Seismic analysis of the soil-foundation interface: physical and numerical modelling of global tilting and local detachment
Rocking foundations offer a potential mechanism for improving seismic performance by allowing controlled uplift and settlement, but uncertainties in soil-foundation interactions limit their widespread use. Current models require complex numerical simulations, which lack accurate representation of the soil-foundation interface.
The main objective of this thesis is to model the transition from local effects (friction, uplift) to the global response of the structure (rocking, sliding, and settlement) under seismic loads, using a combined experimental and numerical approach. Hence, ensure reliable numerical modeling of rocking structures. Key goals include:
• Investigating sensitivity of physical parameters in seismic response of rocking soil-structure systems using machine learning and numerical analysis.
• Developing and conducting both monotonic and dynamic experimental tests to measure the soil-foundation-structure responses in rocking condition.
• Implementing numerical simulations to account for local interaction effects and validate results with experimental results.
Finally, this research aims to propose a reliable experimental and numerical framework for enhancing seismic resilience in engineering design. This thesis will provide the student with practical engineering, along with expertise in laboratory tests and numerical modeling. The results will be published in international and national journals and presented at conferences, advancing research in the soil and structure dynamics field.
Validation of a Model-Free Data Driven Identification approach for ductile fracture behavior modeling
This research proposes a shift from traditional constitutive modeling to a Data-Driven Computational Mechanics (DDCM) framework which has been recently introduced [1]. Instead of relying on complex constitutive equations, this approach utilizes a database of strain-stress states to model material behavior. The algorithm minimizes the distance between calculated mechanical states and database entries, ensuring compliance with equilibrium and compatibility conditions. This new paradigm aims to overcome the uncertainties and empirical challenges associated with conventional methods.
As a corollary tool for simulations DDCM, Data-Driven Identification (DDI) has emerged as a powerful standalone method for identifying material stress responses [2, 3]. It operates with minimal assumptions about while being model-free, this making it particularly suitable for calibrating complex models commonly used in industry.
Key objectives of this research include adapting DDCM strategies for plasticity [4] and fracture [5], enhancing DDI for high-performance computing, and evaluating constitutive equations. The proposed methodology involves collecting full-field measurement maps from an heterogeneous test, utilizing High-Speed cameras and Digital Image Correlation. It will adapt DDCM for ductile fracture scenarios, implement a DDI solver in a high-performance computing framework, and conduct an assessment of a legacy constitutive model without uncertainties. The focus will be on 316L steel, a material widely used in nuclear engineering.
This thesis is the result of a collaboration between several labs at CEA ans Centrale Nantes which are prominent in computational and experimental mechanics, applied mathematics, software engineering and signal processing.
[1] Kirchdoerfer, Trenton, and Michael Ortiz. "Data-driven computational mechanics." Computer Methods in Applied Mechanics and Engineering 304 (2016): 81-101.
[2] Leygue, Adrien, et al. "Data-based derivation of material response." Computer Methods in Applied Mechanics and Engineering 331 (2018): 184-196.
[3] Dalémat, Marie, et al. "Measuring stress field without constitutive equation." Mechanics of Materials 136 (2019): 103087.
[4] Pham D. et al, Tangent space Data Driven framework for elasto-plastic material behaviors, Finite Elements in Analysis and Design, Volume 216, 2023, https://doi.org/10.1016/j.finel.2022.103895.
[5] P. Carrara, L. De Lorenzis, L. Stainier, M. Ortiz, Data-driven fracture mechanics, Computer Methods in Applied Mechanics and Engineering, Volume 372, 2020, https://doi.org/10.1016/j.cma.2020.113390.
Impact of pollution on the dynamics of bubbly flows
In accident conditions, if the core of a nuclear reactor boils, the pollution of the water can have an important role in heat exchanges. The challenge of this thesis is to understand this impact and learn to simulate it, the aim being ultimately to provide reference data for boiling in reactor conditions. To achieve this, this thesis will focus on simulating the transport of a pollutant concentration within bubbly flow. The student will simulate the pollution of interfaces by surfactant molecules, a particular case of pollutant found in most hydraulic systems. This study will be carried out using Direct Numerical Simulations carried out with the TRUST/TrioCFD open-source code. The student will be hosted at the Laboratory of Modeling and Simulation in Fluid Mechanics (LMSF) within a group of researchers and numerous PhD students. In collaboration with the academic world, the student will publish his work and participate in international conferences. We are therefore looking for a student who has completed his studies in computational fluid mechanics (M2 or equivalent). Knowledge of modern C++ language would be a notable advantage. Carrying out an internship prior to the thesis is possible.
