Flow-induced vibrations (FIV) in tube bundles subjected to cross-flow pose significant risks to the safety and performance of nuclear reactor components. These vibrations, driven by mechanisms such as vortex shedding, turbulence fluctuations, and fluid-elastic instabilities, can lead to structural fatigue and failure. Accurately predicting FIV remains a challenge due to the complex interaction between fluid dynamics and structural response. This study investigates the effectiveness of hybrid turbulence models within commercial Computational Fluid Dynamics (CFD) tools to improve the accuracy and computational efficiency of FIV simulations.

Hybrid turbulence models, including Scale-Adaptive Simulation (SAS) and Delayed Detached
Eddy Simulation (DDES), were explored for their ability to balance computational efficiency with the accuracy required for FIV predictions. Simulations were conducted in ANSYS Fluent and STAR-CCM+ across various mesh configurations and flow conditions. The results indicate that DDES, particularly with fine polyhedral meshes and wall y+ < 1, provides improved predictions of flow-induced forces and vibrations. ANSYS Fluent exhibited greater robustness and reliability in this study, particularly for strongly coupled FSI problems in FIV simulations.

This research contributes to the ongoing efforts in refining FIV simulation methodologies by proposing a standardized workflow and offering insights into the applicability of hybrid turbulence modeling. The findings enhance the understanding of FIV phenomena and support the development of improved numerical approaches for the safer design of nuclear reactor components.

This thesis investigates the application of hybrid turbulence models for fluid-structure interaction (FSI) to evaluate phenomena such as vortex-induced vibration (VIV) in tube rods within nuclear reactors. The study aimed to identify and validate the most effective hybrid modeling approaches to improve the accuracy and efficiency of FSI simulations in this critical application. Hybrid RANS-LES models aim to use Reynolds Averaged Navier Stokes (RANS) modeling in regions where the flow is relatively steady and well-predicted by the RANS approach, while switching to Large Eddy Simulation (LES) in regions where turbulence is more dynamic and complex. This switch can either be very explicit and segregate the domain into RANS and LES zones or it can be a subtle modification in the transport equations using either RANS or LES as baseline.

The research began with a detailed literature review, leading to the selection of four promising hybrid turbulence models: Improved Delayed Detached Eddy Simulation (IDDES), Scale Adaptive Simulation (SAS), Partially Averaged Navier–Stokes (PANS), and Struct Epsilon (SE). These models were implemented and tested through numerical case setups to validate their performance against established reference data for crossflow over a circular cylinder at a Reynolds number (Re) of 3900.

A comparative analysis was conducted to assess the performance of the selected models. Based on this analysis, two models were shortlisted for further testing. These models were subjected to additional validation on a rigid body motion case to ensure their reliability and accuracy in FSI applications.

In this validation, the models were used to simulate flow over an elastically mounted rigid cylinder in crossflow to evaluate the frequency and displacement of its oscillation. They were tested at different velocities to determine the amplitude and frequency response. The SE model was observed to be more robust and efficient than IDDES.

In conclusion, the research provides validation, selection and comparison of different hybrid turbulence models for FSI applications. Recommendations for future research are also provided to further refine these modeling approaches.

With the increasing demand for flights, the environmental impact of aviation is on the rise. To tackle this challenge, it is necessary to look into the design of unconventional aircraft configurations and engine design improvements to improve overall fuel efficiency. The Blended Wing Body (BWB) aircraft is one of the most promising unconventional configurations for future airliners. Several studies have looked at the design optimization of the BWB and its engine-integration effects. However, most of these studies were limited by either a lack of coupling between the aerodynamic and propulsion models or restricted design freedom for nacelle design and engine-airframe integration. Thus, there is a need to explore the benefits of aeropropulsive trade-offs using non-axisymmetric nacelle designs and refining its relative placement above the wing.

The work performed in this thesis focuses on realizing these aeropropulsive trade-offs by optimizing the engine and the BWB wing simultaneously using selectively non-axisymmetric engine design variables, including design variables for wing shape and engine placement. To achieve this objective, a free-form deformation (FFD) based parameterization scheme is developed for non-axisymmetric engine parameterization along with design variables for modifying wing shape and relative engine placement. ADflow CFD solver is used for the aerodynamic discipline, and the PyCycle thermodynamic cycle library is used for the propulsion discipline. Coupled aeropropulsive optimizations are performed by extending the MACH-Aero framework using coupled derivatives in an Individual Discipline Feasible (IDF) architecture for gradient-based optimization using the SNOPT optimization algorithm.

A set of studies are performed to establish the robustness and effectiveness of the proposed FFD-based parameterization for engine optimization in isolation. Later, the engine is mounted onto the BWB in an over-the-wing configuration, and the effect of non-axisymmetric engine design is explored in conjunction with engine placement and wing shape modifications. Optimizing the engine location with axisymmetric nacelle and wing shape design variables reduces the wing drag by 11.5% and engine fuel consumption by 2.3% compared to the baseline engine location. Using non-axisymmetric nacelle design variables further drops the fuel consumption by 1.1%. A comparison of this optimized engine-airframe design with an optimized engine in isolation shows that placing the engine in an over-the-wing configuration with the BWB reduces the thrust-specific fuel consumption by 4.4%. Non-axisymmetric nacelle design also improves the quality of engine inflow by eliminating local super velocities at the inlet lip. This study shows the benefits of an OWN configuration for the BWB and quantifies the performance gains from optimizing the engine location with non-axisymmetric nacelles.

