dérivation est guidée par deux ambitions : le nouveau modèle doit d’une part être peu coûteux en temps de calcul et doit d’autre part donner des résultats pertinents à toute échelle de temps. Deux types d’écoulements dominants apparaissent dans ce contexte lorsque le rapport de l’épaisseur sur la longueur de l’aquifère est petit : le premier écoulement apparaît en temps court et est décrit par un problème vertical Richards 1d ; le second correspond aux grandes échelles de temps, la charge hydraulique est alors considérée comme indépendante de la variable verticale. Ces deux types d’écoulements sont donc modélisés de manière appropriée par le couplage d’une équation 1d pour la partie insaturée de l’aquifère et d’une équation 2d pour la partie saturée. Ces équations sont couplées au niveau d’une interface de profondeur h (t,x) en dessous de laquelle l’hypothèse de Dupuit est vérifiée. Le couplage est assuré de telle sorte que la masse globale du système soit conservée. Notons que la profondeur h (t,x) peut être une inconnue du problème ou être fixée artificiellement. Nous prouvons (dans le cas d’aquifères minces) en utilisant des développements asymptotiques que le problème Richards 3d se comporte de la même manière que les modèles de cette classe à toutes les échelles de temps considérées (courte, moyenne et grande). Nous décrivons un schéma numérique pour approcher le modèle couplé non linéaire. Une approximation par éléments finis est combinée à une méthode d’Euler implicite en temps. Ensuite, nous utilisons une reformulation de l’équation discrète en introduisant un opérateur de Dirichlet-to-Neumann pour gérer le couplage non linéaire en temps. Une méthode de point fixe est appliquée pour résoudre l’équation discrète reformulée. Le modèle couplé est testé numériquement dans différentes situations et pour différents types d’aquifère. Pour chacune des simulations, les résultats numériques obtenus sont en accord avec ceux obtenus à partir du problème de Richards original. Nous concluons notre travail par l’analyse mathématique d’un modèle couplant le modèle Richards 3d à celui de Dupuit. Il diffère du premier parce que nous ne supposons plus un écoulement purement vertical dans la frange capillaire supérieure. Ce modèle consiste donc en un système couplé non linéaire d’équation Richards 3d avec une équation parabolique non linéaire décrivant l’évolution de l’interface h (t,x) entre les zones saturées et non saturées de l’aquifère. Les principales difficultés à résoudre sont celles inhérentes à l’équation 3D-Richards, la prise en compte de la frontière libre h (t,x) et la présence de termes dégénérés apparaissant dans les termes diffusifs et dans les dérivées temporellesWe present a class of new efficient models for water flow in shallow unconfined aquifers, giving an alternative to the classical but less tractable 3D-Richards model. Its derivation is guided by two ambitions : any new model should be low cost in computational time and should still give relevant results at every time scale.We thus keep track of two types of flow occurring in such a context and which are dominant when the ratio thickness over longitudinal length is small : the first one is dominant in a small time scale and is described by a vertical 1D-Richards problem ; the second one corresponds to a large time scale, when the evolution of the hydraulic head turns to become independent of the vertical variable. These two types of flow are appropriately modelled by, respectively, a one-dimensional and a two-dimensional system of PDEs boundary value problems. They are coupled along an artificial level below which the Dupuit hypothesis holds true (i.e. the vertical flow is instantaneous below the function h(t,x)) in away ensuring that the global model is mass conservative. Tuning the artificial level, which even can depend on an unknown of the problem, we browse the new class of models. We prove using asymptotic expansions that the 3DRichards problem and eachmodel of the class behaves the same at every considered time scale (short, intermediate and large) in thin aquifers. We describe a numerical scheme to approximate the non-linear coupled model. The standard Galerkin’s finite element approximation in space and Backward Euler method in time are used for discretization. Then we reformulate the discrete equation by introducing the Dirichlet to Neumann operator to handle the nonlinear coupling in time. The fixed point iterative method is applied to solve the reformulated discrete equation. We have examined the coupled model in different boundary conditions and different aquifers. In the every situations, the numerical results of the coupled models fit well with the original Richards problem. We conclude our work by the mathematical analysis of a model coupling 3D-Richards flow and Dupuit horizontal flow. It differs from the first one because we no longer assume a purely vertical flow in the upper capillary fringe. This model thus consists in a nonlinear coupled system of 3D-Richards equation with a nonlinear parabolic equation describing the evolution of the interface h(t,x) between the saturated and unsaturated zones of the aquifer. The main difficulties to be solved are those inherent to the 3D-Richards equation, the consideration of the free boundary h(t,x) and the presence of degenerate terms appearing in the diffusive terms and in the time derivatives
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Qi, Yuan-Wei. « The blow-up of quasi-linear parabolic equations ». Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.253381.