Sub-Grid modelling of interfacial heat and mass transfers applied to condensation of bubble swarms
To assess the safety of nuclear power plants, the CEA develops and uses multi-scale thermohydraulic simulation tools. The application of CFD to two-phase flows is limited because it requires many models that are difficult to determine. Among our other tools, direct numerical simulations (DNS) with resolved interfaces provide reference data inaccessible by experimental means. This is for example the case of bubble swarms, where heat and mass transfers are influenced by complex collective effects.
In order to reduce the cost of these DNS simulations, we recently developed an approach [1] which shows promising results: it consists of coupling a fine resolution of thermal transfers at the liquid-vapor interfaces to a far field calculated on a less resolved mesh. To broaden the application of this method to more industrial cases, it is necessary to take into account collisions between bubbles and to adapt the model to the phase change.
During this thesis, we propose to start with this physical modeling work and its implementation in C++ in our open-source simulation code TRUST/TrioCFD [2]. Next, we will use this new capacity to carry out a parametric study and an in-depth physical analysis of the phenomena which would ultimately lead to an improvement in heat transfer models in industrial codes.
[1] M. Grosso, G. Bois, A. Toutant, Thermal boundary layer modelling for heat flux prediction of bubbles at saturation: A priori analysis based on fully-resolved simulations, International Journal of Heat and Mass Transfer, Vol 222, 2024, https://doi.org/10.1016/j.ijheatmasstransfer.2023.124980
[2] Trio_CFD webpage : http://triocfd.cea.fr/recherche/modelisation-physique/two-phase-flows
A macroscale approach to evaluate the long-term degradation of concrete structures under irradiation
In nuclear power plants, the concrete biological shield (CBS) is designed to be very close of the reactor vessel. It is expected to absorb radiation and acts as a load-bearing structure. It is thus exposed during the lifetime of the plant to high level of radiations that can have consequences on the long term. These radiations may result especially in a decrease of the material and structural mechanical properties. Given its key role, it is thus necessary to develop tools and models, to predict the behaviors of such structures at the macroscopic scale.
Based on the results obtained at a lower scale - mesoscopic simulations, from which a better understanding of the irradiation effect can be achieved and experimental results which are expected to feed the simulation (material properties especially), it is thus proposed to develop a macroscopic methodology to be applied to the concrete biological shield. This approach will include different phenomena, among which radiation-induced volumetric expansion, induced creep, thermal defromations and Mechanical loading.
These physical phenomena will be developed within the frame of continuum damage mechanics to evaluate the mechanical degradation at the macroscopic scale in terms of displacements and damage especially. The main challenges of the numerical developments will be the proposition of adapted evolution laws, and particularly the coupling between microstructural damage and damage at the structural level due to the stresses applied on the structure.
Uncertainty quantification and sensitivity analysis for vibrations of thin structures under axial flow
Fluid-structure interaction (FSI) phenomena are omnipresent in industrial installations where structures are in contact with a flowing fluid that exerts a mechanical load. In the case of slender flexible structures, IFS can induce vibratory phenomena and mechanical instabilities, resulting in large displacement amplitudes. The nuclear industry is confronted with this problem, particularly concerning piping, fuel assemblies, and steam generators. Computation codes are an essential tool that, based on several input parameters, provide access to quantities of interest (output variables) that are often inaccessible experimentally for the prevention and control of vibrations. However, knowledge of input parameters is sometimes limited by a lack of characterization (measurement error or lack of data) or simply by the intrinsically random nature of these parameters.
In this context, this thesis aims to analyze the vibratory response of a thin structure with uncertain geometric characteristics (structure with a curvature defect, localized or global). In particular, we aim to understand how geometric uncertainties affect the stability of the flexible structure.
This characterization will be carried out both theoretically and numerically. As the work progresses, the effect of different uncertainties (linked, for example, to the material characteristics of the structure or the properties of the incident flow) may be considered. Ultimately, the work carried out as part of this thesis will enable us to improve the prediction and control of vibrations of thin structures under axial flow.
