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On the development of a turbulent boundary layer over staggered three dimensional cavities
Get PDFStudies by Gowree et al. have suggested that staggered
three dimensional circular cavities can potentially reduce
the skin friction drag of a turbulent boundary layer developing
over a flat plate. This is true up to a certain
Reynolds number, after which the drag rapidly increases.
The detailed mechanism behind the drag increase and the
possible skin friction drag reduction was not discussed,
due to the limitations in the measurement technique. In
the current experimental campaign the results found by
Gowree et al. have been confirmed, in addition, to more
detailed insight of the turbulent boundary layer over the
first rows of cavities. Through numerical simulations we
aim to give a heuristic explanation of the two distinct behaviours. Here we report some quantitative evidence of
the mechanism that may govern the drag increase, which
is the main focus of the current study. The modification
of the flow field in the presence of the cavities provides a qualitative explanation of the drag reduction benefit which is subjected for further investigation
Time reversal plasmas as a versatile space-time patterning deposition method
Get PDFThe aim of this communication is to discuss the possibility of using the innovative plasma source that we have developed recently, namely the “Space-Time Plasma Steering Source”, as a versatile space-time deposition method. More precisely, we focus on the space-time control capabilities of time reversal plasmas over large and complex 3D surfaces. Simultaneous ignition of several plasmas by time reversal is also experimentally demonstrated. These unprecedented control capabilities of plasmas could lead to the development of an original and versatile method allowing the patterning of complex 2D or 3D structures on large areas
Parameterization using Generative Adversarial Networks for Control Space Reduction in Data Assimilation.
Get PDFThis thesis examines the use of generative adversarial networks (GANs) as a parameterization tool for inverse problems solved with ensemble-based data assimilation methods. Ensemble methods often rely on the assumption of Gaussian distributed parameters in cases where this assumption is not valid, the parameter estimation can be invalid. Parameterization methods allow the transformation of these non-Gaussian parameters into a better suited distribution, and optimally reduce their dimension. Another limitation of ensemble methods is the injection of prior information of the physical relation as a constraint between parameters such as spatial coherence or physical balances. Optimal parameterization should encompass these different properties to facilitate the estimation. The novel approach presented in this work relies on GANs to achieve these objectives. Two application domains are tackled through the present work. In a first application, subsurface reservoir characterization, the objective is to determine geological properties of a numerical reservoir model from the observation of the reservoir dynamical response by the way of data assimilation. Rock facies, that describe the type of rock present in each cell of the numerical model, have to be determined due to their strong influence on the dynamical response. The rock facies spatial distribution is ruled by geological phenomena such as sedimentation and forms well known patterns, like channels, called heterogeneities. The noncontinuous property and their spatial coherence make their characterization by ensemble-based data assimilation algorithms difficult, and requires parameterization. Parameterization is a challenge for numerous heterogeneities, notably channels, due to the numerical cost or the statistical representation of their spatial distribution. A Second application domain is the atmospheric balance in the context of numerical weather prediction. When new observations are available, correction of the atmospheric state is done using ensemblebased data assimilation methods. This correction step can introduce imbalance in the physical state and cause numerical instability during the integration in time of the atmosphere, deteriorating the information brought by the previous observations. The importance of generating or correcting balanced climate, also called initialized atmospheric state, during data assimilation is then a key step in numerical weather prediction. This work aims at presenting the performance of GAN parameterization and its multi-disciplinary applicability to researchers who are not familiar with the domain of deep learning. GAN is an unsupervised deep learning method belonging to the deep generative network family, able to learn a dataset distribution and generate new samples from the learned distribution in an unsupervised way. These synthetic samples are encoded in a low-dimensional latent space that can be sampled from a Gaussian distribution that is suited to perform ensemble data assimilation. Their recent ability to generate complex images led us to consider them as a good candidate for parameterization method. The unsupervised property of this type of parameterization makes it applicable to several diverse domains such as learning the pattern of geological heterogeneities or learning the physical constraints that makes an atmospheric state balanced. This study shows how to train GANs for two different applications : subsurface reservoir and climate data. The use of the parameterization in an ensemble based data assimilation such as ensemble smoother with multiple data assimilation (ES-MDA) is demonstrated for subsurface reservoir. Finally, a posteriori conditioning of the GAN function is examined using derivative free optimization
