Gas/wall collision mechanisms play a key role in Knudsen diffusion process. In particular, the channel wall structure has a major influence in mass transfer. So, we investigate the influence of the wall roughness, anisotropy and porosity on the self-diffusion of helium and neon in nanochannels. Three materials are proposed: graphite and β-cristobalite and amorphous silica. The study makes it possible to analyze, in function of temperature, the correlation between 1/the ballistic/diffusion transition regime of the surface gas transfer, 2/the transition of the bouncing process to a linear increase of the bounce number with time and 3/the shape of the surface residence time distribution characterized by a Fréchet like distribution at short time and an exponential decay at long time. As concerns the amorphous SiO 2 , the bounce must be redefined owing to the transfer inside the material which is dominated by a cage effect. The anisotropy effect on collision process and Knudsen diffusion is analyzed by means of a tensorial computation of the tangential momentum accommodation coefficient and of the mean square displacement. Using the Langevin at the channel scale and the Arya model, the ballistic/diffusion transition time of the mean square displacement is related to the collision frequency and the collision number required for the velocity to be uncorrelated. A stochastic model confirms the molecular dynamics results with β-SiO 2 channel: The behavior of the Knudsen diffusion coefficient according to the Arrhenius law and the influence of collision frequency on transition time.
This chapter presents the activity conducted by the ITPA topical group (TG) on Diagnostics over about the last 15 years. Following a general introduction of the ITER Diagnostics led by their measurement roles, the document is organized in several subchapters detailing the design support, research and development activity conducted by each of the specialist working groups (WGs) of the TG. Please note that the magnetic diagnostics were supported at the TG without a specific WG. Their status is included in the general introduction. In the following some highlights of the subchapter's contents are provided. Recent advances in ITER first wall (FW) diagnostics for the measurements of plasma-metallic wall interaction in support of the ITER research plan are reported. An InfraRed imaging Video Bolometer for ITER has been developed and tested on several tokamaks to measure the radiated power loss. A laser-induced breakdown spectroscopy (LIBS) technique which utilizes a pulsed laser beam to ablate locally by Nucl. Fusion 65 (2025) 113001 Review inconsistencies. Physics-based modeling and parameter relationships provide additional information improving the treatment of ill-posed inversion problems. A coherent combination of all kind of available information within a probabilistic framework allows for improved data analysis results. The concept of integrated data analysis (IDA) in the framework of Bayesian probability theory is outlined and contrasted with conventional data analysis. Components of the probabilistic approach are summarized and specific ingredients beneficial for data analysis at fusion devices are discussed. This paper is part of the Special Issue: On the Path to Tokamak Burning Plasma Operation: A collection of papers prepared by the ITPA Topical Physics Groups reviewing progress in the development of the physics basis for burning plasma operation.
This article discusses the challenges of modeling atmospheric reentry using computational fluid dynamics (CFD) due to its complexity and practical industrial applications. Ablation phenomena caused by high energy make it difficult and time-consuming to use a ''high-fidelity'' CFD method to accurately measure forces and heat flux. As a result, methods based on Newton's theory are used to model aerodynamic forces, incorporating statistical correlations from CFD results to estimate heat fluxes. However, these methods sacrifice accuracy for CPU and engineering time and have difficulty representing realistic physics when complex phenomena occur, such as shock interactions. To bridge the gap between approximate and high-fidelity methods, we propose a new approach using an automatic grid generation method of the octree Cartesian type coupled to a solver solving Euler's equations. To apply the boundary condition accurately we compared two immersed boundary methods: a diffuse interface method and a sharp interface method under hypersonic flow configurations. We present a comparative study of these two formulations on verification and validation phases, including an academic test case and an industrial case on a real reentry spacecraft. The novelty lies in applying these methods to complex cases involving strong discontinuities (attached shocks). After concluding this comparative study, we demonstrate that with adapted formulations and an optimized approach, IB methods can handle complex geometries typical of atmospheric reentry.