Fluid-structure interactions and associated instabilities are present in many fields, whether in aeronautics with the phenomena of wing flutter, in nuclear power with the vibrations of components under flow, in biology for the understanding of underwater animal locomotion, in botany for the understanding of plant growth, in sport for performance optimization, in energy recovery from fluid-excited flexible structures. The thesis will enable the student to acquire a wide range of skills in mathematics, numerical simulation, fluid mechanics and solid mechanics, and to train for research in the field of fluid and solid mechanics, leading ultimately to a career in this field, whether in academia or in applied research and development in numerous fields of interest to scientists and society in general. A 6-month internship subject is also offered as a preamble to the thesis (optional).
Education level: Master 2 / Final year of engineering school.
Required training: continuum mechanics, strength of materials (beam theory)
fluid mechanics, fluid-structure interaction, numerical simulation (finite elements).
Design of asynchronous algorithms for solving the neutron transport equation on massively parallel and heterogeneous architectures
This PhD thesis work aims at designing an efficient solver for the solution to the neutron transport equation in Cartesian and hexagonal geometries for heterogeneous and massively parallel architectures. This goal can be achieved with the design of optimal algorithms with parallel and asynchronous programming models.
The industrial framework for this work is in solving the Boltzmann equation associated to the transportof neutrons in a nuclear reactor core. At present, more and more modern simulation codes employ an upwind discontinuous Galerkin finite element scheme for Cartesian and hexagonal meshes of the required domain.This work extends previous research which have been carried out recently to explore the solving step ondistributed computing architectures which we have not yet tackled in our context. It will require the cou-pling of algorithmic and numerical strategies along with programming model which allows an asynchronousparallelism framework to solve the transport equation efficiently.
This research work will be part of the numerical simulation of nuclear reactors. These multiphysics computations are very expensive as they require time-dependent neutron transport calculations for the severe power excursions for instance. The strategy proposed in this research endeavour will decrease thecomputational burden and time for a given accuracy, and coupled to a massively parallel and asynchronousmodel, may define an efficient neutronic solver for multiphysics applications.
Through this PhD research work, the candidate will be able to apply for research vacancies in highperformance numerical simulation for complex physical problems.
Study of the transitions of flow regimes in post-burnout
Dispersed two-phase flows are part of many fluid systems such as the cooling of nuclear reactors. Depending on the heat flux in the reactor core, the flow rate, the subcooling or the pressure, different flows may occur: single phase, bubbly or annular flows (with a liquid film on the wall and a vapour core).
During a loss of primary coolant accident, the reactor core, containing the fuel rods, increases in temperature until the boiling crisis when the heat flux is high enough. The different regimes of two-phase flows that occur in this type of accident are illustrated in figure 1. A vapour film appears rapidly and thermally insulates the rods, while some liquid remains in the centre of the flow. The rods are dried up, thus their surface are cooled down by the single vapour, and the heat exchange at the wall is reduced [1], which corresponds to the « inverted annular film boiling » flow. When the liquid gradually vaporises, the vapour film thickens and the induced turbulence tends to form waves at the vapour-liquid interface, and to destabilise the interface until the formation of liquid slugs (inverted slug film boiling). Then, the evaporation and fragmentation of these slugs lead to the formation of a dispersed flow with droplets (dispersed film boiling).
The transitions of flow regimes in this configuration are not well-identified [1], [2] although their understanding is significant to study the cooling of a nuclear reactor core. One of the main obstacles in experimental studies is that the walls need to be strongly heated up in order to form and maintain a vapour film, which leads to opaque test sections. Thus, a direct visualisation is particularly complex to obtain, as much as measuring local parameters such as temperature and velocity fields. The experimental results available in the literature on this topic are insufficient to develop a physical model [1], [3], [4], [5].
As a first step towards an accurate identification of the regime transitions, this thesis focuses on the single effect of the hydrodynamics, by coupling experimental and analytical approaches. In order to clarify the physics of the different phenomena, the configuration of a liquid flow inside a gas flow is proposed. Indeed, the interface deformation and the gas and liquid velocities may influence the transition from one regime to another [6], [7]: the smooth interface is therefore perturbed by waves (Kelvin-Helmholtz instabilities) and droplets could be entrained from the interface. A parametric analysis is considered by varying the gas and liquid flow rates and the thickness of the gas film, in order to observe these different phenomena and to understand the influence of each parameter on the regime transitions. An experimental facility has recently been conceived at DM2S/STMF/LE2H to study these transitions by a visualisation of the interface deformations, and may be adapted with new measurements or new methodology if necessary.
