<p>Granular systems are ensembles of inelastic particles which dissipate energy during collisions. Granular systems serve as excellent models for a wide variety of materials such as sand, soils, corn, and powder. A rather remarkable property of granular systems is when excited, whether due to an interstitial fluid or via the boundaries, the granular particlesdisplay fluid-like behavior. As a result, there has been decades of granular research with the overarching goal of formulating a general granular hydrodynamic theory.</p> <p>However, the granular hydrodynamic theory is limited, and the underlying transport coefficients often require modifications which are based on empirical observations, and assuch, are system-specific. It is ideally better to devise a general theory which minimizes the information needed about the systema priori. The main thrust of the work undertaken shown here strives to develop such a model by using kinetic theory as the basis. More specifically, I investigate granular gases via the direct simulation Monte Carlo (DSMC) methodand modify the governing equations. In this thesis, two idealized cases of granular gases areconsidered: the homogeneous cooling state and a boundary-heated gas (or the pure conduc-tion case). In the former, the effects of polydispersity are probed. In the latter, the evolutionof the local hydrodynamics due to strong rarefaction effects are divulged. Additionally, amodified, more generalized constitutive relation for the heat flux is proposed and comparedwith DSMC results. Extensions of the DSMC method for dense granular gases and granulargases composed of non-spherical particles are also discussed.</p>

In this research, subsonic rarefied flows over a flat-plate at constant pressure are investigated using the direct simulation Monte Carlo (DSMC) technique. An infinitely thin plate (either finite or semi-infinite) with zero angle of attack is considered. Flows with a Mach number of 0.102 and 0.4 and a Reynolds number varying between 0.063 and 246 are considered covering most of the transitional regime between the free-molecule and the continuum limits. A two-dimensional DSMC code of G.A. Bird is used to simulate these flows, and the code is modified to examine the effects of various inflow and outflow boundary conditions. It is observed that simulations of the subsonic rarefied flows are sensitive to the applied boundary conditions. Several extrapolation techniques are considered for the evaluation of the flow properties at the inflow and outflow boundaries. Among various alternatives, four techniques are considered in which the solutions are found to be relatively less sensitive. In addition to the commonly used extrapolation techniques, in which the flow properties are taken from the neighboring boundary cells of the domain, a newly developed extrapolation scheme, based on tracking streamlines, is applied to the outflow boundaries, and the best results are obtained using the new extrapolation technique together with the Neumann boundary conditions. With the new technique, the flow is not distorted even when the computational domain is small. Simulations are performed for various freestream conditions and computational domain configurations, and excellent agreement is obtained with the available experimental data.

Thermochemical nonequilibrium widely exists in supersonic combustion, cold plasma and hypersonic flight. The effect can influence heat transfer, surface ablation and aerodynamic loads. One distinct feature of it is the coupling between internal energy excitation and chemical reactions, particularly the vibration-dissociation coupling. The widely used models are empirical and calibrated based on limited experimental data. Advances in theories and computational power have made the first-principle calculation of thermal nonequilibrium reaction rates by methods like quasi-classical trajectory (QCT) almost a routine today. However, the approach is limited by the uncertainties and availability of potential energy surfaces. To the best of our knowledge, there is no study of thermal nonequilibrium transport properties with this approach. Most importantly, non-trivial effort is required to process the QCT data and implement it in flow simulation methods. In this context, the first part of this work establishes the approach to compute transport properties by the QCT method and studies the influence of thermal nonequilibrium on transport properties for N₂-O molecules. The preponderance of the work is the second part, a comprehensive study of the development of a new thermal nonequilibrium reaction model based on reasonable assumptions and approximations. The new model is as convenient as empirical models. By validating against recent QCT data and experimental results, we found the new model can predict nonequilibrium characteristics of dissociation reactions with nearly the same accuracy as QCT calculations do. In general, the results show the potential of the new model to be used as the standard dissociation model for the simulation of thermochemical nonequilibrium flows.

