50 research outputs found
A Generalized Approximate Control Variate Framework for Multifidelity Uncertainty Quantification
We describe and analyze a variance reduction approach for Monte Carlo (MC)
sampling that accelerates the estimation of statistics of computationally
expensive simulation models using an ensemble of models with lower cost. These
lower cost models --- which are typically lower fidelity with unknown
statistics --- are used to reduce the variance in statistical estimators
relative to a MC estimator with equivalent cost. We derive the conditions under
which our proposed approximate control variate framework recovers existing
multi-model variance reduction schemes as special cases. We demonstrate that
these existing strategies use recursive sampling strategies, and as a result,
their maximum possible variance reduction is limited to that of a control
variate algorithm that uses only a single low-fidelity model with known mean.
This theoretical result holds regardless of the number of low-fidelity models
and/or samples used to build the estimator. We then derive new sampling
strategies within our framework that circumvent this limitation to make
efficient use of all available information sources. In particular, we
demonstrate that a significant gap can exist, of orders of magnitude in some
cases, between the variance reduction achievable by using a single low-fidelity
model and our non-recursive approach. We also present initial sample allocation
approaches for exploiting this gap. They yield the greatest benefit when
augmenting the high-fidelity model evaluations is impractical because, for
instance, they arise from a legacy database. Several analytic examples and an
example with a hyperbolic PDE describing elastic wave propagation in
heterogeneous media are used to illustrate the main features of the
methodology
On the Optimization of Approximate Control Variates with Parametrically Defined Estimators
Multi-model Monte Carlo methods, such as multi-level Monte Carlo (MLMC) and
multifidelity Monte Carlo (MFMC), allow for efficient estimation of the
expectation of a quantity of interest given a set of models of varying
fidelities. Recently, it was shown that the MLMC and MFMC estimators are both
instances of the approximate control variates (ACV) framework [Gorodetsky et
al. 2020]. In that same work, it was also shown that hand-tailored ACV
estimators could outperform MLMC and MFMC for a variety of model scenarios.
Because there is no reason to believe that these hand-tailored estimators are
the best among a myriad of possible ACV estimators, a more general approach to
estimator construction is pursued in this work. First, a general form of the
ACV estimator variance is formulated. Then, the formulation is utilized to
generate parametrically-defined estimators. These parametrically-defined
estimators allow for an optimization to be pursued over a larger domain of
possible ACV estimators. The parametrically-defined estimators are tested on a
large set of model scenarios, and it is found that the broader search domain
enabled by parametrically-defined estimators leads to greater variance
reduction
Survey of multifidelity methods in uncertainty propagation, inference, and optimization
In many situations across computational science and engineering, multiple
computational models are available that describe a system of interest. These
different models have varying evaluation costs and varying fidelities.
Typically, a computationally expensive high-fidelity model describes the system
with the accuracy required by the current application at hand, while
lower-fidelity models are less accurate but computationally cheaper than the
high-fidelity model. Outer-loop applications, such as optimization, inference,
and uncertainty quantification, require multiple model evaluations at many
different inputs, which often leads to computational demands that exceed
available resources if only the high-fidelity model is used. This work surveys
multifidelity methods that accelerate the solution of outer-loop applications
by combining high-fidelity and low-fidelity model evaluations, where the
low-fidelity evaluations arise from an explicit low-fidelity model (e.g., a
simplified physics approximation, a reduced model, a data-fit surrogate, etc.)