Dimensionless numbers will be identified or defined from the experimental results to describe the phenomena. Then, the regime transitions will be characterized, based on these dimensionless numbers, in order to establish a diagram of the transitions of flow regimes.
The combination of the results obtained in this thesis will enable to reinforce the physical models used in the system code CATHARE, developed at CEA for thermal-hydraulic studies about nuclear safety. This thesis presents a strong academic interest thanks to an innovative experimental facility and production of original results. Besides, it also presents an interest on the industrial level since it contributes to enhance the expertise of significant phenomena in the demonstration of nuclear reactor safety.
References:
[1] M. Ishii et G. De Jarlais, « Flow visualization study of inverted annular flow of post-dryout heat transfer region », Nuclear Engineering and Design, 1987.
[2] G. De jarlais, M. Ishii, et J. Linehan, « Hydrodynamic stability of inverted annular flow in an adiabatic simulation », Argonne National Laboratory, CONF-830702-9, 1983.
[3] T. G. Theofanous, « The boiling crisis in nuclear reactor safety and performance », International Journal of Multiphase Flow, vol. 6, no 1, p. 69-95, févr. 1980, doi: 10.1016/0301-9322(80)90040-3.
[4] N. Takenaka, T. Fujii, et others, « Flow pattern transition and heat transfer of inverted annular flow », Int. J. Multiphase Flow, 1989.
[5] M. A. El Nakla, D. C. Groeneveld, et S. C. Cheng, « Experimental study of inverted annular film boiling in a vertical tube cooled by R-134a », International Journal of Multiphase Flow, vol. 37, p. 37-75, 2011.
[6] Q. Liu, J. Kelly, et X. Sun, « Study on interfacial friction in the inverted annular film boiling regime », Nuclear Engineering and Design, vol. 375, 2021.
[7] K. K. Fung, « Subcooled and low quality film boiling of water in vertical flow at atmospheric pressure », PhD Thesis, Argonne National Laboratory, 1981.
Hydrogen and ammonia combustion within porous media: experiments and modelling
- Context
Current energy prospects suggest the use of hydrogen (H2) and ammonia (NH3) as carbon-free energy carriers to achieve neutrality by 2050. NH3 offers advantages like high energy density and safe storage but faces combustion challenges such as narrow flammability and high NOx emissions. Interestingly, some H2 can be obtained by partial cracking of NH3 to create blends of more favourable combustion properties, with open questions regarding pollutant emissions and unburnt NH3 content.
- Challenges
Porous burners show promise for safe and low-pollutant combustion of NH3/H2 blends. However, material durability issues and the complexity of flame stabilization pose significant hurdles. Fortunately, recent advances in additive manufacturing enable the precise tailoring of porous matrices, but the experimental characterization remains difficult due to the opacity of the solid matrix.
- Research objectives
The PhD candidate will operate an experimental bench at CEA Saclay to conduct combustion experiments with NH3/H2/N2+air mixtures in various porous burners. Key tasks will include designing new burner geometries, comparing experimental results with numerical simulations, and advancing the modelling of porous burners using 1D Volume-Averaged Models and asymptotic theory. Experimental measurements will include hotwire anemometry, infrared thermometry, output gas composition analysis, chemiluminescence, and laser diagnostics. The porous burners will be manufactured using 3D printing techniques with materials such as stainless steel, inconel, alumina, zirconia, and silicon carbide.
The research aims to develop more robust and efficient porous burners for NH3/H2 combustion, enhancing their practical application in achieving carbon neutrality. The candidate will contribute to advancing the field through experimental data, innovative designs, and improved modelling techniques.
Modeling of the fall of a drop in a volume, in support of the system code CATHARE
This thesis focuses on the study of droplet fall in free volumes, as part of the continuous improvement of the physical models in the CATHARE code, used for safety studies of Pressurized Water Reactors. The current models are based on the work of Ishii and Zuber, who model the fall velocity of droplets in a two-phase fluid. The objective of the thesis is to refine the precision of this model by incorporating additional parameters and validating it through experiments such as those of Dampierre and CARAYDAS. The PhD candidate will be required to develop a more representative mechanistic model, based on experimental data or CFD simulations if necessary. The innovation lies in developing a more accurate model of droplet fall processes, paving the way for specific applications such as spray modeling, and thus contributing to the validation of the CATHARE code in additional fields.