Thesis (M.S.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references (leaves 90-92).

Thesis (Ph. D.)--University of Alabama at Birmingham, 2006. / Description based on contents viewed Jan. 26, 2007; title from title screen. Includes bibliographical references (p. 112-126).

Ce travail de thèse est une contribution à l’analyse de bifurcation des écoulements fluides avec prise en compte des interactions fluide-structure. Les phénomènes d’instabilité en interaction fluide-structure apparaissent dans de nombreux domaines de la vie courante ou industriels comme, par exemple : le flottement d’un drapeau dans le vent ou bien l’écoulement au sein d’échangeurs thermiques sur les sites de production d’énergie, l’écoulement autour des câbles sous-marins pour l’extraction de matières premières ou la fixation des plateformes off-shore, l’écoulement autour des structures aéronautiques ou navales. Dans ces situations, un phénomène complexe de vibration des structures induite par vortex peut se produire. L’objectif de la thèse est de proposer un algorithme permettant l’analyse de stabilité de tels systèmes. Ainsi, le couplage original d’une méthode de perturbation d’ordre élevé (Méthode Asymptotique Numérique - MAN) à une discrétisation spatiale permettant la prise en compte des interactions fluide-structure est proposée. À cet effet, une description purement eulérienne du mouvement est retenue. L’interaction fluide- structure est décrite au moyen d’une méthode de frontières immergées (MFI) à forçage continu (méthode de pénalisation) et discret (méthode Ghost-Cell). La présence d’obstacles au sein de l’écoulement est obtenue au moyen de la méthode de Level-Set. En complément, un intégrateur temporel des équations du mouvement associant la MAN, la MFI et une technique d’homotopie est proposé. L’ensemble de ces algorithmes est appliqué à des problèmes d’écoulement incompressible, à faible nombre de Reynolds, d’un fluide visqueux newtonien en présence d’obstacles solides rigides (fixes ou mobiles). L’analyse de stabilité d’un écoulement dans une conduite avec expansion/contraction soudaine (bifurcation stationnaire), et autour d’un cylindre (bifurcation de Hopf) est traitée. L’analyse transitoire d’un écoulement autour d’un cylindre rigide et mobile est également proposée. Les résultats obtenus permettent d’évaluer la précision et la performance des algorithmes proposés. Ainsi, les résultats de cette thèse permettent de conclure sur le bien-fondé de l’approche et constituent une première étape vers l’analyse de stabilité d’écoulements en présence de structures complexes, représentatifs de situations réelles / This thesis is a first contribution to the bifurcation analysis of fluid flows by taking into account fluid-structure interactions. Instability with fluid-structure interactions appears in many areas of everyday life or industry such as, for example: flag floating in the wind, flow within heat exchangers for energy production, flow around submarine cables for the extraction of raw materials or the fixing of off-shore platforms, flow around aeronautical or naval structures. In these situations, complex vortex-induced vibrations of the structures can occur. The aim of the thesis is to propose an algorithm allowing stability analysis of such systems. Thus, an original coupling of a high order perturbation method (Asymptotic Numerical Method - ANM) to a spatial discretization which takes into account fluid-structure interactions is proposed. For this purpose, a purely Eulerian description of the motion is retained. Fluid-structure interaction is described using an immersed boundary method (IBM) with continuous forcing (penalization method) and discrete (Ghost-Cell method) forcing. The presence of bodies within the flow is obtained by means of the Level-Set method. In addition, a time integrator of the governing equations associating ANM, IBM and homotopy technique is proposed. All these algorithms are applied to analyse incompressible flows, at low Reynolds number, of a Newtonian viscous fluid in the presence of rigid solids (fixed or moving). Bifurcation analysis of flows in a channel with sudden expansion / contraction (stationary bifurcation), or around a cylinder (Hopf bifurcation) are carried out. Transient analysis of a flow around a moving rigid cylinder is also proposed. Our results make it possible to evaluate accuracy and performance of the proposed algorithms. Thus, thesis results allow to conclude on the validity of the proposed approach. Finally, this thesis work constitutes a first step towards flow stability analysis in the presence of complex structures, representative of real situations.