that approximates the same output quantity as the high-fidelity model. The
overall premise of these multifidelity methods is that low-fidelity models are
leveraged for speedup while the high-fidelity model is kept in the loop to
establish accuracy and/or convergence guarantees. We categorize multifidelity
methods according to three classes of strategies: adaptation, fusion, and
filtering. The paper reviews multifidelity methods in the outer-loop contexts
of uncertainty propagation, inference, and optimization.Comment: will appear in SIAM Revie
Global Sensitivity Analysis and Estimation of Model Error, Toward Uncertainty Quantification in Scramjet Computations
The development of scramjet engines is an important research area for
advancing hypersonic and orbital flights. Progress toward optimal engine
designs requires accurate flow simulations together with uncertainty
quantification. However, performing uncertainty quantification for scramjet
simulations is challenging due to the large number of uncertain parameters
involved and the high computational cost of flow simulations. These
difficulties are addressed in this paper by developing practical uncertainty
quantification algorithms and computational methods, and deploying them in the
current study to large-eddy simulations of a jet in crossflow inside a
simplified HIFiRE Direct Connect Rig scramjet combustor. First, global
sensitivity analysis is conducted to identify influential uncertain input
parameters, which can help reduce the systems stochastic dimension. Second,
because models of different fidelity are used in the overall uncertainty
quantification assessment, a framework for quantifying and propagating the
uncertainty due to model error is presented. These methods are demonstrated on
a nonreacting jet-in-crossflow test problem in a simplified scramjet geometry,
with parameter space up to 24 dimensions, using static and dynamic treatments
of the turbulence subgrid model, and with two-dimensional and three-dimensional
geometries.Comment: Preprint 29 pages, 10 figures (26 small figures); v1 submitted to the
AIAA Journal on May 3, 2017; v2 submitted on September 17, 2017. v2 changes:
(a) addition of flowcharts in Figures 4 and 5 to summarize the tools used;
(b) edits to clarify and reorganize certain parts; v3 submitted on February
7, 2018. v3 changes: (a) title; (b) minor edit
Multifidelity Covariance Estimation via Regression on the Manifold of Symmetric Positive Definite Matrices
We introduce a multifidelity estimator of covariance matrices formulated as
the solution to a regression problem on the manifold of symmetric positive
definite matrices. The estimator is positive definite by construction, and the
Mahalanobis distance minimized to obtain it possesses properties which enable
practical computation. We show that our manifold regression multifidelity
(MRMF) covariance estimator is a maximum likelihood estimator under a certain
error model on manifold tangent space. More broadly, we show that our
Riemannian regression framework encompasses existing multifidelity covariance
estimators constructed from control variates. We demonstrate via numerical
examples that our estimator can provide significant decreases, up to one order
of magnitude, in squared estimation error relative to both single-fidelity and
other multifidelity covariance estimators. Furthermore, preservation of
positive definiteness ensures that our estimator is compatible with downstream
tasks, such as data assimilation and metric learning, in which this property is
essential.Comment: 30 pages + 15-page supplemen
Bi-fidelity Stochastic Gradient Descent for Structural Optimization under Uncertainty
The presence of uncertainty in material properties and geometry of a
structure is ubiquitous. The design of robust engineering structures,
therefore, needs to incorporate uncertainty in the optimization process.
Stochastic gradient descent (SGD) method can alleviate the cost of optimization
under uncertainty, which includes statistical moments of quantities of interest
in the objective and constraints. However, the design may change considerably
during the initial iterations of the optimization process which impedes the
convergence of the traditional SGD method and its variants. In this paper, we
present two SGD based algorithms, where the computational cost is reduced by
employing a low-fidelity model in the optimization process. In the first
algorithm, most of the stochastic gradient calculations are performed on the
low-fidelity model and only a handful of gradients from the high-fidelity model
are used per iteration, resulting in an improved convergence. In the second
algorithm, we use gradients from low-fidelity models to be used as control
variate, a variance reduction technique, to reduce the variance in the search
direction. These two bi-fidelity algorithms are illustrated first with a
conceptual example. Then, the convergence of the proposed bi-fidelity
algorithms is studied with two numerical examples of shape and topology
optimization and compared to popular variants of the SGD method that do not use
low-fidelity models. The results show that the proposed use of a bi-fidelity
approach for the SGD method can improve the convergence. Two analytical proofs
are also provided that show the linear convergence of these two algorithms
under appropriate assumptions.Comment: 38 pages, 24 figure
Active Learning with Multifidelity Modeling for Efficient Rare Event Simulation
While multifidelity modeling provides a cost-effective way to conduct
uncertainty quantification with computationally expensive models, much greater
efficiency can be achieved by adaptively deciding the number of required
high-fidelity (HF) simulations, depending on the type and complexity of the
problem and the desired accuracy in the results. We propose a framework for
active learning with multifidelity modeling emphasizing the efficient
estimation of rare events. Our framework works by fusing a low-fidelity (LF)
prediction with an HF-inferred correction, filtering the corrected LF
prediction to decide whether to call the high-fidelity model, and for enhanced
subsequent accuracy, adapting the correction for the LF prediction after every
HF model call. The framework does not make any assumptions as to the LF model
type or its correlations with the HF model. In addition, for improved
robustness when estimating smaller failure probabilities, we propose using
dynamic active learning functions that decide when to call the HF model. We
demonstrate our framework using several academic case studies and two finite
element (FE) model case studies: estimating Navier-Stokes velocities using the
Stokes approximation and estimating stresses in a transversely isotropic model
subjected to displacements via a coarsely meshed isotropic model. Across these
case studies, not only did the proposed framework estimate the failure
probabilities accurately, but compared with either Monte Carlo or a standard
