Loukas F. Kallivokas

Large-scale simulation of elastic wave propagation in heterogeneous media on parallel computers

This paper reports on the development of a parallel numerical methodology for simulating large-scale earthquake-induced ground motion in highly heterogeneous basins. We target large sedimentary basins with contrasts in wavelengths of over an order of magnitude. Regular grid methods prove intractable for such problems. We overcome the problem of multiple physical scales by using unstructured finite elements on locally-resolved Delaunay triangulations derived from octree-based grids. The extremely large mesh sizes require special mesh generation techniques. Despite the method’s multiresolution capability, large problem sizes necessitate the use of distributed memory parallel supercomputers to solve the elastic wave propagation problem. We have developed a system that helps automate the task of writing efficient portable unstrucmred mesh solvers for distributed memory parallel supercomputers. The numerical methodology and software system have been used to simulate the seismic response of the San Fernando Valley in Southern California to an aftershock of the 1994 Northridge Earthquake. We report on parallel performance on the Cray T3D for several models of the basin ranging in size from 35 000 to 77 million tetrahedra. The results indicate that, despite the highly irregular structure of the problem, excellent performance and scalability are achieved.

Earthquake ground motion modeling on parallel computers

We describe the design and discuss the performance of a parallel elastic wave propagation simulator that is being used to model and study earthquake-induced ground motion in large sedimentary basins. The components of the system include mesh generators, a mesh partitioner, a parceler, and a parallel code generator, as well as parallel numerical methods for applying seismic forces, incorporating absorbing boundaries, and solving the discretized wave propagation problem. We discuss performance on the Cray T3D for unstructured mesh wave propagation problems of up to 14 million tetrahedra. By paying careful attention to each step of the process, we obtain excellent performance despite the highly irregular structure of the coefficient matrices of the problem. The mesh generator, partitioner, parceler, and code generator have been incorporated into an integrated toolset/compiler. This system, called Archimedes, automates the solution of unstructured mesh PDE problems on parallel computers, and is being used for other unstructured mesh applications beyond ground motion modeling.

Clamped Plates on Pasternak-Type Elastic Foundation by the Boundary Element Method