In this article, we present a stochastic model for the movement of Helium particles within a graphite channel, focusing on Knudsen diffusion. We develop a semi-Markov model to describe the movement of the particle, derive the stationary distribution of its mean position, and analyze the model's asymptotic properties. To validate the model, we compare its theoretical outcomes with Monte Carlo simulations. As temperature significantly influences on the movement of particles, two situations are studied for high and low temperature. In both cases, theoretical and simulation results by Monte Carlo coincide. Furthermore, we propose estimation methods for the local parameters of the model and demonstrate its application using data from Molecular Dynamics simulations.
Even small fluctuations in the magnetic field are known to impact edge plasma turbulence and transport properties in magnetic confinement fusion devices. Magnetic induction modifies the parallel electric field, and as such, it impacts the parallel current in Ohm's law. In addition, magnetic fluctuations can induce geometrical and topological changes in the magnetic field structure, leading to parallel transport across the equilibrium magnetic surfaces. This paper presents the new drift-reduced fluid electromagnetic model implemented in SOLEDGE3X [Bufferand et al., Nuc. Fus. 2021]. Based on a domain decomposition, a specific numerical scheme is proposed using conservative second-order finite volumes associated with a semi-implicit time advancement. The coupling between the parallel current j ∥ and the parallel electromagnetic potential A ∥ in Ohm's and Ampere's laws is treated using a new toroidally and poloidally staggered grid. While adding A ∥ doubles the size of the vorticity operator to be inverted, numerical tests show that the inclusion of the finite mass of the electrons in the new model acts as an upper limit on the parallel diffusion coefficient, thus improving the conditioning of the matrix at high plasma temperatures when the parallel resistivity η ∥ approaches zero. The changes in the algorithm are verified using the method of manufactured solutions. The implementation is first validated with respect to theoretical linear stability results, recovering the transition from Alfvén to thermal electron waves as the perpendicular wavenumber increased. Comparisons with available results in the literature show a qualitative agreement on a single blob propagation in a limited slab geometry. The first 3D simulations of plasma edge turbulence in TCV demonstrate the capability of the new solver to handle realistic tokamak configurations. The explicit-implicit time integration scheme enables one to compare electrostatic and electromagnetic effects using the same solver, with or without electron inertia and magnetic flutter. This ability opens the way to a better understanding of the impact of the electrostatic and electromagnetic mechanisms on the transport and turbulence properties in realistic tokamak configurations.
This paper proposes a maintenance model for a Proton-Exchanged Membrane Fuel Cells system. Two types of failures are considered: non repairable internal failures and possibly repairable external failures. The maintenance involves replacing one or more cells. Therefore the maintenance can be perfect or imperfect. Moreover after each repair, the maintenance operator improves and the down time due to maintenance is reduced. A semi-Markov process is proposed to model the system's lifetime. In this framework, the reliability measures and the remaining useful lifetime are derived. The long run average maintenance cost is studied. A sensitivity analysis of model parameters and maintenance unit costs is performed and a maintenance optimization strategy is proposed.
Within the 9th European Framework programme, since 2021 EUROfusion is operating five tokamaks under the auspices of a single Task Force called 'Tokamak Exploitation'. The goal is to benefit from the complementary capabilities of each machine in a coordinated way and help in developing a scientific output scalable to future largre machines. The programme of this Task Force ensures that ASDEX Upgrade, MAST-U, TCV, WEST and JET (since 2022) work together to achieve the objectives of Missions 1 and 2 of the EUROfusion Roadmap: i) demonstrate plasma scenarios that increase the success margin of ITER and satisfy the requirements of DEMO and, ii) demonstrate an integrated approach that can handle the large power leaving ITER and DEMO plasmas. The Tokamak Exploitation task force has therefore organized experiments on these two missions with the goal to strengthen the physics and operational basis for the ITER baseline scenario and for exploiting the recent plasma exhaust
This work is devoted to a theoretical and numerical study of the dynamics of a two-phase system vapour bubble in equilibrium with its liquid phase under translational vibrations in the absence of gravity. The bubble is initially located in the container centre. The liquid and vapour phases are considered as viscous and incompressible. Analysis focuses on the vibrational conditions used in experiments with the two-phase system SF 6 in the MIR space station and with the two-phase system para-Hydrogen (p-H 2 ) under magnetic compensation of Earth's gravity. These conditions correspond to small-amplitude high-frequency vibrations. Under vibrations, additionally to the forced oscillations, an average displacement of the bubble to the wall is observed due to an average vibrational attraction force related to the Bernoulli effect. Vibrational conditions for SF 6 correspond to much smaller average vibrational force (weak vibrations) than for p-H 2 (strong vibrations). For weak vibrations, the role of the initial vibration phase is crucial. The difference in the behaviour at different initial phases is explained using a simple mechanical model. For strong vibrations, the average displacement to the wall stops when the bubble reaches a quasi-equilibrium position where the resulting average force is zero. At large vibration velocity amplitudes this position is near the wall where the bubble performs only forced oscillations. At moderate vibration velocity amplitudes the bubble average displacement stops at a finite distance from the wall, then large-scale damped oscillations around this position accompanied by forced oscillations are observed. Bubble shape oscillations and the parametric resonance of forced oscillations are also studied.
