Air cushion vehicle flexible seals operate in a complex and chaotic environment dominated by fluid-structure interaction. An efficient means to explore interdependencies between various governing parameters that affect performance is through high fidelity numerical simulation. As previous numerical efforts have employed separate iterative partitioned solvers, or have implemented simplified physics, the approaches have been complex, computationally expensive, or of limited utility. This research effort performs numerical simulations to verify and validate the commercial multi-physics tool STAR-CCM+ as a stand-alone partitioned approach for fluid-structure interaction problems with or without a free surface. A dimensional analysis is first conducted to identify potential non-dimensional forms of parameters related to seal resistance. Then, an implicit, Reynolds-averaged Navier-Stokes finite volume fluid solver is coupled to an implicit, nonlinear finite element structural solver to successfully replicate benchmark results for an elastic beam in unsteady laminar flow. To validate the implementation as a seal parameter exploratory tool, a planer bow seal model is developed and results are obtained for various cushion pressures and inflow speeds. Previous numerical and experimental results for deflection and resistance are compared, showing good agreement. An uncertainty analysis for inflow velocity reveals an inversely proportional resistance dependency. Using Abaqus/Explicit, methodologies are also developed for a two-way, loosely coupled explicit approach to large deformation fluid-structure interaction problems, with and without a free surface. Following numerous verification and validation problems, Abaqus is ultimately abandoned due to the inability to converge the fluid pressure field and achieve steady state. This work is a stepping stone for future researchers having interests in ACV seal design and other large deformation, fluid-structure interaction problems. By modeling all necessary physics within a verified and validated stand-alone approach, a designer's ability to comprehensively investigate seal geometries and interactions has never been more promising. / Ph. D. / Air cushion vehicles are specialized marine craft that utilize flexible seals to enable improved performance and fully amphibious operation. An efficient means to explore interdependencies between various seal design parameters that affect performance is through computer modeling of the fluid-structure interaction between the seal and the sea. This research effort performs numerical simulations to verify and validate the commercial multi-physics tool STAR-CCM+ as a single computer program for fluid-structure interaction problems occurring on the water surface. A dimensional analysis is first conducted to identify parameters related to seal resistance. Then, a fluid model is coupled to a structural model to successfully replicate benchmark results for a flexible beam in an oscillating fluid flow. To validate the implementation as a seal parameter exploratory tool, a model of an ACV bow seal is developed and results are obtained for various operational conditions and inflow speeds. Previous numerical and experimental results for seal deflection and seal resistance are compared, showing good agreement. This work is a stepping stone for future researchers having interests in ACV seal design and other large deformation, fluid-structure interaction problems. By modeling all necessary physics within a verified and validated stand-alone computer program, a designer’s ability to comprehensively investigate seal geometries and interactions has never been more promising.

