190 research outputs found
SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES
Crack propagation in thin shell structures due to cutting is conveniently simulated
using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell
elements are usually preferred for the discretization in the presence of complex material
behavior and degradation phenomena such as delamination, since they allow for a correct
representation of the thickness geometry. However, in solid-shell elements the small thickness
leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new
selective mass scaling technique is proposed to increase the time-step size without affecting
accuracy. New ”directional” cohesive interface elements are used in conjunction with selective
mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile
shells
Advanced BEM-based methodologies to identify and simulate wave fields in complex geostructures
To enhance the applicability of BEM for geomechanical modeling numerically optimized BEM models, hybrid FEM-BEM models, and parallel computation of seismic Full Waveform Inversion (FWI) in GPU are implemented. Inverse modeling of seismic wave propagation in inhomogeneous and heterogeneous half-plane is implemented in Boundary Element Method (BEM) using Particle Swarm Optimization (PSO). The Boundary Integral Equations (BIE) based on the fundamental solutions for homogeneous elastic isotropic continuum are modified by introducing mesh-dependent variables. The variables are optimized to obtain the site-specific impedance functions. The PSO-optimized BEM models have significantly improved the efficiency of BEM for seismic wave propagation in arbitrarily inhomogeneous and heterogeneous media. Similarly, a hybrid BEM-FEM approach is developed to evaluate the seismic response of a complex poroelastic soil region containing underground structures. The far-field semi-infinite geological region is modeled via BEM, while the near-field finite geological region is modeled via FEM. The BEM region is integrated into the global FEM system using an equivalent macro-finite-element. The model describes the entire wave path from the seismic source to the local site in a single hybrid model. Additionally, the computational efficiency of time domain FWI algorithm is enhanced by parallel computation in CPU and GPU
Computational Scattering Models for Elastic and Electromagnetic Waves in Particulate Media
Numerical models were developed to simulate the propagation of elastic and electromagnetic waves in an arbitrary, dense dispersion of spherical particles. The scattering interactions were modeled with vector multipole fields using pure-orbital vector spherical harmonics, and solved using the full vector form of the boundary conditions. Multiple scattering was simulated by translating the scattered wave fields from one particle to another with the use of translational addition theorems, summing the multiple-scattering contributions, and recalculating the scattering in an iterative fashion to a convergent solution. The addition theorems were rederived in this work using an integral method, and were shown to be numerically equivalent to previously published theorems. Both ordered and disordered collections of up to 5,000 spherical particles were used to demonstrate the ability of the scattering models to predict the spatial and frequency distributions of the transmitted waves. The results of the models show they are qualitatively correct for many particle configurations and material properties, displaying predictable phenomena such as refractive focusing, mode conversion, and photonic band gaps. However, the elastic wave models failed to converge for specific frequency regions, possibly due to resonance effects. Additionally, comparison of the multiple-scattering simulations with those using only single-particle scattering showed the multiple-scattering computations are quantitatively inaccurate. The inaccuracies arise from nonconvergence of the translational addition theorems, introducing errors into the translated fields, which minimize the multiple-scattering contributions and bias the field amplitudes towards single-scattering contributions. The addition theorems are shown to converge very slowly, and to exhibit plateaus in convergence behavior that can lead to false indications of convergence. The theory and algorithms developed for the models are broad-based, and can accommodate a variety of structures, compositions, and wave modes. The generality of the approach also lends itself to the modeling of static fields and currents. Suggestions are presented for improving and implementing the models, including extension to nonspherical particles, efficiency improvements for the algorithms, and specific applications in a variety of fields
[Activity of Institute for Computer Applications in Science and Engineering]
This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, fluid mechanics, and computer science
Distributed Simulations for 3D Ultrasound Computer Tomography
More than 10% of all women in the western world get breast cancer. The Ultrasound Computer Tomography (USCT) project aims to provide a screening method which can detect cancer tumours at 5mm
Three-dimensional modelling and inversion of controlled source electromagnetic data
The marine Controlled Source Electromagnetic (CSEM) method is an important and almost
self-contained discipline in the toolkit of methods used by geophysicists for probing the earth.
It has increasingly attracted attention from industry during the past decade due to its potential
in detecting valuable natural resources such as oil and gas.
A method for three-dimensional CSEM modelling in the frequency domain is presented. The
electric field is decomposed in primary and secondary components, as this leads to a more
stable solution near the source position. The primary field is computed using a resistivity
model for which a closed form of solution exists, for example a homogeneous or layered
resistivity model. The secondary electric field is computed by discretizing a second order
partial differential equation for the electric field, also referred in the literature as the vector
Helmholtz equation, using the edge finite element method. A range of methods for the solution
of the linear system derived from the edge finite element discretization are investigated.
The magnetic field is computed subsequently, from the solution for the electric field, using
a local finite difference approximation of Faraday’s law and an interpolation method. Tests,
that compare the solution obtained using the presented method with the solution computed
using alternative codes for 1D and 3D synthetic models, show that the implemented approach
is suitable for CSEM forward modelling and is an alternative to existing codes.
An algorithm for 3D inversion of CSEM data in the frequency domain was developed and
implemented. The inverse problem is solved using the L-BFGS method and is regularized
with a smoothing constraint. The inversion algorithm uses the presented forward modelling
scheme for the computation of the field responses and the adjoint field for the computation
of the gradient of the misfit function. The presented algorithm was tested for a synthetic
example, showing that it is capable of reconstructing a resistivity model which fits the synthetic
data and is close to the original resistivity model in the least-squares sense.
Inversion of CSEM data is known to lead to images with low spatial resolution. It is well
known that integration with complementary data sets mitigates this problem. It is presented
an algorithm for the integration of an acoustic velocity model, which is known a priori, in the
inversion scheme. The algorithm was tested in a synthetic example and the results demonstrate
that the presented methodology is promising for the improvement of resistivity models
obtained from CSEM data
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