16,519 research outputs found
Generic Regularity of Conservative Solutions to a Nonlinear Wave Equation
The paper is concerned with conservative solutions to the nonlinear wave
equation . For an open dense set of
initial data, we prove that the solution is piecewise smooth in the
- plane, while the gradient can blow up along finitely many
characteristic curves. The analysis is based on a variable transformation
introduced in [7], which reduces the equation to a semilinear system with
smooth coefficients, followed by an application of Thom's transversality
theorem.Comment: 25 page
A nonlinear scalar model of extreme mass ratio inspirals in effective field theory I. Self force through third order
The motion of a small compact object in a background spacetime is
investigated in the context of a model nonlinear scalar field theory. This
model is constructed to have a perturbative structure analogous to the General
Relativistic description of extreme mass ratio inspirals (EMRIs). We apply the
effective field theory approach to this model and calculate the finite part of
the self force on the small compact object through third order in the ratio of
the size of the compact object to the curvature scale of the background (e.g.,
black hole) spacetime. We use well-known renormalization methods and
demonstrate the consistency of the formalism in rendering the self force finite
at higher orders within a point particle prescription for the small compact
object. This nonlinear scalar model should be useful for studying various
aspects of higher-order self force effects in EMRIs but within a comparatively
simpler context than the full gravitational case. These aspects include
developing practical schemes for higher order self force numerical
computations, quantifying the effects of transient resonances on EMRI waveforms
and accurately modeling the small compact object's motion for precise
determinations of the parameters of detected EMRI sources.Comment: 30 pages, 8 figure
A Numerical Slow Manifold Approach to Model Reduction for Optimal Control of Multiple Time Scale ODE
Time scale separation is a natural property of many control systems that can
be ex- ploited, theoretically and numerically. We present a numerical scheme to
solve optimal control problems with considerable time scale separation that is
based on a model reduction approach that does not need the system to be
explicitly stated in singularly perturbed form. We present examples that
highlight the advantages and disadvantages of the method
Pipe Poiseuille flow of viscously anisotropic, partially molten rock
Laboratory experiments in which synthetic, partially molten rock is subjected
to forced deformation provide a context for testing hypotheses about the
dynamics and rheology of the mantle. Here our hypothesis is that the aggregate
viscosity of partially molten mantle is anisotropic, and that this anisotropy
arises from deviatoric stresses in the rock matrix. We formulate a model of
pipe Poiseuille flow based on theory by Takei and Holtzman [2009a] and Takei
and Katz [2013]. Pipe Poiseuille is a configuration that is accessible to
laboratory experimentation but for which there are no published results. We
analyse the model system through linearised analysis and numerical simulations.
This analysis predicts two modes of melt segregation: migration of melt from
the centre of the pipe toward the wall and localisation of melt into
high-porosity bands that emerge near the wall, at a low angle to the shear
plane. We compare our results to those of Takei and Katz [2013] for plane
Poiseuille flow; we also describe a new approximation of radially varying
anisotropy that improves the self-consistency of models over those of Takei and
Katz [2013]. This study provides a set of baseline, quantitative predictions to
compare with future laboratory experiments on forced pipe Poiseuille flow of
partially molten mantle.Comment: 23 pages, 7 figures. Submitted to Geophysical Journal International
on 25 April 2014. Revised after reviewer comments and resubmitted on 20
August 201
A variational framework for flow optimization using semi-norm constraints
When considering a general system of equations describing the space-time
evolution (flow) of one or several variables, the problem of the optimization
over a finite period of time of a measure of the state variable at the final
time is a problem of great interest in many fields. Methods already exist in
order to solve this kind of optimization problem, but sometimes fail when the
constraint bounding the state vector at the initial time is not a norm, meaning
that some part of the state vector remains unbounded and might cause the
optimization procedure to diverge. In order to regularize this problem, we
propose a general method which extends the existing optimization framework in a
self-consistent manner. We first derive this framework extension, and then
apply it to a problem of interest. Our demonstration problem considers the
transient stability properties of a one-dimensional (in space) averaged
turbulent model with a space- and time-dependent model "turbulent viscosity".
We believe this work has a lot of potential applications in the fluid
dynamics domain for problems in which we want to control the influence of
separate components of the state vector in the optimization process.Comment: 30 page
Optimal control of multiscale systems using reduced-order models
We study optimal control of diffusions with slow and fast variables and
address a question raised by practitioners: is it possible to first eliminate
the fast variables before solving the optimal control problem and then use the
optimal control computed from the reduced-order model to control the original,
high-dimensional system? The strategy "first reduce, then optimize"--rather
than "first optimize, then reduce"--is motivated by the fact that solving
optimal control problems for high-dimensional multiscale systems is numerically
challenging and often computationally prohibitive. We state sufficient and
necessary conditions, under which the "first reduce, then control" strategy can
be employed and discuss when it should be avoided. We further give numerical
examples that illustrate the "first reduce, then optmize" approach and discuss
possible pitfalls
A minimization principle for the description of time-dependent modes associated with transient instabilities
We introduce a minimization formulation for the determination of a
finite-dimensional, time-dependent, orthonormal basis that captures directions
of the phase space associated with transient instabilities. While these
instabilities have finite lifetime they can play a crucial role by either
altering the system dynamics through the activation of other instabilities, or
by creating sudden nonlinear energy transfers that lead to extreme responses.
However, their essentially transient character makes their description a
particularly challenging task. We develop a minimization framework that focuses
on the optimal approximation of the system dynamics in the neighborhood of the
system state. This minimization formulation results in differential equations
that evolve a time-dependent basis so that it optimally approximates the most
unstable directions. We demonstrate the capability of the method for two
families of problems: i) linear systems including the advection-diffusion
operator in a strongly non-normal regime as well as the Orr-Sommerfeld/Squire
operator, and ii) nonlinear problems including a low-dimensional system with
transient instabilities and the vertical jet in crossflow. We demonstrate that
the time-dependent subspace captures the strongly transient non-normal energy
growth (in the short time regime), while for longer times the modes capture the
expected asymptotic behavior
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