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Prochaines Soutenances de Thèse

31 Mars 2022 - Numerical modelling of core/edge turbulent transport in tokamak with realistic geometry using an advanced numerical method / PhD defense Manuel SCOTTO D' ABUSCO
Doctorant : Manuel SCOTTO D' ABUSCO

Date : 31 Mars 2022 à 14h00, salle de réunion M2P2

Abstract : Nowadays a challenge remains the design of optimized plasma scenarios for tokamak operation to control the heat flow from the core region to the wall. This calls for the development of efficient and reliable numerical codes with predictive capabilities for plasma simulations. Despite significant progress the last years, predictive capabilities of current transport codes are still acknowledged by the international community as being largely insufficient. Among the new capabilities for the codes expected to get predictive capabilities, the accurate discretization of real geometries of tokamak chambers, the flexibility with respect to the magnetic geometry and the computational efficiency in terms of speed have been yet clearly identified by the fusion community. To solve all these issues, a new approach is proposed in the present work which aims to develop a high-order finite elements code based on hybrid discontinuous Galerkin numerical scheme and an efficient implicit time integration method for solving non isothermal Braginskii reduced fluid equations in versatile tokamak and magnetic equilibrium geometries. The use of such numerical scheme allows to perform simulations with time evolving magnetic configurations, avoiding expensive re-meshing of the computational domain. The first part of the thesis shows the structure and the realization of such a numerical tool. The feasibility of the latter is then investigated through a careful validation and benchmarking operation with SolEdge3X. The physical model is then enriched with self-consistent sources of particle due to plasma recycling and sources of energy due to Ohmic heating. In particular, a fluid model for neutrals is implemented and coupled to the plasma by ionization-recombination-radiation terms. The introduction of the sources allows to perform 2D simulation of a domain of computation which encompass a full poloidal cross section of the tokamk. The capability of such a model of reproducing the main features of a detached plasma is investigated in the second part of this work for the WEST tokamk machine. Then the first core-edge transport simulations of an entire WEST discharge (shot #54487) are shown from the start-up phase to the final plasma landing. The experimental magnetic equilibrium which evolves from a high field side limiter to a X-point configuration is implemented in the simulation together with the experimental gas puff time evolution. Comparisons between experimental interferometry and synthetic simulation data show a remarkable agreement in the plasma center with an accurate prediction of the ramp up and the flat top phase of the central lines integrated density. The agreement is less good however in the vicinity of the X-point. Finally, the time evolution during the discharge of the particles and heat fluxes at the wall, are analyzed showing in particular the energy redistribution between ion and electron during the discharge. The present result are also used to assess the tungsten sputtering, using both a simple cinematic model and the impurity tracker monte-carlo code ERO2.0. The analysis confirms the need to consider full discharge simulation to accurately treat the W source of contamination. The work also demonstrates the interest of developing magnetic equilibrium free solver including efficient time integration to tend toward predictive capabilities in the future fusion operation. 

Jury
Directeur de these  M. Eric SERRE  CNRS / M2P2
CoDirecteur de these  M. Hugo BUFFERAND  CEA-IRFM
Examinateur  M. Mathias GROTH  Aalto University
Examinateur  M. Sebastijan BREZINSEK  Institut für Energie- und Klimaforschung
Examinateur  M. Giorgio GIORGIANI ITER
Rapporteur  M. Nicola VIANELLO  Consorzio RFX
Rapporteur  M. Gravier ETIENNE  Université de Loirrane
Président  Mme Clarisse BOURDELLE  CEA- IRFM
15 Mars 2022 - Influence of Radiative Effects on Buoyancy-induced Flows in High-pressure Compressor Inter-disk Cavities / PhD defense Ahmed HODAIB
Doctorant : Ahmed HODAIB 

