¡@¡@¡@¡@¡@¡@¡@­^¡@¤å¡@ºK¡@­n¡@¡@¡@¡@¡@¡@¡@ Heat transfer of a molten splat to a thin layer rapidly solidified on a cold substrate and the heat transfer coefficient at the bottom surface of a splat is extensively and self-consistently investigated. Rapid freezing in the splat is governed by a nonequilibrium kinetics at the solidification front in contrast to the melting in the substrate simulated by the traditional phase change problem. Solving one-dimensional unsteady heat conduction equations and accounting for distinct properties between phases and splat and substrate, the results show the effects of dimensionless parameters such as the dimensionless kinetic coefficient, stefan number, latent heat ratio, initial, equilibrium melting, and nucleation temperature, and conductivity, density, and specific heat ratios between solid and liquid and splat and substrate on unsteady temperature fields and freezing and melting rates in the splat and substrate and on unsteady variation of Biot number are presented. The unsteady variation of the heat coefficient or Biot number can be divided by five regimes: liquid splat-solid substrate, liquid splat-liquid substrate, solid splat-solid substrate, solid splat-liquid substrate, and the nucleation of the splat. Appropriate choices of dimensionless parameters to control the time for freezing and melting of the splat and substrate and an understanding and estimation of the heat coefficient at the bottom surface of the splat therefore are presented. The velocity distribution function and transport variables of the positive ions and electrons in the collisionless presheath and sheath of a plasma near a wall partially reflecting ions and electrons are determined from a kinetic analysis. Since velocities of the ions and electrons near the wall are highly non-Maxwell-Boltzmann distributions, accurate predictions of transport variables such as density, fluid velocity, mean pressure, fluidlike viscous stress and conduction require kinetic analysis. The result find that dimensionless transport variables of ions and electrons in the presheath and sheath can be exactly expressed in terms of transcendental functions determined by dimensionless independent parameters of ions and electrons reflectivities of the wall, ion-to-electron mass ratio, charge number and electron-to-ion temperature ratio at the presheath edge. The effects of the parameters on transport variables at the wall are also obtained. The computed transport variables in the presheath and sheath show agreement with available theoretical data for a completely absorbing wall.

No description available.

Thesis(M.S.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references (p.30-31).

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 186-200). Also available in electronic version.

We obtain first-order cover pebbling thresholds of the complete graph for Maxwell Boltzmann and Bose Einstein configurations.

Estudos experimentais foram realizados visando a geração de um jato metálico de átomos neutros por ablação a laser em alvos de cobre. Para estudar a composição da pluma gerada por ablação e para estudar o processo de remoção da fração carregada eletricamente da pluma, foi utilizado um dispositivo constituído por um par de eletrodos de extração de íons e por um sensor de pressão de PVDF. O sensor de PVDF responde à pressão das partículas em sua superfície, enquanto o potencial aplicado aos eletrodos extrai os íons da pluma. A dependência da fluência do laser na razão entre as populações dos íons e átomos neutros na região entre 20 a 32 J/cm2 foi investigada e utilizaram-se valores de tensão de extração de até 800 V. Neste intervalo dos parâmetros de estudo, o percentual de átomos neutros varia entre 30 - 8% da população total da pluma. O ajuste de uma função de distribuição Maxwelliana de velocidades foi realizado ao sinal do sensor de PVDF para inferir os parâmetros de velocidade de deriva, que variou de 3,4 a 4,6 km/s e, através do modelo hidrodinâmico encontrado na literatura para o tratamento da expansão da pluma, foi possível obter os valores de temperatura translacional da pluma variando entre 8 e 14×104 K. Constatou-se a existência de um mecanismo interno de aceleração das partículas da pluma, atribuído à formação de camada dupla, que por sua vez gera um potencial acelerador interno. Foi inferido, pela diferença entre a velocidade de deriva e a velocidade térmica, que este potencial acelerador é da ordem de 3 V, mesmo quando não há tensão aplicada aos eletrodos. Foi verificado que a pluma gerada por ablação a laser na condição experimental deste trabalho encontra-se no regime em transição, que para baixas fluências apresenta um comportamento de partícula livre, e em fluências mais altas é possível observar um comportamento de plasma. Pelos resultados obtidos, foi confirmada a geração de um jato metálico neutro que poderá ser utilizado em experimentos de separação isotópica usando lasers.

Thesis (PhD)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Sarah L. Codd. Includes bibliographical references (leaves 119-132).