Développement d'une approche opérationnelle pour l'identification automatique des peupleraies à large échelle par télédétection hypertemporelle. : De l'adaptation de domaine à la création d'un indice spectral dédié
Get PDFLe peuplier cultivé constitue la première essence de feuillus plantée en France. Il joue un rôle économique de premier plan, notamment pour la production de bois-matériau, contreplaqués et emballages légers. Il est également valorisé dans l’industrie papetière. Pourtant, malgré l'importance de la filière, les surfaces couvertes en peupleraies à l'échelle nationale sont encore très incertaines. Selon la source de données utilisée (BD Forêt IGN, cadastre, inventaire forestier), les estimations montrent des écarts de plus de 50 000 ha. La fréquence de mise à jour de ces sources est inadaptée pour suivre le peuplier cultivé dont le cycle de rotation est court (15-20 ans). La télédétection satellitaire est utilisée depuis longtemps pour cartographier les milieux forestiers, qu'il s'agisse de forêts naturelles ou de plantations. Avec l'amélioration constante des caractéristiques spatiale, spectrale et temporelle des capteurs, il est possible d'envisager son appropriation dans un contexte opérationnel, pour un suivi régulier de la ressource sur de grandes étendues. L'objectif de cette thèse est double. Le premier est d'explorer le potentiel des séries temporelles d'images optiques Sentinel-2 pour distinguer automatiquement les peupleraies des autres essences de feuillus en tenant compte de la diversité des contextes populicoles. Le second est de proposer une stratégie de classification à l'échelle nationale en tenant compte de la nonstationnarité spatiale de la réponse spectrale des peupleraies, de l’hétérogénéité des acquisitions, et du nombre limité de données de référence. La démarche adoptée a consisté à étudier différentes techniques d'adaptation de domaine disponibles dans le champ de l'apprentissage automatique. Ces techniques, non supervisées ou semi-supervisées, ont permis de répondre aux contraintes de passage à l'échelle avec un nombre limité d'échantillons de référence supplémentaires. Cette étude a débouché sur la création d'une chaîne de traitement opérationnelle permettant de produire la première carte des plantations de peuplier à l'échelle nationale à partir d'images satellitaires. Elle s'appuie sur un nouvel indice spectral proposé -- le Poplar Index (PI) -- qui exploite les bandes du SWIR et du Red edge des données Sentinel-2. Cet indice, et son évolution annuelle, ont permis de reconnaître les peupleraies avec une précision producteur de près de 95%. Le résultat de ce travail offre à la filière populicole une méthode robuste pour assurer une production annuelle d'une carte des peupleraies avec un niveau de fiabilité adapté
Lunar Orbital Debris Mitigation: Characterisation Of The Environment And Identification Of Disposal Strategies
Get PDFThere is a clear, increasing interest towards the Moon, with the idea of establishing permanent human settlements on its surface and in orbit, assembling and operating a cislunar orbital laboratory, the Lunar Orbital Platform - Gateway (LOP-G), in a L 2 Near Rectilinear Halo Orbit (NRHO). This new scenario will involve multiple actors, including national agencies and private companies, which will insert and operate a high number of spacecraft in the cislunar space. A subject that still remains open and generally unregulated is the end of mission: the majority of past missions have been commanded to impact on the lunar surface, but this option might not be a sustainable long-term solution. In this work, the Circular Restricted 3-Body Problem (CR3BP) is used to study the dynamics of the Earth-Moon system, and several families of periodic orbits are identified and their main dynamical characteristics and behaviours described, with the final aim of creating a cartography of the cislunar space. This review of CR3BP orbits could prove useful both for identifying optimal matches with future applications and provide an estimation of the cislunar space util-
isation and debris environment. Particular focus is given to NRHOs since, in the coming decades, they are likely to become the most targeted orbits for lunar spacecraft and therefore the origin of most artificial debris. Furthermore, disposal strategies available in the cislunar space, such as lunar impact and ejection towards a graveyard orbit, are reviewed and evaluated. To provide a preliminary trade-off for the disposal of planned missions, several scenarios are simulated to assess the optimal mission design ensuring feasibility of the chosen strategy with respect to mission requirements and its sustainability with respect to national and international guidelines
Flash Oxidation Tests to Evidence the Segregation of Boron at the Grain Boundaries of Superalloy Inconel 718