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Ascencio, Pedro. « Adaptive observer design for parabolic partial differential equations ». Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/49454.
Texte intégralRésumé :
This thesis addresses the observer design problem, for a class of linear one-dimensional parabolic Partial Differential Equations, considering the simultaneous estimation of states and parameters from boundary measurements. The design is based on the Backstepping methodology for Partial Differential Equations and extends its central idea, the Volterra transformation, to compensate for the parameters uncertainties. The design steps seek to reject time-varying parameter uncertainties setting forth a type of differential boundary value problems (Kernel-PDE/ODEs) to accomplish its objective, the solution of which is computed at every fixed sampling time and constitutes the observer gains for states and parameters. The design does not include any pre-transformation to some canonical form and/or a finite-dimensional formulation, and performs a direct parameter estimation from the original model. The observer design problem considers two cases of parameter uncertainty, at the boundary: control gain coefficient, and in-domain: diffusivity and reactivity parameters, respectively. For a Luenberger-type observer structure, the problems associated to one and two points of measurement at the boundary are studied through the application of an intuitive modification of the Volterra-type and Fredholm-type transformations. The resulting Kernel-PDE/ODEs are addressed by means of a novel methodology based on polynomial optimization and Sum-of-Squares decomposition. This approach allows recasting these coupled differential equations as convex optimization problems readily implementable resorting to semidefinite programming, with no restrictions to the spectral characteristics of some integral operators or system's coefficients. Additionally, for polynomials Kernels, uniqueness and invertibility of the Fredholm-type transformation are proved in the space of real analytic and continuous functions. The direct and inverse Kernels are approximated as the optimal polynomial solution of a Sum-of-Squares and Moment problem with theoretically arbitrary precision. Numerical simulations illustrate the effectiveness and potentialities of the methodology proposed to manage a variety of problems with different structures and objectives.
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Yi, Zhuobiao. « Identification of General Source Terms in Parabolic Equations ». University of Cincinnati / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1036084593.
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Romutis, Todd. « Numerical Smoothness and Error Analysis for Parabolic Equations ». Bowling Green State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1522150799203255.
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Tsang, Siu Chung. « Preconditioners for linear parabolic optimal control problems ». HKBU Institutional Repository, 2017. https://repository.hkbu.edu.hk/etd_oa/464.
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In this thesis, we consider the computational methods for linear parabolic optimal control problems. We wish to minimize the cost functional while fulfilling the parabolic partial differential equations (PDE) constraint. This type of problems arises in many fields of science and engineering. Since solving such parabolic PDE optimal control problems often lead to a demanding computational cost and time, an effective algorithm is desired. In this research, we focus on the distributed control problems. Three types of cost functional are considered: Target States problems, Tracking problems, and All-time problems. Our major contribution in this research is that we developed a preconditioner for each kind of problems, so our iterative method is accelerated. In chapter 1, we gave a brief introduction to our problems with a literature review. In chapter 2, we demonstrated how to derive the first-order optimality conditions from the parabolic optimal control problems. Afterwards, we showed how to use the shooting method along with the flexible generalized minimal residual to find the solution. In chapter 3, we offered three preconditioners to enhance our shooting method for the problems with symmetric differential operator. Next, in chapter 4, we proposed another three preconditioners to speed up our scheme for the problems with non-symmetric differential operator. Lastly, we have the conclusion and the future development in chapter 5.
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Williams, J. F. « Scaling and singularities in higher-order nonlinear differential equations ». Thesis, University of Bath, 2003. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.275878.
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Nam, Dukjin. « Multiscale numerical methods for some types of parabolic equations ». [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-2993.
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Felsinger, Matthieu [Verfasser]. « Parabolic equations associated with symmetric nonlocal operators / Matthieu Felsinger ». Bielefeld : Universitätsbibliothek Bielefeld, 2013. http://d-nb.info/1042557322/34.
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