Nuclear power is increasingly seen as a potential solution to the decarbonization of the energy sector in the coming decades. However, one of the main causes of downtime for current-generation nuclear reactors is the phenomenon of Grid-To-Rod-Fretting (GTRF) inside the reactor core. This is predominantly caused by Fluid-Structure Interaction (FSI), where the turbulent cooling water flow causes unwanted vibrational behaviour of the long and slender rods where the fission reaction takes place.

To quantify the effects of GTRF, before this thesis, a numerical FSI workflow called NRG-FSIFOAM was developed by Nuclear Research Group (NRG). The fluid modelling consisted of a synthetic turbulence model (AniPFM) developed in OpenFOAM. The structural solution was obtained using a 3D Finite Element Method (FEM) solver implemented in deal.II, while the mapping and coupling between the two solvers was handled by preCICE.

In this context, the objective of the thesis is to propose a simplified and cost-effective structural solver, without compromising the accuracy of the methodology. To this end, an eigenmode-based Reduced-Order Model (ROM) of a 1D beam-element FEM formulation is proposed. To further improve the computational costs, by taking into account the 1D FEM formulation of the structural solver, novel mapping routines between the fluid and the structural grid are also proposed. To ensure a strong coupling between the two domains, an Aitken subiteration algorithm is used. What’s more, to simplify the architecture of the methodology, the new features are directly implemented in OpenFOAM. Thus, the newly proposed workflow is fully contained within OpenFOAM, eliminating the need to additionally use deal.II and preCICE. This newly proposed FSI methodology is called NRG-beamFoam.

The thesis first deals with the individual verification of the structural solver, the mapping routines, and the Aitken subiteration scheme. Subsequently, all of the elements are combined within FSI simulations for the same benchmark case that was used for the validation of NRG-FSIFOAM. It is found that for simulations where an Unsteady Reynolds-Averaged Navier Stokes (URANS) fluid model is used, NRG-beamFoam reduces the computational cost per FSI subiteration by 48% compared to its NRG-FSIFOAM predecessor, while obtaining a 0.5% relative difference in the frequency and the damping ratio characterizing the structural dynamic response. What’s more, using the ROM, the instantaneous deformations of a single rod to turbulent excitation by axial water flow appear to be accurately computed using a total number of degrees of freedom that is reduced by a factor of approximately 103 compared to the FEM solver of NRG-FSIFOAM. Further research is recommended to improve the stability of NRG-beamFoam when coupled with the AniPFM for simulation times in the order of seconds. Furthermore, future studies ought to explore the causes for the small amplitude differences observed between the outputs of the ROM and the FEM solver for FSI simulations.

The use of liquid hydrogen LH2 in aviation offers significant potential for reducing CO2 emissions. However, its unique properties, such as rapid dispersion, wide flammability limits, and low ignition energy, introduce explosion safety concerns in confined spaces like the aircraft tail cone, where LH2 tanks are typically placed. Using a numerical approach, this thesis investigates the blast loads and dynamic response of tail cone structures to a confined hydrogen-air detonation, identified as a worst-case explosion scenario.

The LS-DYNA CESE-Chemistry solver, employing an 8-step finite-rate reaction mechanism, was used to model the detonation blast wave. The structural response is coupled via Immersed Boundary Fluid-Structure Interaction (FSI). The detonation model was validated with literature results from a hydrogen-air detonation experiment, while the structural model was validated with blast-loaded circular plates, both showing good agreement.


Results show that the blast wave propagates in the tail cone with multiple reflections, producing 2–3 distinct pressure peaks. While the first pulse is the primary shock-loading in most locations, the second pulse is more significant at the bulkhead center, amplified by shock convergence effects.

The structural response occurs in four distinct phases: initial deformation, flexural wave propagation, secondary pulse loading, and snapback instability. Large deformations and high plasticity were observed at the bulkhead center and edge, identifying them as critical zones for damage. Parametric studies reveal that deflections increase with ignition distance and decrease with bulkhead thickness or radius. A linear relationship was found between a dimensionless impulse parameter and permanent deformations or plastic strains across varying bulkhead configurations.

The insights gained provide a foundation for designing safer hydrogen-powered aircraft and addressing regulatory gaps in explosion safety. Recommendations for future work include extending the model to 3D geometries, incorporating material failure models, and conducting tailored validation experiments.

In contemporary world, integrated chips(ICs) play a vital role in how various systems function. These are manufactured by lithography machines to print intricate features on the silicon wafers with nanometer precision. As these use high energy radiations(Deep Ultra-violet(DUV) or Extreme Ultra-violet(EUV)), they generate high amount of heat, which needs to be dissipated via cooling circuits. As the circuit can contain bends, change in cross-sectional area, junctions, orifices; these can cause turbulent flow fluctutations. These fluctutations can lead to Flow-Induced Vibrations(FIV), which can affect the accuracy of these machines. Hence, in this thesis, an efficient method to compute the turbulent flow fluctuations is dealt in detail.