In this doctoral work, we adress various problems arising when dealing with multi-physical simulations using a segregated (non-monolithic) approach. We concentrate on a few specific problems and focus on the solution of aeroelastic <p>flutter for linear elastic structures in compressible fl<p>ows, conjugate heat transfer for re-entry vehicles including thermo-chemical reactions and finally, industrial electro-chemical plating processes which often include<p>stiff source terms. These problems are often solved using specifically developed<p>solvers, but these cannot easily be reused for different purposes. We have therefore considered the development of a <p>flexible and reusable software platform for the simulation of multi-physics problems. We have based this<p>development on the COOLFluiD framework developed at the von Karman Institute in collaboration with a group of partner institutions.<p>For the solution of fl<p>uid fl<p>ow problems involving compressible <p>flows, we have used the Finite Volume method and we have focused on the application of the method to moving and deforming computational domains using the Arbitrary Lagrangian Eulerian formulation. Validation on a series of testcases (including turbulent flows) is shown. In parallel, novel time integration<p>methods have been derived from two popular time discretization methods.<p>They allow to reduce the computational effort needed for unsteady fl<p>ow computations.<p>Good numerical properties have been obtained for both methods.<p>For the computations on deforming domains, a series of mesh deformation techniques are described and compared. In particular, the effect of the stiffness definition is analyzed for the Solid material analogy technique. Using<p>the techniques developed, large movements can be obtained while preserving a good mesh quality. In order to account for very large movements for which mesh deformation techniques lead to badly behaved meshes, remeshing is also considered.<p>We also focus on the numerical discretization of a class of physical models that are often associated with <p>fluid fl<p>ows in coupled problems. For the elliptic problems considered here (elasticity, heat conduction and electrochemical<p>potential problems), the implementation of a Finite Element solver is presented. Standard techniques are described and applied for a variety of problems, both steady and unsteady.<p>Finally, we discuss the coupling of the <p>fluid flow solver with the finite element solver for a series of applications. We concentrate only on loosely and strongly coupled algorithms and the issues associated with their use and implementation. The treatment of non-conformal meshes at the interface between two coupled computational domains is discussed and the problem<p>of the conservation of global quantities is analyzed. The software development of a <p>flexible multi-physics framework is also detailed. Then, several coupling algorithms are described and assessed for testcases in aeroelasticity and conjugate heat transfer showing the integration of the <p>fluid and solid solvers within a multi-physics framework. A novel strongly coupled algorithm, based on a Jacobian-Free Newton-Krylov method is also presented and applied to stiff coupled electrochemical potential problems. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Thesis (MScEng)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: The Council for Scientific and Industrial Research has developed a vibration excitation system. The system is designed to excite the rotor blades of an axial compressor in the specified vibration mode and at the specified frequency. The vibration excitation system was tested on Stellenbosch University’s Rofanco compressor test bench. A two-way staggered fluid-structure interaction (FSI) model was created that was capable of simulating the vibration of the rotor blades excited by the system. The results of the FSI model were verified using available experimental data. It was concluded that the FSI model is able to recreate the vibration excited by the system to within the desired level of accuracy. In addition, the results of the FSI model showed that the vibration excitation system should be able to excite the blades in the selected vibration mode and at the selected frequency provided that the excitation frequency is close the natural frequency of the first bending mode. The results also suggested that a transient computational fluid dynamics model should be sufficient for the prediction of the aerodynamic forces acting on the rotor blades. Furthermore, a one-way staggered FSI model should be adequate for calculating the motions of the blades. / AFRIKAANSE OPSOMMING: Die Wetenskaplike en Nywerheidnavorsingsraad het ’n vibrasie-opwekkingstelsel ontwerp om die rotorlemme van ’n aksiaalvloei kompressor in die gespesifiseerde vibrasiemodus en teen die gespesifiseerde frekwensie op te wek. Die vibrasieopwekkingstelsel is met behulp van die Universiteit Stellenbosch se Rofanco kompressortoetsbank getoets. Daarna is ’n tweerigting vloeistof-struktuur-interaksie model geskep om die vibrasie van die rotorlemme, wat deur die stelsel opgewek is, te simuleer. Beskikbare eksperimentele data is gebruik om die resultate van die vloeistof-struktuur-interaksie model te bevestig. Die gevolgtrekking is gemaak dat die model wél die vibrasie van die lemme met die nodige akkuraatheid kan simuleer. Die resultate van die vloeistof-struktuur-interaksie model toon ook dat die stelsel die lemme in die gekose vibrasiemodus en teen die gekose frekwensie behoort te kan opwek, solank die opwekkingsfrekwensie na aan die natuurlike frekwensie van die eerste buigmodus is. Voorts dui die resultate daarop dat ’n berekeningsvloeimeganika model die aërodinamiese laste van die lemme sal kan voorspel. ’n Eenrigting vloeistof-struktuur-interaksie model behoort voldoende te wees om die beweging van die rotorlemme te bereken.