Past Due DATE : FEBRUARY 25-27, 1987 LOCATION: GOLDEN, CO TITLE: INDUSTRY-UNIVERSITY ADVANCED MATERIALS CONFERENCE INFO : DR JEROME G MORSE, ADVANCED MATERIALS INSTITUTE, COLORADO SCHOOL OF MINES, GOLDEN, CO 80401 TEL 303 273 3852 DATE : MARCH 1-5, 1987 LOCATION: HOUSTON, TX ABSTRACT: No Info TITLE: 6TH INTL SYMP ON OFFSHORE MECHANICS AND ARTIC ENGINEERING INFO : AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER 345 E 47TH STREET, NEW YORK, NY 10017 DATE : MARCH 2-6, 1987 LOCATION: BEER-SHEVA, ISRAEL ABSTRACT: Past Due TITLE: 5TH BEER-INTERNATIONAL SEMINAR ON MHD FLOWS AND TURBULENCE INFO : PROF H BRANOVER, CENTER FOR MHD STUDIES, BEN-GURION UNIVERSITY OF THE NEGEV, BEER-SHEVA 85105, ISRAEL DATE : MARCH 18-20, 1937 LOCATION: CAMBRIDGE, UK ABSTRACT: No Info TITLE: EUROMECH 220: MIXING AND CHEMICAL REACTIONS IN TURBULENT FLOWS INFO : PROF K N C BRAY, UNIVERSITY ENGINEERING DEPARTMENT, TRUMPINGTON ST, CAMBRIDGE, CB2 1PZ UK DATE : MARCH 22-26, 1987 LOCATION: SAN ANTONIO, TX ABSTRACT: 8/1/36 TITLE: FOURTH INT. CONF. ON NUMERICAL METHODS IN FRACTURE MECHANICS INFO : M.F. KANNINEN, ENGIN. AND MAT. SCIENCE DIV., SOUTHWEST RESEARCH INST., PO DRAWER 28510, SAN ANTONIO, TX 78284 TEL: 512-522-3248 OATE : APRIL 6-8, 1987 LOCATION: GHENT, BELGIUM ABSTRACT: Past Due TITLE: INTERNATIONAL CONFERENCE ON STABILITY OF PLATE AND SHELL STRUCTURES INFO : D VANDEPITTE, GROTESTEENWEG-NORD 2, B-9710 ZWIJNAARDE, BELGIUM DATE : APRIL 6-8, 1987 LOCATION: MONTEREY, CA ABSTRACT: 8/22/86 TITLE: 28TH AIAA STRUCTURES, STRUCTURAL DYNAMICS, AND MATERIALS CONFERENCE INFO : DR. KEITH T. KEDWARD, ALCOA DEFENSE SYSTEMS, INC., 16761 VIA DEL CAMPO COURT, SAN DIEGO, 92127 TEL: 619-695-2260 DATE : APRIL 6-9, 1987 LOCATION: LONDON, ENGLAND ABSTRACT: 7/1/86 TITLE: 5TH INTERNATIONAL MODAL ANALYSIS CONFERENCE INFO : DOMINICK J. DEMICHELE, UNION COLLEGE, GRADUATE S CONTINUING STUDIES, WELLS HOUSE 1 UNION AVENUE, SCHENECTADY, NY 12308 TEL:518-370-6288 DATE : APRIL 6-10, 1987 LOCATION: BARCELONA, SPAIN ABSTRACT: No Info TITLE: INT CONF ON COMPUTATIONAL PLASTICITY INFO : D R J OWEN, DEPT OF CIVIL ENGINEERING, UNIVERSITY COLLEGE OF SWANSEA, SINGLETON PARK, SWANSEA SA2 8PP, UK DATE : APRIL 7-8, 1987 LOCATION: GLASGOW, SCOTLAND ABSTRACT: No Info TITLE: APPLIED SOLID MECHANICS CONFERENCE INFO : A S TOOTH, DEPARTMENT OF MECHANICS OF MATERIALS, UNIVERSITY OF STRATHCLYDE, 75 MONTROSE ST, GLASGOW Gl 1XJ UK ABSTRACT: 8/22/86 DATE : APRIL 9-10, 1987 LOCATION: MONTEREY, CA TITLE: AIAA DYNAMICS SPECIALISTS CONFERENCE INFO : ANTHONY F. MESSINA, DEPT. 76-12, LOCKHEED-CALIFORNIA CO., P.O. BOX 551, BURBANK, CA 91520 TEL: 818-847-4910 8/22/86 DATE : APRIL 9-10, 1987 LOCATION: MONTEREY, CA TITLE: AIAA DYNAMICS SPECIALISTS CONFERENCE INFO : ANTHONY F. MESSINA, DEPT. 76-12, LOCKHEED-CALIFORNIA CO., P.O. BOX 551, BURBANK, CA 91520 TEL: 818-847-4910 DATE : APRIL 13-15, 1987 LOCATION: UNIVERSITY PARK,PA ABSTRACT: Past Due TITLE: INTERNATIONAL CONFERENCE ON ENVIRONMENTAL DEGRADATION OF MATERIALS INFO : R P MCNITT, 227 HAMMOND BUILDING, UNIVERSITY PARK, PA 16802 OATE : APRIL 13-16, 1987 LOCATION: SAN ANTONIO, TX ABSTRACT: Past Due TITLE: IUTAM SYMPOSIUM ON ADVANCED BOUNDARY ELEMENT METHODS INFO : DR T A CRUSE, SOUTHWEST RESEARCH INSTITUTE, P 0 DRAWER 28510, SAN ANTONIO, TX 78284 TEL 512 684 5111 DATE : APRIL 26-30, 1987 LOCATION: CINCINNATI, OH ABSTRACT: 6/1/86 TITLE: SYM. ON COMPOSITE MATERIALS: FATIGUE AND FRACTURE, 9TH SYM. (ASTM) INFO : PAUL A. LAGACE, MIT, ROOM 33-313, CAMBRIDGE, MA 02139 TEL: 617-253-3628 DATE : APRIL 26-30, 1987 LOCATION: CINCINNATI, OH ABSTRACT: 6/1/86 TITLE: MECHANICAL RELAXATION OF RESIDUAL STRESSES (ASTM) INFO : LEONARD MORDFIN, NATIONAL BUREAU OF STANDARDS, B344 MATERIALS BUILDING, GAITHERSBURG, MD 20899 DATE : APRIL 28-MAY 1, 1987 LOCATION: UXBRIDGE, UK ABSTRACT: Past Due TITLE: MAFELAP 1987:C0NF ON THE MATHEMATICS OF FINITE ELEMENTS 8 APPLICATIONS INFO : SECRETARY, INSTITUTE OF COMPUTATIONAL MATHEMATICS, BRUNEL UNIVERSITY, UXBRIDGE, MIDDLESEX, UB8 3PH UK Journal of Applied Mechanics DECEMBER 1986, Vol. 53/917 Downloaded 12 Aug 2010 to 146.6.92.224. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm

Time‐domain analysis of transient structural acoustics problems based on the finite element method and a novel absorbing boundary element

This paper is concerned with the development of an efficient and accurate finite element procedure for the solution directly in the time domain of transient problems involving structures submerged in an infinite acoustic fluid. The central feature of the procedure is a novel impedance, or, absorbing boundary, element that is used to render the computational domain finite. This element is local in both time and space, and is completely defined by a pair of symmetric stiffness and damping matrices. It thus can be attached directly to the adjoining fluid elements within the computational domain using standard assembly procedures. Due to its local nature, it also preserves the overall structure of the global equations of motion, including symmetry and sparseness. Thus the new impedance element makes it possible to solve complex transient exterior structural acoustics problems via existing finite element software for interior problems by just incorporating this element into current finite element libraries. St...

