The topic of this dissertation lies at the interface between the areas of Harmonic Analysis, Partial Differential Equations, and Geometric Measure Theory, with an emphasis on the study of singular integral operators associated with second and higher order elliptic boundary value problems in non-smooth domains. The overall aim of this work is to further the development of a systematic treatment of second and higher order elliptic boundary value problems using singular integral operators. This is relevant to the theoretical and numerical treatment of boundary value problems arising in the modeling of physical phenomena such as elasticity, incompressible viscous fluid flow, electromagnetism, anisotropic plate bending, etc., in domains which may exhibit singularities at all boundary locations and all scales. Since physical domains may exhibit asperities and irregularities of a very intricate nature, we wish to develop tools and carry out such an analysis in a very general class of non-smooth domains, which is in the nature of best possible from the geometric measure theoretic point of view. The dissertation will be focused on three main, interconnected, themes: A. A systematic study of the poly-Cauchy operator in uniformly rectifiable domains in $\mathbb{C}$; B. Solvability results for the Neumann problem for the bi-Laplacian in infinite sectors in ${\mathbb{R}}^2$; C. Connections between spectral properties of layer potentials associated with second-order elliptic systems and the underlying tensor of coefficients. Theme A is based on papers [16, 17, 18] and this work is concerned with the investigation of polyanalytic functions and boundary value problems associated with (integer) powers of the Cauchy-Riemann operator in uniformly rectifiable domains in the complex plane. The goal here is to devise a higher-order analogue of the existing theory for the classical Cauchy operator in which the salient role of the Cauchy-Riemann operator $\overline{\partial}$ is now played by $\overline{\partial}^m$ for some arbitrary fixed integer $m\in{\mathbb{N}}$. This analysis includes integral representation formulas, higher-order Fatou theorems, Calderón-Zygmund theory for the poly-Cauchy operators, radiation conditions, and higher-order Hardy spaces. Theme B is based on papers [3, 19] and this regards the Neumann problem for the bi-Laplacian with $L^p$ data in infinite sectors in the plane using Mellin transform techniques, for $p\in(1,\infty)$. We reduce the problem of finding the solvability range of the integrability exponent $p$ for the $L^{p}$ biharmonic Neumann problem to solving an equation involving quadratic polynomials and trigonometric functions employing the Mellin transform technique. Additionally, we provide the range of the integrability exponent for the existence of a solution to the $L^{p}$ biharmonic Neumann problem in two-dimensional infinite sectors. The difficulty we are overcoming has to do with the fact that the Mellin symbol involves hypergeometric functions. Finally regarding theme C, based on the ongoing work in [2], the emphasis is the investigation of coefficient tensors associated with second-order elliptic operators in two dimensional infinite sectors and properties of the corresponding singular integral operators, employing Mellin transform. Concretely, we explore the relationship between distinguished coefficient tensors and $L^{p}$ spectral and Hardy kernel properties of the associated singular integral operators. / Mathematics

Este é o primeiro trabalho que apresenta simulações tridimensionais completamente adaptativas de um modelo de campo de fase para um fluido incompressível com densidade de massa constante e viscosidade variável, conhecido como Modelo H. Solucionando numericamente as equações desse modelo em malhas refinadas localmente com a técnica AMR, simulamos computacionalmente escoamentos bifásicos tridimensionais. Os modelos de campo de fase oferecem uma aproximação física sistemática para investigar fenômenos que envolvem sistemas multifásicos complexos, tais como fluidos com camadas de mistura, a separação de fases sob forças de cisalhamento e a evolução de micro-estruturas durante processos de solidificação. Como as interfaces são substituídas por delgadas regiões de transição (interfaces difusivas), as simulações de campo de fase requerem muita resolução nessas regiões para capturar corretamente a física do problema em estudo. Porém essa não é uma tarefa fácil de ser executada numericamente. As equações que caracterizam o modelo de campo de fase contêm derivadas de ordem elevada e intrincados termos não lineares, o que exige uma estratégia numérica eficiente capaz de fornecer precisão tanto no tempo quanto no espaço, especialmente em três dimensões. Para obter a resolução exigida no tempo, usamos uma discretização semi-implícita de segunda ordem para solucionar as equações acopladas de Cahn-Hilliard e Navier-Stokes (Modelo H). Para resolver adequadamente as escalas físicas relevantes no espaço, utilizamos malhas refinadas localmente que se adaptam dinamicamente para recobrir as regiões de interesse do escoamento, como por exemplo, as vizinhanças das interfaces do fluido. Demonstramos a eficiência e a robustez de nossa metodologia com simulações que incluem a separação dos componentes de uma mistura bifásica, a deformação de gotas sob cisalhamento e as instabilidades de Kelvin-Helmholtz. / This is the first work that introduces 3D fully adaptive simulations for a phase field model of an incompressible fluid with matched densities and variable viscosity, known as Model H. Solving numerically the equations of this model in meshes locally refined with AMR technique, we simulate computationally tridimensional two-phase flows. Phase field models offer a systematic physical approach to investigate complex multiphase systems phenomena such as fluid mixing layers, phase separation under shear and microstructure evolution during solidification processes. As interfaces are replaced by thin transition regions (diffuse interfaces), phase field simulations need great resolution in these regions to capture correctly the physics of the studied problem. However, this is not an easy task to do numerically. Phase field model equations have high order derivatives and intricate nonlinear terms, which require an efficient numerical strategy that can achieve accuracy both in time and in space, especially in three dimensions. To obtain the required resolution in time, we employ a semi-implicit second order discretization scheme to solve the coupled Cahn-Hilliard/Navier-Stokes equations (Model H). To resolve adequatly the relevant physical scales in space, we use locally refined meshes which adapt dynamically to cover special flow regions, e.g., the vicinity of the fluid interfaces. We demonstrate the efficiency and robustness of our methodology with simulations that include spinodal decomposition, the deformation of drops under shear and Kelvin-Helmholtz instabilities.

