A method for generating skewed random numbers using two overlapping uniform distributions

The objective of this work was to implement and evaluate a method for generating skewed random numbers using a combination of uniform random numbers. The method provides a simple and accurate way of generating skewed random numbers from the specified first three moments without an a priori specification of the probability density function. We describe the procedure for generating skewed random numbers from unifon-n random numbers, and show that it accurately produces random numbers with the desired first three moments over a range of skewness values. We also show that in the limit of zero skewness, the distribution of random numbers is an accurate approximation to the Gaussian probability density function. Future work win use this method to provide skewed random numbers for a Langevin equation model for diffusion in skewed turbulence

A Real-Time Atmospheric Dispersion Modeling System

This paper describes a new 3-D multi-scale atmospheric dispersion modeling system and its on-going evaluation. This system is being developed for both real-time operational applications and detailed assessments of events involving atmospheric releases of hazardous material. It is part of a new, modernized Department of Energy (DOE) National Atmospheric Release Advisory Center (NARAC) emergency response computer system at Lawrence Livermore National Laboratory. This system contains coupled meteorological data assimilation and dispersion models, initial versions of which were described by Sugiyama and Chan (1998) and Leone et al. (1997). Section 2 describes the current versions of these models, emphasizing new features. This modeling system supports cases involving both simple and complex terrain, and multiple space and time scales from the microscale to mesoscale. Therefore, several levels of verification and evaluation are required. The meteorological data assimilation and interpolation algorithms have been previously evaluated by comparison to observational data (Sugiyama and Chan, 1998). The non-divergence adjustment algorithm was tested against potential flow solutions and wind tunnel data (Chan and Sugiyama, 1997). Initial dispersion model results for a field experiment case study were shown by Leone et al. (1997). A study in which an early, prototype version of the new modeling system was evaluated and compared to the current NARAC operational models showed that the new system provides improved results (Foster et al., 1999). In Section 3, we show example results from the current versions of the models, including verification using analytic solutions to the advection-diffusion equation as well as on-going evaluation using microscale and mesoscale dispersion field experiments

Collective dynamics of colloids at fluid interfaces

The evolution of an initially prepared distribution of micron sized colloidal particles, trapped at a fluid interface and under the action of their mutual capillary attraction, is analyzed by using Brownian dynamics simulations. At a separation \lambda\ given by the capillary length of typically 1 mm, the distance dependence of this attraction exhibits a crossover from a logarithmic decay, formally analogous to two-dimensional gravity, to an exponential decay. We discuss in detail the adaption of a particle-mesh algorithm, as used in cosmological simulations to study structure formation due to gravitational collapse, to the present colloidal problem. These simulations confirm the predictions, as far as available, of a mean-field theory developed previously for this problem. The evolution is monitored by quantitative characteristics which are particularly sensitive to the formation of highly inhomogeneous structures. Upon increasing \lambda\ the dynamics show a smooth transition from the spinodal decomposition expected for a simple fluid with short-ranged attraction to the self-gravitational collapse scenario.Comment: 13 pages, 12 figures, revised, matches version accepted for publication in the European Physical Journal

Conformational dynamics and internal friction in homopolymer globules: equilibrium vs. non-equilibrium simulations

We study the conformational dynamics within homopolymer globules by solvent-implicit Brownian dynamics simulations. A strong dependence of the internal chain dynamics on the Lennard-Jones cohesion strength ε and the globule size N [subscript G] is observed. We find two distinct dynamical regimes: a liquid-like regime (for ε ε[subscript s] with slow internal dynamics. The cohesion strength ε[subscript s] of this freezing transition depends on N G . Equilibrium simulations, where we investigate the diffusional chain dynamics within the globule, are compared with non-equilibrium simulations, where we unfold the globule by pulling the chain ends with prescribed velocity (encompassing low enough velocities so that the linear-response, viscous regime is reached). From both simulation protocols we derive the internal viscosity within the globule. In the liquid-like regime the internal friction increases continuously with ε and scales extensive in N [subscript G] . This suggests an internal friction scenario where the entire chain (or an extensive fraction thereof) takes part in conformational reorganization of the globular structure.American Society for Engineering Education. National Defense Science and Engineering Graduate Fellowshi

