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Two unsteady solutions of a homogeneous equilibrium model in a porous medium
No full textInternational audienceWe present in this note two reference one-dimensional unsteady solutions of a HEM model, considering possible heat source terms, and focusing on a steady porous medium. The model is recalled first, then details on exact solutions are provided. Several Equations Of State are discussed
Écoulements de suspensions fluide-particules dominés par la sédimentation
No full textInternational audienc
Les Cahiers de RECITS n°12 - 2025: Aux sources de la démocratie technique : la créativité technique
No full textInternational audienceLe concept de démocratie s’enracine sur diverses périodes et relève de multiples origines géographiques (Athènes, Moyen-Orient, Chine…), autrement dit « la » démocratie est beaucoup plus ancienne, et probablement beaucoup plus multiforme, que ce qu’on imagine. En effet, très souvent, « la » démocratie est considérée comme une réalité unique et universelle or cette dernière s’avère plurielle, loin d’être pensée de la même façon selon les femmes ou les hommes, selon les générations, dans un même milieu. et c’est même cette diversité qui la structure, et qui, en un mot, lui permet de jouer un rôle fondamental dans la société. Quant à « la » technique, elle reste également omniprésente dans les activités de l’espèce humaine, et cela au moins depuis des dizaines de millénaires. Très rapidement, de premiers groupes humains conçoivent et reconçoivent leurs outils issus de cheminements techniques plus diversifiés que ce qu’on imagine habituellement. C’est dire si la variété mais également la trajectoire de ces choix techniques demeurent sur le temps long, inhérentes de l’espèce humaine… et ne relèvent en rien d’une exclusivité du contemporain !La session du XIVe congrès de la Société Française d’Histoire des Sciences et des Techniques qui s’est tenue à Bordeaux en avril 2023 visait à approfondir la connaissance des conditions favorisant la créativité technique et à identifier des moments où le monde de la technique est « interpelé » voire « percuté » par le concept de démocratie technique. C’est une certitude, il n’existe pas d’universalité, ni de solutions techniques à l’échelle planétaire, et c’est bien cette quête de la diversité, de cheminements singuliers, d’acceptation de logiques disruptives qui permettra d’asseoir un peu plus les fondements d’une véritable démocratie technique. N’est-ce pas là, le meilleur héritage que nous pourrons transmettre aux générations à venir
Path-following methods for quasi-static crack propagation simulated by variational phase field approach
No full textEURAD state-of-the-art report: thermo-hydro-mechanical behaviour at high temperature of host clay formations
No full textInternational audienceMost safety cases for radioactive waste disposal concepts consider a temperature limit of 90°C in the clay host rock. Being able to tolerate higher temperature would have significant advantages. For this reason, part of the EURAD-HITEC project aimed at determining the influence of temperature above 90°C on clay host rock properties, trying to establish the possible extent of elevated temperature damage in the near and far field of clay host rock formations and the consequences of any such damage. Three clay formations considered to host radioactive waste repositories in Europe were the focus of the studies: the Boom Clay, the Callovo-Oxfordian claystone and the Opalinus Clay. A summary of the background knowledge about the thermo-hydro-mechanical behaviour of these clay host rocks is first presented. Then, the experimental and modelling activities carried out in the framework of the EURAD-HITEC project concerning these materials have been synthesised. The laboratory tests analysed the impact of temperature on the short- and long-term behaviour of the clay host rock and the self-sealing processes. Hydro-mechanical couplings between peak pore water pressure, temperature, permeability and confining stress were identified. The results confirmed that the claystone keeps its good mechanical and retention properties even when heated up to 100°C. Provided that the clay content of the samples is high enough, self-sealing was an efficient mechanism whatever the experimental conditions, although temperature may have a delaying effect. Poro-elastic models were used to model generic cases of a high-level waste repository, and consistent results were obtained by the different codes and teams, which shows the robustness of the modelling approach used to design the repositories. Two heating tests, performed in the HADES (Belgium) and MHM (France) underground research laboratories, were selected as benchmarks for the modelling activities. The evolutions of temperature and pore pressure were well modelled in the far field with a poro-elastic approach, but more advanced models are needed to take into account the processes occurring around the tunnels (e.g., modification of hydraulic properties within the EDZ, creep). The modelling of laboratory experiments showed the importance of a good understanding of the tests setup and of the boundary conditions
Averaged Steklov Eigenvalues, Inside Outside Duality and Application to Inverse Scattering
No full textInternational audienceWe introduce a new family of artificial backgrounds corresponding to averaged impedance boundary conditions formulated in an abstract framework. These backgrounds are used to define a finite number of averaged Steklov eigenvalues, which are associated with inverse scattering problems from inhomogeneous media. We prove that these special eigenvalues can be determined from full-aperture, fixed-frequency far-fields using the inside-outside duality method. We then show and numerically demonstrate how this method can be used to reconstruct averaged values of the refractive index.</div
