The purpose of this study was to contribute to an analytical framework for creating and validating simulations. This was achieved by investigating viewer response to changes of a single-image variable (field-of-view). Eight images of two different test sites, with fields-of-view that ranged from 40 to 120 degrees, were judged by 47 people. Image representativeness was analyzed by evaluating viewer response of the images to the actual scene. Statistical analysis revealed that simulations with fields-of-view > 40 and ≤ 90 degrees may best represent the test sites, regardless of the scenes viewshed. Preference ratings for the same images were analyzed and compared with response to representativeness. The results of the study reveal no relationship between image preference and the perceived accuracy of the images to represent the test sites. Implications of the findings suggest further research is needed on methods of estimating the validity of visual simulations. / Master of Landscape Architecture

Star cluster formation involves the conversion of molecular gas into stars inside giant molecular clouds (GMCs). Such a process involves many dynamical evolution mechanisms, including mergers between smaller star clusters (subclusters) on which we focus in this thesis. We take results of simulations performed by Howard et al. 2018 (H18) which found that young massive cluster (YMC) formation is heavily dependant on the process of subcluster mergers, and we simulate said mergers at higher resolution. Subclusters inside such GMC simulations are modelled using the sink particle prescription which does not resolve individual star particles or gas parcels inside the subcluster they represent. We employ a more controlled method in simulating subcluster mergers to better understand the response of the stellar and gas components of a subcluster from the merger process. To do this, we take the parameters of the sink particles created in H18 and set up spheres of stars and gas. We use the AMUSE framework to couple the N-body evolution of the stars to the smoothed particle hydrodynamics (SPH) evolution of the gas such that both components of a given cluster can realistically react to each other. We model 15 of these mergers and find that once the velocity at which the two clusters collide (collisional velocity) exceeds $\approx 10$kms$^{-1}$, the resultant cluster is not monolithic (i.e. it still contains two separate stellar components) while all other simulations merge into one monolithic stellar and gas component cluster. We also find that, regardless of the collisional velocity of masses of the component clusters, all resultant clusters lose a fraction of their stellar and gas mass. This fraction is directly proportional to the collisional velocity and is a discrepancy between the sink particle prescription (where all mass is contained inside a constant sink particle accretion radius) and real cluster mergers. A further discrepancy we find is that all simulations result in a cluster whose outermost regions are expanding and that the rate of this expansion is somewhat proportional to the collisional velocity of the merger. These results point to the inaccuracy of the sink particle prescription and allow us to develop tools to improve on it in future simulations. Next, we fit commonly used analytical density profiles to both the stellar and gas component of our resultant clusters and find that, while they do not provide particularly excellent fits, they provide constraints on what is an acceptable fit. Lastly, we analyze the amount by which gas with potentially star forming densities increase due to the merger and we find that all mergers increase their star forming gas mass fraction by roughly 50 per cent implying that mergers may be an effective tool for triggering star formation. / Thesis / Master of Science (MSc)

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Dust grains are a crucial component of disks around young stellar systems where current observations and theory show that planets form. Dust grains must grow 10 orders of magnitude in size to become planets. However, one of the early steps in this growth phase faces stringent theoretical constraints. The metre barrier relates to two well-studied physical mechanisms which inhibit grain growth beyond centimetre sizes. We report on numerical studies which probe these early stages of planet formation including instabilities that promote dust concentration such as the streaming instability (SI). We explore several different SPH models for dusty gas evolution. We find the linear SI is difficult for SPH to capture because it begins with perturbations below the 1% level. We also employ the Athena 3rd order Eulerian code which has been used to study the SI in the linear phase and the non-linear or saturated phase. We present numerical confirmations of recent analytical predictions of enhancements to the SI growth rates caused by the dust settling to the disk midplane in the earliest stages of the protoplanetary disk evolution. Symmetric analytical predictions for SI growth are not directly relevant to the non-axisymetric, planar geometry of the saturated, non-linear phase. We lay the ground work to explore this in future work. / Thesis / Master of Science (MSc)