Date de soutenance : le 15 mars 2022 à 15h00 ; Amphi 3 Centrale Marseille

Abstract  : In aircraft engines, a secondary air flow is obtained from an intermediate compressor stage, to be used to cool the turbine disks. This flow passes through the high-pressure compressor inter-disk cavities (Farthing et al., ASME J. Turbomach., 1992). A better understanding of this complex buoyancy-induced flow is essential to determine the thermal stresses, the radial growth of the blades, due to thermal expansion, and the temperature rise of the air used for cooling. Besides, to be able to determine the optimum clearance between the rotating blades and the surrounding casing, in order to improve the engine performance. This convective flow is not only unsteady and three-dimensional, it is unstable. Due to high temperature differences, the flow and heat transfer give rise to a strongly conjugate problem: the flow is affected by the temperature of the disks, and vice versa (Owen & Long, ASME J. Turbomach., 2015). The compressible Navier-Stokes equations, coupled with the energy equation and perfect gas law, are solved in the framework of the Low Mach Number (LMN) approximation, allowing a reduction of computational costs by filtering the high-speed waves while keeping a good accuracy by considering the compressibility effects (Motheau & Abraham, J. Comput. Phys., 2016). A fourth-order compact spatial discretisation scheme combined with parallelised Fourier method is implemented on a staggered grid. A second-order semi-implicit scheme is introduced for time integration. A two-step algorithm is developed for the solution of the LMN equations. In a first step, the thermodynamic variables are calculated through an iterative process, and used to compute the velocity divergence. In a second step, the variable density continuity and momentum equations are solved using a projection-type method. A parallelized iterative domain decomposition technique is implemented for the simulation of the three-dimensional flow and heat transfer in a T-shape model cavity. The parallelisation of the resulting computational code is performed through a hybrid MPI/OpenMP approach. Spatial and temporal accuracies of the proposed algorithm are checked on a manufactured solution in a simplified configuration. Then, the algorithm is applied to study the flow and heat transfer in an idealised compressor inter-disk cavity, while considering conduction inside the walls, to allow for a more accurate thermal balance. The results are compared with data available in the literature based on local Nusselt numbers. A parametric study is done for a range of the two main parameters governing the flow, according to Farthing et al. (ASME J. Turbomach., vol. 114, pp. 229-236 and pp. 237-246, 1992): the temperature difference and the Rossby number. To include surface radiation exchanges, the discrete radiative heat transfer equation is solved based on the zonal method. The adequacy of the proposed Low Mach number approach is shown, compared to Boussinesq approximation. Moreover, the validity of neglecting the gravitational acceleration with respect to the centrifugal acceleration in the equations is discussed. Then, the definition of an effective Rayleigh number is established, where both centrifugal and gravitational accelerations are taken into account in the buoyancy terms. The results reveal that the flow exhibits a Poiseuille-Rayleigh-Bénard-like instability, and that this effective Rayleigh number governs the flow structure and the heat transfer in the whole cavity, and hence the stability of the flow. In the end, it is shown that radiative exchanges become more significant the more we get closer to the inner radius of the cavity, in agreement with the results reported by Tang & Owen (ASME J. Turbomach., 2021). It is observed that the temperature profiles at the upstream and downstream disks approach each other, when radiation is considered, where the upstream disk temperatures increase. 

Jury
Directeur de these M. Anthony RANDRIAMAMPIANINA Aix Marseille Université 
Rapporteur M. Gary D. LOCK University of Bath, UK
Rapporteur M. Artur TYLISZCZAK Czestochowa University of Technology, Poland
Président  M. Pierre SAGAUT Aix-Marseille Université
Examinateur M. Innocent MUTABAZI Normandie Université
Examinateur M. Stéphane ABIDE Université de Perpignan Via Domitia
CoDirecteur de these Mme Isabelle RASPO Aix-Marseille Université
CoDirecteur de these M. Stéphane VIAZZO Aix-Marseille-Université
4 Février 2022 - Lattice-Boltzmann methods for compressible flows / PhD defense Gabriel Farag
Doctorant : Fabriel FARAG

Date de soutenance : le 4 février 2022 à 14h00 ; Amphi 3 Centrale Marseille

Abstract  : Since the late 1970's, computational fluid dynamics solvers became essentials due to increasingly complex applications requiring fluid solutions. The small scales necessary for industrial applications often need a very fine grid or very small timestep. This dramatically increases the computational cost of nowadays simulations. To design more computationally efficient solvers, a popular approach is to use Lattice-Boltzmann methods. Originating from the kinetic theory of gases, this method have gained a tremendous popularity among fluid dynamicists due to its cheap and easily implemented collide & stream algorithm. However, its intrinsic assumptions confines classical Lattice-Boltzmann solvers to weakly compressible flows. Yet, some compressible models have been proposed. The purpose of this manuscript is to improve the robustness as well as accuracy of compressible Lattice-Boltzmann models. To this end, the Lattice-Boltzmann method is fully reinterpreted as a numerical scheme. This allows a straightforward and parsimonious derivation of the equivalent Navier-Stokes-Fourier system using the sole assumption of a negligible timestep. Using this formalism, the order of accuracy is shown to depend on the collision kernel, as well as the mechanical constitutive model. Various models are investigated and we show that the Knudsen number is not the sole parameter controlling the consistency with the Navier-Stokes-Fourier model. Additionally, capabilities of the entropy equation to model low supersonic flows is explained through standard shock wave theory arguments. A MUSCL-Hancock scheme is employed to discretize the entropy equation and improve both stability and accuracy compared to previous schemes. Equipped with this new formalism, a compressible pressure-based model is proposed and validated on various supersonic test cases. Then, we unify all compressible models proposed by our group under a single formalism and investigate the differences and optimal choices for the various degrees of freedom of our family of models. Finally, this unified model is validated on high supersonic smooth flows and low supersonic shocked flows. 