<p> The Virtual Super-resolution Optics with Reconfigurable Swarms (VISORS) mission  aims to produce high-resolution images of solar release sites in the solar corona using a  distributed telescope. The collected data will be used to investigate the existence of underlying  energy release mechanisms. The VISORS telescope is composed of two spacecraft flying in a  formation configuration. The optics spacecraft (OSC) hosts the optic system, while the detector  spacecraft (DSC) is located behind the OSC in alignment with the Sun and houses a detector.  The two modes of operation for the CubeSats are Science Operations Mode and Standby Mode.  In Science Operations Mode, the two spacecraft are at a close distance which may make the plume impingement an issue. The cold gas thruster propulsion systems in both the OSC and  DSC use R-236fa (HFC) refrigerant. The plume from the system is modeled using SPARTA  Direct Simulation Monte Carlo (DSMC) Simulator while the refrigerant itself is modeled using  an equivalent particle that closely matches viscosity and specific heat. This work aims to  investigate plume propagation for two different flow inputs. The DSMC simulations are  performed with the input parameters acquired using the isentropic relations and CFD simulations  of the 2D axisymmetric nozzle flow. Additionally, the DSMC results are compared to the  Boynton-Simons, Roberts-South, and Gerasimov analytical plume models. </p>

The inductively coupled plasma mass spectrometer (ICP-MS) has been used in laboratories for many years. The majority of the improvements to the instrument have been done empirically through trial and error. A few fluid models have been made, which have given a general description of the flow through the mass spectrometer interface. However, due to long mean free path effects and other factors, it is very difficult to simulate the flow details well enough to predict how changing the interface design will change the formation of the ion beam. Towards this end, Spencer et al. developed FENIX, a direct simulation Monte Carlo algorithm capable of modeling this transitional flow through the mass spectrometer interface, the transitional flow from disorganized plasma to focused ion beam. Their previous work describes how FENIX simulates the neutral ion flow. While understanding the argon flow is essential to understanding the ICP-MS, the true goal is to improve its analyte detection capabilities. In this work, we develop a model for adding analyte to FENIX and compare it to previously collected experimental data. We also calculate how much ambipolar fields, plasma sheaths, and electron-ion recombination affect the ion beam formation. We find that behind the sampling interface there is no evidence of turbulent mixing. The behavior of the analyte seems to be described simply by convection and diffusion. Also, ambipolar field effects are small and do not significantly affect ion beam formation between the sampler and skimmer cones. We also find that the plasma sheath that forms around the sampling cone does not significantly affect the analyte flow downstream from the skimmer. However, it does thermally insulate the electrons from the sampling cone, which reduces ion-electron recombination. We also develop a model for electron-ion recombination. By comparing it to experimental data, we find that significant amounts of electron-ion recombination occurs just downstream from the sampling interface.

Cette thèse traite, dans une première partie, de la propagation unidimensionnelle de fronts de réactions exothermiques, à différentes échelles de description. Dans une approche macroscopique, la quantité de chaleur dégagée par la réaction vient coupler l'équation de convection-réaction-diffusion et les équations de l'hydrodynamique. Ce travail montre l'existence d'un domaine interdit de vitesses de propagation pour un front d'onde chimique stationnaire. Il met en évidence une transition entre une propagation principalement déterminée par les processus de réaction-diffusion, pour de faibles chaleurs de réaction, et une propagation principalement déterminée par les équations de l'hydrodynamique et l'équation d'état du fluide, pour une quantité de chaleur plus importante. Cette bifurcation est illustrée dans les cas d'un gaz parfait et d'un fl uide de van der Waals. La simulation microscopique de la dynamique des particules par la méthode 'Direct Simulation Monte Carlo' (DSMC) permet de retrouver ces résultats pour un gaz dilué. Dans une seconde partie, cette thèse développe un modèle de précipitation d'aiguilles de gypse à partir de grains d'hémihydrate de sulfate de calcium ainsi qu'un algorithme de simulation de la prise du plâtre à une échelle submicrométrique. Les résultats de simulation sont comparés à ceux issus d'une approche déterministe et d'une approche stochastique par une équation maîtresse. En dégageant un ensemble de paramètres ajustables et interprétables physiquement, le modèle permet de proposer une explication de l'effet d'un traitement industriel con dentiel améliorant la cinétique de formation et la morphologie du matériau final.

Le traitement des conditions aux limites et la modélisation fine des interactions de sous-maille ont été abordés dans cette thèse. La formulation caractéristique des conditions aux limites a été analysée et une nouvelle procédure 3D-NSCBC est proposée qui autorise la prise en compte de l’évolution de la vitesse et de la pression dans le plan des frontières, afin d’introduire le caractère tridimensionnel de l’écoulement dans les conditions limites. Des nouvelles formulations pour resoudre le couplage des ondes caractéristiques au niveau des arêtes et des coins ont été développées. Dans le cadre de la Simulation des Grandes Échelles, pour reproduire correctement la dynamique de la turbulence à la paroi et pour mieux prendre en compte l'anisotropie du tenseur des contraintes de sous-maille, un modèle structural fondé sur l'hypothèse de similarité est développé pour des écoulements modérément compressibles et validé sur la simulation d'un jet rond en impaction sur une paroi plane. / The treatment of boundary conditions and sub-grid scale interactions’ modeling, with particular attention to the asymptotic behavior near the wall, were addressed in this thesis. The characteristic formulation of boundary conditions has been analyzed and a novel procedure 3D-NSCBC is proposed, which, accounting for the evolution of velocity and pressure on the boundary planes, allows a better representation of the three-dimensional character of the flow at the boundary. New formulations to solve characteristic wave coupling on edges and corners are developed. Within the framework of the Large-Eddy Simulation, in order to give a correct reproduction of near-wall turbulence dynamics and in order to better account for the sub-grid scale stress tensor’s anisotropy, a structural model based on the similarity hypothesis has been developed for weakly compressible flows and validated on the simulation of a round jet impinging over a flat plane.