<p>The computational modeling of ocean waves and ocean-faring devices poses numerous challenges. Among these are the need to stably and accurately represent both the fluid-fluid interface between water and air as well as the fluid-structure interfaces arising between solid devices and one or more fluids. As techniques are developed to stably and accurately balance the interactions between fluid and structural solvers at these boundaries, a similarly pressing challenge is the development of algorithms that are massively scalable and capable of performing large-scale three-dimensional simulations on reasonable time scales. This dissertation introduces two separate methods for approaching this problem, with the first focusing on the development of sophisticated fluid-fluid interface representations and the second focusing primarily on scalability and extensibility to higher-order methods.</p><p>We begin by introducing the narrow-band gradient-augmented level set method (GALSM) for incompressible multiphase Navier-Stokes flow. This is the first use of the high-order GALSM for a fluid flow application, and its reliability and accuracy in modeling ocean environments is tested extensively. The method demonstrates numerous advantages over the traditional level set method, among these a heightened conservation of fluid volume and the representation of subgrid structures.</p><p> </p><p>Next, we present a finite-volume algorithm for solving the incompressible Euler equations in two and three dimensions in the presence of a flow-driven free surface and a dynamic rigid body. In this development, the chief concerns are efficiency, scalability, and extensibility (to higher-order and truly conservative methods). These priorities informed a number of important choices: The air phase is substituted by a pressure boundary condition in order to greatly reduce the size of the computational domain, a cut-cell finite-volume approach is chosen in order to minimize fluid volume loss and open the door to higher-order methods, and adaptive mesh refinement (AMR) is employed to focus computational effort and make large-scale 3D simulations possible. This algorithm is shown to produce robust and accurate results that are well-suited for the study of ocean waves and the development of wave energy conversion (WEC) devices.</p> / Dissertation

Le domaine de l'interaction fluide-structure regroupe l'étude de tous les phénomènes présentant le couplage du mouvement d'une structure avec celui d'un fluide. La gamme des phénomènes étudiés est très étendue, allant de l'étude de cylindres vibrants dans des écoulements comme c'est le cas dans l'industrie nucléaire, à des structures vibrantes dans des écoulements turbulents, en passant par des phénomènes de surface libre dans des réservoirs. Cependant, la complexité des phénomènes étudiés se répercute par des coûts de calculs prohibitifs, ce qui nous amène à rechercher des modèles réduits dont le temps de calcul serait plus réaliste. Dans cette thèse, on va présenter les différents modèles d'interaction fluide-structure et on va mettre en avant le modèle adopté dans notre étude. La réduction du modèle ainsi que l'optimisation des structures vont être introduites dans un contexte de couplage. En introduisant les incertitudes, l'étude fiabiliste de même qu'une approche d'optimisation basée fiabilité vont être proposées. Les différentes méthodologies adoptées vont être validées numériquement et comparées expérimentalement / The domain of the fluid-structure interaction includes the study of all phenomena presenting the coupling of the motion of a structure with the one of a fluid. The range of the phenomena being studied is very extensive, going from the study of vibrating cylinders in the flow as is the case in the nuclear industry, to vibrating structures in turbulent flows, through the free surface phenomena in reservoirs. However, the complexity of the phenomena studied is reflected by the cost of the prohibitive calculations, which leads us to look for models with the computation time would be more realistic. In this thesis, we will present different models of fluid-structure interaction and we will put forward the model adopted in our study. Reducing the model as well as the optimization of the structures will be introduced into a coupling setting. By introducing uncertainties, the reliability study as well as an optimization based reliability approach will be proposed. The different methodologies adopted will be validated numerically and experimentally compared