Jury
Directeur de these M. Pierre BOIVIN CNRS / M2P2
CoDirecteur de these  M. Guillaume CHIAVASSA  Centrale Marseille
Rapporteur M. Rémi ABGRALL Univertität Zürich
Rapporteur M. Jonas LATT Université de Genève
Examinateur Mme Paola CINNELLA Sorbonne Université
Examinateur M. Manfred KRAFCZYK Technische Universität Braunschweig
Examinateur M. Pierre SAGAUT Aix-Marseille Université / M2P2
15 Décembre 2021 - Study of the energy potential for a water supply network / Soutenance de thèse Gautier HYPOLITE
Doctorant : Gautier HYPOLITE

Date de soutenance :  mercredi 15 Décembre 2021 à 14:00 (Amphithéâtre du CEREGE / Technopôle de l'Arbois-Méditerranée, BP80, 13545 Aix-en-Provence)

Abstract : In order to reduce fossil fuels consumption for heating and cooling, different heat sources can be considered. Given theamount of water they carry, water supply systems can play this role and appear to have a high thermal potential. To date, this source has not been used: the main problem is to optimize the sizing of the equipment according to the temporal variability of water flow, water temperature, and the heat (or cold) demand. A first task is to evaluate the available thermal energy. For this purpose, a model based on a minimum number of measurements has been developed. It allows to determine the annual evolution of the temperature and the flow at each point of the network. Temporal variations of water demand and soil surface temperature are taken into account. The ground surface temperature is obtained by satellite measurements. Water flow, soil temperature and water temperature measurements in the network are performed to validate the models and the soil thermal properties. A simulation of the water system hydraulic and thermal behavior is performed for the year 2018 and compared to these measurements. The impact on the water temperature of adding several heat exchanges to the network is then evaluated with this model. In this study, the potential of a raw water system (composed of 5000 km of pipes, and transporting 200 million cubic meters of water per year in the south of France) is studied. As the temperature, the flow rate and heat demand are highly time dependent, a method has been developed to optimize the sizing and location of the exchange systems. This method is based on minimizing the entropy generation in the heat exchanger between the water pipes and the users. The dynamic behavior of a simple heat exchanger (concentric tube) between the network and the user is modeled (pressure profile and fluids and wall temperature calculation). The value of entropy generation due to temperature difference and pressure drop in the exchanger is obtained in transient operation, this value is used as an objective function for the optimization. The results based on the cooling of a data center show that the entropy gain is significant when the optimal size of the heat exchanger is chosen. The use of the raw water network connected to a reversible heat pump for heating and cooling a building has also been studied and results in a high gain compared to an air source heat pump. 

Jury :
Directeur de these M. Jean-Henry FERRASSE Aix Marseille Université
Rapporteur M. Clausse MARC INSA LYON
Rapporteur M. Francois LANZETTA Unversité de Franche-Conté
Examinateur Mme Nathalie MAZET Université de Perpignan
CoDirecteur de these M. Olivier BOUTIN Aix Marseille Université
Examinateur M. Sylvain SERRA LaTEP
26 November 2021 - Ultrafiltration as urban wastewater tertiary treatment for water reuse at semi-industrial scale / Thesis defense Jiaqi YANG
Doctorant : Jiaqi YANG 

Date de soutenance :   Vendredi 26 November 2021 à 10h (Grand Amphithéâtre du CEREGE - Site de l'Arbois) 

Abstract : Water reuse is a sustainable development strategy that benefits society and future generations. In this study, a semi-industrial ultrafiltration (UF) pilot plant established at the outlet of a wastewater treatment plant was studied to assess its feasibility and sustainability for non-potable water reuse. The optimization of operating conditions made it possible to support reliable and sustainable filtration performance, the operating conditions were optimized through comparative analysis in terms of water quality, permeability variation, irreversible fouling management, and water recovery rate. The best conditions were J80t40BW1/3 (flux of 80 L·h−1·m−2, filtration cycle time of 40 min, 1 air backwash followed by 3 classical backwashes), J60t60BW1/4 and J60t60BW1/3. The long-term study on condition J60t60BW1/3 provides sustainable and adaptable filtration performance regardless of the temperature and feed water quality variation. In addition, the air backwashes enabled excellent reversibility of membrane fouling, which was approximately 1.25 to 2 times higher than of classic backwashes in average. The quality of the UF permeate was good enough to be reused in non-potable purposes as it met reuse guidelines of the World Health Organization, reuse standards of France, and the most recent EU regulation for agricultural irrigation. A specific study of membrane cleaning has shown that the addition of NaClO in backwash water can greatly increase cleaning efficiency of air backwashes. Finally, the calculation of the capital expenditure (CAPEX) and operational expenditure (OPEX) of the UF system under optimized conditions gives a profitable net unit price for water production. Through this thesis, UF is confirmed to be a reliable tertiary treatment for water reuse and the results give operational indications for the industrial scale and provides proposals for the management of membrane fouling by air backwash with chemical assistance. 

Jury :
Annabelle COUVERT (Examinateur) / Professeur des Universités, ISCR, ENSC Rennes
Lionel ERCOLEI (Membre invité) /Directeur de l’Innovation, Société des Eaux de Marseille Métropole
Marc HÉRAN (Rapporteur) / Professeur des Universités, IEM, Université de Montpellier
Stéphanie LABORIE (Rapporteur) / Maître de Conférences HDR, TBI, INSA Toulouse
Mathias MONNOT (Co-Directeur de Thèse) / Maître de Conférences, M2P2, Aix-Marseille Université
Philippe MOULIN (Directeur de Thèse) / Professeur des Universités, M2P2, Aix-Marseille Université
Patrick SAUVADE (Membre Invité) / Product manager, Aquasource, Toulouse

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