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Bourne, Emily. "Non-uniform numerical schemes for the modelling of turbulence in the 5D GYSELA code". Electronic Thesis or Diss., Aix-Marseille, 2022. http://www.theses.fr/2022AIXM0412.
Pełny tekst źródłaThis thesis lies within the context of fusion plasma simulations and it has a double objective: (i) develop new scalable numerical methods, adapted to the semi-lagrangian scheme used in the 5D gyrokinetic GYSELA code, capable of solving the problem of large fluctuations and temperature variations at the edge of the plasma, and (ii) take into account more realistic magnetic configurations than the concentric circles currently simulated by the code. I present a new approach for quadrature using splines, which limits the condition number for the procurement of such quadrature coefficients. I present a local spline method where derivatives are transported between patches, and show its stability for semi-lagrangian advection. The semi-lagrangian method based on non-uniform splines on a Vlasov-Poisson 1D-1V model is used for studies of the plasma sheath. The existing VOICE code (which is a mini version of GYSELA), designed to study such problems, has been modified and optimised on a GPU to operate on a non-uniform mesh. Co-variant and contra-variant transformation matrices of a new realistic magnetic configuration were derived and implemented in the code to allow the 5D Vlasov equations to take into account new geometry. The inclusion of this new magnetic configuration has been successfully numerically validated on the linear benchmarks used for GAM studies. In parallel, a test platform for the 2D Poisson solver was developed in order to numerically compare this spline finite elements solver to two other multi-grid solvers: (i) a solver using finite volumes on a uniform cartesian mesh with embedded boundaries, and (ii) a solver using finite differences on a logical mesh
Banon, Navarro Alejandro. "Gyrokinetic large Eddy simulations". Doctoral thesis, Universite Libre de Bruxelles, 2012. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209592.
Pełny tekst źródłaDu point de vue théorique, la turbulence plasma est décrite par les équations gyrocinétiques, un ensemble d équations aux dérivées partielles non linéaires couplées. Par suite des très différentes échelles spatiales mises en jeu dans des conditions expérimentales réelles, une simulation numérique directe et complète (DNS) de la turbulence gyrocinétique est totalement hors de portée des plus puissants calculateurs actuels, de sorte que démontrer la faisabilité d’une alternative permettant de réduire l’effort numérique est primordiale. En particulier, les simulations de grandes échelles (”Large-Eddy Simulations” - LES) constituent un candidat pertinent pour permettre une telle r éduction. Les techniques LES ont initialement été développées pour les simulations de fluides turbulents à haut nombre de Reynolds. Dans ces simulations, les plus grandes échelles sont explicitement simulées numériquement, alors que l’influence des plus petites est prise en compte via un modèle implémenté dans le code.
Cette thèse présente les premiers développements de techniques LES dans le cadre des équations gyrocinétiques (GyroLES). La modélisation des plus petites échelles est basée sur des bilans d’énergie libre. En effet, l’energie libre joue un rôle important dans la théorie gyrocinétique car elle en est un invariant non lin éaire bien connu. Il est démontré que sa dynamique partage de nombreuses propriétés avec le transfert d’energie dans la turbulence fluide. En particulier, il est montré l’existence d’une cascade d énergie libre, fortement locale et dirigée des grandes échelles vers les petites, dans le plan perpendiculaire â celui du champ magnétique ambiant.
La technique GyroLES est aujourd’hui implantée dans le code GENE et a été testée avec succès pour les instabilités de gradient de température ionique (ITG), connues pour jouer un rôle crucial dans la micro-turbulence gyrocinétique. A l’aide des GyroLES, le spectre du flux de chaleur obtenu dans des simulations à très hautes résolutions est correctement reproduit, et ce avec un gain d’un facteur 20 en termes de coût numérique. Pour ces raisons, les simulations gyrocinétiques GyroLES sont potentiellement un excellent candidat pour réduire l’effort numérique des codes gyrocinétiques actuels.
/ Anomalous transport due to plasma micro-turbulence is known to play an important role in confinement properties of magnetically confined fusion plasma devices such as ITER. Indeed, plasma turbulence is strongly connected to the energy confinement time, a key issue in thermonuclear fusion research. Plasma turbulence is described by the gyrokinetic equations, a set of nonlinear partial differential equations. Due to the various scales characterizing the turbulent fluctuations in realistic experimental conditions, Direct Numerical Simulations (DNS) of gyrokinetic turbulence remain close to the computational limit of current supercomputers, so that any alternative is welcome to decrease the numerical effort. In particular, Large-Eddy Simulations (LES) are a good candidate for such a decrease. LES techniques have been devised for simulating turbulent fluids at high Reynolds number. In these simulations, the large scales are computed explicitly while the influence of the smallest scales is modeled.
