Second order convergence of a modified MAC scheme for Stokes interface problems

Stokes flow equations have been implemented successfully in practice for simulating problems with moving interfaces. Though computational methods produce accurate solutions and numerical convergence can be demonstrated using a resolution study, the rigorous convergence proofs are usually limited to particular reformulations and boundary conditions. In this paper, a rigorous error analysis of the marker and cell (MAC) scheme for Stokes interface problems with constant viscosity in the framework of the finite difference method is presented. Without reformulating the problem into elliptic PDEs, the main idea is to use a discrete Ladyzenskaja-Babuska-Brezzi (LBB) condition and construct auxiliary functions, which satisfy discretized Stokes equations and possess at least second order accuracy in the neighborhood of the moving interface. In particular, the method, for the first time, enables one to prove second order convergence of the velocity gradient in the discrete $\ell^2$-norm, in addition to the velocity and pressure fields. Numerical experiments verify the desired properties of the methods and the expected order of accuracy for both two-dimensional and three-dimensional examples

Superconvergence, Superaccuracy, and Stability of the Discontinuous Galerkin Finite Element Method

This thesis is concerned with the investigation of the superconvergence, superaccuracy, and stability properties of the discontinuous Galerkin (DG) finite element method in one and two dimensions. We propose a novel method for the analysis of these properties. We apply the DG method to a model linear advection problem to derive a PDE which is satisfied by the numerical solution itself. This PDE is equivalent to the original advection equation but with a forcing term that is proportional to the jump in the numerical solution at the cell interfaces. We then use classical Fourier analysis to determine the solutions to this PDE with particular temporal frequencies. We find that these Fourier modes are completely determined on each cell by the inflow into that cell and a certain rational function of the mode's frequency. By using local expansions of these modes, we prove several local superconvergence properties of the DG method, as well as superaccurate errors in terms of dissipation and dispersion. Next, by considering a uniform mesh and assuming periodic boundary conditions, we investigate the spectrum of the method. In particular, we show that the spectrum can be partitioned into physical and non-physical modes. The physical modes advect with high-order accuracy while the non-physical modes decay exponentially quickly in time. Using these results we establish several global superconvergence properties of the method on uniform meshes. Finally, we also propose a new family of schemes which can been viewed as a modified version of the DG scheme. We extend our analysis to these new schemes we construct schemes with significantly larger stable CFL numbers than the classic DG method. We demonstrate through some numerical examples that these modified schemes can be effective in capturing fine structures of the numerical solution when compared with the DG scheme with equivalent computational effort.4 month

Solution-Acceleration Strategies for High-Order Unstructured Methods

The design of next-generation aircraft relies on computational fluid dynamics (CFD) to minimize testing requirements at reduced cost and risk. However, current industry reliance on Reynolds-averaged Navier-Stokes (RANS)-based CFD is limited in predicting transitional and turbulent flows. Large-eddy simulation (LES) offers accuracy where RANS methods fail, but can have prohibitive computational cost. To address this, we propose a high-order CFD framework to advance flux reconstruction (FR) methods toward industrial-scale simulations. FR is a family of high-order, unstructured schemes that provide accuracy at reduced cost per degree-of-freedom (DOF) compared to low-order methods, with proven potential for LES. We develop practical strategies to reduce the computational cost of FR methods for explicit and implicit formulations. Due to the low cost per time step, explicit time stepping is typically used in FR methods. However, stability constraints prohibitively limit time-step sizes in numerically stiff problems. Hence, implicit time stepping is preferred in these cases, but it requires solving large, nonlinear systems and can be computationally expensive. This thesis introduces optimal Runge-Kutta methods to alleviate stability limits and reduce wall-clock times by approximately half in moderately low stiffness problems. For increased stiffness, we hybridize implicit FR methods using a trace variable, which allows a reduction of the implicit system via static condensation, decreasing implicit time stepping costs, especially at higher orders. Hybridization with both discontinuous (HFR) and continuous function spaces (EFR) is suitable for advection and advection-diffusion type problems within the FR method and enables significant speedup gains over standard FR. We incorporate polynomial adaptation to the hybridized framework, varying the solution polynomial’s degree locally within each element, which results in an overall reduction in DOF and significant speedup gains in a two-dimensional problem against standard polynomial-adaptive formulations. Finally, we combine implicit-explicit (IMEX) time stepping with hybridization to tackle geometry-induced numerical stiffness. The resulting method reduces computational cost at least fifteen times over explicit methods in a multi-element airfoil problem at Reynolds 1.7 million. Our proposed framework enables substantial reductions in both moderate and high stiffness problems, thus advancing high-order methods toward large industrial-scale problems

