7,544 research outputs found
Fujiwhara interaction of tropical cyclone scale vortices using a weighted residual collocation method
The fundamental interaction between tropical cyclones was investigated
through a series of water tank experiements by Fujiwhara [20, 21, 22]. However,
a complete understanding of tropical cyclones remains an open research
challenge although there have been numerous investigations through measurments
with aircrafts/satellites, as well as with numerical simulations. This article
presents a computational model for simulating the interaction between cyclones.
The proposed numerical method is presented briefly, where the time integration
is performed by projecting the discrete system onto a Krylov subspace. The
method filters the large scale fluid dynamics using a multiresolution
approximation, and the unresolved dynamics is modeled with a Smagorinsky type
subgrid scale parameterization scheme. Numerical experiments with Fujiwhara
interactions are considered to verify modeling accuracy. An excellent agreement
between the present simulation and a reference simulation at Re = 5000 has been
demonstrated. At Re = 37440, the kinetic energy of cyclones is seen
consolidated into larger scales with concurrent enstrophy cascade, suggesting a
steady increase of energy containing scales, a phenomena that is typical in
two-dimensional turbulence theory. The primary results of this article suggest
a novel avenue for addressing some of the computational challenges of mesoscale
atmospheric circulations.Comment: 24 pages, 11 figures, submitte
Projection-free approximate balanced truncation of large unstable systems
In this article, we show that the projection-free, snapshot-based, balanced
truncation method can be applied directly to unstable systems. We prove that
even for unstable systems, the unmodified balanced proper orthogonal
decomposition algorithm theoretically yields a converged transformation that
balances the Gramians (including the unstable subspace). We then apply the
method to a spatially developing unstable system and show that it results in
reduced-order models of similar quality to the ones obtained with existing
methods. Due to the unbounded growth of unstable modes, a practical restriction
on the final impulse response simulation time appears, which can be adjusted
depending on the desired order of the reduced-order model. Recommendations are
given to further reduce the cost of the method if the system is large and to
improve the performance of the method if it does not yield acceptable results
in its unmodified form. Finally, the method is applied to the linearized flow
around a cylinder at Re = 100 to show that it actually is able to accurately
reproduce impulse responses for more realistic unstable large-scale systems in
practice. The well-established approximate balanced truncation numerical
framework can therefore be safely applied to unstable systems without any
modifications. Additionally, balanced reduced-order models can readily be
obtained even for large systems, where the computational cost of existing
methods is prohibitive
Multiscale modelling couples patches of two-layer thin fluid flow
The multiscale gap-tooth scheme uses a given microscale simulator of
complicated physical processes to enable macroscale simulations by computing
only only small sparse patches. This article develops the gap-tooth scheme to
the case of nonlinear microscale simulations of thin fluid flow. The microscale
simulator is derived by artificially assuming the fluid film flow having two
artificial layers but no distinguishing physical feature. Centre manifold
theory assures that there exists a slow manifold in the two-layer fluid film
flow. Eigenvalue analysis confirms the stability of the microscale simulator.
This article uses the gap-tooth scheme to simulate the two-layer fluid film
flow. Coupling conditions are developed by approximating the values at the
edges of patches by neighbouring macroscale values. Numerical eigenvalue
analysis suggests that the gap-tooth scheme with the developed two-layer
microscale simulator empowers feasible computation of large scale simulations
of fluid film flows. We also implement numerical simulations of the fluid film
flow by the gap-tooth scheme. Comparison between a gap-tooth simulation and a
microscale simulation over the whole domain demonstrates that the gap-tooth
scheme feasibly computes fluid film flow dynamics with computational savings.Comment: 8 figures and 1 tabl
Projection-free approximate balanced truncation of large unstable systems
In this article, we show that the projection-free, snapshot-based, balanced truncation method can be applied directly to unstable systems. We prove that even for unstable systems, the unmodified balanced proper orthogonal decomposition algorithm theoretically yields a converged transformation that balances the Gramians (including the unstable subspace). We then apply the method to a spatially developing unstable system and show that it results in reduced-order models of similar quality to the ones obtained with existing methods. Due to the unbounded growth of unstable modes, a practical restriction on the final impulse response simulation time appears, which can be adjusted depending on the desired order of the reduced-order model. Recommendations are given to further reduce the cost of the method if the system is large and to improve the performance of the method if it does not yield acceptable results in its unmodified form. Finally, the method is applied to the linearized flow around a cylinder at Re = 100 to show that it actually is able to accurately reproduce impulse responses for more realistic unstable large-scale systems in practice. The well-established approximate balanced truncation numerical framework therefore can be safely applied to unstable systems without any modifications. Additionally, balanced reduced-order models can readily be obtained even for large systems, where the computational cost of existing methods is prohibitive
A note on robust preconditioners for monolithic fluid-structure interaction systems of finite element equations
In this note, we consider preconditioned Krylov subspace methods for discrete
fluid-structure interaction problems with a nonlinear hyperelastic material
model and covering a large range of flows, e.g, water, blood, and air with
highly varying density. Based on the complete factorization of the
coupled system matrix, the preconditioner is constructed in form of
, where , and are proper
approximations to , and , respectively. The inverse of the
corresponding Schur complement is approximated by applying one cycle of a
special class of algebraic multigrid methods to the perturbed fluid
sub-problem, that is obtained by modifying corresponding entries in the
original fluid matrix with an explicitly constructed approximation of the exact
perturbation coming from the sparse matrix-matrix multiplications
Modeling of Transitional Channel Flow Using Balanced Proper Orthogonal Decomposition
We study reduced-order models of three-dimensional perturbations in
linearized channel flow using balanced proper orthogonal decomposition (BPOD).
