Exponential-time differencing schemes for low-mass DPD systems

Several exponential-time differencing (ETD) schemes are introduced into the method of dissipative particle dynamics (DPD) to solve the resulting stiff stochastic differential equations in the limit of small mass, where emphasis is placed on the handling of the fluctuating terms (i.e., those involving the random forces). Their performances are investigated numerically in some test viscometric flows. Results obtained show that the present schemes outperform the velocity-Verlet algorithm regarding both the satisfaction of equipartition and the maximum allowable time step

Strongly overdamped dissipative particle dynamics for fluid-solid systems

In this paper, a numerical scheme is used to study strongly-overdamped Dissipative Particles Dynamics (DPD) systems for the modelling of fluid-solid systems. In the scheme, the resultant set of algebraic equations for the velocities are directly solved in an iterative manner. Different test problems, e.g., viscometric flows, particulate suspensions and flows past a periodic square array of cylinders, are used to verify the proposed method. In the simulation of particulate suspensions, a new simple model for massless suspended particles is presented. A DPD fluid in the overdamped limit is shown to possess several attractive properties including much faster dynamic response and near-incompressibility

The long range dispersion of radioactive particulates

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982.Microfiche copy available in Archives and ScienceBibliography: leaves 194-196.by Joshua Michael Aaron Ryder Wurman.M.S

Theoretical issues in Numerical Relativity simulations

In this thesis we address several analytical and numerical problems related with the general relativistic study of black hole space-times and boson stars. We have developed a new centered finite volume method based on the flux splitting approach. The techniques for dealing with the singularity, steep gradients and apparent horizon location, are studied in the context of a single Schwarzschild black hole, in both spherically symmetric and full 3D simulations. We present an extended study of gauge instabilities related with a class of singularity avoiding slicing conditions and show that, contrary to previous claims, these instabilities are not generic for evolved gauge conditions. We developed an alternative to the current space coordinate conditions, based on a generalized Almost Killing Equation. We performed a general relativistic study regarding the long term stability of Mixed-State Boson Stars configurations and showed that they are suitable candidates for dark matter models.En esta tesis abordamos varios problemas analíticos y numéricos relacionados con el estudio de agujeros negros relativistas y modelos de materia oscura. Hemos desarrollado un nuevo método de volúmenes finitos centrados basado en el enfoque de la división de flujo. Discutimos las técnicas para tratar con la singularidad, los gradientes abruptos y la localización del horizonte aparente en el contexto de un solo agujero negro de Schwarzschild, en simulaciones tanto con simetría esférica como completamente tridimensionales. Hemos extendido el estudio de una familia de condiciones de foliaciones evitadoras de singularidad y mostrado que ciertas inestabilidades no son genéricas para condiciones de gauge dinámicas. Desarrollamos una alternativa a las prescripciones actuales basada en una Almost Killing Equation generalizada. Hemos realizado también un estudio con respecto a la estabilidad a largo plazo de configuraciones de Mixed-State Boson Stars, el cual sugiere que estas podrían ser candidatas apropiadas para modelos de materia oscura

Cooperative Non-Equilibrium Dynamics in a Thermal Rydberg Ensemble

This thesis reports the investigation of cooperative non-equilibrium dynamics in a thermal Rydberg ensemble. Cooperative behaviour arises due to resonant dipole-dipole interactions between highly excited Rydberg atoms. In order to transfer atomic population from the ground state to the Rydberg state in a caesium vapour, a three-photon excitation scheme is developed. This scheme has a number of benefits over traditional two-photon Rydberg excitation as each transition utilises in- expensive high-power diode lasers. The process of developing the excitation scheme produces a number of important results, including an excited-state polarisation spectroscopy technique and the observation of coherent three-photon electromagnetically induced transparency. The optical response and atomic dynamics of the interacting ensemble can be separated into two distinct phases. When the Rydberg number density is low and the interactions are negligible, the system can be described by the behaviour of a single atom. However, when the Rydberg number density is high, resonant dipole-dipole interactions result in a significant modification of the ensemble properties. This cooperative many-body phase cannot be described by the behaviour of a single atom. In the frequency domain, the interactions produce an excitation-dependent cooperative energy shift that is observed using probe transmission spectroscopy. In the time domain, the interactions result in a cooperative enhancement of the atomic decay rate that is analysed using fluorescence spectroscopy. At the transition between the single-body and many-body phase, a first-order non-equilibrium phase transition occurs. This is observed spatially along the length of the excitation region as a sharp switch in the emitted fluorescence. The first-order phase transition is also observed in the temporal response of the ensemble through critical slowing down. The divergence of the switching time to steady state follows a universal scaling law for phase transitions and the determined critical exponent is in excellent agreement with previous work on non-equilibrium phase transitions

On the core flow and turbulent boundary layer in a curved duct

Curved ducts for transporting fluids are commonplace in industry and nature and, whilst many previous studies have concentrated on viscous responses to curvature, there are configurations and/or occasions when the fluid flow is sufficiently rapid that an inviscid approximation is more appropriate. One such occasion is the rapid flow of compressed air through an ejector in a food-sorting machine, which occurs over a typical timescale of 3 milliseconds. The particulars of the current project are connected with the interests of a manufacturer of food-sorting machines, and consequently variations around a specific geometry are studied in this thesis. Using asymptotic analysis and computational techniques, the thesis first studies the steady inviscid response of the flow to the curvature of the duct. Part I concentrates on the core-flow response to the curvature, extending a recent study to include compressibility. Bulk properties of the flow are derived analytically, along with far-downstream predictions of the flow behaviour and the response close to sharp corners of the cross-section. These results are supported by weakly and fully non-linear computational solutions of the steady three-dimensional Euler equations. Part II then studies the growth and development of the turbulent boundary layer in a curved duct. It is observed that there is an apparent lack of experimental data for the level of curvature specified by the industrial application. Entry region analysis suggests the turbulent boundary layer velocities split into a component matching the core flow and a turbulent component. Solutions are found for the turbulent components in the entry region. Three downstream regimes of the major parameters are also identified and described. One, the quasi-straight regime, is studied in detail numerically and comparisons with experiments are made. Increasing curvature effects are discussed by means of the other two regimes