Climate-Induced Changes in River Hydrological and Thermal Conditions in a Large Basin: Implications for Diadromous Fish Migration
No full textInternational audienceClimate-induced changes in river conditions, such as water temperature and current velocity, can disrupt species migration patterns by altering the conditions encountered during migration. We assessed these impacts across the Loire River basin in Europe by compiling long-term fish passage data for three diadromous fish species at fishways, alongside reconstructed time series of water temperature and current velocity. We considered both water temperature and current velocity on days when migration was observed as ''suitable conditions'' for the species' upstream migration. Our findings for the 1963–2019 period revealed that trends in the frequency of suitable conditions for upstream migration were species-specific. For Atlantic salmon, particularly fall migrants, the frequency of suitable conditions has declined over time in certain areas. In contrast, the frequency of suitable migration conditions for both allis shad and sea lamprey has shown an overall increase across the basin. This study offers policy-makers and environmental managers valuable insights for developing effective, spatially explicit strategies to preserve diadromous fish populations amid ongoing climate change
The non-intrusive reduced basis two-grid method applied to sensitivity analysis
No full textInternational audienceThis paper deals with the derivation of Non-Intrusive Reduced Basis (NIRB) techniques for sensitivity analysis, more specifically the direct and adjoint state methods. For highly complex parametric problems, these two approaches may become too costly ans thus Reduced Basis Methods (RBMs) may be a viable option. We propose new NIRB two-grid algorithms for both the direct and adjoint state methods in the context of parabolic equations. The NIRB two-grid method uses the HF code solely as a “black-box”, requiring no code modification. Like other RBMs, it is based on an offline-online decomposition. The offline stage is time-consuming, but it is only executed once, whereas the online stage employs coarser grids and thus, is significantly less expensive than a fine HF evaluation. On the direct method, we prove on a classical model problem, the heat equation, that HF evaluations of sensitivities reach an optimal convergence rate in L∞(0, T ; H10(Ω)), and then establish that these rates are recovered by the NIRB two-grid approximation. These results are supported by numerical simulations. We then propose a new procedure that further reduces the computational costs of the online step while only computing a coarse solution of the state equations. On the adjoint state method, we propose a new algorithm that reduces both the state and adjoint solutions. All numerical results are run with the model problem as well as a more complex problem, namely the Brusselator system
A multi-objective optimization to characterize the diffusion of nanocavities in tungsten
No full textInternational audienceWe characterize the diffusion properties of nanocavities and their uncertainties by designing a multi-objective optimization approach. In this work, the nanocavity diffusion on the 0.3–4 nm size range is the input of a multi-scale simulation that is adjusted to reproduce experimental results of a systematic study of nanocavity growth with temperature up to 1773 K. Under irradiation, in the material microstructure, the damage evolution results from a complicated interplay of the defects and their clusters (formed from the vacancies and self-interstitials created) which diffuse, recombine and grow. The simulation of the whole experiment, based on an Object Kinetic Monte Carlo algorithm, can take several hours per condition which is a strong limitation for the optimization scheme. We describe the method that succeeds for our problem. Starting from a rough and random sampling of the space of parameters, we then consider that each simulation is one point of the hypersurface in the high dimensional space formed by the optimized parameters and objectives. We iteratively improve the characterization of this hypersurface where the objectives are optimum thanks to a systematic search of patterns formed by points on the coordinate planes. The non-dominated solutions, i.e. the equally good solutions, also named the Pareto front, are finally characterized. They draw two “valleys” in the subspace of parameters, delimiting the uncertainties on the searched diffusion properties, which cannot be reduced with the experimental data and the model in their current form
Regularization of softening plasticity with the cumulative plastic strain-rate gradient
No full textInternational audienceWe propose a novel variational framework to regularize softening plasticity problems. Specifically, we modify the plastic dissipation potential term by adding a contribution depending on the cumulative plastic strain-rate gradient. We formulate the evolution of the so-obtained strain-rate gradient plasticity model with an incremental variational principle. The timediscretized evolution equations are deduced from the corresponding first-order optimality conditions. To investigate the model, the problem of a bar in traction is studied. Analytical solutions are explicitly derived, and characterized by exponential localization profiles. Contrary to other regularization strategies, no spurious spreading of the plastic localization band is observed. A first numerical implementation in 1D and 2D plane strain conditions is proposed based on conic programming solvers and validated against the analytical predictions. Numerical results on plane strain von Mises plasticity show that the proposed framework leads to mesh-independent results and efficient control of plastic localization bands