Στην παρούσα διδακτορική διατριβή μελετάμε τρία Προβλήματα που αναφέρονται στην Ψευδο-Ευκλείδεια Γεωμετρία. Στα δύο πρώτα Κεφάλαια, Κεφάλαιο 1 και Κεφάλαιο 2 αναφέρουμε γνωστά αποτελέσματα και περιγράφουμε βασικές έννοιες της Ρημάννιας και Ψευδό - Ρημάννιας Γεωμετρίας. Στο Κεφάλαιο 3 μελετάμε επιφάνειες εκ περιστροφής στον τρισδιάστατο Lorentz - Minkowski χώρο ικανοποιώντας δοσμένη γεωμετρική συνθήκη. Στο Κεφάλαιο 4 βρίσκουμε όλες τις κανονικές μορφές του τελεστή σχήματος των τρισδιάστατων υπερεπιφανειών τύπου (-, +, -) του τετρασδιάστατου Ψευδο - Ευκλείδειου χώρου τύπου (-, +, -, +). Τέλος, στο Κεφάλαιο 5 μελετάμε τη σχέση που υπάρχει μεταξύ των διαρμονικών και ελαχιστικών υπερεπιφανειών που αναφέρθηκαν στο Κεφάλαιο 4, χρησιμοποιώντας τον τελεστή σχήματός τους. Ειδικότερα, αποδεικνύουμε ότι κάθε τέτοια διαρμονική υπερεπιφάνεια είναι ελαχιστική. / In the present PH.D. thesis we study three problems referred in the pseudo-Euclidean geometry. In the first two chapters, Chapter 1 and Chapter 2, we review known results and describe the basic notions of the Riemannian and Pseudo-Riemannian geometry. In Chapter 3, we study surfaces of revolution of the three dimensional Lorentz-Minkowski space satisfying given geometric condition. In Chapter 4, we find all the canonical forms of the shape operator of the three dimensional hypersurfaces of signature (-, +, -) of the four dimensional pseudo-Euclidean space of signature (-, +, -, +). Finally, in Chapter 5, we study the relation which exists between the biharmonic and minimal hypersurfaces referred in Chapter 4, by using their shape operator. Precisely, we prove that every such biharmonic hypersurface is minimal.

Este é o primeiro trabalho que apresenta simulações tridimensionais completamente adaptativas de um modelo de campo de fase para um fluido incompressível com densidade de massa constante e viscosidade variável, conhecido como Modelo H. Solucionando numericamente as equações desse modelo em malhas refinadas localmente com a técnica AMR, simulamos computacionalmente escoamentos bifásicos tridimensionais. Os modelos de campo de fase oferecem uma aproximação física sistemática para investigar fenômenos que envolvem sistemas multifásicos complexos, tais como fluidos com camadas de mistura, a separação de fases sob forças de cisalhamento e a evolução de micro-estruturas durante processos de solidificação. Como as interfaces são substituídas por delgadas regiões de transição (interfaces difusivas), as simulações de campo de fase requerem muita resolução nessas regiões para capturar corretamente a física do problema em estudo. Porém essa não é uma tarefa fácil de ser executada numericamente. As equações que caracterizam o modelo de campo de fase contêm derivadas de ordem elevada e intrincados termos não lineares, o que exige uma estratégia numérica eficiente capaz de fornecer precisão tanto no tempo quanto no espaço, especialmente em três dimensões. Para obter a resolução exigida no tempo, usamos uma discretização semi-implícita de segunda ordem para solucionar as equações acopladas de Cahn-Hilliard e Navier-Stokes (Modelo H). Para resolver adequadamente as escalas físicas relevantes no espaço, utilizamos malhas refinadas localmente que se adaptam dinamicamente para recobrir as regiões de interesse do escoamento, como por exemplo, as vizinhanças das interfaces do fluido. Demonstramos a eficiência e a robustez de nossa metodologia com simulações que incluem a separação dos componentes de uma mistura bifásica, a deformação de gotas sob cisalhamento e as instabilidades de Kelvin-Helmholtz. / This is the first work that introduces 3D fully adaptive simulations for a phase field model of an incompressible fluid with matched densities and variable viscosity, known as Model H. Solving numerically the equations of this model in meshes locally refined with AMR technique, we simulate computationally tridimensional two-phase flows. Phase field models offer a systematic physical approach to investigate complex multiphase systems phenomena such as fluid mixing layers, phase separation under shear and microstructure evolution during solidification processes. As interfaces are replaced by thin transition regions (diffuse interfaces), phase field simulations need great resolution in these regions to capture correctly the physics of the studied problem. However, this is not an easy task to do numerically. Phase field model equations have high order derivatives and intricate nonlinear terms, which require an efficient numerical strategy that can achieve accuracy both in time and in space, especially in three dimensions. To obtain the required resolution in time, we employ a semi-implicit second order discretization scheme to solve the coupled Cahn-Hilliard/Navier-Stokes equations (Model H). To resolve adequatly the relevant physical scales in space, we use locally refined meshes which adapt dynamically to cover special flow regions, e.g., the vicinity of the fluid interfaces. We demonstrate the efficiency and robustness of our methodology with simulations that include spinodal decomposition, the deformation of drops under shear and Kelvin-Helmholtz instabilities.

