Implicit/explicit transition for numerical simulation of Fluid-Structure Interaction problems treated by immersed boundary techniques

In many industrial sectors, rapid transient phenomena are involved in accident scenarios. An example in the nuclear industry is the Loss of Primary Coolant Accident, in which an expansion wave propagates through the primary circuit of a Pressurized Water Reactor, potentially vaporizing the primary fluid and causing structural damage. Nowadays, the simulation of these fast transient phenomena relies mainly on "explicit" time integration algorithms, as they enable robust and efficient treatment of these problems, which are generally highly non-linear. Unfortunately, because of the stability constraints imposed on time steps, these approaches struggle to calculate steady-state regimes. Faced with this difficulty, in many cases, the kinematic quantities and internal stresses of the steady state of the system under consideration at the time of occurrence of the simulated transient phenomenon are neglected.

Furthermore, the applications in question involve solid structures interacting with the fluid, undergoing large-scale deformation and possibly fragmenting. A immersed boundary technique known as MBM (Mediating Body Method [1]) recently developed at the CEA enables structures with complex geometries and/or undergoing large deformations to be processed efficiently and robustly. However, this coupling between fluid and solid structure has only been considered in the context of "fast" transient phenomena treated by "explicit" time integrators.

The final objective of the proposed thesis is to carry out a nominal regime calculation followed by a transient calculation in a context of fluid/immersed-structure interaction. The transient phase of the calculation is necessarily based on "explicit" time integration and involves the MBM fluid/structure interaction technique. In order to minimize numerical disturbances during the transition between nominal and transient regimes, the calculation of the nominal regime should be based on the same numerical model as the transient calculation, and therefore also rely on an adaptation of the MBM method.

Recent work defined an efficient and robust strategy for calculating steady states for compressible flows, based on "implicit" time integration. However, although generic, this approach has so far only been tested in the case of perfect gases, and in the absence of viscosity.

On the basis of this initial work, the main technical challenges of this thesis are 1) to validate and possibly adapt the methodology for more complex fluids (in particular water), 2) to introduce and adapt the MBM method for fluid-structure interaction in this steady-state calculation strategy, 3) to introduce fluid viscosity, in particular within the framework of the MBM method initially developed for non-viscous fluids. At the end of this work, implicit/explicit transition demonstration calculations with fluid-structure interaction will be implemented and analyzed.

An internship can be arranged in preparation for thesis work, depending on the candidate's wishes.

[1] Jamond, O., & Beccantini, A. (2019). An embedded boundary method for an inviscid compressible flow coupled to deformable thin structures: The mediating body method. International Journal for Numerical Methods in Engineering, 119(5), 305-333.

In situ 3D visualization and modeling of grain growth during solidification of 316L steel in welding and additive manufacturing processes

CEA is currently carrying out R&D studies to assess the potential of Additive Manufacturing (AM) processes using wire deposition (WAAM and WLAM) for 316L steel, a material used in the manufacture of a large number of components. These processes are similar to the welding techniques currently used in the manufacture and repair of parts for the nuclear industry. Microstructures with a strong crystallographic texture are often obtained after welding or additive manufacturing, leading to highly anisotropic mechanical behaviors, and the prediction of these microstructures is also a key element in ensuring the reliability of non-destructive testing of parts manufactured in this way.

The aim of the thesis, which will be based on a coupled experimental/simulation approach, is to gain a better understanding of the main physical phenomena involved in solidification, in particular grain growth.

To this end, an original approach to characterizing these phenomena will be conducted on the basis of an innovative instrumented test, with the aim of obtaining a high-resolution quasi-3D view of the molten zone during solidification. The results of the experimental approach will enrich the physical models of solidification, already implemented in a 3D CA-FE (Cellular Automaton-Finite Element) model, combining a Cellular Automata (CA) approach and thermal or multiphysics modeling (FE) of the molten bath, to simulate the solidification microstructures resulting from additive manufacturing and welding processes.

Head-on Reflections of High-Speed Combustion Waves: Experimental and Numerical Investigation and Mitigation Measures.

This thesis focuses on the analysis of hydrogen safety in industries, particularly in cases of accidents where hydrogen is released or generated, such as in nuclear power plants. The interest in hydrogen safety has increased with the use of fuel cells for mobility. In compartmentalized buildings, flammable atmospheres can form, leading to explosions that compromise safety. Flame dynamics are influenced by boundary conditions, especially confined geometries that accelerate the flames. This phenomenon can result in a deflagration-to-detonation transition, causing significant damage to structures through shock waves and combustion waves. Research shows that certain geometric configurations and hydrogen mixtures produce higher pressures, even with low hydrogen concentrations. Three key questions are raised: the influence of geometry on pressure and impulse, the optimal hydrogen concentration, and the possibility of mitigating these effects with sound-absorbing coatings. To answer these questions, experiments and simulations will be conducted to understand and model these phenomena, providing practical tools for safety engineers.

