Efficient boundary integral solution for acoustic wave scattering by irregular surfaces

The left-right operator splitting method is studied for the efficient calculation of acoustic fields scattered by arbitrary rough surfaces. Here the governing boundary integral is written as a sum of left- and right-going components, and the solution expressed as an iterative series, expanding about the predominant direction of propagation. Calculation of each term is computationally inexpensive both in time and memory, and the field is often accurately captured using one or two terms. The convergence and accuracy are examined by comparison with exact solution for smaller problems, and a series of much larger problems are tackled. The method is also immediately applicable to other scatterers such as waveguides, of which examples are given

Developing web services in a computational grid environment

Grid and Web services are both hot topics today. In this paper, we will present some ongoing work and planned future work at the Cambridge eScience Centre. After an introduction to these technologies in the context of Grid applications development, we describe two use-cases: a database of results in computational fluid dynamics (CFD) and a small computational Grid for aircraft engineering design. As Grid services are moving towards Web services, we continue to make use of the Globus Toolkit v2.4 (GT2.4), without adopting the Open Grid Services Architecture (OGSA) wholesale. In our scenario, GT2.4 integrates distributed computing resources including HPC and clusters while Web services wrap the scientific code as a service.Proceedings of the IEEE International Conference on Services Computing, Shanghai, September 200

Random scattering by rough surfaces with spatially varying impedance

A method is given for evaluating electromagnetic scattering by an irregular surface with spatially-varying impedance. This uses an operator expansion with respect to impedance variation and allows examination of its effects and the resulting modification of the field scattered by the rough surface. For a fixed rough surface and randomly varying impedance, expressions are derived for the scattered field itself, and for the coherent field with respect to impedance variation for both flat and rough surfaces in the form of effective impedance conditions

Recovery of rough surface in ducting medium from grazing angle scattered wave

A method is developed for rough surface reconstruction using fields scattered at grazing angles in a medium with a linearly varying refractive index and Neumann boundary condition. This regime represents a ducting medium, bounded by a perfectly conducting surface with a TM incident field or an acoustically hard surface. This significantly extends the iterated marching method, based upon the parabolic integral equation for forward-scattered field components [Chen and Spivack, J. Opt. Soc. Am. A 35, 504–513 (2018)]. The approach, which uses a fixed frequency, is able to accurately recover multiscale surfaces and is found to be robust with respect to measurement noise and localized perturbations.</jats:p

Rough surface reconstruction from phaseless single frequency data at grazing angles

We develop a method for the reconstruction of a perfectly reflecting rough surface from phaseless measurements of a field arising from single frequency scattering at grazing angles. Formulations are given for both Dirichlet and Neumann boundary conditions, and numerical experiments are presented in which close agreement is found with the exact solution. The approach is a marching method based on the parabolic integral equation, which recovers the surface progressively in range, and is iterated a small number of times to produce the final result. It is found that the approach is robust with respect to spatially localized perturbations and to measurement noise

Rough surface scattering via two-way parabolic integral equation

This paper extends the parabolic integral equation method, which is very effective for forward scattering from one-dimensional rough surfaces, to include backscatter. This is done by applying left-right splitting to a modified two-way governing integral operator, to express the solution as a series of Volterra operators; this series describes successively higher-order surface interactions between forward and backward going components, and allows highly efficient numerical evaluation. This and equivalent methods such as ordered multiple interactions have been developed for the full Helmholtz integral equations, but not previously applied to the parabolic Green’s function. Equations are derived for both Dirichlet and Neumann boundary conditions (TE and TM).The first author is grateful to BAE Systems for data and financial support during formative stages of the work

Statistical moments for rough surface scatter from two-way parabolic integral equation at low grazing angles

The moments of a plane wave scattered at low grazing angles from a one-dimensional perfectly reflecting rough surface are considered. The mean intensity and autocorrelation of the scattered field and the corresponding angular spectrum are obtained to second order in surface height. The derivations are based on an operator expansion of the extended (two-way) parabolic integral equation solution. The resulting operator series describes successively higher-order surface interactions between forward and backward going components. The expressions derived may be regarded as backscatter corrections to those obtained via the standard (one-way) parabolic integral equation method

Stress accumulation versus shape flattening in frustrated, warped-jigsaw particle assemblies

Geometrically frustrated assembly has emerged as an attractive paradigm for understanding and engineering assemblies with self-limiting, finite equilibrium dimensions. We propose and study a novel 2D particle based on a so-called "warped jigsaw" (WJ) shape design: directional bonds in a tapered particle favor curvature along multi-particle rows that frustrate 2D lattice order. We investigate how large-scale intra-assembly stress gradients emerge from the microscopic properties of the particles using a combination of numerical simulation and continuum elasticity. WJ particles can favor anisotropic ribbon assemblies, whose lateral width may be self-limiting depending on the relative strength of cohesive to elastic forces in the assembly, which we show to be controlled by the range of interactions and degree of shape misfit. The upper limits of self-limited size are controlled by the crossover between two elastic modes in assembly: the accumulation of shear with increasing width at small widths giving way to unbending of preferred row curvature, permitting assembly to grow to unlimited sizes. We show that the stiffness controlling distinct elastic modes is governed by combination and placement of repulsive and attractive binding regions, providing a means to extend the range of accumulating stress to sizes that are far in excess of the single particle size, which we corroborate via numerical studies of discrete particles of variable interactions. Lastly, we relate the ground-state energetics of the model to lower and upper limits on equilibrium assembly size control set by the fluctuations of width along the ribbon boundary.Comment: 18 pages, 9 figures, 2 appendice

Subseafloor life and its biogeochemical impacts

Subseafloor microbial activities are central to Earth’s biogeochemical cycles. They control Earth’s surface oxidation and major aspects of ocean chemistry. They affect climate on long timescales and play major roles in forming and destroying economic resources. In this review, we evaluate present understanding of subseafloor microbes and their activities, identify research gaps, and recommend approaches to filling those gaps. Our synthesis suggests that chemical diffusion rates and reaction affinities play a primary role in controlling rates of subseafloor activities. Fundamental aspects of subseafloor communities, including features that enable their persistence at low catabolic rates for millions of years, remain unknown

Infant BMI or Weight-for-Length and Obesity Risk in Early Childhood

Weight-for-length (WFL) is currently used to assess adiposity under 2 years. We assessed WFL- versus BMI-based estimates of adiposity in healthy infants in determining risk for early obesity