The models are obtained from three-dimensional simulations in physical space as
opposed to the traditional single-wavenumber approach, and are therefore better
able to capture the effects of localized disturbances or localized actuators.
In order to assess the performance of the models, we consider the impulse
response and frequency response, and variation of the Reynolds number as a
model parameter. We show that the BPOD procedure yields models that capture the
transient growth well at a low order, whereas standard POD does not capture the
growth unless a considerably larger number of modes is included, and even then
can be inaccurate. In the case of a localized actuator, we show that POD modes
which are not energetically significant can be very important for capturing the
energy growth. In addition, a comparison of the subspaces resulting from the
two methods suggests that the use of a non-orthogonal projection with adjoint
modes is most likely the main reason for the superior performance of BPOD. We
also demonstrate that for single-wavenumber perturbations, low-order BPOD
models reproduce the dominant eigenvalues of the full system better than POD
models of the same order. These features indicate that the simple, yet accurate
BPOD models are a good candidate for developing model-based controllers for
channel flow.Comment: 35 pages, 20 figure
An integrated data-driven computational pipeline with model order reduction for industrial and applied mathematics
In this work we present an integrated computational pipeline involving
several model order reduction techniques for industrial and applied
mathematics, as emerging technology for product and/or process design
procedures. Its data-driven nature and its modularity allow an easy integration
into existing pipelines. We describe a complete optimization framework with
automated geometrical parameterization, reduction of the dimension of the
parameter space, and non-intrusive model order reduction such as dynamic mode
decomposition and proper orthogonal decomposition with interpolation. Moreover
several industrial examples are illustrated
An Image-Based Fluid Surface Pattern Model
This work aims at generating a model of the ocean surface and its dynamics
from one or more video cameras. The idea is to model wave patterns from video
as a first step towards a larger system of photogrammetric monitoring of marine
conditions for use in offshore oil drilling platforms. The first part of the
proposed approach consists in reducing the dimensionality of sensor data made
up of the many pixels of each frame of the input video streams. This enables
finding a concise number of most relevant parameters to model the temporal
dataset, yielding an efficient data-driven model of the evolution of the
observed surface. The second part proposes stochastic modeling to better
capture the patterns embedded in the data. One can then draw samples from the
final model, which are expected to simulate the behavior of previously observed
flow, in order to determine conditions that match new observations. In this
paper we focus on proposing and discussing the overall approach and on
comparing two different techniques for dimensionality reduction in the first
stage: principal component analysis and diffusion maps. Work is underway on the
second stage of constructing better stochastic models of fluid surface dynamics
as proposed here.Comment: a reduced version in Portuguese appears in proceedings of the XVI EMC
- Computational Modeling Meeting (Encontro de Modelagem Computacional), 201
A probabilistic decomposition-synthesis method for the quantification of rare events due to internal instabilities
We consider the problem of probabilistic quantification of dynamical systems
that have heavy-tailed characteristics. These heavy-tailed features are
associated with rare transient responses due to the occurrence of internal
instabilities. Here we develop a computational method, a probabilistic
decomposition-synthesis technique, that takes into account the nature of
internal instabilities to inexpensively determine the non-Gaussian probability
density function for any arbitrary quantity of interest. Our approach relies on
the decomposition of the statistics into a `non-extreme core', typically
Gaussian, and a heavy-tailed component. This decomposition is in full
correspondence with a partition of the phase space into a `stable' region where
we have no internal instabilities, and a region where non-linear instabilities
lead to rare transitions with high probability. We quantify the statistics in
the stable region using a Gaussian approximation approach, while the
non-Gaussian distributions associated with the intermittently unstable regions
of the phase space are inexpensively computed through order-reduction methods
that take into account the strongly nonlinear character of the dynamics. The
probabilistic information in the two domains is analytically synthesized
through a total probability argument. The proposed approach allows for the
accurate quantification of non-Gaussian tails at more than 10 standard
deviations, at a fraction of the cost associated with the direct Monte-Carlo
simulations. We demonstrate the probabilistic decomposition-synthesis method
for rare events for two dynamical systems exhibiting extreme events: a
two-degree-of-freedom system of nonlinearly coupled oscillators, and in a
nonlinear envelope equation characterizing the propagation of unidirectional
water waves
On matrix-free computation of 2D unstable manifolds
Recently, a flexible and stable algorithm was introduced for the computation
of 2D unstable manifolds of periodic solutions to systems of ordinary
differential equations. The main idea of this approach is to represent orbits
in this manifold as the solutions of an appropriate boundary value problem. The
boundary value problem is under determined and a one parameter family of
solutions can be found by means of arclength continuation. This family of
orbits covers a piece of the manifold. The quality of this covering depends on
the way the boundary value problem is discretised, as do the tractability and
accuracy of the computation. In this paper, we describe an implementation of
the orbit continuation algorithm which relies on multiple shooting and
Newton-Krylov continuation. We show that the number of time integrations
necessary for each continuation step scales only with the number of shooting
intervals but not with the number of degrees of freedom of the dynamical
system. The number of shooting intervals is chosen based on linear stability
analysis to keep the conditioning of the boundary value problem in check. We
demonstrate our algorithm with two test systems: a low-order model of shear
flow and a well-resolved simulation of turbulent plane Couette flow
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