High-mast light poles are used frequently to illuminate large areas such as motorways and parking lots. These poles are extremely tall with respect to their cross-section, reaching heights of more than 40 meters. These structures undergo a strong aeroelastic response due to wind, oftentimes resulting in fatigue cracking at the base. The purpose of the research is to better understand the effects of wind-induced vibrations of tall flexible structures using a combination of computational fluid dynamics and structural finite element codes. Field results of existing high-mast poles will be used to calibrate and verify the theoretical modeling.Periodic vortex shedding is observed to occur on these structures at certain wind velocities. The shedding of vortices causes pressure differences across the pole, resulting in a net driving force perpendicular to the direction of the wind. When the frequency of shedding, and thus the driving force, matches the natural frequency of the pole, excitation of the structure can be significant. This phenomenon is called lock-in. Poles that are repeatedly subjected to wind at lock-in velocity may suffer excessive deformation and fatigue damage. The aeroelastic response is especially significant, since the damping of the structural system is so small.In order to model the fluid-structure interaction, OpenFOAM libraries were compiled into a single application that combined a structural dynamic finite element code along with a mesh movement algorithm. The loosely coupled system applies the driving forces (integrated pressures) to the structure in a conventional serial staggered procedure. The coupling of the two domains and the mesh deformation calculationswere software written by the author. The 3-field solution formulation is implemented using a mesh movement algorithm based on a pseudo-elastic approach. Incompressible flow is assumed, as the lock-in velocities for the first three natural frequency modes ofthe pole are relatively low. Large Eddy Simulation is used for turbulence modeling.In conjunction with the University of Wyoming, two existing steel hexadecagonal high-mast poles in Wyoming, USA, were instrumented with accelerometers and anemometers. These data were used to calibrate and verify the structural, stiffness, damping, and response characteristics.A series of 14 simulations were run that increased in the difficulty of the domain being simulated. Different aspects of the pole aparatus were investigated individually, such as the taper and angle of incidence of flow. An atmospheric boundary layer model was incorporated. The final case resulted in the simulation of a 16-sided tapered pole subject to flow from an atmospheric boundary layer inlet, incorporating large eddy simulation turbulence modeling. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

La caractérisation des mécanismes de transfert entre un écoulement fluide incompressible et une structure solide constitue l’objectif principal de ce présent mémoire. A partir d’un solveur développé au sein de l’équipe, les travaux se sont plus particulièrement focalisés sur les stratégies de couplage avec un solveur solide, afin de traiter à la fois les échanges énergétiques et les mouvements de la structure. Dans notre approche, le modèle traitant la partie solide est le solveur ASTER et une attention particulière a été portée sur la stratégie de couplage à mettre en place.Dans la partie couplage fluide/structure, des cas de référence ont été réalisés avec une complexité croissante et l’intégration de la problématique des frontières immergées a été étudiée. En effet, alors que la modélisation avec des frontières immergées semble ne pas perturber les traitements côté fluide, les changements d’état de la topologie induit par le mouvement du solide dans le domaine de calcul génèrent des discontinuités dans les forces fluides estimées sur la structure. Ces dernières peuvent être plus ou moins amorties par l’introduction de techniques hybrides dans les traitements aux frontières immergées.Malgré ses quelques limitations, le solveur est capable de traiter de grande déformation assurant un fonctionne robuste et rapide pour la caractérisation des mécanismes fortement couplés. Pour le souligner, une application sur des écoulements anisothermes au sein d’une cavité représentant une cellule frigorifique a été réalisée dans le cadre du projet de recherche OSEO. A notre connaissance, les traitements réalisés ont pour la première fois permis de quantifier l’effet des ouvrants (dans les phases d’ouverture et fermeture des portes de la cellule) sur les écoulements et les échanges thermiques. Une telle modélisation permet alors de proposer des améliorations de la géométrie en cours d’analyse. / Characterization of heat transfer mechanisms between an incompressible fluid flow and solid structure is the main objective of the proposed work. From a solver developed within the team, we particularly focused on strategies for coupling with a solid solver to address both energy and structure motion. In our approach, the model treating solid part is the solver ASTER and specific attention was paid to the coupling strategy. In the fluid / structure coupling part, reference cases were performed with increasing complexity and immersed boundaries was investigated. The change in topology for the Immersed Boundary Method enhances here and there some numerical instability and the latter can be more or less damped by hybrid techniques in the treatments to submerged borders.Despite such limitations, the solver is able to handle large deformation ensuring robust and fast analysis for the characterization of strongly coupled mechanisms. To emphasize such a point, isothermal flow in a cavity representing a cold-cell was conducted as part of the research project OSEO. To our knowledge, the processing performed for the first time quantified the effect of opening (in the opening and closing of the doors of the cell phases) on the flow and heat exchange. Such modeling is then used to suggest improvements to the geometry being analyzed.