Generally, at ASML, experimental methods or numerical methods like Direct Numerical Simulation(DNS) or Large Eddy Simulation(LES) are used to compute turbulent fluctuations. However, these are time consuming, complex and computationally expensive. So, in this thesis a Reynolds Averaged Navier Stokes(RANS) based stochastic method has been developed from existing methods to generate turbulent velocity fluctuations for ASML applications. This method can account for multiple turbulence characteristics like spatial correlation, velocity-time correlation and anisotropy that are essential for computing FIV. In addition, a pressure Poisson equation solver has been developed to compute turbulent pressure fluctuations from velocity fluctuations. Further, the developed method has been validated for two academic test-cases, homogeneous isotropic turbulence(experiment) and turbulent channel flow(DNS). The method was found to generate turbulent fluctuations adhering to the energy spectrum imposed; the exponential time correlation applied and account for anisotropy based on the Reynolds Stress Tensor given as input. Finally, the method has been applied to a test-case, sharp-bend and compared against LES results. It was found that the method agrees well to the energy spectrum imposed, however it deviates from the LES energy spectrum. Nonetheless, the capability of turbulent fluctuation generation from RANS at lesser computational effort than LES/DNS has been demonstrated, even though there can be improvements made to the energy spectrum given as input to the developed method.

This thesis investigates the aeroelastic behaviour of a flexible wing with minitabs and their effectiveness in alleviating the peak gust loads. Minitabs, which are small deployable surfaces located on the upper surface of the wing, were examined through a series of static and dynamic Fluid-Structure Interaction (FSI) simulations, which evaluated the aerodynamic and structural responses when deployed and stowed. The transient response of deployment of the minitabs indicated a greater load reduction capability compared to their steady-state performance. Notably, the peak alleviation of the minitabs is achieved when the gust peak lies within the aerodynamic reaction time of the minitab. Evaluating the deployment of minitabs for various gust profiles further demonstrates that minitabs performed better in response to sharp, high-amplitude gusts, while their efficacy diminishes with longer gust lengths. When deployed at the ideal moment, the minitabs were found to reduce peak gust loads by an average of 5%. As a consequence of the mid-flight deployment and stowing of the minitabs, simulations revealed significant aeroelastic effects, characterized primarily by first bending mode oscillations at approximately 1 Hz. A sweep analysis of the deployment and stowing times revealed that delaying the actuation of the minitab beyond the ideal deployment or stowing time leads to an increased amplitude oscillations and extended recovery time to steady state conditions. The study evaluated the performance of the minitabs in response to various gust profiles characterized by different gust lengths. The minitabs demonstrated improved performance in alleviating the gust peak loads from short, sharp gusts. However, the effectiveness in reducing the peak loads diminished for gusts of longer gust length and subsequent longer recovery time. The study also identified gusts with frequencies close to the natural first bending frequency of the wing structure as critical cases, where resonant behaviour emerges, leading to large amplitude oscillations and prolonged recovery times. While minitabs effectively reduce peak lift and bending moments induced by gusts, they also introduce lower amplitude oscillations in the bending and torsion moments that persist over multiple cycles. While the peak moments do not exceed a magnitude that would raise a concern to the structural integrity, the subsequent prolonged multi-cycle oscillations in the moments raises concerns about potential fatigue life issues for the internal structure. In conclusion, while this research confirms the potential of minitabs as a viable load alleviation method, it emphasizes the need for further investigation. Comprehensive aeroelastic studies on various minitab configurations, geometries and placements over the wing will provide a complete understanding of the aeroelastic effects of the minitabs and determine an ideal configuration to maximize the load alleviation capability. Additionally, the original actuation method of the minitab involves the rotating them from a vortex generator position to the minitab position. This thesis simplified the actuation to a ON/OFF procedure. Further research is required to incorporate this motion to evaluate any aeroelastic effects resulting from this actuation motion.