Les canopées sont des assemblées compactes de plantes dont l'étude concerne de nombreuses problématiques environnementales. Des applications technologiques sont également envisageables. Les précédents travaux se sont principalement focalisés sur les écoulements au-dessus des canopées, considérées comme des rugosités de fond. La présence d'un point d'inflexion dans le profil de vitesses dans le fluide autorise le développement d'instabilités de type couche de mélange à l'interface. De plus, la prise en compte de la flexibilité des plantes complique le problème, car leur forme est modifiée par le courant pour réduire la traînée exercée sur elles --c'est le phénomène de reconfiguration-- mais elles ont également une dynamique propre qui peut éventuellement influencer l'écoulement. La démarche envisagée dans cette thèse est essentiellement expérimentale. Elle cherche à comprendre la réaction des tiges à différents types d'écoulements, afin d'étudier les effets collectifs inhérents à la canopée, et d'identifier les mécanismes communs qui en sont à l'origine. On utilise pour cela des tiges modèles très simples dans un canal étroit, ce qui assure une configuration quasi 2D et facilite les observations. Dans un premier temps, on étudie la réaction statique de la canopée à un écoulement établi. L'effet de la densité est très clair tant que les plantes sont assez proches, sinon elles se comportent comme si elles étaient seules. Ensuite, la canopée est soumise à un écoulement oscillant (houle), et, de la même façon, on étudie la différence de réaction entre une tige seule et une tige incluse dans une canopée. La troisième partie s'intéresse à la dynamique de la canopée soumise à un écoulement unidirectionnel, permettant le développement d'instabilités au sommet de la canopée. Le régime de grandes ondulations cohérentes de la canopée, apparenté au ‘monami' de la littérature, est caractérisé / Vegetation in rivers is often considered as a source of water resistance which slows down the water conveyance. It is also one of the main component for river equilibrium, insofar as it prevents body erosion by providing bed stabilization, it plays a vital role during floods. It is crucial for sediment transport, water quality and also shelter to provide the necessary habitat for the biodiversity of aquatic species. It is then useful to understand the mechanical behaviour of aquatic canopies resulting from the interaction between vegetation and a water flow. From land-use planning to river management, such a knowledge would also shed light upon plant biomechanics and improve bio-inspired engineering.Traditionally, studies on aquatic vegetation explored its influence on flow properties, like velocity distribution, wake dynamics, turbulence, water conveyance and sediment transport by considering it simply as a rigid or flexible roughness element.This thesis is an experimental work which aims at understanding how a model canopy reacts to a water flow depending on the canopy geometry and the flow conditions. Three different series of experiments are performed.First, the effect of density on the canopy reconfiguration and the corresponding drag reduction is investigated. The drag acting on the canopy, and also on individual sheets, is systematically measured. A strong sheltering effect exists as long as the spacing is smaller than a critical value depending on the sheet width.Then, the canopy is submitted to a wave flow to test its sensibility to a determined frequency. Each stem is found to act like a forced oscillator with a strong resonance at natural frequency (modified by canopy density).Finally, a parallel free flow allows mixing layer instabilities to develop above the canopy. Different behaviour are observed and characterized, especially the large coherent waving called 'monami'

Oil and gas exploitation in deep waters has become more than just a profit business to be a daily necessity, since the world energy matrix is based on fossil components. Risers are offshore structures that are intimately linked with oil and gas exploitation and those are subjected to a great variety of effects in field, e.g., marine currents, Vortex Induced Vibration (VIV), heave motion caused by gravitational waves, non-linear contact with the sea floor, and many others. Riser dynamics is essentially non-linear and experimental tests in real scale are almost impossible due to a great variety of control parameters acting concomitantly. Small-scale models are a better experimental approach. Nevertheless, there are many structural and hydrodynamical parameters to be evaluated. Considering only vertical risers in the present work, Galerkin\'s modal decomposition is used in order to reduce the dynamics of a vertical flexible cylinder to a few linear modes in which the majority of energy and information are contained. From the modal analysis, added mass and structural parameters damping of a vertical flexible cylinder using data obtained from free-decay tests performed both