3,699 research outputs found
Backstepping PDE Design: A Convex Optimization Approach
Abstract\u2014Backstepping design for boundary linear PDE is
formulated as a convex optimization problem. Some classes of
parabolic PDEs and a first-order hyperbolic PDE are studied,
with particular attention to non-strict feedback structures. Based
on the compactness of the Volterra and Fredholm-type operators
involved, their Kernels are approximated via polynomial
functions. The resulting Kernel-PDEs are optimized using Sumof-
Squares (SOS) decomposition and solved via semidefinite
programming, with sufficient precision to guarantee the stability
of the system in the L2-norm. This formulation allows optimizing
extra degrees of freedom where the Kernel-PDEs are included
as constraints. Uniqueness and invertibility of the Fredholm-type
transformation are proved for polynomial Kernels in the space
of continuous functions. The effectiveness and limitations of the
approach proposed are illustrated by numerical solutions of some
Kernel-PDEs
Pullback attractor for a dynamic boundary non-autonomous problem with Infinite delay
In this work we prove the existence of solution for a p-Laplacian non-autonomous problem with dynamic boundary and infinite delay. We ensure the existence of pullback attractor for the multivalued process associated to the non-autonomous problem we are concerned. Finally, we also prove the existence of a more general attractor for the problem known as D-pullback attractor.Conselho Nacional de Desenvolvimento Científico e TecnológicoMinisterio de Economía y CompetitividadFondo Europeo de Desarrollo RegionalJunta de Andalucí
Towards developing robust algorithms for solving partial differential equations on MIMD machines
Methods for efficient computation of numerical algorithms on a wide variety of MIMD machines are proposed. These techniques reorganize the data dependency patterns to improve the processor utilization. The model problem finds the time-accurate solution to a parabolic partial differential equation discretized in space and implicitly marched forward in time. The algorithms are extensions of Jacobi and SOR. The extensions consist of iterating over a window of several timesteps, allowing efficient overlap of computation with communication. The methods increase the degree to which work can be performed while data are communicated between processors. The effect of the window size and of domain partitioning on the system performance is examined both by implementing the algorithm on a simulated multiprocessor system
Parametric amplification of optical phonons
Amplification of light through stimulated emission or nonlinear optical
interactions has had a transformative impact on modern science and technology.
The amplification of other bosonic excitations, like phonons in solids, is
likely to open up new remarkable physical phenomena. Here, we report on an
experimental demonstration of optical phonon amplification. A coherent
mid-infrared optical field is used to drive large amplitude oscillations of the
Si-C stretching mode in silicon carbide. Upon nonlinear phonon excitation, a
second probe pulse experiences parametric optical gain at all wavelengths
throughout the reststrahlen band, which reflects the amplification of
optical-phonon fluctuations. Starting from first principle calculations, we
show that the high-frequency dielectric permittivity and the phonon oscillator
strength depend quadratically on the lattice coordinate. In the experimental
conditions explored here, these oscillate then at twice the frequency of the
optical field and provide a parametric drive for lattice fluctuations.
Parametric gain in phononic four wave mixing is a generic mechanism that can be
extended to all polar modes of solids, as a new means to control the kinetics
of phase transitions, to amplify many body interactions or to control
phonon-polariton waves
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