Vertical axis wind turbines (VAWTs) are popular design solutions for omnidirectional wind and limited installation area, particularly in urban areas, due to their advantages of low noise, low energy cost, and lower sensitivity to turbulence. They are often found in either of two distinct designs: the lift-driven Darrieus, with high aerodynamic power, and the drag-lift-driven Savonius, which excels in its start-up performance. The Darrieus- Savonius combined vertical axis wind turbine (hybrid VAWT) emerges from combining the advantages of Darrieus and Savonius. However, the hybrid VAWT has complex fluid dynamics, with multiple scales and interactions. Therefore, a comprehensive analysis and modeling of hybrid VAWTs can benefit from a multi-fidelity approach.
The choice of numerical methods, in either Eulerian or Lagrangian reference frames, holds paramount importance in simulating VAWTs, each method offering distinct ad- vantages and limitations. Through high-fidelity Eulerian unsteady Reynolds averaged Navier Stokes (URANS) simulations, insights into airflow patterns, and turbulence phe- nomena across varied operating conditions are gained. Conversely, Lagrangian models such as the vortex particle method (VPM) can enable efficient analyses of vortical struc- tures and wake interactions. While URANS simulation offers high-fidelity representa- tions of complex flow phenomena and allows for precise optimization of turbine design parameters, it demands significant computational resources and expertise. In contrast, VPM excels in capturing flow features efficiently but may struggle to accurately repre- sent boundary layer effects and near-wall flows. Consequently, the integration of both the Eulerian URANS and the Lagrangian VPM (hybrid method) is crucial for achieving comprehensive, reliable, and cost-effective simulations of VAWT performance. In this thesis, the hybrid VAWT is investigated using multi-fidelity numerical tools to estimate rotor/blade aerodynamics, computational efficiency and accuracy. Eulerian (U)RANS simulations in OpenFOAM and Lagrangian VPM are first applied to different types of VAWTs, followed by the application of the hybrid method in the hybrid VAWT case.
The goal of this thesis is also to investigate the flow features and power performance of the hybrid VAWT with multi-fidelity methods. In the context of the Eulerian reference frame, this thesis advances the knowledge of hybrid VAWT aerodynamics in several as- pects. The Darrieus and Savonius parts in a hybrid VAWT are modeled as uniform force fields to exclude the effects of structural and operational parameters on the power losses of the wind turbines. The results show that the hybrid configuration cannot show a sig- nificant power increase, and it is only beneficial for the startup performance. The vor- tex dynamics behind the hybrid VAWT are analyzed in different attachment angles and tip speed ratios. The blade-vortex interaction is characterized and correlated with the torque generation of the Darrieus blade. Results show that the Darrieus blade torque in- crease is dependent on the interaction with the shed vortex from the advanced Savonius blade.
In the context of Lagrangian reference frame, both Savonius and hybrid VAWT con- ix
x
figurations are employed to assess the computational efficiency and accuracy of the vor- tex particle method. In the case of Savonius rotor simulations using the vortex method, the Savonius is defined as a rotor with two trailing edges because it has no clear lead- ing/trailing edge like an airfoil, named double-trailing-edge-wake-modeling vortex par- ticle method (DTVPM). Results show that a maximum power coefficient is achieved at a tip speed ratio of approximately 0.8, consistent with experimental findings. Further- more, the process of trailing-edge vortex generation and detachment is effectively cap- tured. A comparative analysis between Eulerian URANS simulations and Lagrangian DTVPM reveals that DTVPM offers a more efficient simulation of Savonius rotors with- out the need for empirical parameters. Notably, DTVPM demonstrates remarkable com- putational speed, with simulations being approximately 20 to 104 times faster than par- allel URANS simulations over five revolutions. This significant reduction in computa- tional time underscores the potential of DTVPM to enhance existing engineering mod- els for wind energy applications. In the case of hybrid VAWT simulations, this thesis extends the application of VPM to hybrid VAWTs and introduces a viscous correction to improve simulation accuracy. By incorporating the airfoil polar, the proposed La- grangian DTVPM effectively predicts comparable force variations to Eulerian URANS simulations. Importantly, the computational efficiency of URANS and DTVPM for hy- brid VAWTs is compared, revealing that serial DTVPM simulations are approximately 20 times faster than parallel URANS simulations over ten revolutions. This notable increase in computational speed highlights the potential of DTVPM to provide efficient and ac- curate simulations for hybrid VAWTs, facilitating further advancements in wind energy technology.
In the context of the hybrid Eulerian-Lagrangian reference frame, this study aims to enhance accuracy and efficiency in analyzing complex flow phenomena. Through con- ducting various scenarios of hybrid VAWT using the hybrid method, this study concludes with the demonstration of the hybrid solver’s capability in simulating hybrid VAWT aero- dynamics. The final goal of this thesis is to ascertain an efficient model for hybrid VAWT and determine the limits of individual Eulerian and Lagrangian methods while consid- ering specific flow features of VAWTs. Overall, this study contributes to comprehensive insights into the correlation of blade-vortex interaction and torque variation. The mod- eling challenge in the hybrid VAWT simulation is studied and a hybrid model is suggested to understand the performance and flow features of hybrid VAWTs.@en

Developments in computational capabilities and the always increasing demand for higher performance of internal flow applications has meant that Computational Fluid Dynamics (CFD) has become an essential tool within the design process. A key point of interest is to couple the fluid solver with numerical optimisation techniques in order to obtain a more automated design process that can handle a vast amount of different design aspects. The open source CFD suite SU2 has emerged as an enabler for aerodynamic shape optimisation involving a large number of design variables, due to its efficient, accurate and flexible discrete adjoint solver. 
For the adjoint-based aerodynamic design optimisation of internal flow applications the deformation of the volumetric mesh has to be performed in an robust and efficient manner. Often small wall clearance gaps and periodic domains are encountered in internal flow domains, which could potentially lead to the deterioration of the mesh. Sliding boundary node methods can be applied in order to maintain the mesh quality in case of small wall clearance gaps. Additionally, periodic boundaries can be displaced in a periodic manner following the applied deformation in order to prevent low quality cells near the periodic interface. Therefore, it would be of interest to implement the sliding boundary node methods and periodic conditions in a Radial Basis Function (RBF) interpolation method, one of the most robust mesh deformation methods available. 
Additionally, the computational efficiency should be considered, since high computational times should be prevented for large and complex three-dimensional cases with a high number of design variables. 
The aim of this thesis project is therefore to develop a robust and computationally efficient mesh deformation method suitable within the discrete adjoint optimisation framework of SU2 for internal flow applications by means of developing an implementation of the RBF interpolation method including sliding boundary node algorithms, periodic boundary conditions and data reductions methods.
The sliding is achieved by replacing the interpolation condition for the sliding nodes with a planar slip condition. Or alternatively, by freely displacing the sliding nodes based on the known deformation and subsequently projecting the nodes back onto the boundary. The periodic displacement of the boundaries is ensured by making the distance function of the RBF periodic. The periodic nodes are then treated as internal nodes to allow them to move.
The developed RBF-SliDe tool is able to generate higher minimum mesh qualities compared with the regular RBF interpolation method. The sliding of the boundary nodes reduces the degree of skewing of the mesh elements in case of drastic deformations, resulting in a higher minimum mesh quality. Furthermore, the introduction of the periodic displacement prevents low quality skewed or compressed mesh elements, as the periodic boundaries move along with the deformation. 
The Aachen turbine stator blade is considered as a realistic three-dimensional test case. For this stator blade an optimised geometry was available, which was obtained with an adjoint-based aerodynamic optimisation performed with SU2. Therefore, the resulting minimum mesh quality is compared to the one obtained with the more conventional linear elasticity equation method as used in SU2. The minimum mesh quality obtained with the RBF-SliDe tool is nearly three times higher compared to the minimum mesh quality of the linear elasticity equations methods. This highlights the potential of the periodic sliding RBF interpolation method in terms of preserving the mesh quality.