in water and in air are evaluated. Finally, a modal Mathieu-Hill oscillator with non-linear damping is constructed and, based on aStrutt diagram, modal stability under parametric resonance is discussed. / Exploração de óleo e gás em bacias de águas profundas tem-se tornado mais do que apenas uma economia lucrativa, para ser uma necessidade diária, já que a matriz energética mundial está baseada em componentes fósseis. Risers são estruturas offshore ligadas intimamente com a exploração de óleo e gás e essas estão sujeitas a uma grande variedade de efeitos na operação, e.g., correntes marítimas, Vibrações Induzidas por Vórtices (VIV), movimento de heave causado por ondas gravitacionais, contato não-linear com o solo marinho, entre outros. Dinâmica de risers é essencialmente não-linear e testes experimentais em escala real são praticamente impossíveis devido a uma enorme variedade de parâmetros de controle agindo concomitantemente. Modelos em escala reduzida são uma abordagem experimental mais conveniente. Não obstante, há muitos parâmetros estruturais e hidrodinâmicos a serem determinados. Considerando apenas risers verticais no trabalho presente, a decomposição modal de Galerkin é usada a fim de reduzir a dinâmica de um cilindro fléxivel vertical a alguns modos lineares em que a maior parte da energia e informação estão contidos. A partir da análise modal, parâmetros de massa adicional e amortecimento estrutural de um cilindro flexível vertical são obtidos usando testes de decaimento livre conduzidos na água e no ar. Finalmente, um oscilador modal de Mathieu-Hill com amortecimento não-linear é proposto e, baseado em um diagrama de Strutt, a estabilidade modal sob excitação de ressonânica paramétrica é discutida.

Esta tese apresenta o desenvolvimento e aplicação de modelos de turbulência, transição laminar-turbulenta e de interações fluido-estrutura ao escoamento externo em cilindro rígido estacionário e em vibrações induzidas por vórtices. Tais desenvolvimentos foram realizados no código ReFRESCO, baseado em técnicas de dinâmica de fluidos computacional (CFD). Realizou-se um estudo quanto ao desempenho do modelo k- SST em extensa faixa de números de Reynolds, segundo o qual se identificaram as deficiências de modelagem para este escoamento. A modelagem adaptativa das escalas (SAS) e o modelo de transição por correlações locais (LCTM), ambos combinados ao SST, melhoraram a aderência aos resultados experimentais para este escoamento, em uma contribuição original deste trabalho. A aplicação de técnicas de verificação e validação possibilitou a estimação de incertezas e erros para os modelos e números de Reynolds e também de identificada como outra contribuição deste trabalho. A combinação da modelagem em SST, SAS e LCTM com movimentos impostos de realizada para números de Reynolds moderados, diferentes frequências e amplitudes de vibração, algo que poucas publicações abordam em detalhes. Com relação aos movimentos livres, este trabalho traz contribuições com a aplicação dos modelos SST e SAS ao estudo de vibrações induzidas por vórtices em dois graus de liberdade, baixa razão de massa e números de Reynolds moderados, mais altos do que normalmente observados na literatura. Por fim, a investigação da importância relativa de efeitos da turbulência aos casos de movimentos livres e impostos, com relação ao caso de cilindro estacionário, comprovou a conjetura formulada na parte inicial deste trabalho, no que tange à escolha do modelo de turbulência em determinadas aplicações. Tal escolha mostrou-se menos decisiva no caso do cilindro em movimento imposto e ainda menos nos movimentos livres, em comparação ao caso estacionário, uma vez que a resposta em movimentos do corpo filtra grande parte dos efeitos turbulentos de ordem superior. Esta observação mostra-se relevante, uma vez que pode permitir simplificações na modelagem e aplicação de ferramentas de CFD em uma classe importante de projetos de engenharia. / This thesis presents the development, implementation and application of turbulence and laminar-turbulent transition models and fuid-structure capabilities to address the vortexshedding and vortex-induced vibrations of a rigid cylinder. These numerical developments have been carried out in the computational fuid dynamics (CFD) code ReFRESCO. In the current work, an investigation of the performance of the turbulence modeling with k- SST in a broad range of Reynolds numbers is carried out identifying its modeling deficiencies for this fow. The implementation and systematic application of the scale adaptive simulations (SAS) and the local correlation transition model (LCTM), both combined with the SST, have improved the agreement with experimental results for the cylinder ow, in a novel contribution of this