Parametric instability processes are thought to produce Stimulated Electromagnetic Emissions (SEE) during ionospheric heating experiments. The phenomenon is primarily attributed to plasma turbulence excited by the high frequency HF heater in the altitude region where the pump frequency <i>ω</i>₀ is near the plasma upper hybrid frequency <i>ω_uh</i>. In this study, parametric instability processes thought to produce SEE are studied using theoretical and electrostatic Particle-In-Cell PIC simulation models. The simulation plasma is driven with a uniform oscillating electric field directed nearly perpendicular to the background geomagnetic field {B} to consider interactions when <i>ω_uh</i> is near electron cyclotron harmonics <i>nΩ_ce</i>. The pump frequency and amplitude are varied to consider the effects on the simulation electric field power spectrum. In this study, theoretical predictions and numerical simulations are used to study the three-wave decay instability process thought to be responsible for the generation of the down-shifted sidebands, the downshifted peak DP and the downshifted maximum DM. In particular, the lower hybrid decay instability LHDI and the ion cyclotron decay instability ICDI are studied in detail. The theory is used to provide the angular regime, with respect to the direction perpendicular to the magnetic field, at which the sidebands develop as well as the frequency and wavenumber regimes of both the LHDI and the ICDI. The effect of the temperature ratio <i>T_e/T_i</i> for both instabilities is discussed. A comparison between the theoretical predictions, the simulation electric field power spectrum and the experimental observations are presented in this study. Time evolution of both the LHDI and the ICDI is also investigated. The theoretical predictions are also used to investigate the cascading of the LHDI and the ICDI. The spectra show consistencies with the experimental observations. A four-wave parametric decay instability process thought to be responsible for SEE broad up-shifted sideband spectral features is discussed as well. Many theoretical results are presented, in which the effect of stepping the heater frequency closer to the upper hybrid frequency on the angle of maximum growth <i>θ_max</i>, the growth rate γ and on both the frequency and wavenumber regimes of the four-wave process is investigated. The simulation electric field power spectrum showed a large amplitude up-shifted sideband and a much smaller amplitude down-shifted sideband, consistent with the experimental observations. Comparisons between the theoretical predictions, the simulation electric field power spectrum and the experimental observations are discussed in detail. The time evolution of the four-wave process is one important aspect that is also presented in this study. The development of density irregularities, cavities and particle heating is also analyzed and investigated in this study. / Ph. D.

Dans le cadre du projet INTERMARC (INTERaction dans les Moteurs à Allumage Commandé), la tâche du CORIA a consisté à produire une base de données à l'échelle RANS (provenant de données DNS) afin de tester, valider et modifier le modèle d'interaction développée par IFPen. Ce modèle vise l'ajout d'une composante d'interaction, phénomène non pris en compte par les lois de paroi actuelles.Ce projet repose sur l'interaction forte entre les différents protagonistes présents. Le CORIA et le CETHIL ont travaillé ensemble à la réalisation d'une base de données pour tester les modèles initiaux proposés par IFPen, puis en fonction des résultats obtenus, à itérer avec IFPen pour modifier et améliorer les modèles. Ces tests ont inclus des simulations 2D laminaires, 2D turbulentes, et 3D turbulentes. / Under the INTERMARC project (Flame wall interaction in spark ignition engines), CORIA's job was to produce a database to RANS scale (from DNS data) to test, validate and modify the interaction model developed by IFPEN. This model aims the addition of the interaction phenomena, non-captured by the current wall laws. This project is based on the strong interaction between the different actors. The CORIA and the CETHIL have worked together in the creation of the database, where the experimental data were also used to validate the resuslts of the DNS code.CORIA then used this database to test the original model proposed by IFPPEN, then according to the results obtained, CORIA iterated with IFPEN to modify and improve the models. These tests included laminar 2D simulations, 2D turbulent and 3D turbulent simulations.