Due to its immense potential to harvest larger amounts of energy from the wind at a higher capacity factor, offshore wind energy is the key to reach the goal of a more sustainable future. The wind energy research community has been focusing all its efforts to bring the new concept of floating offshore wind turbines (FOWT) into the market. By installing the turbine on top of a floating platform the restrictions of offshore turbines regarding the sea depth are lifted and numerous new sites now become available for the wind energy market.
Due to the motions that FOWT experience during their operation several questions arise regarding their aerodynamics, structural integrity and control efficiency. Since aerodynamic models are utilized in almost every field involved in the design of a wind farm, its particularly important to make sure that the current industrial aerodynamic codes, like BEM, are able to capture the unsteady aerodynamics of this complex system. High-fidelity CFD codes and experiments is the only way to truly assess the performance of BEM.
Surge motion is recognized by all authors as one of the most important motions imposed to FOWT. Thus, in the present thesis the impact of surge motion in the aerodynamics of FOWT was investigated by employing a high-fidelity blade-resolved CFD model in OpenFoam. After the extensive validation of the CFD model with the high-fidelity experiment of the UNAFLOW project, two test cases were examined. One with low surge frequency/amplitude which was employed to test whether momentum theory can accurately predict the induction of FOWT and one with an extreme surge frequency/amplitude which was used to test whether propeller and vortex ring states occur during the operation of FOWT - rendering BEM invalid.
In the first test case, Ferreira-Micallef induction computation model was utilized to extract the induction field from the CFD field data. After extensively comparing the induction computed by Ferreira-Micallef model and momentum theory it became apparent that due to the 2D nature of both models it was not possible to draw any significant conclusions about the accuracy of momentum theory. Specifically, due to the existence of 3D radial flow during the operation of FOWT Ferreira-Micallef model is not reliable and can not be used as a basis to compute the error of momentum theory. Having said that, in the mid-span region of the blade and when the FOWT is operating in the mild surge conditions, 3D flow was not present and a comparison of induction fields between momentum theory and Ferreira-Micallef model yielded very similar results. In the second extreme surge test case, two observations were made. First, even though thrust was acquiring negative values during the surge motion of the rotor, the stream-tube did not seem to experience any propeller state since it was expanding at all times. Second, in all the 3D iso-surface q-criterion scenes that were produced there was no large vortex ring visible in the wake, thus there was no indication that vortex ring state occurs for a FOWT surging under these harsh surge conditions.
The present thesis contributes in the better understanding of FOWT aerodynamics and provides clarity on the ambiguous research topic of whether propeller/vortex ring states occur during their operation. Furthermore, the high-fidelity blade-resolved CFD model which was developed can be used as a starting point for several future research efforts.

A popular class of methods for solving Fluid-Structure Interactions (FSI) problems computationally are the moving mesh deformation methods. These methods generally involve deforming the fluid domain so that it conforms to the structure as it undergoes changes. An efficient and robust moving mesh technique involves interpolation of displacements using Radial Basis Functions (RBFs). But it is noticed that large deformations in the computational domain restrict the applicability of RBF methods due to resulting poor mesh quality. This further worsens the accuracy of the simulation by introducing additional numerical errors. The standard practice to overcome this deficiency has been to introduce the computationally expensive step of re-meshing the entire domain.
The current thesis aims to tackle this issue of poor mesh quality caused due to large deformations of the structure by introducing localized corrections in regions of poor quality. But deforming the mesh, then calculating the quality and then applying corrections would become an expensive operation of its own. Therefore, firstly a methodology is devised to predict the state of the mesh after the deformation apriori to performing the deformation step. It is found that the gradient information from the deformation functions can be utilized to accurately predict the various algebraic mesh quality metrics. The algorithm is tested on 1D and 2D meshes and grid convergence studies are performed to validate the predicted quality with the actual quality post-deformation. The theoretical framework for the 3D prediction algorithm is also laid out.
Once the mesh quality is predicted, based on a user-defined threshold, if the quality is deemed to be poor in a certain region then localized corrections are implemented to improve the mesh quality. These corrections are introduced in the form of enrichment functions on additional control points within the domain. The gradient of the deformation function is constrained using these functions such that a good quality and smooth mesh deformation is obtained post-deformation. Multiple enrichment functions are initially tested in 1D in a global sense. The findings from this analysis are then carried forward to analyze where to introduce these additional control points for local corrections and the nature of the correction itself.
Finally, the methodology is tested on 2D unstructured grids. It is found that the imposition of gradients is more complicated in the 2D sense as global information can not be used directly as in the 1D case. Therefore, a parametric study is performed to analyze if a logical conclusion can be made with regard to the imposition of the gradients. It is found that such a logical trend is not clear, therefore the strategy is modified such that the interpolation coefficients are imposed explicitly instead based on data available from eigen decomposition of the deformation gradients. It is found that this strategy performs admirably when compared to the results obtained from the standard RBF model. In conclusion, the method significantly improves the robustness of the mesh deformation process by ensuring that large deformations can be accommodated in the domain without huge computational costs.

