Interaction of a Turbulent Wind with Ocean Surface Waves - Numerical Modeling

Wave-modeling can be categorized in terms of different scales and theoretical frameworks. This dissertation focuses on the numerical modeling of wind-wave generation and its effects on wave growth and propagations. As categorized by scales and methodologies, wind-wave modeling in this dissertation covers two main topics: 1) Large-scale modeling: wind-wave development in real seas. As a phase-average model, SWAN is employed to study the wind-wave environment in the Persian Gulf and Qatar. The wind-wave generation is parameterized as source terms in a spectral model. The special wind condition, called shamal, is particularly investigated. An experimental tower is installed around Doha Port, and by using video imagery, the in situ wave features are extracted and compared. 2) Small-scale modeling: de-tailed wave development using CFD (Computational Fluid Dynamics). A curvilinear surface-fitted moving grid model for three-dimensional Navier-Stokes equations is developed and used to simulate linear and non-linear waves with fully nonlinear surface conditions. Also, by simplifying it to a fixed rectilinear grid based on Cartesian formulations, a DNS (Direct Numerical Simulation) model is developed with an air-water fully-coupled domain and improved coupled interface conditions. By using this DNS model, the detail of wind-wave generation is investigated from still water and the applied top shear wind. For the second topic, the CFD problems are solved by an in-house numerical tool, SPX. SPX is a general PDE (Partial Differential Equations) framework, developed by using C++1y (shortened form of C++11/14/17), currently aiming at the structural domain. It is designed by modern software methodologies, such as generic programming, meta-programming and object-oriented programming. In addition, concept-based generic programming, an ongoing advanced software technology, is first introduced into the PDE numerical tool design. By using these modern design methodologies, all significant components used for solving PDE, particularly for fluid and wave problems, are all implemented in SPX. These components include high-performance numerical array, implicit solvers, grids, differential basis and operators, time integrators, and system infrastructures such as serializations and timer. On structured domain, a general PDE can be expressed by the arbitrary combination of any general differential operator and any arithmetic operator, which is the most challenging part of SPX design. This research proposes a general stencil operator design that integrates with the concept-based expression template. It is successfully demonstrated that the proposed design can automatically deduce the resulting stencils to represent the resulting field operator by giving an arbitrary PDE expression at any given grid point. With the deduced stencils, the user-defined PDE expression is therefore, numerically-solvable by using any solver. In consequence, SPX can be easily applied to any user-defined PDE problem on structural grids with arbitrary user-specified numerical components. Its design shows high flexibility and re-usability without sacrificing efficiency. The development of SPX, therefore, justifies the success of C++-Concept applications on the large-scale numerical framework design