Water hammer represents a complex hydraulic phenomenon with significant consequences on the proper functioning and safety of operation for pipe and conduit systems. The complexity and intricate physics of water hammer translated into significant difficulties associated firstly, with finding a proper solution for understanding the mechanism of its occurrence and, secondly, relating to proposing technically and economically viable design methods and devices that would help reduce and mitigate water hammer effects. In this context, the present thesis deals with the numerical modeling of the transient behaviour of water pipe segments. Following an extensive literature review of the state-of-the-art on the water hammer mechanisms and past work on experimental, analytical and numerical analysis of this phenomenon, a three dimensional numerical model of the water hammer in a pipe which considers the fluid-structure interaction (FSI) is developed using a Finite Element Method – Finite Volume Method (FEM-FVM) technique. Structural and fluid computational results based on rapid and slow gate closure scenarios are compared with existing closed-form solutions of the water hammer. A parametric study is also performed on a simply supported pipe segment to determine the influence of various design parameter. A systematic sensitivity analysis was conducted and a ranking mechanism was established for the importance of each parameter on the fluid fields and structural response. A first comparative analysis is conducted on horizontally and vertically bent elevated pipe segments to quantify the influence of the bend angle on the results. A second comparative analysis is performed on a horizontally bent segment buried in soil to determine the influence of the pipe interaction with the soil on the response. It is observed that the thickness, span, initial velocity and bend angle had a significant impact on the pressure and structural response. The presence of soil was observed to have a significant benefit in decreasing the von-Mises stresses.

In this dissertation, a continuum sensitivity method is developed for efficient and accurate computation of design derivatives for nonlinear aeroelastic structures subject to transient<br />aerodynamic loads. The continuum sensitivity equations (CSE) are a set of linear partial<br />differential equations (PDEs) obtained by differentiating the original governing equations of<br />the physical system. The linear CSEs may be solved by using the same numerical method<br />used for the original analysis problem. The material (total) derivative, the local (partial)<br />derivative, and their relationship is introduced for shape sensitivity analysis. The CSEs are<br />often posed in terms of local derivatives (local form) for fluid applications and in terms of total<br />derivatives (total form) for structural applications. The local form CSE avoids computing<br />mesh sensitivity throughout the domain, as required by discrete analytic sensitivity methods.<br />The application of local form CSEs to built-up structures is investigated. The difficulty<br />of implementing local form CSEs for built-up structures due to the discontinuity of local<br />sensitivity variables is pointed out and a special treatment is introduced. The application<br />of the local form and the total form CSE methods to aeroelastic problems are compared.<br />Their advantages and disadvantages are discussed, based on their derivations, efficiency,<br />and accuracy. Under certain conditions, the total form continuum method is shown to be<br />equivalent to the analytic discrete method, after discretization, for systems governed by a<br />general second-order PDE. The advantage of the continuum sensitivity method is that less<br />information of the source code of the analysis solver is required. Verification examples are<br />solved for shape sensitivity of elastic, fluid and aeroelastic problems. / Ph. D.

This work details the development of a computational tool that can accurately model strongly-coupled fluid-structure-interaction (FSI) problems, with a particular focus on thin-walled structures undergoing large, geometrically non-linear deformations, which has a major interest in, amongst others, the aerospace and biomedical industries. The first part of this work investigates improving the efficiency with which a stable and robust in-house code, Elemental, models thin structures undergoing dynamic fluid-induced bending deformations. Variations of the existing finite volume formulation as well as linear and higher-order finite element formulations are implemented. The governing equations for the solid domain are formulated in a total Lagrangian or undeformed conguration and large geometrically non-linear deformations are accounted for. The set of equations is solved via a single-step Jacobi iterative scheme which is implemented such as to ensure a matrix-free and robust solution. Second-order accurate temporal discretisation is achieved via dual-timestepping, with both consistent and lumped mass matrices and with a Jacobi pseudo-time iteration method employed for solution purposes. The matrix-free approach makes the scheme particularly well-suited for distributed memory parallel hardware architectures. Three key outcomes, not well documented in literature, are highlighted: the issue of shear locking or sensitivity to element aspect ratio, which is a common problem with the linear Q4 finite element formulation when subjected to bending, is evaluated on the finite volume formulations; a rigorous comparison of finite element vs. finite volume methods on geometrically non-linear structures is done; a higher-order finite volume solid mechanics procedure is developed and evaluated. The second part of this work is concerned with fluid-structure interaction (FSI) modelling. It considers the implementation and coupling of a higher order finite element structural solver with the existing finite volume fluid-flow solver in Elemental. To the author’s knowledge, this is the first instance in which a strongly-coupled hybrid finite element–finite volume FSI formulation is developed. The coupling between the fluid and structural components with non-matching nodes is rigorously assessed. A new partitioned fluid-solid interface coupling methodology is also developed, which ensures stable partitioned solution for strongly-coupled problems without any additional computational overhead. The solver is parallelised for distributed memory parallel hardware architectures. The developed technology is successfully validated through rigorous temporal and mesh independent studies of representative two-dimensional strongly-coupled large-displacement FSI test problems for which analytical or benchmark solutions exist. / Dissertation (MEng)--University of Pretoria, 2012. / Mechanical and Aeronautical Engineering / unrestricted

Please read the abstract (Summary) in the 00front part of this document / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

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