This work is devoted to a theoretical and numerical study of the dynamics of a two-phase system vapour bubble in equilibrium with its liquid phase under translational vibrations in the absence of gravity. The bubble is initially located in the container centre. The liquid and vapour phases are considered as viscous and incompressible. Analysis focuses on the vibrational conditions used in experiments with the two-phase system SF 6 in the MIR space station and with the two-phase system para-Hydrogen (p-H 2 ) under magnetic compensation of Earth's gravity. These conditions correspond to small-amplitude high-frequency vibrations. Under vibrations, additionally to the forced oscillations, an average displacement of the bubble to the wall is observed due to an average vibrational attraction force related to the Bernoulli effect. Vibrational conditions for SF 6 correspond to much smaller average vibrational force (weak vibrations) than for p-H 2 (strong vibrations). For weak vibrations, the role of the initial vibration phase is crucial. The difference in the behaviour at different initial phases is explained using a simple mechanical model. For strong vibrations, the average displacement to the wall stops when the bubble reaches a quasi-equilibrium position where the resulting average force is zero. At large vibration velocity amplitudes this position is near the wall where the bubble performs only forced oscillations. At moderate vibration velocity amplitudes the bubble average displacement stops at a finite distance from the wall, then large-scale damped oscillations around this position accompanied by forced oscillations are observed. Bubble shape oscillations and the parametric resonance of forced oscillations are also studied.
Fluid-structure interaction (FSI) occurs in a wide range of contexts, from aeronautics to biological systems. To numerically address this challenging type of problem, various methods have been proposed, particularly using implicit coupling when the fluid and the solid have the same density, i.e., the density ratio is equal to 1. Aiming for a computationally efficient approach capable of handling strongly coupled dynamics and/or realistic conditions, we present an alternative to the implicit formulation by employing a fully explicit algorithm. The Lattice Boltzmann Method (LBM) is used for the fluid, with the finite element method (FEM) utilized for the structure. The Immersed Boundary Method (IBM) is applied to simulate moving and deforming boundaries immersed in fluid flows. The novelty of this work lies in the combination of Laplacian smoothing at the fluid/solid interface, an improved collision model for the LBM, and a reduction of non-physical frequencies on the structure mesh. The use of these adaptations results in a solver with remarkable stability properties, a primary concern when dealing with explicit coupling. We validate the numerical framework on several challenging test cases of increasing complexity, including 2D and 3D configurations, density ratio of 1, and turbulent conditions.
In degradation modeling, stochastic processes often do not meet the classical properties necessary for traditional goodness-of-fit tests. This paper presents an initial investigation into employing the ACH depth function and its potential in degradation model selection. We commence by presenting various stochastic processes as degradation models and their selection criteria. Subsequently, we delve into the ACH depth function, highlighting its potential in this context. Through simulated data, we assess the application of this functional depth measure for model selection. The methodology's validity is further reinforced by its application to real-world data, underscoring its effectiveness.