In this thesis, we present for the first time the development of the LES for gyrokinetics (GyroLES). The modeling of the smallest scales is based on free energy diagnostics. Indeed, free energy plays an important role in gyrokinetic theory, since it is known to be a nonlinear invariant. It is shown that its dynamics share many properties with the energy transfer in fluid turbulence. In particular, one finds a (strongly) local, forward (from large to small scales) cascade of free energy in the plane perpendicular to the background magnetic field.
The GyroLES technique is implemented in the gyrokinetic code Gene and successfully tested for the ion temperature gradient instability (ITG), since ITG is suspected to play a crucial role in gyrokinetic micro-turbulence. Employing GyroLES, the heat flux spectra obtained from highly resolved direct numerical simulations are recovered. It is shown that the gain of GyroLES runs is 20 in terms of computational time. For this reason, Gyrokinetic Large Eddy Simulations can be considered a serious candidate to reduce the numerical cost of gyrokinetic simulations.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished
Weidl, Martin S. "Cosmic-ray propagation in simulations of cross-helical plasma turbulence". Diss., Ludwig-Maximilians-Universität München, 2015. http://nbn-resolving.de/urn:nbn:de:bvb:19-184214.
Pełny tekst źródłaTurbulenz ist in astrophysikalischen Plasmen allgegenwärtig. Viele solche Systeme weisen eine sogenannte Kreuz-Helizität auf, also eine von Null verschiedene Korrelation zwischen Geschwindigkeits- und Magnetfeld-Fluktuationen. In einer anisotropen Magnetfeldgeometrie, z. B. im Sonnenwind oder dem interstellaren Medium, deutet die Kreuz-Helizität auf ein Ungleichgewicht zwischen Alfven-Wellen, die sich in Richtung des gemittelten Feldes ausbreiten, und solchen, die in die Gegenrichtung propagieren, hin. Obwohl dieses Ungleichgewicht die stochastische Beschleunigung und Streuung, die geladene Teilchen in einem Plasma erfahren, dramatisch beeinflusst, wurde es in bisherigen numerischen Studien über turbulenten Teilchentransport gemeinhin außer Acht gelassen. In dieser Arbeit nun werden rechnergestützte Simulationen von magnetohydrodynamischer Turbulenz präsentiert, in denen die Energie und die Kreuz-Helizität kontrolliert werden können, ohne jedoch kinetische oder magnetische Helizität als unerwünschte Nebenwirkung zu erzeugen. Die Stärke des mittleren Magnetfeldes bestimmt dabei die Anisotropie des Gleichgewichtszustandes. Die Simulationen erfüllen in allen Parameterbereichen die Vorhersagen, die theoretische Modelle für realistische Plasmaturbulenz treffen. Die Diffusion kosmischer Strahlung in turbulenten Plasmen wird häufig im Rahmen der quasilinearen Theorie unter Heranziehung eines stark vereinfachten Turbulenzspektrums berechnet. Indem die Trajektorien von Testteilchen in dynamischen Turbulenzsimulationen mit Kreuz-Helizität berechnet werden, lassen sich quasilineare Ergebnisse für die Beschleunigungsrate geladener Teilchen nachprüfen. Theorie und numerische Simulation stimmen für Teilchen mit der Alfven-Geschwindigkeit gut überein, solange resistive Effekte vernachlässigt werden können. Weiterhin werden aus der quasilinearen Theorie berechnete Diffusionskoeffizienten mit numerisch ermittelten Streuraten für Testteilchen nach einer Gyroperiode in stark anisotropen Feldkonfigurationen verglichen, wobei der Schwerpunkt erneut beim Einfluss der Kreuz-Helizität liegt. Für alle verwendeten Werte der Kreuz-Helizität ergibt sich eine exzellente Übereinstimmung zwischen Simulationsergebnis und Vorhersage. Schließlich wird die Rolle des magnetischen Moments, einer adiabatischen Invarianten bei der Bewegung geladener Teilchen in einem Magnetfeld, für die Streuung über Zeitskalen von mehreren Gyroperioden erläutert.
Gracio, Bilro Castela Maria Luis. "Direct Numerical Simulations of plasma-assisted ignition in quiescent and turbulent flow conditions". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLC042/document.