No description available.

<p>As the small satellite market has grown from a niche of the space economy to a full commercial force,  microthrusters remain an area of significant growth in the space industry as new technologies mature. The \textit{Film-Evaporation Microelectricalmechanical Tunable Array} (FEMTA) is one such device. FEMTA is \textit{microelectricalmechanical system} (MEMS) device that harnesses the microcapillary action of water and vacuum boiling to generate thrust. The water propellant is not chemically altered at all by the process; it is simply evaporated. This technology has been tested in relevant laboratory environments, and a suborbital flight opportunity in 2023 as a payload on a Blue Origin New Shepard rocket  will grant FEMTA a demonstration in a space environment. The flight will provide 150 seconds of weightlessness at the zenith of the suborbital flight path before the booster returns to land. During weightlessness, the experiment will be exposed to the ambient environment allowing for a full capability test of the thruster. The experiment is meant to demonstrate the propellant management system for FEMTA in 0G and measure the thrust produced by a FEMTA thruster.</p> <p><br></p> <p>The propellant management system portion of the experiment consists of an oversized version of the subsystem intended for use in the thruster. The propellant management system uses a hydrofluoroether to inflate a diaphragm to ensure constant wetting of the propellant tank exit and nozzle inlet. The experiment will take tank pressure data and flow sensor data to understand the system's behavior. The system is duplicated for redundancy and to double the possible data. This system requires further testing before being prepared for launch, vibrational testing, thermal testing, and vacuum testing. </p> <p><br></p> <p>The 0G thrust experiment and plume analysis portion of the experiment consists of numerical modeling and a novel thrust measurement approach. \textit{Direct Simulation Monte Carlo} (DSMC) is being applied to understand the pressure, density, and temperature distributions of the plume of water vapor produced by the FEMTA thruster. The FEMTA nozzle environment is challenging to simulate with computational fluid dynamics  or DSMC due to chaotic transient effects and because both the continuum and molecular regimes must be considered. The current analysis consisted of a two-dimensional model and investigated the effect of meniscus location and contact angle on thrust generated.</p> <p><br></p> <p>It is not possible to use traditional thrust measurement devices (sensitive torsional thrust stands or microsensors intended for use on small satellites) for microthrusters on a rocket booster. Two  novel approaches for performing thrust measurement in the range of 100 microNewtons have been investigated. The first approach ionizes the FEMTA thruster plume and analyzes the plasma by optical emission spectroscopy. The theory states that the relative intensity of a given wavelength observed correlates to the density of the species in the plasma. The density of water would be directly correlated to the thrust generated by FEMTA during the experiment, as more water is evaporated as thrust is increased. This method is no longer being considered for the suborbital experiment but did yield promising results. </p> <p><br></p> <p>The second approach employs a d'Arsonval meter, a photo-interrupt, and an Arduino controller. The d'Arsonval meter consists of a stationary permanent magnet with a moving coil and a pointer. Increasing the voltage in the coil causes a torque on the system due to the magnetic field induced by the permanent magnet. This torque causes a deflection of the pointer that is proportional to the voltage applied. The flag of the sensor will be placed in the path of the gas jet from the thruster. The force caused by the jet pressure will move the flag. An Arduino controller will vary the voltage to hold the flag in place. As the mass flow rate increases, the reaction force required to hold the flag in place will increase. This sensor can be calibrated using an analog cold gas system that passes various gases (air nitrogen, argon, etc.) through an orifice nozzle at mass flow rates that are set by a mass flow rate controller. DSMC analysis has been performed to understand the flow field and flow properties and how they directly relate to the force experienced by the flag sensor. </p> <p>An undergraduate course has supported parts of the work described in this dissertation. This course has applied the Vertically Integrated Projects approach to project-based learning. This method and its results were analyzed and lessons learned as well as a blueprint for future application of this method to other small satellite projects are discussed.</p>