Simulations aux grandes échelles de l’écoulement diphasique dans un brûleur aéronautique par une approche Euler-Lagrange

Les turbines à gaz aéronautiques doivent satisfaire des normes d'émissions polluantes toujours en baisse. La formation de polluants est directement liée à la qualité du mélange d’air et de carburant en amont du front de flamme. Ainsi, leur réduction implique une meilleure prédiction de la formation du spray et de son interaction avec l'écoulement turbulent gazeux. La simulation aux grandes échelles (SGE) semble un outil numérique approprié pour étudier ces mécanismes. Le but de cette thèse est d’évaluer l'impact de modèles d'injection simplifiés sur la SGE de l'écoulement diphasique évaporant d’une configuration complexe. La configuration cible choisie est un brûleur aéronautique installé sur le banc expérimental MERCATO. Le banc expérimental est equipé d’un système d’injection d'air vrillé et d’un système d'injection liquide avec un atomiseur pressurisé swirlé représentatif de foyers aéronautiques réels. Dans un premier temps, un modèle d'injection simplifié pour atomiseurs pressurisés swirlés négligeant les effets de l'atomisation sur la dynamique du spray est présenté. L'objectif principal de ce modèle réside dans la reproduction de conditions d’injection similaires pour des traitements Eulériens et Lagrangiens de la phase particulaire. Dans un second temps, la composante Lagrangienne de ce modèle d'injection est combinée à un modèle d'atomisation secondaire de la litérature pour permettre une prise en compte partielle des phénomènes de pulvérisation liquide. Les SGE de l'écoulement diphasique évaporant de la configuration MERCATO présentées comportent deux aspects. Premièrement, différents modèles d’injection sont évalués pour quantifier leur impact sur la dynamique de la phase particulaire. Deuxièmement, une comparaison de simulations Euler-Euler et Euler-Lagrange reposant sur un modèle d'injection unifié est effectuée. ABSTRACT : Aeroautical gas turbines need to satisfy growingly stringent demands on pollutant emission. Pollutant emissions are directly related to the quality of fuel air mixing prior to combustion. Therefore, their reduction relies on a more accurate prediction of spray formation and interaction of the spray with the gaseous turbulent flowfield. Large-Eddy Simulation (LES) seems an adequate numerical tool to predict these mechanisms. The objective of this thesis is to evaluate the impact of simplified injection methods on the LES of the evaporating two-phase flow inside a complex geometry. The chosen target configuration is an aeronautical combustor installed on the MERCATO test-rig. The experimental setup includes an air-swirler injection system and a pressureswirl atomizer typical of realistic aeronautic combustors. In a first step, a simplified injection model for pressure swirl atomizers neglecting the impact of liquid disintegration on spray dynamics is presented. The main objective of this model lies in the reproduction of similar injection conditions for Eulerian and Lagrangian representations of the dispersed phase. In a second step, the Lagrangian injection method is combined to a secondary breakup model of the literature to partly account for the liquid disintegration process. The presented LES’s of the evaporating two-phase flow inside the MERCATO geometry consider two different aspects. First, the impact of injection modeling on spray dynamics is assessed. Second, Euler-Euler and Euler-Lagrange simulations relying on the common simplified injection model are compared

Aeronautical engineering: A cumulative index to a continuing bibliography (supplement 274)

This publication is a cumulative index to the abstracts contained in supplements 262 through 273 of Aeronautical Engineering: A Continuing Bibliography. The bibliographic series is compiled through the cooperative efforts of the American Institute of Aeronautics and Astronautics (AIAA) and the National Aeronautics and Space Administration (NASA). Seven indexes are included: subject, personal author, corporate source, foreign technology, contract number, report number, and accession number

MS FT-2-2 7 Orthogonal polynomials and quadrature: Theory, computation, and applications

Quadrature rules find many applications in science and engineering. Their analysis is a classical area of applied mathematics and continues to attract considerable attention. This seminar brings together speakers with expertise in a large variety of quadrature rules. It is the aim of the seminar to provide an overview of recent developments in the analysis of quadrature rules. The computation of error estimates and novel applications also are described

Generalized averaged Gaussian quadrature and applications

A simple numerical method for constructing the optimal generalized averaged Gaussian quadrature formulas will be presented. These formulas exist in many cases in which real positive GaussKronrod formulas do not exist, and can be used as an adequate alternative in order to estimate the error of a Gaussian rule. We also investigate the conditions under which the optimal averaged Gaussian quadrature formulas and their truncated variants are internal