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.

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 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.