Το κύριο αντικείμενο της διατριβής συνίσταται στη μελέτη της γεωμετρίας των τρισδιάστατων H-μετρικών πολλαπλοτήτων επαφής, ή, ισοδύναμα, των μετρικών πολλαπλοτήτων επαφής για τις οποίες το διανυσματικό πεδίο ξ είναι πεδίο ιδιοδιανυσμάτων του τελεστή Ricci Q. Συγκεκριμένα, αποδεικνύεται ότι μια τρισδιάστατη H-μετρική πολλαπλότητα επαφής [Μ, (η, ξ, φ, g)] χαρακτηρίζεται γεωμετρικά από μια συνθήκη που εμπλέκει τον τανυστή καμπυλότητας της Μ και τρεις διαφορίσιμες συναρτήσεις κ, μ και ν της Μ. Η συνθήκη αυτή οδηγεί στην εισαγωγή μιας νέας κλάσης μετρικών πολλαπλοτήτων επαφής: τις (κ, μ, ν)-πολλαπλότητες επαφής. Το ενδιαφέρον με τις (κ, μ, ν)-πολλαπλότητες επαφής είναι ότι για διάσταση μεγαλύτερη του τρία εκφυλίζονται στις (κ, μ)-πολλαπλότητες επαφής, δηλαδή, οι συναρτήσεις κ, μ είναι σταθερές και η συνάρτηση ν είναι η μηδενική συνάρτηση. Αντιθέτως, αποδεικνύεται ότι τέτοιες μετρικές πολλαπλότητες επαφής υπάρχουν στη διάσταση τρία. Ένα άλλο από τα προβλήματα που εξετάζονται σ' αυτή τη διατριβή είναι ο χαρακτηρισμός των διαρμονικών καμπυλών του Legendre και των αντι-αναλλοίωτων επιφανειών εμβυθισμένων σε τρισδιάστατες (κ, μ, ν)-πολλαπλότητες επαφής. Συγκεκριμένα, αποδεικνύεται ότι οι διαρμονικές καμπύλες του Legendre είναι οι γεωδαισιακές αυτών των χώρων. Επιπλέον, αποδεικνύεται ότι οι διαρμονικές και χωρίς ελαχιστικά σημεία αντι-αναλλοίωτες επιφάνειες που είναι εμβυθισμένες σε τρισδιάστατες γενικευμένες (κ, μ)-πολλαπλότητες επαφής και των οποίων το μέτρο του διανυσματικού πεδίου της μέσης καμπυλότητας είναι σταθερό, είναι τοπικά Ευκλείδειες. / The main object of this Doctoral Thesis is the study of the geometry of 3-dimensional H-contact metric manifolds, or, equivalently, the contact metric manifolds whose the vector field ξ is an eigenvector of the Ricci operator Q. More precisely, it is proved that 3-dimensional H-contact metric manifolds [M, (η, ξ, φ, g)] are geometrically characterized by a specific curvature condition and three differentiable functions κ, μ and ν of M. This condition leads to the introduction of a new class of contact metric manifolds: the (κ, μ, ν)-contact metric manifolds. It is remarkable that for dimension greater than three, such manifolds are reduced to (κ, μ)-contact metric manifolds, i.e. the functions κ, μ are constants and the function ν is the zero function. On the contrary, in three dimension (κ,μ,ν)-contact metric manifolds exist. Another problem which is studied is the classification of biharmonic Legendre curves and anti-invariant surfaces immersed in 3-dimensional (κ, μ, ν)-contact metric manifolds. It is proved that biharmonic Legendre curves in 3-dimensional (κ, μ, ν)-contact metric manifolds are necessarily geodesics. Furthermore, it is proved that biharmonic and without minimal points anti-invariant surfaces immersed in 3-dimensional generalized (κ, μ)-contact metric manifolds with constant norm of the mean curvature vector field, are locally flat.