5 research outputs found
A practical factorization of a Schur complement for PDE-constrained Distributed Optimal Control
A distributed optimal control problem with the constraint of a linear
elliptic partial differential equation is considered. A necessary optimality
condition for this problem forms a saddle point system, the efficient and
accurate solution of which is crucial. A new factorization of the Schur
complement for such a system is proposed and its characteristics discussed. The
factorization introduces two complex factors that are complex conjugate to each
other. The proposed solution methodology involves the application of a parallel
linear domain decomposition solver---FETI-DPH---for the solution of the
subproblems with the complex factors. Numerical properties of FETI-DPH in this
context are demonstrated, including numerical and parallel scalability and
regularization dependence. The new factorization can be used to solve Schur
complement systems arising in both range-space and full-space formulations. In
both cases, numerical results indicate that the complex factorization is
promising
Full-waveform inversion in three-dimensional PML-truncated elastic media
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 derive
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.Comment: Submitted Journal Pape
Space-time least-squares Petrov-Galerkin projection for nonlinear model reduction
This work proposes a space-time least-squares Petrov-Galerkin (ST-LSPG)
projection method for model reduction of nonlinear dynamical systems. In
contrast to typical nonlinear model-reduction methods that first apply
(Petrov-)Galerkin projection in the spatial dimension and subsequently apply
time integration to numerically resolve the resulting low-dimensional dynamical
system, the proposed method applies projection in space and time
simultaneously. To accomplish this, the method first introduces a
low-dimensional space-time trial subspace, which can be obtained by computing
tensor decompositions of state-snapshot data. The method then computes
discrete-optimal approximations in this space-time trial subspace by minimizing
the residual arising after time discretization over all space and time in a
weighted -norm. This norm can be defined to enable complexity reduction
(i.e., hyper-reduction) in time, which leads to space-time collocation and
space-time GNAT variants of the ST-LSPG method. Advantages of the approach
relative to typical spatial-projection-based nonlinear model reduction methods
such as Galerkin projection and least-squares Petrov-Galerkin projection
include: (1) a reduction of both the spatial and temporal dimensions of the
dynamical system, (2) the removal of spurious temporal modes (e.g., unstable
growth) from the state space, and (3) error bounds that exhibit slower growth
in time. Numerical examples performed on model problems in fluid dynamics
demonstrate the ability of the method to generate orders-of-magnitude
computational savings relative to spatial-projection-based reduced-order models
without sacrificing accuracy.Comment: Accepted to the SIAM Journal on Scientific Computin
Accelerating design optimization using reduced order models
Although design optimization has shown its great power of automatizing the
whole design process and providing an optimal design, using sophisticated
computational models, its process can be formidable due to a computationally
expensive large-scale linear system of equations to solve, associated with
underlying physics models. We introduce a general reduced order model-based
design optimization acceleration approach that is applicable not only to design
optimization problems, but also to any PDE-constrained optimization problems.
The acceleration is achieved by two techniques: i) allowing an inexact linear
solve and ii) reducing the number of iterations in Krylov subspace iterative
methods. The choice between two techniques are made, based on how close a
current design point to an optimal point. The advantage of the acceleration
approach is demonstrated in topology optimization examples, including both
compliance minimization and stress-constrained problems, where it achieves a
tremendous reduction and speed-up when a traditional preconditioner fails to
achieve a considerable reduction in the number of linear solve iterations.Comment: 20 pages, 5 figures, 5 tables, 5 algorithm
Efficient space-time reduced order model for linear dynamical systems in Python using less than 120 lines of code
A classical reduced order model (ROM) for dynamical problems typically
involves only the spatial reduction of a given problem. Recently, a novel
space-time ROM for linear dynamical problems has been developed, which further
reduces the problem size by introducing a temporal reduction in addition to a
spatial reduction without much loss in accuracy. The authors show an order of a
thousand speed-up with a relative error of less than 0.00001 for a large-scale
Boltzmann transport problem. In this work, we present for the first time the
derivation of the space-time Petrov-Galerkin projection for linear dynamical
systems and its corresponding block structures. Utilizing these block
structures, we demonstrate the ease of construction of the space-time ROM
method with two model problems: 2D diffusion and 2D convection diffusion, with
and without a linear source term. For each problem, we demonstrate the entire
process of generating the full order model (FOM) data, constructing the
space-time ROM, and predicting the reduced-order solutions, all in less than
120 lines of Python code. We compare our Petrov-Galerkin method with the
traditional Galerkin method and show that the space-time ROMs can achieve
O(100) speed-ups with O(0.001) to O(0.0001) relative errors for these problems.
Finally, we present an error analysis for the space-time Petrov-Galerkin
projection and derive an error bound, which shows an improvement compared to
traditional spatial Galerkin ROM methods.Comment: 24 pages, 18 figure