In the evolving landscape of nuclear energy, ensuring the safety and efficiency of nuclear reactors remains paramount, particularly with the increasing demands for energy and a concurrent rise in global temperatures. A significant aspect of nuclear safety involves maintaining the integrity of the fuel rods, which are susceptible to Turbulence Induced Vibrations (TIV) resulting from axial flows of the coolant liquid. TIV can instigate severe repercussions including structural damage such as fatigue, wear, and stress corrosion cracking, posing substantial threats to reactor safety. Despite the historical attention this phenomenon has garnered since the 1950s, conventional semi-empirical methods offer limited predictive accuracy and do not facilitate extrapolations for multi-rod scenarios effectively. 
Recent developments have turned to Fluid-Structure Interaction (FSI) simulations as a powerful tool to study fuel rods’ behavior under TIV effects, capitalizing on the increase of computational power available today. While Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) offer more accurate predictions, their computational demands make them unsuitable for complex FSI simulations. This has led to a preference for Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, despite them underpredicting the displacement amplitudes of the vibrations. 
Evolving from this shortfall, the current study directs its focus on a recently developed Anisotropic Pressure Fluctuation Model (AniPFM). This model generates a synthetic velocity fluctuations field, which is used to solve for the pressure fluctuations. The use of this model together with URANS poses as a possible way to inexpensively simulate the excitation mechanisms of TIV of fuel rods. While previous research has highlighted the potential of this model, it is important to note the considerable level of uncertainty still associated with it. Additionally, there are parameters, definitions and constants whose impacts on the model are not yet fully understood or even explored. This calls for a comprehensive research to fine-tune the model, optimize its performance and further validate it. This is precisely the goal of this study, carried out through the analysis of two pure flow and two FSI cases. Hypotheses were formulated and tested in pure flow scenarios before being further validated in FSI cases. Key advancements were made by optimizing the time correlation method used on the generated velocity fluctuations, which significantly reduced the model’s uncertainty. This method was then calibrated using DNS data of turbulent channel flow. Further calibration was undertaken, this time in the parameters part of the modelling of the turbulent kinetic energy spectrum, to address the overprediction of pressure fluctuations near the wall observed in the baseline model. Moreover, the turbulent annular flow was used as the second flow only case, providing more complexity compared to channel flow, by adding curvature, as well as another opportunity to test the hypothesis made. 
Furthermore, the hypotheses underwent additional validation via FSI simulations, through a brass beam in turbulent axial flow, showing a substantial decrease in the average difference from the experimental data to 19% from a previous 68%, over the range of inflow velocities considered. Notably, the calibrated AniPFM surpassed LES in accuracy while requiring fewer computational resources. The results obtained are promising, but further validation is needed. This thesis also outlines and lays the foundation for further validation work, through the setup and initial simulations of a flexible cantilever rod in turbulent axial water flow.

The Lattice Boltzmann Method (LBM) is an appealing framework to apply to unsteady, incompressible, low Reynolds number flow due to its simplicity and potential for massive parallelisation. The immersed boundary method is often used in conjunction with the LBM to simulate flow around curved, moving boundaries in the interior of the fluid domain. In the immersed boundary method, the boundary is imposed on a non-conforming grid by applying an external forcing at the boundary. The distribution of this external forcing is calculated based on interpolation of nearby fluid nodes, and the force field is then distributed over a number of nearby fluid nodes.

The immersed interface method is an approach similar to the immersed boundary method, but imposes this applied force field directly on the solution field through jump conditions. It has yet to be correctly applied to the LBM framework. In this thesis, a proposal of an immersed interface method in the LBM is made, and its implementation is validated and compared against the immersed boundary method. To aid in this, a solver capable of solving fluid-structure interaction is developed.

It is shown that the immersed interface method in the LBM has significant benefits compared to the immersed boundary method, in particular with regards to reducing numerical oscillations in the spatial variation of the boundary force distribution, as well appearing to yield a slight increase in overall accuracy at the same level of grid refinement.

Due to the complicated design of nuclear reactors, many complex phenomena can cause points of failure. In the case of nuclear fuel rods, the axial flow that cools the rods can induce vibrations due to the turbulent nature of the flow. Turbulence-induced pressure fluctuations create small but significant vibration amplitudes, which in turn can cause structural effects such as material fatigue and fretting wear. For this reason, turbulence-induced vibrations have been the subject of many studies, with a recent focus on the development of computational methods, so called Fluid-Structure Interaction (FSI) simulations. While scale-resolving methods can predict pressure fluctuations directly, they are typically too expensive for industrial nuclear applications in FSI. Instead, an Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach coupled with a pressure fluctuation model can be used, to reduce the computational cost. While showing promising results, this approach generally underestimated the vibration amplitudes.

For this reason, an improved pressure fluctuation model, called AniPFM (Anisotropic Pressure Fluctuation Model), was developed. It models velocity fluctuations based on existing methods for synthetic turbulence. In turn, these velocity fluctuations are used to obtain the pressure fluctuations. AniPFM improves the prediction of the pressure fluctuations in three ways. First, whereas previous iterations could only represent the turbulence as isotropic, in the current model anisotropic Reynolds stresses can be embodied. Second, only the scales that can be resolved on the grid are represented by the velocity fluctuations, causing a more realistic distribution of energy along the different wavelengths. Finally, time correlation is introduced based on the transport and decorrelation of turbulence.

