An axisymmetric time-domain spectral-element method for full-wave simulations: Application to ocean acoustics

The numerical simulation of acoustic waves in complex 3D media is a key topic in many branches of science, from exploration geophysics to non-destructive testing and medical imaging. With the drastic increase in computing capabilities this field has dramatically grown in the last twenty years. However many 3D computations, especially at high frequency and/or long range, are still far beyond current reach and force researchers to resort to approximations, for example by working in 2D (plane strain) or by using a paraxial approximation. This article presents and validates a numerical technique based on an axisymmetric formulation of a spectral finite-element method in the time domain for heterogeneous fluid-solid media. Taking advantage of axisymmetry enables the study of relevant 3D configurations at a very moderate computational cost. The axisymmetric spectral-element formulation is first introduced, and validation tests are then performed. A typical application of interest in ocean acoustics showing upslope propagation above a dipping viscoelastic ocean bottom is then presented. The method correctly models backscattered waves and explains the transmission losses discrepancies pointed out in Jensen et al. (2007). Finally, a realistic application to a double seamount problem is considered.Comment: Added a reference, and fixed a typo (cylindrical versus spherical

NASA Thesaurus Supplement: A three part cumulative supplement to the 1982 edition of the NASA Thesaurus (supplement 3)

The three part cumulative NASA Thesaurus Supplement to the 1982 edition of the NASA Thesaurus includes Part 1, Hierarchical Listing, Part 2, Access Vocabulary, and Part 3, Deletions. The semiannual supplement gives complete hierarchies for new terms and includes new term indications for entries new to this supplement

Solutions to range-dependent benchmark problems by the finite-difference method

Author Posting. © Acoustical Society of America, 1990. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 87 (1990): 1527-1534, doi:10.1121/1.399452.An explicit second-order finite-difference scheme has been used to solve the elastic-wave equation in the time domain. Solutions are presented for the perfect wedge, the lossless penetrable wedge, and the plane parallel waveguide that have been proposed as benchmarks by the Acoustical Society of America. Good agreement with reference solutions is obtained if the media is discretized at 20 gridpoints per wavelength. There is a major discrepancy (up to 20 dB) in reference-source level because the reference solutions are normalized to the source strength at 1 m in the model, but the finite-difference solutions are normalized to the source strength at 1 m in a homogeneous medium. The finite-difference method requires computational times between 10 and 20 h on a super minicomputer without an array processor. The method has the advantage of providing phase information and, when run for a pulse source, of providing insight into the evolution of the wave field and energy partitioning. More complex models, including velocity gradients and strong lateral heterogeneities, can be solved with no additional computational effort. The method has also been formulated to include shear wave effects.This work was supported by the Office of Naval Research under Contract No. N00014-87-K-0007

NASA Thesaurus Supplement: A three part cumulative supplement to the 1982 edition of the NASA Thesaurus (supplement 2)

The three part cumulative NASA Thesaurus Supplement to the 1982 edition of the NASA Thesaurus includes: part 1, hierarchical listing; part 2, access vocabulary, and part 3, deletions. The semiannual supplement gives complete hierarchies for new terms and includes new term indications for terms new to this supplement

Interlinking With Models for Predicting Weather and Climate Change

Meteorology has employed automatic computing machines since the early days of electronic computers. From the 1950s on, a large body of models used for “in silico” experiments (numerical simulation) has been built up, together with an international infrastructure of measuring, modeling, and testing. These outstanding developments— unique in science—led not only to an increasing standardization in developing and applying models but also to deepening the interlinking between modeling and generating evidence. The article explores needs and strategies for evaluating scientific results based on mass data output devices

Report on the Office of Naval Research Shallow-Water Acoustic Workshop 1-3 October 1996

The results of an unclassified workshop on Shallow Water Acoustics, jointly sponsored by ONR and DARPA, are presented. The workshop was held on October 1-3, 1996 at the Naval Research Laboratory, Stennis Space Center, and included 83 participants specializing in ocean acoustics, geology and geophysics, physical oceanography, and other disciplines relevant to shallow water research. The goal of the workshop was to help determine the current status of and future directions for shallow water acoustics research. The report summarizes the deliberations and recommendations of the workshop, and includes detailed report from the three working groups (bottom, water column, and modeling and signal processing) as well as from the workshop moderator (Dr. James Lynch, WHOI).Funding was provided by the Office of Naval Research through Contract No. N00014-96-1-1031. Supported by ONR and DARPA

Direct finite element simulation of turbulent flow for marine based renewable energy

In this article we present a computational framework for simulation of turbulent flow in marine based renewable energy applications. In particular, we focus on floating structures and rotating turbines. This work is an extension to multiphase turbulent flow, of our existing framework of residual based turbulence modeling for single phase turbulent incompressible flow. We illustrate the framework in four examples: a regular wave test where we compare against an exact solution, the standard MARIN wave impact benchmark with experimental validation data, a vertical axis turbine with complex geometry from an existing turbine, and finally a prototype simulation of decay test in a coupled moving boundary rigid-body and two-phase fluid simulation.IEA-OES Task 1

The GeoClaw software for depth-averaged flows with adaptive refinement

Many geophysical flow or wave propagation problems can be modeled with two-dimensional depth-averaged equations, of which the shallow water equations are the simplest example. We describe the GeoClaw software that has been designed to solve problems of this nature, consisting of open source Fortran programs together with Python tools for the user interface and flow visualization. This software uses high-resolution shock-capturing finite volume methods on logically rectangular grids, including latitude--longitude grids on the sphere. Dry states are handled automatically to model inundation. The code incorporates adaptive mesh refinement to allow the efficient solution of large-scale geophysical problems. Examples are given illustrating its use for modeling tsunamis, dam break problems, and storm surge. Documentation and download information is available at www.clawpack.org/geoclawComment: 18 pages, 11 figures, Animations and source code for some examples at http://www.clawpack.org/links/awr10 Significantly modified from original posting to incorporate suggestions of referee