work. The application of verification and validation technique has allowed the estimation of numerical errors and uncertainties for the diferent models. That is also identified as a contribution of this thesis. The combination of SST modeling with imposed motions is carried out as well as with the SAS and LCTM for moderate Reynolds numbers, diferent vibration frequencies and amplitudes, which is considered novel, as few publications address this issue in extent. Regarding the free-moving cylinder capabilities, the present work brings contributions with the application of SST and SASSST with free-moving cylinder for the study of VIV of two degrees of-freedom, low mass ratio and moderate Reynolds numbers, higher than commonly seen in the literature. Finally, the investigation of the relative importance of turbulence effects on the freemoving cylinder and the imposed-motions case, with respect to the fixed case is carried out. A natural conjecture that has been raised early on this work and proved correct is that, for engineering applications, the choice of turbulence modeling strategy is less decisive when the cylinder is moving with prescribed motion and even less stringent, for free motions as the body response filters most of the higher order turbulence effects. That is a relevant observation as it might allow modeling simplifications and the application of CFD tools to a range of engineering problems.

Les animaux volants et flottants ont développé des façons efficaces de produire l'écoulement de fluide qui génère les forces désirées pour leur locomotion. Cette thèse est placée dans ce contexte interdisciplinaire et utilise des simulations numériques pour étudier ces problèmes d'interaction fluides-structure, et les applique au vol des insectes et à la nage des poissons. Basée sur les travaux existants sur les obstacles mobiles rigides, une méthode numérique a été développée, permettant également la simulation des obstacles déformables et fournissant une polyvalence et précision accrues dans le cas des obstacles rigides. Nous appliquons cette méthode d'abord aux insectes avec des ailes rigides, où le corps et d'autres détails, tels que les pattes et les antennes, peuvent être inclus. Après la présentation de tests de validation détaillée, nous procédons à l'étude d'un modèle de bourdon dans un écoulement turbulent pleinement développé. Nos simulations montrent que les perturbations turbulentes affectent les insectes volants d'une manière différente de celle des avions aux ailes fixées et conçues par l'humain. Dans le cas de ces derniers, des perturbations en amont peuvent déclencher des transitions dans la couche limite, tandis que les premiers ne présentent pas de changements systématiques dans les forces aérodynamiques. Nous concluons que les insectes se trouvent plutôt confrontés à des problèmes de contrôle dans un environnement turbulent qu'à une détérioration de la production de force. Lors de l‘étape suivante, nous concevons un modèle solide, basé sur une équation de barre monodimensionnelle, et nous passons à la simulation des systèmes couplés fluide–structure. / Flying and swimming animals have developed efficient ways to produce the fluid flow that generates the desired forces for their locomotion. These bio-inspired problems couple fluid dynamics and solid mechanics with complex geometries and kinematics. The present thesis is placed in this interdisciplinary context and uses numerical simulations to study these fluid--structure interaction problems with applications in insect flight and swimming fish. Based on existing work on rigid moving obstacles, using an efficient Fourier discretization, a numerical method has been developed, which allows the simulation of flexible, deforming obstacles as well, and provides enhanced versatility and accuracy in the case of rigid obstacles. The method relies on the volume penalization method and the fluid discretization is still based on a Fourier discretization. We first apply this method to insects with rigid wings, where the body and other details, such as the legs and antennae, can be included. After presenting detailed validation tests, we proceed to studying a bumblebee model in fully developed turbulent flow. Our simulations show that turbulent perturbations affect flapping insects in a different way than human-designed fixed-wing aircrafts. While in the latter, upstream perturbations can cause transitions in the boundary layer, the former do not present systematical changes in aerodynamic forces. We conclude that insects rather face control problems in a turbulent environment than a deterioration in force production. In the next step, we design a solid model, based on a one--dimensional beam equation, and simulate coupled fluid--solid systems.