From simulating decaying homogeneous isotropic turbulence, it was found that this time correlation method gives a significant improvement over previous methods. From turbulent channel flow simulations, the results show that for anisotropic turbulence, the pressure fluctuations are overestimated, but they are still within a reasonable range of 10% compared to high-resolution data. AniPFM doubles the cost of simulation, compared to a URANS simulation. Even though that is a steep increase, the cost is still much lower than scale-resolving methods.

Finally, the fluid-structure interaction of a brass beam in turbulent water is simulated, which showed the ability of the AniPFM to predict turbulence-induced vibrations. The AniPFM showed errors w.r.t. the replicated experiment that were in a similar range as LES calculations, while using less computational resources. The AniPFM simulations gave an error range of 15-60% w.r.t. experimental data over the full range of simulated flow velocities, whereas a previously used pressure fluctuation model underestimated the RMS amplitude by a factor of six. While further validation is ongoing, the AniPFM has demonstrated its potential for cheaper simulations of turbulence-induced vibrations in industrial nuclear applications.

In this thesis report, a new radial basis function (RBF) interpolation mesh deformation method is investigated that is applicable on periodic domains that contain small clearance gaps. In the field of fluid structure interaction, there is a strong interaction between a structure and the fluid in which this structure is immersed. When performing simulations of this interaction, the fluid mesh has to adapt to the changing domain which occurs due to the structure deforming. The adaption of the fluid mesh can be done with an RBF interpolation method. Although this is considered to be one of the most robust methods available, there are specific cases in which the method can still yield some poor quality deformed meshes. This is for example the case when small clearance gaps are present such as the region between a blade and the shroud in turbomachinery. Then it becomes beneficial to allow boundary nodes to slide to improve the mesh quality.
In turbomachinery, periodic domains are also often encountered, for example in a gas turbine. Here, the same blade is constantly repeated, allowing to use one single blade in computations which will have a domain with periodic boundaries. In general, the larger the number of blades, the smaller the distance between the blade and the periodic mesh boundary will be. Hence, also here small clearance gaps are present. As the blade might undergo a large deformation, it is possible for the blade to even intersect the mesh boundary if this boundary is kept fixed, which would lead to a degenerate mesh.
The aim of this thesis is to avoid this scenario by making adaptions to the RBF interpolation method that allow the periodic boundaries of the mesh to be displaced with the deforming blade. Evidently, this has to be done in a periodic manner, keeping both periodic boundaries equal. This will yield a higher mesh quality of the deformed mesh as it will reduce the skewness and avoid the occurrence of degenerate cells. Two different methods for periodic boundary displacement are investigated in this report. Both methods work for both translational and rotational periodic boundaries and several two-dimensional (2D) and three-dimensional (3D) meshes are used as test cases.
The first method is the periodic distance method which is based on making the distance between the boundary nodes periodic. This method is very similar to the regular RBF method and only requires a regular Euclidean distance function to be replaced by a periodic distance function. This means that the computational cost is also similar to an interpolation matrix of size N_m x N_m. This method shows good results for both 2D and 3D meshes and both types of periodic boundaries. Apart from giving the periodic boundaries an identical displacement, it also allows for a smooth transition from one mesh to another.
The second method is based on equal displacement. Hence, the RBF interpolation matrix is adapted to include conditions imposing equal displacement of the periodic boundary nodes. These extra conditions yield a larger interpolation matrix of size N_m+N_p x N_m+N_p. This means the computational cost for this method is higher than for the periodic distance or the regular RBF method. This method also shows good results in terms of deformed mesh quality but does not feature the same smoothness over the interface between two meshes and creates small kinks at the interface.
Even though RBF interpolation can be relatively low in computational cost compared to other mesh deformation methods that require mesh connectivity information, it still becomes computationally expensive for large 3D problems with a high number of nodes. Therefore, a greedy algorithm is also used for larger problems which allows for a reduced amount of nodes selected to perform the interpolation.
In conclusion, the periodic displacement method used with RBF interpolation shows great potential for different types of problem sizes and periodic boundaries and can potentially make the simulation process of periodic problems much more efficient.

The advent of global warming has brought an increased interest in non-conventional sources of energy, one of which is nuclear energy. Threatening the almost year-round functioning of nuclear power plants are Flow-Induced Vibrations (FIV). One such mechanism, Vortex-Induced Vibration (VIV), holds importance in areas of cross flow in nuclear power plants. To make fail-safe
designs, computational analysis in the domain of Fluid Structure Interactions (FSI) has been increasing over the past two decades. The thesis work aims to add to the body of knowledge by making predictions for an in-line two-cylinder configuration, set up as part of a benchmark proposed by the Organization for Economic Co-operation and Development (OECD), using the commercial code Simcenter STARCCM+ (v2020.3.1).

The main objective of this study is to test the efficacy of the URANS framework in predicting VIV which is strongly correlated with the objective of the OECD to propose recommendations for the Best Practice Guidelines. To do so, it is desired to shortlist the most appropriate turbulence model for the final application and point out gaps in the prediction of the same. The thesis work is thus carried out in three phases: code validation, turbulence model selection and final application. Key results from this study reveal the ‘Standard K-Epsilon
Low Re: Cubic’ model to be the most apt for the final application. Furthermore, gaps are also identified in the application of URANS to predict VIV resonance conditions, the primary of which is the overprediction of the vortex shedding frequency.

