940 research outputs found
Conceptual Trends and Implications for Risk Research. Report of the Task Force Meeting: Risk and Policy Analysis Under Conditions of Uncertainty, November 25-27,1985
The Task Force focused on the uncertainties in decision systems for choosing or modifying technologies intended to improve human well-being. The challenge was to delineate an international research agenda to assist communities to venture into the future with greater confidence in technological innovation.
The Task Force recommended research in three interrelated areas: (1) Protocols -- Development of procedural advice for the integrated assessment of the contribution of technologies to environmental and economic achievements, and the associated uncertainties. (2) Case Studies -- Integrated assessments involving local or regional clusters of technologies and decision-making bodies, and investigation of ecosystem effects, economic effects, and effects on human well-being, as well as the structure and performance of institutions. (3) Educational Materials -- Development of educational materials to support integrated assessments
Convergence of iterative split-operator approaches for approximating nonlinear reactive transport problems
Abstract Numerical solutions to nonlinear reactive solute transport problems (NRTPs) are often computed using split-operator (SO) approaches, which separate the transport and reaction processes. This uncoupling introduces an additional source of numerical error, known as the splitting error. The iterative split-operator (ISO) algorithm removes the splitting error through iteration. Although the ISO algorithm is often used, there has been very little analysis of its convergence behavior. This work uses theoretical analysis and numerical experiments to investigate the convergence rate of the ISO approach for solving NRTPs. We show that under certain assumptions regarding smoothness, the convergence rate of the ISO algorithm applied NRTPs is O(Δ ). We demonstrate that the theoretical convergence rate can be achieved in practice if the numerical solution of the transport and reaction steps are carried out with sufficient accuracy. We also show that accurate estimation of the lagged operator in each step is crucial to obtaining the theoretical convergence rate
An analysis of polynomial chaos approximations for modeling single-fluid-phase flow in porous medium systems
We examine a variety of polynomial-chaos-motivated approximations to a stochastic form of a steady state groundwater flow model. We consider approaches for truncating the infinite dimensional problem and producing decoupled systems. We discuss conditions under which such decoupling is possible and show that to generalize the known decoupling by numerical cubature, it would be necessary to find new multivariate cubature rules. Finally, we use the acceleration of Monte Carlo to compare the quality of polynomial models obtained for all approaches and find that in general the methods considered are more efficient than Monte Carlo for the relatively small domains considered in this work. A curse of dimensionality in the series expansion of the log-normal stochastic random field used to represent hydraulic conductivity provides a significant impediment to efficient approximations for large domains for all methods considered in this work, other than the Monte Carlo method
Dense non-aqueous phase liquids at former manufactured gas plants: Challenges to modeling and remediation
The remediation of dense non-aqueous phase liquids (DNAPLs) in porous media continues to be one of the most challenging problems facing environmental scientists and engineers. Of all the environmentally relevant DNAPLs, tars in the subsurface at former manufactured gas plants (FMGP’s) pose one of the biggest challenges due to their complex chemical composition and tendency to alter wettability. To further our understanding of these complex materials, we consulted historic documentation to evaluate the impact of gas manufacturing on the composition and physicochemical nature of the resulting tars. In the recent literature, most work to date has been focused in a relatively narrow portion of the expected range of tar materials, which has yielded a bias toward samples of relatively low viscosity and density. In this work, we consider the dissolution and movement of tars in the subsurface, models used to predict these phenomena, and approaches used for remediation. We also explore the open issues and detail important gaps in our fundamental understanding of these extraordinarily complex systems that must be resolved to reach a mature level of understanding
Optimal design for problems involving flow and transport phenomena in saturated subsurface systems
Abstract Estimation problems arise routinely in subsurface hydrology for applications that range from water resources management to water quality protection to subsurface restoration. Interest in optimal design of such systems has increased over the last two decades and this area is considered an important and active area of research. In this work, we review the state of the art, assess important challenges that must be resolved to reach a mature level of understanding, and summarize some promising approaches that might help meet some of the challenges. While much has been accomplished to date, we conclude that more work remains before comprehensive, efficient, and robust solution methods exist to solve the most challenging applications in subsurface science. We suggest that future directions of research include the application of direct search solution methods, and developments in stochastic and multi-objective optimization. We present a set of comprehensive test problems for use in the research community as a means for benchmarking and comparing optimization approaches
Complete genome sequences of Campylobacter jejuni strains RM3196 (233.94) and RM3197 (308.95) isolated from patients with Guillain-Barré syndrome
Infections with Campylobacter jejuni subsp. jejuni are a leading cause of foodborne gastroenteritis and the most prevalent infection preceding Guillain-Barré syndrome (GBS). This study describes the genomes of C. jejuni subsp. jejuni HS:41 strains RM3196 (233.94) and RM3197 (308.95) that were isolated from patients with GBS in Cape Town, South Africa
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
Efficient steady-state solution techniques for variably saturated groundwater flow
We consider the simulation of steady-state variably saturated groundwater flow using Richards' equation (RE). The difficulties associated with solving RE numerically
are well known. Most discretization approaches for RE lead to nonlinear systems
that are large and difficult to solve. The solution of nonlinear systems for steady-
state problems can be particularly challenging, since a good initial guess for the
steady-state solution is often hard to obtain, and the resulting linear systems may
be poorly scaled. Common approaches like Picard iteration or variations of Newton's method have their advantages but perform poorly with standard globalization
techniques under certain conditions.
Pseudo-transient continuation has been used in computational fluid dynamics for some time to obtain steady-state solutions for problems in which Newton's method
with standard line-search strategies fails. It combines aspects of backward Euler time
integration and Newton's method to select intermediate estimates of the steady-state solution. Here, we examine the use of pseudo-transient continuation as well as
Newton's method combined with standard globalization techniques for steady-state
problems in heterogeneous domains. We investigate the methods' performance with
direct and preconditioned Krylov iterative linear solvers. We then make recommendations for robust and efficient approaches to obtain steady-state solutions for RE
under a range of conditions
Computing gravitational waves from slightly nonspherical stellar collapse to black hole: Odd-parity perturbation
Nonspherical stellar collapse to a black hole is one of the most promising
gravitational wave sources for gravitational wave detectors. We numerically
study gravitational waves from a slightly nonspherical stellar collapse to a
black hole in linearized Einstein theory. We adopt a spherically collapsing
star as the zeroth-order solution and gravitational waves are computed using
perturbation theory on the spherical background. In this paper we focus on the
perturbation of odd-parity modes. Using the polytropic equations of state with
polytropic indices and 3, we qualitatively study gravitational waves
emitted during the collapse of neutron stars and supermassive stars to black
holes from a marginally stable equilibrium configuration. Since the matter
perturbation profiles can be chosen arbitrarily, we provide a few types for
them. For , the gravitational waveforms are mainly characterized by a
black hole quasinormal mode ringing, irrespective of perturbation profiles
given initially. However, for , the waveforms depend strongly on the
initial perturbation profiles. In other words, the gravitational waveforms
strongly depend on the stellar configuration and, in turn, on the ad hoc choice
of the functional form of the perturbation in the case of supermassive stars.Comment: 31 pages, accepted for publication in Phys. Rev. D, typos and minor
errors correcte
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