3,001 research outputs found
Variational integrators for degenerate Lagrangians, with application to point vortices
We develop discrete mechanics and variational integrators
for a class of degenerate Lagrangian systems,
and apply these integrators to a system of
point vortices. Excellent numerical behavior is observed.
A longer term goal is to use these integration
methods in the context of control of mechanical
systems, such as coordinated groups of underwater
vehicles. In fact, numerical evidence given
in related problems, such as those in [2] shows that
in the presence of external forces, these methods
give superior predictions of energy behavior
Repeatable method of thermal stress fracture test of brittle materials
Method heats specimens slowly and with sufficient control so that the critical temperature gradient in the specimens cannot occur before temperature equilibrium is reached
A computer software system for integration and analysis of grid-based remote sensing data with other natural resource data. Remote Sensing Project
A computer-based information system is described designed to assist in the integration of commonly available spatial data for regional planning and resource analysis. The Resource Analysis Program (RAP) provides a variety of analytical and mapping phases for single factor or multi-factor analyses. The unique analytical and graphic capabilities of RAP are demonstrated with a study conducted in Windsor Township, Eaton County, Michigan. Soil, land cover/use, topographic and geological maps were used as a data base to develope an eleven map portfolio. The major themes of the portfolio are land cover/use, non-point water pollution, waste disposal, and ground water recharge
pMSSM Benchmark Models for Snowmass 2013
We present several benchmark points in the phenomenological Minimal
Supersymmetric Standard Model (pMSSM). We select these models as experimentally
well-motivated examples of the MSSM which predict the observed Higgs mass and
dark matter relic density while evading the current LHC searches. We also use
benchmarks to generate spokes in parameter space by scaling the mass parameters
in a manner which keeps the Higgs mass and relic density approximately
constant.Comment: 10 pages, 6 figure
Learning Bilinear Models of Actuated Koopman Generators from Partially-Observed Trajectories
Data-driven models for nonlinear dynamical systems based on approximating the
underlying Koopman operator or generator have proven to be successful tools for
forecasting, feature learning, state estimation, and control. It has become
well known that the Koopman generators for control-affine systems also have
affine dependence on the input, leading to convenient finite-dimensional
bilinear approximations of the dynamics. Yet there are still two main obstacles
that limit the scope of current approaches for approximating the Koopman
generators of systems with actuation. First, the performance of existing
methods depends heavily on the choice of basis functions over which the Koopman
generator is to be approximated; and there is currently no universal way to
choose them for systems that are not measure preserving. Secondly, if we do not
observe the full state, then it becomes necessary to account for the dependence
of the output time series on the sequence of supplied inputs when constructing
observables to approximate Koopman operators. To address these issues, we write
the dynamics of observables governed by the Koopman generator as a bilinear
hidden Markov model, and determine the model parameters using the
expectation-maximization (EM) algorithm. The E-step involves a standard Kalman
filter and smoother, while the M-step resembles control-affine dynamic mode
decomposition for the generator. We demonstrate the performance of this method
on three examples, including recovery of a finite-dimensional Koopman-invariant
subspace for an actuated system with a slow manifold; estimation of Koopman
eigenfunctions for the unforced Duffing equation; and model-predictive control
of a fluidic pinball system based only on noisy observations of lift and drag
Model Reduction for Nonlinear Systems by Balanced Truncation of State and Gradient Covariance
Data-driven reduced-order models often fail to make accurate forecasts of
high-dimensional nonlinear dynamical systems that are sensitive along
coordinates with low-variance because such coordinates are often truncated,
e.g., by proper orthogonal decomposition, kernel principal component analysis,
and autoencoders. Such systems are encountered frequently in shear-dominated
fluid flows where non-normality plays a significant role in the growth of
disturbances. In order to address these issues, we employ ideas from active
subspaces to find low-dimensional systems of coordinates for model reduction
that balance adjoint-based information about the system's sensitivity with the
variance of states along trajectories. The resulting method, which we refer to
as covariance balancing reduction using adjoint snapshots (CoBRAS), is
analogous to balanced truncation with state and adjoint-based gradient
covariance matrices replacing the system Gramians and obeying the same key
transformation laws. Here, the extracted coordinates are associated with an
oblique projection that can be used to construct Petrov-Galerkin reduced-order
models. We provide an efficient snapshot-based computational method analogous
to balanced proper orthogonal decomposition. This also leads to the observation
that the reduced coordinates can be computed relying on inner products of state
and gradient samples alone, allowing us to find rich nonlinear coordinates by
replacing the inner product with a kernel function. In these coordinates,
reduced-order models can be learned using regression. We demonstrate these
techniques and compare to a variety of other methods on a simple, yet
challenging three-dimensional system and a nonlinear axisymmetric jet flow
simulation with state variables
Analysis of amplification mechanisms and cross-frequency interactions in nonlinear flows via the harmonic resolvent
We propose a framework that elucidates the input-output characteristics of
flows with complex dynamics arising from nonlinear interactions between
different time scales. More specifically, we consider a periodically
time-varying base flow, and perform a frequency-domain analysis of periodic
perturbations about this base flow; the response of these perturbations is
governed by the harmonic resolvent, which is a linear operator similar to the
harmonic transfer function introduced by Wereley (1991). This approach makes it
possible to explicitly capture the triadic interactions that are responsible
for the energy transfer between different time scales in the flow. For
instance, perturbations at frequency are coupled with perturbations at
frequency through the base flow at frequency . We draw
a connection with resolvent analsyis, which is a special case of the harmonic
resolvent when evaluated about a steady base flow. We show that the left and
right singular vectors of the harmonic resolvent are the optimal response and
forcing modes, which can be understood as full spatio-temporal signals that
reveal space-time amplification characteristics of the flow. We illustrate the
method on examples, including a three-dimensional system of ordinary
differential equations and the flow over an airfoil at near-stall angle of
attack
Learning Nonlinear Projections for Reduced-Order Modeling of Dynamical Systems using Constrained Autoencoders
Recently developed reduced-order modeling techniques aim to approximate
nonlinear dynamical systems on low-dimensional manifolds learned from data.
This is an effective approach for modeling dynamics in a post-transient regime
where the effects of initial conditions and other disturbances have decayed.
However, modeling transient dynamics near an underlying manifold, as needed for
real-time control and forecasting applications, is complicated by the effects
of fast dynamics and nonnormal sensitivity mechanisms. To begin to address
these issues, we introduce a parametric class of nonlinear projections
described by constrained autoencoder neural networks in which both the manifold
and the projection fibers are learned from data. Our architecture uses
invertible activation functions and biorthogonal weight matrices to ensure that
the encoder is a left inverse of the decoder. We also introduce new
dynamics-aware cost functions that promote learning of oblique projection
fibers that account for fast dynamics and nonnormality. To demonstrate these
methods and the specific challenges they address, we provide a detailed case
study of a three-state model of vortex shedding in the wake of a bluff body
immersed in a fluid, which has a two-dimensional slow manifold that can be
computed analytically. In anticipation of future applications to
high-dimensional systems, we also propose several techniques for constructing
computationally efficient reduced-order models using our proposed nonlinear
projection framework. This includes a novel sparsity-promoting penalty for the
encoder that avoids detrimental weight matrix shrinkage via computation on the
Grassmann manifold
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