Thermohydraulic modelisation of a steam generator and chemical species propagation

Steam generators are essential components of nuclear reactors whose main function is heat exchange. The chemical species present in steam generators are the cause of many parasitic phenomena (clogging, fouling, sludge deposition, etc.). Numerical simulation of species transport, taking into account the migration of chemical species and exchanges between species, both intra- and inter-phase, will allow a better understanding and better management of these problems. Numerical resolution of species transport systems presents real difficulties, in particular the management of the appearance and total disappearance of certain species, high void rates, as well as rapidly excessive calculation times.

While relying on the new code for nuclear components developed at STMF, the thesis will address the following three main scientific issues:
• Upstream, the analysis of numerical methods allowing in particular the management of evanescence, as mentioned above, and thermo-hydraulic modeling at high void rates. For this, we will rely on the PolyMAC and PolyVEF numerical schemes, already implemented in the component code.
• The physical modeling of a steam generator in the new component code, via the addition (in C++) of correlations specific to steam generators, the completion of the state laws already available, etc..
• The determination of the major chemical species to be transported, in order to be able to take into account both thermo-hydraulics and chemistry. The algorithmic coupling between thermo-hydraulics and chemistry, taking into account feedback, being the long-term objective.

While benefiting from the existing parallelization of the component code, the thermo-hydraulic and chemical modeling will be done taking into account the constraints on computation times.

Improving the predictivity of large eddy simulations using machine learning guided by high-fidelity simulations

This thesis aims to explore the application of machine learning techniques to improve turbulence modeling and numerical simulations in fluid mechanics. More specifically, we are interested in the application of artificial neural networks (ANNs) for large eddy simulation. The latter is a modeling approach that focuses on the direct resolution of large turbulent structures, while modeling small scales by a subgrid-scale model. It requires a certain ratio of total kinetic energy to be resolved. However, this ratio may be difficult to achieve for industrial simulations due to the high computational cost, leading to under-resolved simulations. We aim to improve the latter by focusing work along two main axes: 1) Using ANNs to build generic sub-mesh models that outperform analytical models and compensate for coarse spatial discretization; 2) Training ANNs to learn wall models. One of the main challenges is the ability of the new models to generalize correctly in configurations different from those used during training. Thus, taking into account the different sources and quantification of uncertainties plays a vital role in improving the reliability and robustness of machine-learned models.

Numerical simulation of the impact between immersed structures in a compressible liquid using immersed boundary type approaches.

Many industrial systems involve structures immersed in dense fluids. Examples include the submarine industry, or, more specifically, certain 4th generation nuclear reactors using coolant fluids such as sodium or salt mixtures. The effect of the interaction of the surrounding fluid on the contact forces between structures is a phenomenon of primary importance, particularly during accidental transient scenarios that can generate large displacements of structures whose residual integrity must be demonstrated for safety purposes.

In the context of this thesis, we are particularly interested in modeling the rapid impact of a structural fragment immersed in a fluid against a wall, resulting, for example, from an explosive phenomenon in a nuclear reactor vessel cooled by sodium. In this context, the sodium, modeled as a compressible fluid, is treated numerically using a volume-finite approach. The reactor's internal structures are treated using a finite-element approach. In order to deal with large structural displacements and possible fracturing, “immersed boundary” techniques are used for fluid-structure interaction.

The aim of this thesis is to define an innovative numerical method to better simulate the fluid film between two structures that come into contact in this context. Initially, we will focus on identifying the physical characteristics of the flow at the level of the fluid film (compressibility, viscosity, etc.) that have the greatest influence on the kinematics of the structures. Secondly, the main challenge of this thesis will be to improve current numerical methods in order to represent the flow characteristics of the fluid film as accurately as possible.

The proposed thesis will be carried out at CEA Saclay, in close collaboration with the EM2C laboratory at CentraleSupélec, within the environment of the Université Paris-Saclay. The PhD student will be immersed in a team with recognized expertise in transient simulations of fluid-structure interaction.