Pełny tekst źródłaPlasma-assisted combustion has received increasing attention in both plasma and combustion communities. Nanosecond Repetitively Pulsed (NRP) discharges are a promising and efficient technique to initiate and control combustion processes particularly when conventional ignition systems are rather ineffective or too energy costly. Even though a promising technique, the phenomena occurring in NRP discharges-assisted combustion are still poorly understood. The numerical studies presented in the literature are limited to 1-D and 2-D simulations in quiescent conditions. The problem complexity increases in practical configurations as ignition phenomena are also controlled by the flow and mixing field characteristics in and around the discharge channel. Direct Numerical Simulations (DNS) is a powerful research tool to understand these plasma/combustion/flow interactions. However, the computational cost of fully coupled detailed non-equilibrium plasma and combustion chemistry, and high Reynolds number simulations is prohibitive. This thesis presents a model to describe the effects of non-equilibrium plasma discharges in the set of equations governing the combustion phenomena. Based on the results reported in the literature, the model is constructed by analyzing the channels through which the electric energy is deposited. The two main channels by which the electrons produced during the discharge impact the reactive mixture are considered: 1) the excitation and the subsequent relaxation of the electronic states of nitrogen molecules, which leads to an ultrafast increase of the gas temperature and dissociation of species; and 2) the excitation and relaxation of vibrational states of nitrogen molecules which causes a much slower gas heating. This high level model of NRP discharges allows DNS studies of plasma-assisted combustion / ignition in high turbulent Reynolds number. The complex physics underlying plasma-assisted ignition by multiple discharges in both quiescent and turbulent flow conditions are discussed in the present thesis
Cerri, Silvio Sergio [Verfasser]. "Plasma turbulence in the dissipation range - theory and simulations / Silvio Sergio Cerri". Ulm : Universität Ulm. Fakultät für Naturwissenschaften, 2016. http://d-nb.info/108198595X/34.
Pełny tekst źródłaManas, Pierre. "Gyrokinetic simulations of turbulent impurity transport in tokamaks". Thesis, Aix-Marseille, 2015. http://www.theses.fr/2015AIXM4745/document.
Pełny tekst źródłaUnderstanding impurity transport in the core of tokamak plasmas is central to achieving controlled fusion. Indeed impurities are ubiquitous in these devices and their presence in the core are detrimental to plasma confinement (fuel dilution, Bremsstrahlung). Recently, specific attention was given to the convective mechanism related to the gradient of the toroidal rotation to explain experimental flat/hollow impurity profiles in the plasma core. In this thesis, up-to-date modelling tools (NEO for neoclassical transport and GKW for turbulent transport) including the impact of toroidal rotation are used to study both the neoclassical and turbulent contributions to impurity fluxes. A comparison of the experimental and modelled carbon density peaking factor (R/LnC) is performed for a large number of baseline and hybrid H-mode plasmas (increased confinement regimes) with modest to high toroidal rotation from the European tokamak JET. Confrontation of experimental and modelled carbon peaking factor yields two main results. First roto-diffusion is found to have a nonnegligible impact on the carbon peaking factor at high values of the toroidal rotation frequency gradient. Second, there is a tendency to overpredict the experimental R/LnC in the core inner region where the carbon density profiles are hollow. This disagreement between experimental and modelled R/LnC, closely related to the collisionality, is also observed for the momentum transport channel which hints at a common parallel symmetry breaking mechanism lacking in the simulations
Baschetti, Serafina. "A new modelling of the cross-field transport in diverted edge plasma : application to 2D transport simulations with SolEdge2D-EIRENE". Electronic Thesis or Diss., Ecole centrale de Marseille, 2019. http://www.theses.fr/2019ECDM0009.
Pełny tekst źródłaSteady-state operations of the next-generation fusion device ITER will require the development of reliable numerical tools to estimate key engineering parameters suitable for technological constraints at reasonable computational cost.So-called transport codes fulfil this requirement since they rely on 2D fluid equations averaged over time fluctuations, similarly to Reynolds Averaged Navier-Stokes models commonly used for engineering applications in the neutral fluid community. Furthermore, transport codes can gather most of the physical ingredients ruling the edge plasma behaviour, as well as realistic magnetic topology and wall geometry. However, their predictability is limited by a crude description of turbulent fluxes perpendicular to the magnetic field lines. In the plasma community, a special concern is devoted to acquire a detailed understanding of these fluxes, since they strongly impact on the power extraction and the confinement of plasma over extended periods of time. In transport codes though, turbulent fluxes, which are assumed diffusive, are crudely determined by either homogeneous, or ad-hoc diffusive coefficients, or feedback-loop procedures applied a-posteriori on experimental data.Motivated by these issues, in this work we introduce step-by-step a new approach with the aim to self-consistently estimate the distribution of turbulent fluxes in transport codes, when steady-state plasmas are concerned. The underlying strategy is inspired by the work done from the 60’s in neutral turbulence and adapted here to plasma for fusion applications.The first key concept is the Boussinesq assumption. It consists in assuming a colinearity between the Reynolds stress tensor - which represents the contribution of turbulence to the mean flow - and the mean rate of strain tensor - expressed by the gradient of the mean velocity through a coefficient: the so-called eddy-viscosity. The second concept is to express this new eddy viscosity coefficient as a function of characteristic turbulence quantities. We have focused here on the most popular in Computational Fluid Dynamics, the κ-ε model, where transport equations for the averaged kinetic turbulent energy and the turbulence dissipation rate are designed semi-empirically. Steady-state κ and ε allow for a self-consistent estimation of the eddy-viscosity coefficient, thus including the impact of turbulence in steady-state mean flows. We propose a κ-ε -like model where two transport equations for turbulent kinetic energy and its dissipation rate are derived algebraically, including the physics of the linear interchange instability. For the numerical implementation, we exploit the flexibility of the transport package SolEdge2D-EIRENE, developed for many years through the collaboration of the IRFM at the CEA and the laboratory M2P2 at Aix-Marseille University.Since the new model is semi-empirical, it presents some free parameters to be closed. In this work, we have proposed different approaches. In particular, in order to increase the predictive capabilities of the model, a reference scaling law for the width of the heat-flux profile in the scrape-off layer has been assumed, empirically determined from the experimental measurements of the outer target heat load in various machines. The new model is integrated in SolEdge2D-EIRENE for simulations with diverted plasma in TCV and WEST-like geometries, for L-mode discharges. Steady-state results are discussed and shown to favourably compare with experimental data at both the outer mid-plane and the outer divertor. Moreover, self-consistent distributions of diffusivities are shown to exhibit poloidal asymmetries consistently with the ballooned distribution of cross-field transport due to the interchange instability and observed at the same conditions in both first-principle codes and experiments
Ben, Hassan Saïdi Ismaïl. "Numerical simulations of the shock wave-boundary layer interactions". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS390/document.
