Modélisation numérique d'un actionneur plasma de type décharge à barrière diélectrique par la méthode de dérive-diffusion

RÉSUMÉ Les actionneurs au plasma de type décharge à barrière diélectrique (DBD) sont un dispositif proposé pour le contrôle actif de l’écoulement en vue d’améliorer les performances des avions et des turbomachines. Ces actionneurs sont essentiellement constitués de deux électrodes séparées par une couche de matériel diélectrique et produisent un effet sur l’écoulement en convertissant l’électricité directement en force d’actionnement. En raison des coûts pour réaliser des expériences dans des conditions réalistes, il existe un besoin pour développer un modèle numérique prédisant la force d’actionnement et l’effet de divers paramètres sur cette dernière. En effet, cette force d’actionnement est notamment affectée par les conditions atmosphériques (température, pression et humidité), la vitesse du fluide neutre, la tension appliquée sur les électrodes (amplitude, fréquence et forme d’onde) et la géométrie de l’actionneur. Dans cette perspective, l’objectif de ce mémoire est d’implémenter un modèle plasma pour actionneur DBD ayant le potentiel de prédire l’effet de ces différents paramètres sur la force d’actionnement. En modélisation d’actionneur DBD, deux types d’approche sont proposés, à savoir modélisation à faible ordre (ou phénoménologique) et modélisation à ordre élevé (ou scientifique). Toutefois, une étude critique, présentée dans ce mémoire, a révélé que l’approche phénoménologique est basée sur des hypothèses erronées et n’a pas la robustesse nécessaire pour prédire une force d’actionnement sans calibration artificielle pour chaque cas spécifique. Ainsi, l’approche choisie pour modéliser la force d’actionnement est un modèle scientifique de type dérive-diffusion prenant en compte quatre espèces chimiques (électrons, ions positifs, ions négatifs et éléments neutres). Ce modèle a été choisi, car les résultats obtenus avec ce dernier sont consistants avec les observations expérimentales. De plus, il possède un bon potentiel d’amélioration pour tenir compte de l’effet de la pression, de la température et de l’humidité de l’air en plus de nécessiter qu’un temps de calcul raisonnable. Le modèle sélectionné a été indépendamment implémenté en langage C++ et validé pour différents cas de test. Le modèle a été ensuite utilisé pour simuler l’effet de la force d’actionnement sur la transition laminaire-turbulente d’un profil d’aile afin de valider la performance des simulations CFD utilisant ce modèle. Les résultats obtenus montrent que ce modèle prédit mieux l’effet de la force d’actionnement sur l’écoulement qu’un modèle phénoménologique pour un cas pratique en aérospatial.----------ABSTRACT Dielectric barrier discharge (DBD) plasma actuator is a proposed device for active for control in order to improve the performances of aircraft and turbomachines. Essentially, these actuators are made of two electrodes separated by a layer of dielectric material and convert electricity directly into flow. Because of the high costs associated with experiences in realistic operating conditions, there is a need to develop a robust numerical model that can predict the plasma body force and the effects of various parameters on it. Indeed, this plasma body force can be affected by atmospheric conditions (temperature, pressure, and humidity), velocity of the neutral flow, applied voltage (amplitude, frequency, and waveform), and by the actuator geometry. In that respect, the purpose of this thesis is to implement a plasma model for DBD actuator that has the potential to consider the effects of these various parameters. In DBD actuator modelling, two types of approach are commonly proposed, low-order modelling (or phenomenological) and high-order modelling (or scientific). However a critical analysis, presented in this thesis, showed that phenomenological models are not robust enough to predict the plasma body force without artificial calibration for each specific case. Moreover, there are based on erroneous assumptions. Hence, the selected approach to model the plasma body force is a scientific drift-diffusion model with four chemical species (electrons, positive ions, negative ions, and neutrals). This model was chosen because it gives consistent numerical results comparatively with experimental data. Moreover, this model has great potential to include the effect of temperature, pressure, and humidity on the plasma body force and requires only a reasonable computational time. This model was independently implemented in C++ programming language and validated with several test cases. This model was later used to simulate the effect of the plasma body force on the laminar-turbulent transition on airfoil in order to validate the performance of this model in practical CFD simulation. Numerical results show that this model gives a better prediction of the effect of the plasma on the fluid flow for a practical case in aerospace than a phenomenological model

A new positive relationship between pCO2 and stomatal frequency in Quercus guyavifolia (Fagaceae): a potential proxy for palaeo-CO2 levels

Background and Aims The inverse relationship between atmospheric CO2 partial pressure (pCO2) and stomatal frequency in many species of plants has been widely used to estimate palaeoatmospheric CO2 (palaeo-CO2) levels; however, the results obtained have been quite variable. This study attempts to find a potential new proxy for palaeo-CO2 levels by analysing stomatal frequency in Quercus guyavifolia (Q. guajavifolia, Fagaceae), an extant dominant species of sclerophyllous forests in the Himalayas with abundant fossil relatives. Methods Stomatal frequency was analysed for extant samples of Q. guyavifolia collected from17 field sites at altitudes ranging between 2493 and 4497 m. Herbarium specimens collected between 1926 and 2011 were also examined. Correlations of pCO2-stomatal frequency were determined using samples from both sources, and these were then applied to Q. preguyavaefolia fossils in order to estimate palaeo-CO2 concentrations for two late-Pliocene floras in south-western China. Key Results In contrast to the negative correlations detected for most other species that have been studied, a positive correlation between pCO2 and stomatal frequency was determined in Q. guyavifolia sampled from both extant field collections and historical herbarium specimens. Palaeo-CO2 concentrations were estimated to be approx. 180-240 ppm in the late Pliocene, which is consistent with most other previous estimates. Conclusions A new positive relationship between pCO2 and stomatal frequency in Q. guyavifolia is presented, which can be applied to the fossils closely related to this species that are widely distributed in the late-Cenozoic strata in order to estimate palaeo-CO2 concentrations. The results show that it is valid to use a positive relationship to estimate palaeo-CO2 concentrations, and the study adds to the variety of stomatal density/index relationships that available for estimating pCO2. The physiological mechanisms underlying this positive response are unclear, however, and require further researc

A CMOS compatible ultracompact silicon photonic optical add-drop multiplexer with misaligned Sidewall Bragg gratings

We experimentally and via simulations demonstrate ultracompact single-stage and cascaded optical add-drop multiplexers using misaligned sidewall Bragg grating in a Mach-Zehnder interferometer for the silicon-on-insulator platform. The single-stage configuration has a device footprint of 400 μm × 90 μm, and the cascaded configuration has a footprint of 400 μm × 125 μm. The proposed designs have 3-dB bandwidths of 6 nm and extinction ratios of 25 dB and 51 dB, respectively, and have been fabricated for the transverse electric mode. A minimum lithographic feature size of 80 nm is used in our design, which is within the limitation of 193 nm deep ultraviolet lithography

Isobutyl acetate: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy and ab initio calculations

The high-resolution vacuum ultraviolet photoabsorption spectrum of isobutyl acetate, C6H12O2, is presented here and was measured over the energy range 4.3–10.8 eV (290–115 nm). Valence and Rydberg transitions with their associated vibronic series have been observed in the photoabsorption spectrum and are assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. The measured photoabsorption cross sections have been used to calculate the photolysis lifetime of this ester in the Earth’s upper atmosphere (20–50 km). Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl acetate and are compared with a photoelectron spectrum (from 9.5 to 16.7 eV), recorded for the first time. Vibrational structure is observed in the first photoelectron band of this molecule