Multidisciplinary design optimization (MDO) refers to the process of designing systems characterized by the interaction of multiple interconnected disciplines. High-fidelity MDO usually requires large computational resources due to the computational cost of achieving multidisciplinary consistent solutions by coupling high-fidelity physics-based solvers. Gradient-based minimization algorithms are generally applied to find local minima, due to their efficiency in solving problems with a large number of design variables. This represents a limitation to performing global MDO and integrating black-box type analysis tools, usually not providing gradient information. The latter issues generally inhibit a wide use of MDO in complex industrial applications. An architecture named multi-criterion adaptive sampling MDO (MCAS-MDO) is presented in the current research for complex simulation-based applications. This research aims at building a global derivative-free optimization tool able to employ high-fidelity/expensive black-box solvers for the analysis of the disciplines. MCAS-MDO is a surrogate-based architecture featuring a variable level of coupling among the disciplines and is driven by a multi-criterion adaptive sampling (MCAS) assessing coupling and sampling uncertainties. MCAS uses the dynamic radial basis function surrogate model to identify the optimal solution and explore the design space through parallel infill of new solutions. The MCAS-MDO is tested versus a global derivative-free multidisciplinary feasible (MDF) approach, which solves fully-coupled multidisciplinary analyses, for two analytical test problems. Evaluation metrics include number of function evaluations required to achieve the optimal solution and sample distribution. The MCAS-MDO outperforms the MDF showing a faster convergence by clustering refined function evaluations in the optimum region. The architecture is applied to a steady fluid-structure interaction (FSI) problem, namely the design of a tapered three-dimensional carbon fiber-reinforced plastic hydrofoil for minimum drag. The objective is the design of shape and composite material layout subject to hydrodynamic, structural, and geometrical constraints. Experimental data are available for the original configuration of the hydrofoil and allow validating the FSI analysis, which is performed coupling computational fluid dynamics, solving the Reynolds averaged Navier-Stokes equations, and finite elements, solving the structural equation of elastic motion. Hydrofoil forces, tip displacement, and tip twist are evaluated for several materials providing qualitative agreement with the experiments and confirming the need for the two-way versus one-way coupling approach in case of significantly compliant structures. The free-form deformation method is applied to generate shape modifications of the hydrofoil geometry. To reduce the global computational expense of the optimization, a design space assessment and dimensionality reduction based on the Karhunen–Loève expansion (KLE) is performed off-line, i.e. without the need for high-fidelity simulations. It provides with a selection of design variables for the problem at hand through basis rotation and re-parametrization. By using the KLE, an efficient design space is identified for the current problem and the number of design variables is reduced by 92%. A sensitivity analysis is performed prior to the optimization to assess the variability associated with the shape design variables and the composite material design variable, i.e. the fiber orientation. These simulations are used to initialize the surrogate model for the optimization, which is carried out for two models: one in aluminum and one in composite material. The optimized designs are assessed by comparison with the original models through evaluation of the flow field, pressure distribution on the body, and deformation under the hydrodynamic load. The drag of the aluminum and composite material hydrofoils is reduced by 4 and 11%, respectively, increasing the hydrodynamic efficiency by 4 and 7%. The optimized designs are obtained by evaluating approximately 100 designs. The quality of the results indicates that global derivative-free MDO of complex engineering applications using expensive black-box solvers can be achieved at a feasible computational cost by minimizing the design space dimensionality and performing an intelligent sampling to train the surrogate-based optimization.