Pełny tekst źródłaSituations where an incident shock wave impinges upon a boundary layer are common in the aeronautical and spatial industries. Under certain circumstances (High Mach number, large shock angle...), the interaction between an incident shock wave and a boundary layer may create an unsteady separation bubble. This bubble, as well as the subsequent reflected shock wave, are known to oscillate in a low-frequency streamwise motion. This phenomenon, called the unsteadiness of the shock wave boundary layer interaction (SWBLI), subjects structures to oscillating loads that can lead to damages for the solid structure integrity.The aim of the present work is the unsteady numerical simulation of (SWBLI) in order to contribute to a better understanding of the SWBLI unsteadiness and the physical mechanism causing these low frequency oscillations of the interaction zone.To perform this study, an original numerical approach is used. The one step Finite Volume approach relies on the discretization of the convective fluxes of the Navier Stokes equations using the OSMP scheme developed up to the 7-th order both in space and time, the viscous fluxes being discretized using a standard centered Finite-Difference scheme. A Monotonicity-Preserving (MP) constraint is employed as a shock capturing procedure. The validation of this approach demonstrates the correct accuracy of the OSMP scheme to predict turbulent features and the great efficiency of the MP procedure to capture discontinuities without spoiling the solution and with an almost negligible additional cost. It is also shown that the use of the highest order tested of the OSMP scheme is relevant in term of simulation time and accuracy compromise. Moreover, an order of accuracy higher than 2-nd order for approximating the diffusive fluxes seems to have a negligible influence on the solution for such relatively high Reynolds numbers.By simulating the 3D unsteady interaction between a laminar boundary layer and an incident shock wave, we suppress the suspected influence of the large turbulent structures of the boundary layer on the SWBLI unsteadiness, the only remaining suspected cause of unsteadiness being the dynamics of the separation bubble. Results show that only the reattachment point oscillates at low frequencies characteristic of the breathing of the separation bubble. The separation point of the recirculation bubble and the foot of the reflected shock wave have a fixed location along the flat plate with respect to time. It shows that, in this configuration, the SWBLI unsteadiness is not observed.In order to reproduce and analyse the SWBLI unsteadiness, the simulation of a shock wave turbulent boundary layer interaction (SWTBLI) is performed. A Synthetic Eddy Method (SEM), adapted to compressible flows, has been developed and used at the inlet of the simulation domain for initiating the turbulent boundary layer without prohibitive additional computational costs. Analyses of the results are performed using, among others, the snapshot Proper Orthogonal Decomposition (POD) technique. For this simulation, the SWBLI unsteadiness has been observed. Results suggest that the dominant flapping mode of the recirculation bubble occurs at medium frequency. These cycles of successive enlargement and shrinkage of the separated zone are shown to be irregular in time, the maximum size of the recirculation bubble being submitted to discrepancies between successive cycles. This behaviour of the separation bubble is responsible for a low frequency temporal modulation of the amplitude of the separation and reattachment point motions and thus for the low frequency breathing of the separation bubble. These results tend to suggest that the SWBLI unsteadiness is related to this low frequency dynamics of the recirculation bubble; the oscillations of the reflected shocks foot being in phase with the motion of the separation point
Weidl, Martin S. [Verfasser], i Harald [Akademischer Betreuer] Lesch. "Cosmic-ray propagation in simulations of cross-helical plasma turbulence / Martin S. Weidl. Betreuer: Harald Lesch". München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2015. http://d-nb.info/1074358694/34.
