HPC two-phase simulations with lattice Boltzmann methods and adaptative mesh refinement
CEA/STMF develops computational fluid dynamics (CFD) codes in thermohydraulics that aim to quantify mass and energy transfers in nuclear cycle systems such as reactors and management devices of radioactive wastes. This thesis focuses on Lattice Boltzmann Methods (LBM) adapted to Adaptive Mesh Refinement (AMR) inside a generic computing environment based on Kokkos and executable on multi-GPU supercomputers. The proposed work consists in developing LB methods in the Kalypsso-lbm code to simulate coupled partial differential equations (PDEs) modelling incompressible two-phase and multi-component flows such as those encountered in downstream cycle devices. Once the developments have been completed, they will be validated with reference solutions. They will allow a comparison of various interpolation methods between blocks of different sizes in the AMR mesh. A discussion will be held on the refinement and de-refinement criteria that will be generalized for these new PDEs. Finally, benchamrks of performance will quantify the contribution of AMR for 3D simulations when the reference simulation is performed on a static and uniform mesh. This work will use supercomputers which are already operational (e.g., Topaze-A100 from CEA-CCRT), as well as the future exascale supercomputer Alice Recoque depending on the progress of its installation.
Modeling the impact of defects in Steel–Concrete Structures. Identification of critical defects through metamodeling and optimization algorithms
To meet growing constructability challenges, steel–concrete (SC) structures are emerging as a promising alternative to conventional reinforced concrete structures. These elements are composed of infill concrete, two external steel plates, and steel shear studs that ensure composite action. While such structures present a clear interest due to their overall mechanical behavior, the presence of the steel plates prevents visual inspection of the concrete casting quality. It is therefore essential to characterize the impact of possible defects. This is the context of the proposed PhD research. Building upon recent results obtained in the laboratory, the goal is to develop a numerical framework to account for defects in steel–concrete structures. The thesis will be structured in several stages: validation of a modeling strategy for the mechanical behavior of defect-free SC structures, introduction of defects in the simulations and assessment of the applicability of the numerical approach, development of a metamodel and sensitivity analysis, and identification of critical defect configurations through optimization algorithms. One of the operational objectives of this doctoral work is to provide a tool capable of identifying critical defect configurations (size, position, and number) with respect to a given target quantity of interest (such as loss of strength or reduction in average stiffness). The research will therefore rely on the use and further development of state-of-the-art numerical tools in the fields of finite element modeling, optimization techniques, sensitivity analysis, and metamodeling. The thesis will be carried out within a rich collaborative environment, notably in partnership with EDF.
Effects of structural heterogeneities on air flow through reinforced concrete walls
The containment building represents the third barrier to confinement in nuclear power plants. Its role is to protect the environment in the event of a hypothetical accident by limiting releases to the outside. Its function is therefore closely linked to its tightness. Traditionally, the estimation of the leakage rate is based on a sound knowledge of transfer properties (such as permeability), combined with a chained (thermo-)hydro-mechanical simulation approach. While the mechanical behavior of the structure is now broadly well understood, progress is still needed in the comprehension and quantification of fluid flow. This is particularly true in the presence of heterogeneities (cracks, honeycombs, construction joints, reinforcements, cables, etc.), which represent situations that can locally disturb permeability. This is the context of the present PhD topic.
The work will consist, through a methodology combining experimental testing and numerical simulation, in improving the representation of fluid flow by explicitly accounting for the impact of heterogeneities. An initial analysis will define an experimental plan, which will then be carried out. The results will be analyzed in order to empirically characterize the influence of each type of heterogeneity tested on transfer properties. A simulation approach, exploiting the experimental findings, will then be developed using finite element and discrete methods. Finally, the applicability of the methodology to a real-scale structure will be assessed, while explicitly accounting for uncertainties regarding the presence and impact of such heterogeneities (probabilistic approach).The PhD will therefore rely on state-of-the-art experimental and numerical tools and methods, and will be conducted in a rich collaborative context (CEA, ASNR, EDF).
Numerical Simulation of Fluid–Structure Interactions with Contact under Flow Using a Penalized Direct Forcing Method
This PhD work is part of the study of the dynamics of fuel assemblies subjected to axial flow and external mechanical excitation, particularly of seismic type. The objective is to develop an innovative numerical approach capable of accurately predicting the three-dimensional dynamic response of one or several assemblies, while accounting for the coupled effects between the fluid flow and mechanical loads. This problem is particularly complex due to the need to consider large displacements, potential contacts between structures, and strong interactions with the surrounding fluid.