L’estimation de la charge d’un satellite et du risque de décharge nécessite dans certains cas la prise en compte dans les modèles numériques d’échelles spatiales très différentes. En particulier, les interconnecteurs présents à la surface des générateurs solaires d’un satellite sont susceptibles de modifier son équilibre électrostatique lors de missions spatiales rencontrant un environnement plasma dense. Une modélisation classique de cet effet nécessiterait le maillage d’éléments à des échelles submillimétriques,sur un satellite de plusieurs dizaines de mètres d’envergure, ce qui rendrait la simulation extrêmement onéreuse en temps de calcul. De plus, ces interconnecteurs sont parfois fortement chargés positivement par rapport à l’environnement, ce qui empêche l’application du modèle de Maxwell-Boltzmann classiquement utilisé pour les populations d’électrons. Dans une première partie, nous avons développé une méthode itérative de type Patch adaptée à la résolution du problème non-linéaire de Poisson-Boltzmann pour la simulation du plasma spatial. Cette méthode numérique multigrille permet la simulation de l’impact d’éléments de petite taille à la surface d’un satellite complet. Dans une seconde partie, nous avons développé un schéma correctif permettant d’utiliser le modèle de Maxwell-Boltzmann pour la population d’électrons, malgré la présence de surfaces satellites chargées positivement, en y ajoutant un terme de correction calculé à l’aide de la méthode Particle-in-Cell. Nous avons montré que ce schéma permet, tout en limitant le coût en calculs, de déterminer avec précision les courants collectés par les surfaces du satellites, qu’elles soient chargées négativement ou positivement. / The numerical simulation of spacecraft charging can require to resolve widely different geometrical scales. In particular, solar array interconnects on the surface of solar panels have a major impact ona satellite electrostatic equilibrium. A classical model of this effect would require a mesh refined tosub-millimetre scales, on a spacecraft spanning several dozen metres, which would make the simulation computationally expensive. Moreover, the solar array interconnects can have a large positive potentialrelative to the space plasma, preventing the use of the classical Maxwell-Boltzmann model for theelectrons in the plasma. In a first part, we have developed an iterative patch method to solve thenonlinear Poisson-Boltzmann equation used in plasma simulations. This multigrid numerical scheme allows to resolve the impact of small-scale components on the surface of a complete spacecraft. In asecond part, we have developed a corrective scheme for the Maxwell-Boltzmann model to account for the presence of charged surfaces in the simulation. We have shown that this simple model is able to precisely compute the currents collected by the spacecraft surfaces.

<p align="justify">Ce travail consiste à étudier des systèmes constitués par un grand nombre de grains, auxquels de l’énergie cinétique est fournie, et à étudier leurs similarités et leurs différences avec des fluides traditionnels. Je me concentre principalement sur la nature de non-équilibre de ces fluides granulaires, en montrant que, même si les méthodes de méchanique statistique y sont applicables, leurs propriétés sont très différentes de celles de systèmes à l’équilibre ou proches de l’équilibre :</p><p><p><ul><li>Les fluides granulaires présentent des phénomènes de transport qui n’ont pas d’équivalent dans des fluides moléculaires, tels qu’un couplage spécifique entre flux de chaleur et gradient de densité.<p><li>Leur distribution de vitesse est en général différente de la distribution de Maxwell-Boltzmann, et présente une surpopulation pour les grandes vitesses. <p><li>Dans le cas de mélanges, différentes espèces de grains sont en général caractérisées par des énergies cinétiques différentes, i.e. ces systèmes sont sujet à une non-equipartition de leur énergie.<p><li>Ces fluides ont tendance à former des inhomogénéités spatiales spontanément. Cette propriété est illustrée en étudiant l’expérience du Demon de Maxwell appliquée aux systèmes granulaires.</ul><p><p align="justify">Chacune de ces particularités est discutée en détail dans des chapitres distincts, où l’on applique différentes méthodes de méchanique statistique (équation de Boltzmann, transition de phase, mean field models…) et où l’on vérifie les prédictions théoriques par simulations numériques (MD, Monte Carlo…).</p> / Doctorat en sciences, Spécialisation physique / info:eu-repo/semantics/nonPublished

This dissertation details our experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution, and cooling of its motion towards the quantum ground state. We have trapped glass microspheres in water, air and vacuum with optical tweezers. We developed a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution. We studied the Brownian motion of a trapped microsphere in air over a wide range of pressures. We measured the instantaneous velocity of a Brownian particle. Our results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. We are currently developing a new detection system to measure the instantaneous velocity of a Brownian particle in water. In vacuum, we have used active feedback to cool the three center-of-mass vibration modes of a trapped microsphere from room temperature to millikelvin temperatures with a minimum mode temperature of 1.5 mK, which corresponds to the reduction of the root mean square (rms) amplitude of the microsphere from 6.7 nm to 15 pm for that mode. The mean thermal occupation number of that mode is reduced from about 6.8$\times 10^8$ at 297 K to about 3400 at 1.5 mK. / text