Pełny tekst źródłaMonnier, Arnaud. "Interactions entre perturbations magnétiques macroscopiques et turbulence microscopique dans un modèle 3D d'un plasma de tokamak". Thesis, Aix-Marseille, 2013. http://www.theses.fr/2013AIXM4773/document.
Pełny tekst źródłaIn this thesis, the interaction between tokamak edge plasma and resonant magnetic perturbation (RMPs) is studied. It is mainly used to mitigate quasi-periodic relaxations in enhanced confinement regime. This regime allows to obtain good conditions for nuclear fusion. Introduction of a RMP in a tokamak plasma has been observed to modified the magnetic topology at the edge and decrease the relaxation amplitude up to complete suppression. Previous works studied the RMP effect on a plasma with relaxations, via numerical simulations. The model used for that consider the electrostatic approximation, where the magnetic topology does not evolve in time. In this thesis, the study is done with an edge plasma model taking into account magnetic fluctuations via the numerical code EMEDGE3D. This code has been modified to include the resonant magnetic perturbation. Comparison with reduced models has been carried out on the RMP penetration and the effect of sheared velocity on it. Then a RMP has been induced in a stable plasma, with or without imposed sheared rotation. A condition on the sheared velocity has been identified to avoid the screening effect, that would prevent the RMP penetration, analytically and in numerical simulations. This works has been repeated in a turbulent plasma in presence or not of transport barrier (sheared velocity). The turbulent plasma generate an effective RMP amplification, while the transport barrier is affected by locked convective cells due to the RMP
Varennes, Robin. "Flow drive in tokamak plasmas : competition and synergies between turbulence and neoclassical effects". Electronic Thesis or Diss., Aix-Marseille, 2022. http://www.theses.fr/2022AIXM0558.
Pełny tekst źródłaThe generation of flows in tokamak plasmas is a crucial subject, especially since their control via external momentum injection in future reactors will be near impossible.However, flows play a major role in the stability and performance of a fusion plasma.In this thesis, the generation of the flow perpendicular to the field lines, associated with the radial electric field, is studied in two experimentally relevant contexts.First, the effect of a 3D perturbation of the magnetic field such as the one caused by the modulation coming from the finite number of toroidal coils, also called "ripple", is studied. Such a perturbation affects the toroidal velocity of the plasma, itself generated spontaneously by the turbulence.Numerous experimental studies on different tokamaks have shown that these effects strongly impact the toroidal velocity of the plasma.Using a theoretical model and simulations, the competition and synergy between turbulence and ripple have been observed and quantified.Preliminary studies show that the effect of the ripple on the flows will not be negligible at the edge in ITER.In a second step, recent experiments on the WEST tokamak showing that the radial electric field is sensitive to the winding rate of the magnetic field lines, called "safety factor", are investigated numerically via gyrokinetic simulations.As observed experimentally, these simulations show that the radial electric field increases as the safety factor and the turbulent intensity decrease.The main effect comes from the turbulent energy transfer varying with the safety factor, which favors either very low frequency flows called "zonal flows", or higher frequency flows called "GAMs"
Caschera, Elisabetta. "Global confinement properties in global, flux-driven, gyrokinetic simulations". Electronic Thesis or Diss., Aix-Marseille, 2019. http://www.theses.fr/2019AIXM0509.
Pełny tekst źródłaUnderstanding and predicting the performance of a fusion reactor in terms of confinement is one of the missing milestones for the availability of fusion energy. The predictions for the design of future reactors such as ITER are based on the extrapolation of empiricalscaling laws. We investigate global confinement properties of turbulent heat transport in a Tokamak with first principle simulations. The research is carried on two main topics: the scaling properties of plasma confinement and the effect of the plasma boundary on the turbulent transport. An important result is obtained when reproducing the global scaling for the energy confinement time with numerical simulations. However the scaling properties are found to brake at the local level. The boundary condition of the code has been modified to mimic the experimental Scrape-Off Layer at the plasma edge. Additional physics is now accessible, such as Kelvin-Helmholtz-like instability at separatrix and edge subcritical turbulence
Hariri, Farah. "FENICIA : un code de simulation des plasmas basé sur une approche de coordonnées alignées indépendante des variables de flux". Thesis, Aix-Marseille, 2013. http://www.theses.fr/2013AIXM4760/document.