Plasmas involving strong electron drift in crossed electric and magnetic fields are of great interest for a number of applications such as space propulsion and material processing plasma sources. Specific applications include Hall thrusters, which are high efficiency, low thrust propulsion systems used on many missions for satellite orbit corrections and for future planned interplanetary missions, as well as magnetrons of various configurations used in plasma deposition devices. Similar conditions also exist in the E-layer of the ionosphere and on the Sun. Despite many successful applications of Hall thrusters and other Hall plasma sources, some aspects of their operation are still poorly understood. A particularly important problem is the anomalous electron transport, which greatly exceeds classical collisional values. Hall plasma devices exhibit numerous turbulent fluctuations in a wide frequency range and it is believed that fluctuations resulting from plasma instabilities are likely one of the main causes of the observed anomalous transport. Plasma turbulence also affects many other important processes such as electron injection, location of the ionization region and wall erosion among others that influence the operation and efficiency of Hall thrusters. In Hall thrusters, the E0xB0 flow is made unstable due to gradients in the plasma density, temperature and magnetic field. The gradient drift instabilities are long wavelength instabilities that propagate in the azimuthal direction. A fluid theory of these unstable modes is proposed. It is shown that a full account of the compressibility of the electron flow in inhomogeneous magnetic field leads to quantitative modifications of the previously obtained instability criteria and characteristics of the unstable modes. The ExB drift also drives ion sound type instabilities in Hall thrusters. The reactive/dissipative response of the closure current to the thruster walls drives these negative energy modes. A model for this type of instabilities is proposed and analyzed for typical Hall thruster conditions. It is shown how wall parameters modify the characteristic growth rate and frequency of the unstable modes and the related anomalous transport. Nonlinear phenomena are important to understand different aspects of the Hall thruster plasma dynamics. A nonlinear fluid model for the typical Hall thruster plasma is proposed. The model takes into account electron inertia, electron collisions with neutrals, density gradients as well as various nonlinear terms that arise from the electron drift and nonlinear polarization that were included via the gyroviscous cancellation. The proposed model includes the long wavelength and the low hybrid modes destabilized by density gradients and collisions. This system of fluid equations was implemented using the computational framework BOUT++ from which a set of nonlinear simulations of plasma turbulence was performed. It is shown from these first principles nonlinear simulations that small scale low hybrid oscillations result in an anomalous electron current significantly exceeding the classical collisional current.

The overall objective of this thesis has been to develop accurate computational methods for the diffusion and adsorption of small gases in zeolites. Firstly, the effect of the zeolite framework flexiblity on the single component and binary diffusion of various gases were discussed. Results indicate that for tight fitting molecules the rigid framework approximation can produce order(s) of magnitude difference in diffusivities as compared to the simulations performed with a fully flexible framework. We proposed two simple methods in which the flexible structure of a zeolite is approximated as a set of discrete rigid snapshots. Both methods are orders of magnitude more efficient than the simulations with the fully flexible structure. Secondly, we use a combined classical and quantum chemistry based approach to systematically develop the force fields based on DFT calculations for interactions of simple molecules like CH4, N2, linear alkanes, and linear alkenes in zeolites. We used a higher level of theory known as the DFT/CC method to correct DFT energies that were used in the periodic DFT calculations to develop force fields. Our results show that DFT-derived force fields give good predictions of macroscopic properties like adsorption isotherms in zeolites. The force fields are transferrable across zeolites and hence can be further used to screen materials for different storage and separation applications.

The thesis considers and examines methods of surface propagation, where the normal velocity of the surface depends on the local curvature and the gradient of the surface. Such fronts occur in many different physical situations from the growth of crystals to the spreading of flames. A number of different methods are considered to find solutions to these physical problems. First the motion is modelled by partial differential equations and numerical methods are developed for solving these equations. The numerical methods involve characteristic, finite differences and transformation of the equations. Stability of the solutions is also briefly considered. Secondly the fronts are modelled by using a cellular approach which subdivides space into regions of small cells. The fronts are assumed to propagate through the region according to stochastic rules. Monte-Carlo simulations are carried out using this approach. Results of the simulations are carried out in two-dimensions and three-dimensions for a number of interesting physical examples.