Military fighter pilots spend most of their working hours performing training missions, simultaneously carried out by multiple pilots with different qualifications. Scheduling these missions is typically done manually, which is a time-consuming task resulting in far from optimal schedules. This study proposes a model that schedules three types of training missions and assigns pilots to these missions for a full-year training program. The proposed methodology consists of a two-stage Mixed Integer Program. Both stages are solved with a Brand and Bound algorithm. Additionally, the second stage implements a Rolling Horizon approach. A case study considering two Royal Netherlands Air Force fighter squadrons is used to test the model. The results show an increase of more than 15% for the average amount of training completed by all pilots, compared to the current scheduling process. The solutions are obtained in a few minutes, demonstrating the suitability to use the model in practice and reduce the time spent on constructing schedules.

The constant impingement of sand particles on the wetted surfaces of a centrifugal dredge pump causes the performance of the pump to deteriorate. Therefore, components or complete pumps have to be replaced after some time. The use of numerical models to estimate the erosion wear in centrifugal dredge pumps can give insight into the principal erosion zones and the magnitude of the erosion in those zones. This information can be used to increase the erosion resistance of pump components. In addition, instructions for the maintenance of the pump can be composed based on the expected erosion wear. In that way, the efficiency of dredge processes can be increased considerably. This Master's Thesis project is carried out by the author during a 12-month period at Damen Dredging Equipment in Nijkerk, The Netherlands. The focus of the project is on developing, validating and demonstrating a numerical model capable of estimating the erosion wear due to slurry flow on the impeller blades of a centrifugal dredge pump by using Computational Fluid Dynamics (CFD). Within the CFD framework, it is found that the appropriate model for calculating the flow of the slurry (water with sand particles) is the Eulerian-Lagrangian method. The numerical computations are performed using the commercial software ANSYS Fluent. For the turbulence of the water, the k-ω SST turbulence model is used. The collisions of the particles with the wall are modeled using the Grant-Tabakoff model, whereas the linear soft sphere model is used for the inter-particle collisions. Using the aforementioned combination of models several studies are conducted, starting with a validation study using a submerged impinging jet benchmark. From that study, it is found that the fluid and particle velocity fields can be calculated with reasonable accuracy. For the same type of problem, the numerical erosion profile is compared to experimental values. It appeared that there is a considerable dependency of the erosion profile on the material hardness. In addition, the result of the four-way coupled method is influenced to a large extent by the values of the collision model parameters. By comparing the resulting erosion profile from the two-way coupled model with the experimental erosion, a good qualitative agreement is found. However, the magnitude of the erosion is overpredicted by this model. In addition, a verification and validation study was performed for the impeller. For the latter, new experiments were conducted in a facility that is available within the company Damen Dredging Equipment. By measuring a number of points before and after a 56 hour experiment, the resulting erosion could be determined. The four-way coupled computation for the impeller could not be fully completed within this project. However, the two-way coupled numerical model showed good correspondence with the experimental results. The currently available model is capable of predicting the maximum erosion zones on the impeller blades. This information could for example be used to improve the design of the impeller or to get a first order estimation of the lifetime of an impeller. The applicability of the numerical model that is developed during the current project can be extended by for instance including the pump volute.

A key challenge in the Airborne Wind Energy industry is the minimisation of the time and energy required for the transfer and return phases inherent to the power cycles of ground-generator-based kite power systems. This thesis contributes to solving this problem by exploring the use of bleed air spoilers during the return phase in terms of their potential to improve the average cycle performance. To this end, a mathematical model is first introduced to evaluate the individual phases of the characteristic power pumping cycles in a general sense. This model is further used to demonstrate that a given kite configuration with the capacity to selectively lower its glide slope and/or resultant aerodynamic loading could improve the net power output of the system by around 30%. A CFD-based simulation environment for investigations of the effects induced by bleed air spoilers is developed using Simcenter STAR-CCM+. To establish the reliability of the setup, a first study revolves around its validation with experimental data from independent wind tunnel tests on bleed air spoilers effects on ram-air parachutes. The second CFD study is based on a standard kite geometry employed by SkySails Power GmbH. Two parameter studies testing the influence of spoiler size and chordwise position are carried out to investigate how bleed air spoilers are best used to reach the desired aerodynamic effects. The results show that bleed-air spoilers should ideally be applied in the vicinity of the suction peak and that relatively small spoiler geometries can already entail a major effect on the resultant flow field. It is predicted that the evaluated configurations would improve the system's overall performance by 9 to 18.3%, mostly due to substantial reductions in the glide ratio.

The purpose of this report is to design and iterate all the subsystems of a drone for Mars exploration. Based on a design option analysis and tradeoff, the vehicle is designed to be a VTOL tilt rotor. The mission need statement is: Enable largescale targeted exploration of the atmosphere and surface of currently inaccessible areas of Mars. This mission statement resulted in two expedition types: collect and return expeditions in which soil samples are collected, and remote sensing expeditions where visual mapping, height mapping, gas analyzing and dust composition data is collected and. The project objective statement is: Design a semiautonomous unpiloted atmospheric vehicle that can assist human Martian exploration by observing remote areas and collecting atmosphere and soil samples from difficulttoreach places...