Pełny tekst źródłaThe primary thrust of this work is the development and implementation of a new approach to the problem of field-aligned coordinates in magnetized plasma turbulence simulations called the FCI approach (Flux-Coordinate Independent). The method exploits the elongated nature of micro-instability driven turbulence which typically has perpendicular scales on the order of a few ion gyro-radii, and parallel scales on the order of the machine size. Mathematically speaking, it relies on local transformations that align a suitable coordinate to the magnetic field to allow efficient computation of the parallel derivative. However, it does not rely on flux coordinates, which permits discretizing any given field on a regular grid in the natural coordinates such as (x, y, z) in the cylindrical limit. The new method has a number of advantages over methods constructed starting from flux coordinates, allowing for more flexible coding in a variety of situations including X-point configurations. In light of these findings, a plasma simulation code FENICIA has been developed based on the FCI approach with the ability to tackle a wide class of physical models. The code has been verified on several 3D test models. The accuracy of the approach is tested in particular with respect to the question of spurious radial transport. Tests on 3D models of the drift wave propagation and of the Ion Temperature Gradient (ITG) instability in cylindrical geometry in the linear regime demonstrate again the high quality of the numerical method. Finally, the FCI approach is shown to be able to deal with an X-point configuration such as one with a magnetic island with good convergence and conservation properties
Viré, Axelle. "Study of the dynamics of conductive fluids in the presence of localised magnetic fields: application to the Lorentz force flowmeter". Doctoral thesis, Universite Libre de Bruxelles, 2010. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/210062.
Pełny tekst źródłaThis interaction is the object of magnetohydrodynamics, a discipline which covers a wide range of applications, from electromagnetic processing to plasma- and astro-physics.
In this dissertation, the attention is restricted to turbulent liquid metal flows, typically encountered in steel and aluminium industries. Velocity measurements in such flows are extremely challenging because liquid metals are opaque, hot and often corrosive. Therefore, non-intrusive measurement devices are essential. One of them is the Lorentz force flowmeter. Its working principle is based on the generation of a force acting on a charge, which moves in a magnetic field. Recent studies have demonstrated that this technique can measure efficiently the mean velocity of a liquid metal. In the existing devices, however, the measurement depends on the electrical conductivity of the fluid.
In this work, a novel version of this technique is developed in order to obtain measurements that are independent of the electrical conductivity. This is particularly appealing for metallurgical applications, where the conductivity often fluctuates in time and space. The study is entirely numerical and uses a flexible computational method, suitable for industrial flows. In this framework, the cost of numerical simulations increases drastically with the level of turbulence and the geometry complexity. Therefore, the simulations are commonly unresolved. Large eddy simulations are then very promising, since they introduce a subgrid model to mimic the dynamics of the unresolved turbulent eddies.
The first part of this dissertation focuses on the quality and reliability of unresolved numerical simulations. The attention is drawn on the ambiguity that may arise when interpretating the results. Owing to coarse resolutions, numerical errors affect the performances of the discrete model, which in turn looses its physical meaning. In this work, a novel implementation of the turbulent strain rate appearing in the models is proposed. As opposed to its usual discretisation, the present strain rate is in accordance with the discrete equations of motion. Two types of flow are considered: decaying turbulence located far from boundaries, and turbulent flows between two parallel and infinite walls. Particular attention is given to the balance of resolved kinetic energy, in order to assess the role of the model.
The second part of this dissertation deals with a novel version of Lorentz force flowmeters, consisting in one or two coils placed around a circular pipe. The forces acting on each coil are recorded in time as the liquid metal flows through the pipe. It is highlighted that the auto- or cross-correlation of these forces can be used to determine the flowrate. The reliability of the flowmeter is first investigated with a synthetic velocity profile associated to a single vortex ring, which is convected at a constant speed. This configuration is similar to the movement of a solid rod and enables a simple analysis of the flowmeter. Then, the flowmeter is applied to a realistic three-dimensional turbulent flow. In both cases, the influence of the geometrical parameters of the coils is systematically assessed.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished
Debliquy, Olivier. "Pseudo-spectral methods applied to hydrodynamic and magnetohydrodynamic turbulence". Doctoral thesis, Universite Libre de Bruxelles, 2004. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211110.
Pełny tekst źródłais no real prospect of a simple analytic theory. Scientists have therefore regarded the numerical simulation as an alternative to compute the relevant properties of turbulent flows. In this context, our thesis aims at developing and using accurate computational methods, namely pseudo-spectral methods, for studying hydrodynamic (1st part) and magnetohydrodynamic (2nd part) turbulence.
In the hydrodynamic part, Chapter I introduces the governing equations of fluid mechanics as well as the main issues related to the numerical study of turbulent flows. In particular, the Direct Numerical Simulations (DNS) of turbulence, in which accurate numerical solutions of the Navier-Stokes equations are obtained, are shown to be limited to moderately turbulent flows.
Chapter II introduces the Large Eddy Simulation (LES) technique which aims at simulating highly turbulent flows and which is based on a separation of scales.
In practice, it consists of simulating the large - resolved - scales of the flow explicitly while modelling the small - unresolved - scales. Two different approaches for modelling the kinetic energy of the unresolved scales are proposed and their respective advantages and drawbacks are discussed.