Potential growth of electric power production from Imperial Valley geothermal resources

The growth of geothermal electric power operations in Imperial Valley, California is projected over the next 40 years. With commercial power forecast to become available in the 1980's, the scenario considers three subsequent growth rates: 40, 100, and 250 MW per year. These growth rates, along with estimates of the total resource size, result in a maximum level of electric power production ranging from 1000 to 8000 MW to be attained in the 2010 to 2020 time period. Power plant siting constraints are developed and used to make siting patterns for the 400- through 8000-MW level of power production. Two geothermal technologies are included in the scenario: flashed steam systems that produce cooling water from the geothermal steam condensate and emit noncondensable gases to the atmosphere; and high pressure, confined flow systems that inject the geoghermal fluid back into the ground. An analysis of the scenario is made with regard to well drilling and power plant construction rates, land use, cooling water requirements, and hydrogen sulfide emissions

Langevin equation modeling of convective boundary layer dispersion assuming homogeneous, skewed turbulence

Vertical dispersion of material in the convective boundary layer, CBL, is dramatically different than in natural or stable boundary layers, as has been shown by field and laboratory experiments. Lagrangian stochastic modeling based on the Langevin equation has been shown to be useful for simulating vertical dispersion in the CBL. This modeling approach can account for the effects of the long Lagrangian time scales (associated with large-scale turbulent structures), skewed vertical velocity distributions, and vertically inhomogeneous turbulent properties found in the CBL. It has been recognized that simplified Langevin equation models that assume skewed but homogeneous velocity statistics can capture the important aspects of dispersion from sources the the CBL. The assumption of homogeneous turbulence has a significant practical advantage, specifically, longer time steps can be used in numerical simulations. In this paper, we compare two Langevin equations models that use the homogeneous turbulence assumption. We also compare and evaluate three reflection boundary conditions, the method for determining a new velocity for a particle that encounters a boundary. Model results are evaluated using data from Willis and Deardorff`s laboratory experiments for three different source heights

Comparison of reflection boundary conditions for langevin equation modeling of convective boundary layer dispersion

Lagrangian stochastic modeling based on the Langevin equation has been shown to be useful for simulating vertical dispersion of trace material in the convective boundary layer or CBL. This modeling approach can account for the effects of the long velocity correlation time scales, skewed vertical velocity distributions, and vertically inhomogeneous turbulent properties found in the CBL. It has been recognized that Langevin equation models assuming skewed but homogenous velocity statistics can capture the important aspects of diffusion from sources in the CBL, especially elevated sources. We compare three reflection boundary conditions using two different Langevin-equation-based numerical models for vertical dispersion in skewed, homogeneous turbulence. One model, described by Ermak and Nasstrom (1995) is based on a Langevin equation with a skewed random force and a linear deterministic force. The second model, used by Hurley and Physick (1993) is based on a Langevin equation with a Gaussian random force and a non-linear deterministic force. The reflection boundary conditions are all based on the approach described by Thompson and Montgomery (1994)

Implementation of a random displacement method (RDM) in the ADPIC model framework

The objective of this work was to implement a 3-D Lagrangian stochastic (also called random walk or Monte Carlo) diffusion method in the framework of the operational ADPIC (Atmospheric Diffusion Particle-In-Cell) code. The Random Displacement Method, RDM, presented here and implemented in the ADPIC code, calculates atmospheric dispersion in a purely Lagrangian, grid-independent manner. Some of the benefits of this approach compared to the previously-used ``particle-in-cell, gradient diffusion`` method are (a) a sub-grid diffusion approximation is no longer needed, (b) numerical accuracy of the diffusion calculation is improved because particle displacement does not depend on the resolution of the Eulerian grid used to calculate species concentration, and (c) adaptation to other grid structures for the input wind field does not affect the diffusion calculation. In addition, the RDM incorporates a unique and accurate treatment of particle interaction with the surface