Reynolds-Averaged Navier-Stokes Based Ice Accretion For Aircraft Wings

RÉSUMÉ Cette thèse aborde l'un des enjeux actuels de la sécurité aérienne pour augmenter les capacités de simulation de givrage pour la prédiction des formes complexes 2D et 3D de verglas sur les surfaces des aéronefs. Durant les années 1980 et 1990, le domaine de l'aérogivrage numérique s’est développé pour soutenir la conception et la certification des aéronefs volant dans des conditions givrantes. Les technologies multidisciplinaires utilisées dans ces codes étaient : l'aérodynamique (méthode des panneaux), le calcul des trajectoires des gouttelettes (méthode Lagrangienne), le module thermodynamique (modèle Messinger) et le module de géométrie (accumulation de glace). Ceux-ci sont intégrés dans un module quasi-stationnaire pour simuler le processus d'accumulation de glace en fonction du temps (procédure à plusieurs pas de temps). Les objectifs de la présente recherche visent à améliorer le module aérodynamique en passant de Laplace à un solveur d’équations de Navier-Stokes moyennée (RANS). Les avantages sont nombreux. Tout d'abord, le modèle physique permet le calcul des effets visqueux dans le module aérodynamique. Deuxièmement, la solution du programme d’aérogivrage fournit directement les moyens pour caractériser les effets aérodynamiques du givrage, comme la perte de portance et la traînée accrue. Troisièmement, l'utilisation d'une approche de volumes finis pour résoudre les équations aux dérivées partielles (PDE) permet des analyses rigoureuses de convergence en maillage et en temps. Enfin, les approches développées en 2D peuvent être facilement transposées aux problèmes 3D. La recherche a été réalisée en trois étapes principales, chacune fournissant des aperçus des approches numériques globales. La réalisation la plus importante vient de la nécessité de développer des algorithmes de génération de maillage spécifiquement pour assurer des solutions réalisables en plusieurs étapes de calculs très complexes d’aéro-givrage. Les contributions sont présentées dans l’ordre chronologique de leurs réalisations. D'abord, un nouveau cadre de simulation de glace bidimensionnel basé sur un code RANS, CANICE2D-NS, est développé. Un code RANS à maillage à simple bloc de l’université de Liverpool (nommé SMB) fournit la solution aérodynamique en utilisant le modèle de turbulence Spalart-Allmaras. L'outil commercial ICEM CFD est utilisé pour le remaillage du profil glacé pour le lissage du domaine.----------ABSTRACT This thesis addresses one of the current issues in flight safety towards increasing icing simulation capabilities for prediction of complex 2D and 3D glaze ice shapes over aircraft surfaces. During the 1980’s and 1990’s, the field of aero-icing was established to support design and certification of aircraft flying in icing conditions. The multidisciplinary technologies used in such codes were: aerodynamics (panel method), droplet trajectory calculations (Lagrangian framework), thermodynamic module (Messinger model) and geometry module (ice accretion). These are embedded in a quasi-steady module to simulate the time-dependent ice accretion process (multi-step procedure). The objectives of the present research are to upgrade the aerodynamic module from Laplace to Reynolds-Average Navier-Stokes equations solver. The advantages are many. First, the physical model allows accounting for viscous effects in the aerodynamic module. Second, the solution of the aero-icing module directly provides the means for characterizing the aerodynamic effects of icing, such as loss of lift and increased drag. Third, the use of a finite volume approach to solving the Partial Differential Equations allows rigorous mesh and time convergence analysis. Finally, the approaches developed in 2D can be easily transposed to 3D problems. The research was performed in three major steps, each providing insights into the overall numerical approaches. The most important realization comes from the need to develop specific mesh generation algorithms to ensure feasible solutions in very complex multi-step aero-icing calculations. The contributions are presented in chronological order of their realization. First, a new framework for RANS based two-dimensional ice accretion code, CANICE2D-NS, is developed. A multi-block RANS code from U. of Liverpool (named PMB) is providing the aerodynamic field using the Spalart-Allmaras turbulence model. The ICEM-CFD commercial tool is used for the iced airfoil remeshing and field smoothing. The new coupling is fully automated and capable of multi-step ice accretion simulations via a quasi-steady approach. In addition, the framework allows for flow analysis and aerodynamic performance prediction of the iced airfoils. The convergence of the quasi-steady algorithm is verified and identifies the need for an order of magnitude increase in the number of multi-time steps in icing simulations to achieve solver independent solutions

A Modular Approach to Large-scale Design Optimization of Aerospace Systems.

Gradient-based optimization and the adjoint method form a synergistic combination that enables the efficient solution of large-scale optimization problems. Though the gradient-based approach struggles with non-smooth or multi-modal problems, the capability to efficiently optimize up to tens of thousands of design variables provides a valuable design tool for exploring complex tradeoffs and finding unintuitive designs. However, the widespread adoption of gradient-based optimization is limited by the implementation challenges for computing derivatives efficiently and accurately, particularly in multidisciplinary and shape design problems. This thesis addresses these difficulties in two ways. First, to deal with the heterogeneity and integration challenges of multidisciplinary problems, this thesis presents a computational modeling framework that solves multidisciplinary systems and computes their derivatives in a semi-automated fashion. This framework is built upon a new mathematical formulation developed in this thesis that expresses any computational model as a system of algebraic equations and unifies all methods for computing derivatives using a single equation. The framework is applied to two engineering problems: the optimization of a nanosatellite with 7 disciplines and over 25,000 design variables; and simultaneous allocation and mission optimization for commercial aircraft involving 330 design variables, 12 of which are integer variables handled using the branch-and-bound method. In both cases, the framework makes large-scale optimization possible by reducing the implementation effort and code complexity. The second half of this thesis presents a differentiable parametrization of aircraft geometries and structures for high-fidelity shape optimization. Existing geometry parametrizations are not differentiable, or they are limited in the types of shape changes they allow. This is addressed by a novel parametrization that smoothly interpolates aircraft components, providing differentiability. An unstructured quadrilateral mesh generation algorithm is also developed to automate the creation of detailed meshes for aircraft structures, and a mesh convergence study is performed to verify that the quality of the mesh is maintained as it is refined. As a demonstration, high-fidelity aerostructural analysis is performed for two unconventional configurations with detailed structures included, and aerodynamic shape optimization is applied to the truss-braced wing, which finds and eliminates a shock in the region bounded by the struts and the wing.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111567/1/hwangjt_1.pd

The Dune framework: Basic concepts and recent developments

This paper presents the basic concepts and the module structure of the Distributed and Unified Numerics Environment and reflects on recent developments and general changes that happened since the release of the first Dune version in 2007 and the main papers describing that state Bastian etal. (2008a, 2008b). This discussion is accompanied with a description of various advanced features, such as coupling of domains and cut cells, grid modifications such as adaptation and moving domains, high order discretizations and node level performance, non-smooth multigrid methods, and multiscale methods. A brief discussion on current and future development directions of the framework concludes the paper