Monte Carlo methods for sensitivity to geometry parameters in reactor physics

The Monte Carlo method is considered to be the most accurate approach for simulating neutron transport in a reactor core, since it requires no or very few approximations and can easily handle complex geometric shapes (no discretisation is involved). A particular challenge for Monte Carlo simulation in reactor physics applications is to calculate the impact of a small model change: formally, this involves calculating the derivative of an observable with respect to a given parameter. In a Monte-Carlo code, the statistical uncertainty is considerably amplified when calculating a difference between similar values. Consequently, several Monte Carlo techniques have been developed to estimate perturbations directly. However, the question of calculating perturbations induced by a change in reactor geometry remains fundamentally an open problem. The aim of this thesis is to investigate the advantages and shortcomings of existing geometric perturbation methods and to propose new ways of calculating the derivatives of reactor parameters with respect to changes in its geometry. The challenge is twofold. Firstly, it will be necessary to design algorithms that can efficiently calculate the geometric perturbation itself. Secondly, the proposed approaches will have to be adapted to high-performance computing environments.

Influence of delayed neutron precursors losses resulting from fission gas evacuation on molten salt reactors dynamics

Over the past twenty years, molten salt reactors (MSRs) have been the focus of renewed interest in the international nuclear community (national programs, start-ups, including one from the CEA). Modern MSR concepts feature a system for evacuating fission gases, which accumulate in the expansion tank. Some of these gases will consist of radionuclides that are delayed neutron precursors, which will therefore be lost for the fission chain reaction. This should further reduce the effective fraction of delayed neutrons in these reactors, already reduced by the circulation of the fuel salt outside the critical zone. The aim of this thesis is to assess the extent of this reduction, and its influence on reactor dynamics.
Such an assessment may involve numerical simulations that take into account 1) a differentiation of delayed neutron precursor groups into “liquid phase groups” and “gas phase groups”, and 2) two-phase flow models (where each type of group joins its corresponding phase). In order to differentiate the groups, we need to evaluate the “liquid” and “gas” fractions for each of them, based for example on the branching ratios of the nuclear evaluations and knowledge of the chemical elements joining each of the phases. Once this has been done, simulations can be carried out with the CATHARE “system” code (already able to use two-phase models) and the TRUST-NK “core” code (whose two-phase calculation functions may require further development) to assess the influence of precursor loss on reactor dynamics.

Development of multiscale and multiview correlation techniques for monitoring large-scale dynamic tests

Experimental data obtained on large-scale specimens plays an important role in the study of structural integrity. Detailed interpretations of these tests require extensive instrumentation of the models. In addition to conventional data acquisition systems, digital image correlation (DIC) techniques can be used to measure displacement fields and extract quantities of interest (e.g. damage field). The aim of this thesis is to develop a multi-view, multi-scale digital image correlation (DI2M) technique for monitoring large-scale dynamic tests. We will focus on the behavior of reinforced concrete structures subjected to dynamic loading. The finite element model updating (FEMU) technique will be used to identify non-linear phenomena in the process zone around cracks. FEMU will be coupled with DI2M analyses, which can also be used to measure boundary conditions. The use of DI techniques to calculate acceleration fields will also be studied. A numerical framework will be proposed for performing modal analysis based on calculated fields. Ultimately, these tools could be integrated into a test/calculation dialogue procedure, providing precise information on the mechanical properties of structural elements and their evolution (e.g. damage) induced by seismic loading.

Design and optimization of an innovative breeding blanket concept for a compact high heat flux nuclear fusion reactor

Skills:
Technical: heat transfer, structural mechanics, hydraulics, materials, numerical simulation
Non-technical: writing, interpersonal skills, English

Prerequisites: this thesis will be preceded by a 6-month internship. Contact the supervisor for more details about the topic.

Context:
This PhD focuses on the design and optimization of an innovative breeding blanket for compact nuclear fusion reactors. Nuclear fusion offers a promising solution to produce clean and sustainable energy. However, it requires the continuous production of tritium, a rare isotope, through breeding blankets surrounding the plasma. These blankets must also extract the generated heat. In compact reactors, technical constraints are increased due to extremely high heat fluxes and severe thermal and neutron conditions.

The PhD will take place within the Design, Calculations, and Realizations Office at CEA Saclay, a recognized player in the development of breeding blankets at the European level. This office has designed several concepts, such as HCLL (Helium Cooled Lithium Lead) and BCMS (Breeder and Coolant Molten Salt), two types of blankets based on helium or molten salt cooling systems.

PhD description:
The research program will take place over three years. The first year will focus on studying existing blankets, identifying the constraints of compact reactors, selecting appropriate materials and heat transfer fluids, and developing a preliminary design of the blanket. The following years will be dedicated to multiphysics modelling (thermal, mechanical, neutron), followed by iterative optimization of the concept to improve its performance.

Perspectives:
The results of this PhD will have a significant impact on the development of compact fusion reactors by ensuring tritium production and structural integrity. This work could also open new avenues for future research on even more advanced breeding blankets, contributing to the growth of sustainable and commercially viable fusion energy.

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