286 research outputs found
Local discontinuous Galerkin methods for fractional ordinary differential equations
This paper discusses the upwinded local discontinuous Galerkin methods for
the one-term/multi-term fractional ordinary differential equations (FODEs). The
natural upwind choice of the numerical fluxes for the initial value problem for
FODEs ensures stability of the methods. The solution can be computed element by
element with optimal order of convergence in the norm and
superconvergence of order at the downwind point of each
element. Here is the degree of the approximation polynomial used in an
element and () represents the order of the one-term
FODEs. A generalization of this includes problems with classic 'th-term
FODEs, yielding superconvergence order at downwind point as
. The underlying mechanism of the
superconvergence is discussed and the analysis confirmed through examples,
including a discussion of how to use the scheme as an efficient way to evaluate
the generalized Mittag-Leffler function and solutions to more generalized
FODE's.Comment: 17 pages, 7 figure
Structure-Preserving Model-Reduction of Dissipative Hamiltonian Systems
Reduced basis methods are popular for approximately solving large and complex
systems of differential equations. However, conventional reduced basis methods
do not generally preserve conservation laws and symmetries of the full order
model. Here, we present an approach for reduced model construction, that
preserves the symplectic symmetry of dissipative Hamiltonian systems. The
method constructs a closed reduced Hamiltonian system by coupling the full
model with a canonical heat bath. This allows the reduced system to be
integrated with a symplectic integrator, resulting in a correct dissipation of
energy, preservation of the total energy and, ultimately, in the stability of
the solution. Accuracy and stability of the method are illustrated through the
numerical simulation of the dissipative wave equation and a port-Hamiltonian
model of an electric circuit
Structure Preserving Model Reduction of Parametric Hamiltonian Systems
While reduced-order models (ROMs) have been popular for efficiently solving
large systems of differential equations, the stability of reduced models over
long-time integration is of present challenges. We present a greedy approach
for ROM generation of parametric Hamiltonian systems that captures the
symplectic structure of Hamiltonian systems to ensure stability of the reduced
model. Through the greedy selection of basis vectors, two new vectors are added
at each iteration to the linear vector space to increase the accuracy of the
reduced basis. We use the error in the Hamiltonian due to model reduction as an
error indicator to search the parameter space and identify the next best basis
vectors. Under natural assumptions on the set of all solutions of the
Hamiltonian system under variation of the parameters, we show that the greedy
algorithm converges with exponential rate. Moreover, we demonstrate that
combining the greedy basis with the discrete empirical interpolation method
also preserves the symplectic structure. This enables the reduction of the
computational cost for nonlinear Hamiltonian systems. The efficiency, accuracy,
and stability of this model reduction technique is illustrated through
simulations of the parametric wave equation and the parametric Schrodinger
equation
IMEX evolution of scalar fields on curved backgrounds
Inspiral of binary black holes occurs over a time-scale of many orbits, far
longer than the dynamical time-scale of the individual black holes. Explicit
evolutions of a binary system therefore require excessively many time steps to
capture interesting dynamics. We present a strategy to overcome the
Courant-Friedrichs-Lewy condition in such evolutions, one relying on modern
implicit-explicit ODE solvers and multidomain spectral methods for elliptic
equations. Our analysis considers the model problem of a forced scalar field
propagating on a generic curved background. Nevertheless, we encounter and
address a number of issues pertinent to the binary black hole problem in full
general relativity. Specializing to the Schwarzschild geometry in Kerr-Schild
coordinates, we document the results of several numerical experiments testing
our strategy.Comment: 28 pages, uses revtex4. Revised in response to referee's report. One
numerical experiment added which incorporates perturbed initial data and
adaptive time-steppin
Deep convolutional neural networks for estimating porous material parameters with ultrasound tomography
We study the feasibility of data based machine learning applied to ultrasound
tomography to estimate water-saturated porous material parameters. In this
work, the data to train the neural networks is simulated by solving wave
propagation in coupled poroviscoelastic-viscoelastic-acoustic media. As the
forward model, we consider a high-order discontinuous Galerkin method while
deep convolutional neural networks are used to solve the parameter estimation
problem. In the numerical experiment, we estimate the material porosity and
tortuosity while the remaining parameters which are of less interest are
successfully marginalized in the neural networks-based inversion. Computational
examples confirms the feasibility and accuracy of this approach
Waves 2005 Conference
Held at Brown University, Providence, RI, June 20--24, 2005International audienc
Fast prediction and evaluation of gravitational waveforms using surrogate models
[Abridged] We propose a solution to the problem of quickly and accurately
predicting gravitational waveforms within any given physical model. The method
is relevant for both real-time applications and in more traditional scenarios
where the generation of waveforms using standard methods can be prohibitively
expensive. Our approach is based on three offline steps resulting in an
accurate reduced-order model that can be used as a surrogate for the
true/fiducial waveform family. First, a set of m parameter values is determined
using a greedy algorithm from which a reduced basis representation is
constructed. Second, these m parameters induce the selection of m time values
for interpolating a waveform time series using an empirical interpolant. Third,
a fit in the parameter dimension is performed for the waveform's value at each
of these m times. The cost of predicting L waveform time samples for a generic
parameter choice is of order m L + m c_f online operations where c_f denotes
the fitting function operation count and, typically, m << L. We generate
accurate surrogate models for Effective One Body (EOB) waveforms of
non-spinning binary black hole coalescences with durations as long as 10^5 M,
mass ratios from 1 to 10, and for multiple harmonic modes. We find that these
surrogates are three orders of magnitude faster to evaluate as compared to the
cost of generating EOB waveforms in standard ways. Surrogate model building for
other waveform models follow the same steps and have the same low online
scaling cost. For expensive numerical simulations of binary black hole
coalescences we thus anticipate large speedups in generating new waveforms with
a surrogate. As waveform generation is one of the dominant costs in parameter
estimation algorithms and parameter space exploration, surrogate models offer a
new and practical way to dramatically accelerate such studies without impacting
accuracy.Comment: 20 pages, 17 figures, uses revtex 4.1. Version 2 includes new
numerical experiments for longer waveform durations, larger regions of
parameter space and multi-mode model
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