A simple impedance-infinite element for the finite element solution of the three-dimensional wave equation in unbounded domains

This paper is concerned with the development of an efficient and accurate impedance-infinite element that can be used either in the frequency- or directly in the time-domain for the modeling and solution of problems described by the scalar three-dimensional wave equation in infinite or semi-infinite domains. The infinite domain is truncated and the effect of the truncated infinite region is simulated by the introduction of an absorbing boundary condition prescribed on the truncation boundary. A systematic procedure for the construction of a family of such conditions of increasing accuracy and complexity is developed with explicit formulas given for approximations up to second order. A central feature of this high-order approximation is that it can be expressed, within the context of a finite element formulation, as a set of local infinite elements located at the boundary of the computational domain, with each element defined by a pair of symmetric, time-invariant, stiffness and damping matrices. This makes it possible to incorporate readily the new local boundary element into finite element software developed for purely interior regions, for applications involving steady-state harmonic or transient excitations. Whereas the theory has been developed formally for arbitrary smooth boundary surfaces, here details are provided for ellipsoidal and spherical boundaries. Thus far, only the latter has been implemented and tested in problems involving cavities and rigid scatterers of spherical and cubic shape. Numerical experiments in both the frequency- and time-domain attest to the efficacy and accuracy of the proposed new element.

The Case for Prediction-Based Best-Effort Real-Time Systems

We propose a prediction-based best-effort real-time service to support distributed, interactive applications in shared, unreserved computing environments. These applications have timing requirements, but can continue to function when deadlines are missed. In addition, they expose two kinds of adaptability: tasks can be run on any host, and their resource demands can be adjusted based on user-perceived quality. After defining this class of applications, we describe a significant example, an earthquake visualization tool, and show how it could benefit from the service. Finally, we present evidence that the service is feasible in the form of two studies of algorithms for host load prediction and for predictive task mapping.

Full-waveform inversion in three-dimensional PML-truncated elastic media

Abstract We are concerned with high-fidelity subsurface imaging of the soil, which commonly arises in geotechnical site characterization and geophysical explorations. Specifically, we attempt to image the spatial distribution of the Lame parameters in semi-infinite, three-dimensional, arbitrarily heterogeneous formations, using surficial measurements of the soil’s response to probing elastic waves. We use the complete waveform response of the medium to drive the inverse problem, by using a partial-differential-equation (PDE)-constrained optimization approach, directly in the time-domain, to minimize the misfit between the observed response of the medium at select measurement locations, and a computed response corresponding to a trial distribution of the Lame parameters. We discuss strategies that lend algorithmic robustness to our proposed inversion scheme. To limit the computational domain to the size of interest, we employ perfectly-matched-layers (PMLs). In order to resolve the forward problem, we use a recently developed hybrid finite element approach, where a displacement–stress formulation for the PML is coupled to a standard displacement-only formulation for the interior domain, thus leading to a computationally cost-efficient scheme. Time-integration is accomplished by using an explicit Runge–Kutta scheme, which is well-suited for large-scale problems on parallel computers. We verify the accuracy of the material gradients obtained via our proposed scheme, and report numerical results demonstrating successful reconstruction of the two Lame parameters for both smooth and sharp profiles.

Stable localized symmetric integral equation method for acoustic scattering problems

An energy‐based infinite boundary element integral equation method is developed for the solution of two‐ or three‐dimensional time harmonic fluid scattering problems. This method is essentially based on a domain decomposition that insures the validity for all frequencies, and uses a hypersingular operator that can be integrated readily by standard procedures for single layers. It leads to a set of sparse, symmetric discretized equations. Numerical experiments for a rigid circular cylindrical scatterer subjected to a plane incident wave confirm the stability of the new procedure, and serve to assess its accuracy for wave numbers ranging from 0 to 30, both directly on the scatterer and in the far field.

The inverse medium problem in 1D PML-truncated heterogeneous semi-infinite domains

We discuss a total wavefield-based inversion approach for the reconstruction of the material profile of heterogeneous semi-infinite domains, directly in the time domain, based on surficial measurements of the domains response to prescribed wave illumination. The ability to recover the in-depth profile of moduli/wave velocities typically associated with geotechnical site characterization applications is of particular interest. We address four key issues associated with the wavefield-based inversion: (a) to limit the semi-infinite extent of the heterogeneous physical domain, a perfectly-matched-layer (PML) is introduced at the truncation interface; (b) to account for the introduction of the PML, we use a mixed unsplit-field PML formulation for the coupled PML-regular-domain problem; (c) to tackle the inversion, we adopt a partial-differential-equation (PDE)-constrained optimization framework that formally leads to a classic Karush–Kuhn–Tucker (KKT) system comprising the initial-value state, final-value adjoint and time-independent control problems; and (d) to narrow the feasibility space and alleviate the inherent solution multiplicity, we discuss Tikhonov and Total Variation regularization schemes, endowed with a regularization factor continuation algorithm. We also limit the total observation time to optimally account for the domains heterogeneity during inversion iterations. We report on the theory and results that lead efficiently to the reconstruction of both sharp and smooth profiles in one dimension.

A Framework for Online Inversion-Based 3D Site Characterization

Our goal is to develop the capability for characterizing the three-dimensional geological structure and mechanical properties of individual sites and complete basins in earthquake-prone regions. Toward this end we present a framework that integrates in situ field testing, observations of earthquake ground motion, and inversion-based modeling.