Chapter III is devoted the study of the mixing-layer using both DNS and LES. It consists of an inhomogeneous turbulent flow which has been studied experimentally and for which well-documented measurements are available. A highly accurate DNS mimicking the same experiment has been produced. It allows to study the inhomogeneity and anisotropy properties of this flow. Also, LES of the same flow, using different models, have been evaluated. In Chapter IV, we explore a pseudo-spectral method to investigate turbulence in a pipe. In this case, the method has to take into account two additional difficulties: i) the presence of the boundary and ii) the axis singularity. We detail how to circumvent these issues.
The second part of the thesis is devoted to magnetohydrodynamic (MHD) turbulence. It concerns phenomena where electrically conducting flows interact with electromagnetism and for which governing equations are derived in Chapter V. In Chapter VI, a detailed analysis of the energy transfers between the magnetic and velocity fields is performed thanks to a high resolution database of homogeneous MHD turbulence. It provides some insights to understand the physics of the nonlinear interactions and is also a valuable diagnostic in the framework of LES modelling. Finally, the inhomogeneous configuration studied in Chapter III has been extended to MHD. Several statistics related to the kinetic and magnetic energies are measured and LES of this flow are performed and presented in Chapter VII.
Doctorat en sciences, Spécialisation physique
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Labit, Benoit. "Transport de chaleur électronique dans un tokamak par simulation numérique directe d'une turbulence de petite échelle". Phd thesis, Université de Provence - Aix-Marseille I, 2002. http://tel.archives-ouvertes.fr/tel-00261562.
Pełny tekst źródłaLa thèse proposée ici cherche à déterminer la pertinence d'un modèle fluide non linéaire, électromagnétique, tridimensionnel, basé sur une instabilité particulière pour décrire les pertes de chaleur par le canal électronique et de déterminer les dépendances du transport turbulent associé en fonction de paramètres adimensionnels, dont β et ρ*. L'instabilité choisie est une instabilité d'échange générée par le gradient de température électronique (Electron Temperature Gradient (ETG) driven turbulence en anglais). Ce modèle non linéaire est construit à partir des équations de Braginskii. Le code de simulation développé est global au sens qu'un flux de chaleur entrant est imposé, laissant les gradients libres d'évoluer.
A partir des simulations non linéaires, nous avons pu mettre en évidence trois caractéristiques principales pour le modèle ETG fluide: le transport de chaleur turbulente est essentiellement électrostatique; les fluctuations de potentiel et de pression forment des structures radialement allongées; le niveau de transport observé est beaucoup plus faible que celui mesuré expérimentalement.
L'étude de la dépendance du transport de chaleur en fonction du rapport de la pression cinétique à la pression magnétique a montré un faible impact de ce paramètre mettant ainsi en défaut la loi empirique d'Ohkawa. En revanche, il a été montré sans ambiguïté le rôle important du rayon de Larmor électronique normalisé dans le tranport de chaleur: le temps de confinement est inversement proportionnel à ce paramètre. Enfin, la faible dépendance du transport de chaleur turbulent en fonction du cisaillement magnétique et de l'inverse du rapport d'aspect a été mise en évidence.
Bien que le niveau de transport observé dans les simulations soit plus faible que celui mesuré expérimentalement, nous avons tenté une confrontation directe avec un choc de Tore Supra. Ce tokamak est particulièrement bien désigné pour étudier les pertes de chaleur électronique. En conservant la plupart des paramètres d'un choc bien référencé de Tore Supra, la simulation non linéaire obtenue donne un seuil en gradient de température proche de la valeur expérimentale. Le niveau de transport observé est plus faible d'un facteur cinquante environ que le transport mesuré. Un paramètre important qui n'a pu être conservé est le rayon de Larmor normalisé.
La limitation en ρ* devra être franchie afin de confirmer ces résultats. Enfin une rigoureuse confrontation avec des simulations girocinétiques permettra de disqualifier ou non l'instabilité ETG pour rendre compte des pertes de chaleur observées.
Mots-clés: fusion thermonucléaire, tokamak, plasma, turbulence ETG, simulations numériques
Zarzoso, David. "Description de l'interaction entre les particules energetiques et les ondes dans les plasmas de fusion". Phd thesis, Ecole Polytechnique X, 2012. http://pastel.archives-ouvertes.fr/pastel-00765271.
Pełny tekst źródłaKrutkin, Oleg. "Theoretical analysis and full-wave simulations combined in the development of the synthetic Doppler reflectometry diagnostics for tokamaks". Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0042.