Get PDFIn this study very short oxidation tests, called flash oxidation tests, were carried out at high temperature on samples of superalloy Inconel 718 containing some boron. Following these tests, a specific oxidation pattern was evidenced: spots were observed on the oxide film, these spots being discontinuously distributed along the grain boundaries. Whether outside or inside the spots, the oxide film exhibited the same three oxide layers which were likely rutile oxide (Cr,Nb,Ti)O2, chromia oxide Cr2O3 and spinel oxide (Ni,Fe)(Cr,Fe)2O4. However, inside the spots the oxide film was characterized by thicker oxide layers, a slightly different chemical composition of the spinel layer, and vitreous oxides at the top. Moreover, all the intergranular (Nb,Mo)2CrB2 borides of the microstructure were involved in this specific oxidation pattern, thus leading to the suspicion of high boron concentrations within the spots. FEG-EPMA analyses confirmed that, during the oxidation process, boron atoms diffused from the matrix to the oxide film, especially within the spots. Actually, the spots revealed areas where boron was highly concentrated just before oxidation. Such areas originated from the non-equilibrium segregation of boron near grain boundaries, which is a classical phenomenon when cooling and/or reheating a boron-containing alloy. Thus, the spots of the oxide film highlighted the grain boundary segments where boron segregated the most, confirming that the segregation of boron was not uniform within the grain boundary network
Combined XPS / TEM study of the chemical composition and structure of the passive film formed on additive manufactured 17-4PH stainless steel
Get PDFhe structure and chemical composition of the passive film formed on 17-4PH martensitic stainless steel (MSS) manufactured by powder bed laser beam melting (LBM) were investigated combining XPS and TEM analyses, by comparison to a conventional l 7-4PH. The structure of the passive film was found similar for both MSSs, with, on the matrix, a duplex layer composed of Fe and Cr oxy-hydroxide followed by an intermediate layer enriched in metallic compounds. A particular Nb oxide film formed on the NbC precipitates merging to the surface for both MSSs. Significant differences in thickness and Cr distribution were observed between both MSSs
Theoretical analysis of the electrochemical systems used for the application of direct current/voltage stimuli on cell cultures
Get PDFEndogenous electric fields drive many essential functions relating to cell proliferation, motion, differentiation and tissue development. They are usually mimicked in vitro by using electrochemical systems to apply direct current or voltage stimuli to cell cultures. The many studies devoted to this topic have given rise to a wide variety of experimental systems, whose results are often difficult to compare. Here, these systems are analysed from an electrochemical standpoint to help harmonize protocols and facilitate optimal understanding of the data produced.
The theoretical analysis of single-electrode systems shows the necessity of measuring the Nernst potential of the electrode and of discussing the results on this basis rather than using the value of the potential gradient. The paper then emphasizes the great complexity that can arise when high cell voltage is applied to a single electrode, because of the possible occurrence of anode and cathode sites. An analysis of two-electrode systems leads to the advice to change experimental practices by applying current instead of voltage. It also suggests that the values of electric fields reported so far may have been considerably overestimated in macro-sized devices. It would consequently be wise to revisit this area by testing considerably lower electric field values
Oxygen-reducing bidirectional microbial electrodes designed in real domestic wastewater
Get PDFMicrobial electrodes were designed in domestic wastewaters to catalyse the oxidation of organic matter (anode) and the reduction of oxygen (cathode) alternately. The successive aeration phases (cathode) enhanced the anodic efficiency, resulting in current densities of up to 6.4 Am-2 without the addition of any substrate. Using nitrogen during the anodic phases affected the microbial populations and the electrodes showed a lower ability to subsequently turn to O2 reduction than the microbial anodes formed in open-to-air conditions did. No strong difference was observed between internal and external biofilm, both of which showed a very large variety of taxa in terms of abundance as well as variance. They comprised a mix of aerobic and anaerobic species, many of which have already been identified separately in bioelectrochemical systems. Such a large diversity, which had not been observed in aerobic bidirectional bioelectrodes so far, can explain the efficiency and robustness observed here.
Keywords: Electroactive biofilm; Bioanode; Microbial snorkel; Microbial Fuel Cell; Microbial population
Aided Inertial Estimation of Wing Shape
Get PDFAdvanced large-wing-span aircraft result in more structural flexibility and the potential for instability or poor handling qualities. These shortcomings call for stability augmentation systems that entail active structural control. Consequently, the in-flight estimation of wing shape is beneficial for the control of very flexible aircraft. This paper proposes a new methodology for estimating flexible structural states based on extended Kalman filtering by exploiting ideas employed in aided inertial navigation systems. High-bandwidth-rate gyro angular velocities at different wing stations are integrated to provide a short-term standalone inertial shape estimation solution, and additional low-bandwidth aiding sensors are then employed to bound diverging estimation errors. The proposed filter implementation does not require a flight dynamics model of the aircraft, facilitates the often tedious Kalman filtering tuning process, and allows for accurate estimation under large and nonlinear wing deflections. To illustrate the approach, the technique is verified by means of simulations using sighting devices as aiding sensors, and an observability study is conducted. In contrast to previous work in the literature based on stereo vision, a sensor configuration that provides fully observable state estimation is found using only one camera and multiple rate gyros for Kalman filtering update and prediction phases, respectively