variance reduction method, it also required only a small fraction of the calls
to the HF model
Multi-scale variance reduction methods based on multiple control variates for kinetic equations with uncertainties
The development of efficient numerical methods for kinetic equations with
stochastic parameters is a challenge due to the high dimensionality of the
problem. Recently we introduced a multiscale control variate strategy which is
capable to accelerate considerably the slow convergence of standard Monte Carlo
methods for uncertainty quantification. Here we generalize this class of
methods to the case of multiple control variates. We show that the additional
degrees of freedom can be used to improve further the variance reduction
properties of multiscale control variate methods.Comment: arXiv admin note: text overlap with arXiv:1810.1084
Multidelity approaches for design under uncertainty
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2013.This electronic version was submitted and approved by the author's academic department as part of an electronic thesis pilot project. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from department-submitted PDF version of thesis.Includes bibliographical references (p. 113-117).Uncertainties are present in many engineering applications and it is important to account for their effects during engineering design to achieve robust and reliable systems. One approach is to represent uncertainties as random inputs to the numerical model of the system and investigate the probabilistic behaviour of the model outputs. However, performing optimization in this setting can be computationally expensive, requiring many evaluations of the numerical model to compute the statistics of the system metrics, such as the mean and the variance of the system performance. Fortunately, in many engineering applications, there are one or more lower fidelity models that approximate the original (high-fidelity) numerical model at lower computational costs. This thesis presents rigorous multifidelity approaches to leverage cheap low-fidelity models and other approximations of the expensive high-fidelity model to reduce the computational expense of optimization under uncertainty. Solving an optimization under uncertainty problem can require estimates of the statistics at many different design points, incurring a significant number of expensive high-fidelity model evaluations. The multifidelity estimator is developed based on the control variate method to reduce the computational cost of achieving a specified root mean square error in the statistic estimate by making use of the correlation between the outputs of the expensive high-fidelity model and the outputs of the cheap low-fidelity model. The method optimally relegates some of the computational load to the low-fidelity model based on the relative model evaluation cost and the strength of the correlation. It has demonstrated 85% computational savings in an acoustic horn robust optimization example. When the model is sufficiently smooth in the design space in the sense that a small change in the design variables produces a small change in the model outputs, it has an autocorrelation structure that can be exploited by the control variate method. The information reuse estimator is developed to reduce the computational cost of achieving a specified root mean square error in the statistic estimate by making use of the correlation between the high-fidelity model outputs at one design point and those at a previously visited design point. As the optimization progresses towards the optimum in the design space, the steps taken in the design space often become shorter, increasing the correlation and making the information reuse estimator more efficient. To further reduce the computational cost, the combined estimator is developed to incorporate the features of both the multifidelity estimator and the information reuse estimator. It has demonstrated 90% computational savings in the acoustic horn robust optimization example. The methods developed in this thesis are applied to two practical aerospace applications. In conceptual aircraft design, there are often uncertainties about the future developments of the underlying technologies. The information reuse estimator can be used to efficiently generate a Pareto front to study the trade off between the expected performance and the risk induced by the uncertainties in the different aircraft designs. In a large-scale wing robust optimization problem with uncertainties in material properties and flight conditions, the combined estimator demonstrated a reasonable solution turnaround time of 9.7 days on a 16-processor desktop machine, paving the way to a larger scale wing optimization problem with distributed uncertainties to account for degradation or damage.by Leo Wai-Tsun Ng.Ph.D
Fast Adjoint-assisted Multilevel Multi delity Method for Uncertainty Quanti cation of the Aleatoric Kind
PhDIn this thesis an adjoint-based multilevel multi delity Monte Carlo (MLMF) method is proposed, analysed, and demonstrated using test problems. Firstly, a multifi delity framework using the approximate function evaluation [1] based on the adjoint error correction of Giles et al. [2] is employed as a low fidelity model. This multifi delity framework is analysed using the method proposed by Ng and Wilcox [3]. The computational cost reduction and accuracy is demonstrated using the viscous Burgers' equation subject to uncertain boundary condition. The multi fidelity framework is extended to include multilevel meshes using the MLMF of Geraci [4] called the FastUQ. Some insights on parameters affecting computational cost are shown. The implementation of FastUQ in Dakota toolkit is outlined. As a demonstration, FastUQ is used to quantify uncertainties in aerodynamic parameters due to surface variations caused by manufacturing process. A synthetic model for surface variations due to manufacturing process is proposed based on Gaussian process. The LS89 turbine cascade subject to this synthetic disturbance model at two o -design conditions is used as a test problem. Extraction of independent random modes and truncation using a goal-based principal component analysis is shown. The analysis includes truncation for problems involving multiple QoIs and test conditions. The results from FastUQ are compared to the state-of-art SMLMC method and the approximate function evaluation using adjoint error correction called the inexpensive Monte Carlo method (IMC). About 70% reduction in computational cost compared to SMLMC is achieved without any loss of accuracy. The approximate model based on the IMC has high deviations for non-linear and sensitive QoI, namely the total-pressure loss. FastUQ control variate effectively balances the low fi delity model errors and additional high fidelity evaluations to yield accurate results comparable to the high fidelity model.This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 642959