Pełny tekst źródłaPlasma turbulence is nowadays believed to be responsible for the anomalous transport and consequently the degradation of discharge conditions in magnetic confined fusion devices, such as tokamaks. Since a good energy confinement time is crucial for achieving a positive energy yield, understanding and control of turbulent processes is currently one of the major goals of the Magnetic Confinement Fusion research. To study the plasma turbulence, experimental tools that are able to provide information about its characteristics are necessary. Such tools include microwave diagnostics and, in particular, Doppler reflectometry and radial correlation Doppler reflectometry. While these non-invasive diagnostics benefit from the simplicity of the setup, there are a number of unresolved issues when it comes to the interpretation of the experimental data. Issues such are small-angle scattering and plasma curvature effects limit the range of applicability of the simple interpretation of the measurements, while nonlinear scattering effects make it inapplicable altogether. These problems make it necessary to validate the interpretation of experimental data. Thus, the primary goal of this thesis was to create a synthetic Doppler reflectometry and radial correlation Doppler reflectometry diagnostic for the interpretation of the FT-2 tokamak experimental results. This goal is achieved by applying full-wave IPF-FD3D code to the results of gyrokinetic plasma modelling with ELMFIRE code to obtain the synthetic signals, which are then benchmarked with experimental measurements. The synthetic diagnostic is also used for a more general study of the possibility of nonlinear effects influencing the experimental measurements. Finally, the secondary goal of this thesis was to perform an analytical research of plasma curvature effects, nonlinear scattering and a novel technique for turbulence structures’ characterization. The first principles analytical study was performed by considering the Helmholtz equation and obtaining an analytical expression for the experimental signals. The results for the latter two topics were numerically validated with the partial use of specially developed linear numerical model and the full-wave IPF-FD3D code
Wahl, Jean-Baptiste. "The Reduced basis method applied to aerothermal simulations". Thesis, Strasbourg, 2018. http://www.theses.fr/2018STRAD024/document.
Pełny tekst źródłaWe present in this thesis our work on model order reduction for aerothermal simulations. We consider the coupling between the incompressible Navier-Stokes equations and an advection-diffusion equation for the temperature. Since the physical parameters induce high Reynolds and Peclet numbers, we have to introduce stabilization operators in the formulation to deal with the well known numerical stability issue. The chosen stabilization, applied to both fluid and heat equations, is the usual Streamline-Upwind/Petrov-Galerkin (SUPG) which add artificial diffusivity in the direction of the convection field. We also introduce our order reduction strategy for this model, based on the Reduced Basis Method (RBM). To recover an affine decomposition for this complex model, we implemented a discrete variation of the Empirical Interpolation Method (EIM) which is a discrete version of the original EIM. This variant allows building an approximated affine decomposition for complex operators such as in the case of SUPG. We also use this method for the non-linear operators induced by the shock capturing method. The construction of an EIM basis for non-linear operators involves a potentially huge number of non-linear FEM resolutions - depending on the size of the sampling. Even if this basis is built during an offline phase, we usually can not afford such expensive computational cost. We took advantage of the recent development of the Simultaneous EIM Reduced basis algorithm (SER) to tackle this issue
Solminihac, Florence de. "Effets de perturbations magnétiques sur la dynamique de la barrière de transport dans un Tokamak : modélisation et simulations numériques". Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4743/document.
Pełny tekst źródłaIn this PhD thesis we study the impact of resonant magnetic perturbations on the transport barrier dynamics in a tokamak. In this goal we have performed turbulence tridimensional numerical simulations in the edge plasma of a tokamak, which reproduced the experimental results observed in different tokamaks. In the improved confinement regime (H mode), the transport barrier is not stable : it does relaxation oscillations, which share common features with the ``Edge Localized Modes'' (ELMs). These ELMs both have advantages and drawbacks. On the one hand, they enable to push away the impurities present in the plasma core. But on the other hand, the thermal load induced on the wall during an ELM can damage the first wall materials. For this reason, they must be controlled. This PhD thesis belongs to the frame of the ITER project, which is today in construction in France. On ITER the ELMs control will be compulsory due to the quantity of energy released. Among the different ways of controlling the ELMs, the resonant magnetic perturbations (RMPs) seem promising. These resonant magnetic perturbations are created by external coils. We consider the TEXTOR tokamak case and we consider two configurations for the external coils : first, a resonant magnetic perturbation with several harmonics, which enables to have a stochastic zone at the plasma edge when the magnetic island chains overlap ; then, a resonant magnetic perturbation with a single harmonic, which therefore creates a single magnetic island chain. In this PhD thesis, we focus on the non-axisymmetric equilibrium created in the plasma by the resonant magnetic perturbation
Lalescu, Cristian. "Test particle transport in turbulent magnetohydrodynamic structures". Doctoral thesis, Universite Libre de Bruxelles, 2011. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209908.
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Doctorat en Sciences
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Püschel, Moritz Johannes [Verfasser]. "Electromagnetic effects in gyrokinetic simulations of plasma turbulence / vorgelegt von Moritz Johannes Püschel". 2009. http://d-nb.info/993942067/34.
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