869 research outputs found
Homogenization of Parabolic Equations with a Continuum of Space and Time Scales
This paper addresses the issue of the homogenization of linear divergence form parabolic operators in situations where no ergodicity and no scale separation in time or space are available. Namely, we consider divergence form linear parabolic operators in with -coefficients. It appears that the inverse operator maps the unit ball of into a space of functions which at small (time and space) scales are close in norm to a functional space of dimension . It follows that once one has solved these equations at least times it is possible to homogenize them both in space and in time, reducing the number of operation counts necessary to obtain further solutions. In practice we show under a Cordes-type condition that the first order time derivatives and second order space derivatives of the solution of these operators with respect to caloric coordinates are in (instead of with Euclidean coordinates). If the medium is time-independent, then it is sufficient to solve times the associated elliptic equation in order to homogenize the parabolic equation
Polyharmonic homogenization, rough polyharmonic splines and sparse super-localization
We introduce a new variational method for the numerical homogenization of
divergence form elliptic, parabolic and hyperbolic equations with arbitrary
rough () coefficients. Our method does not rely on concepts of
ergodicity or scale-separation but on compactness properties of the solution
space and a new variational approach to homogenization. The approximation space
is generated by an interpolation basis (over scattered points forming a mesh of
resolution ) minimizing the norm of the source terms; its
(pre-)computation involves minimizing quadratic (cell)
problems on (super-)localized sub-domains of size .
The resulting localized linear systems remain sparse and banded. The resulting
interpolation basis functions are biharmonic for , and polyharmonic
for , for the operator -\diiv(a\nabla \cdot) and can be seen as a
generalization of polyharmonic splines to differential operators with arbitrary
rough coefficients. The accuracy of the method ( in energy norm
and independent from aspect ratios of the mesh formed by the scattered points)
is established via the introduction of a new class of higher-order Poincar\'{e}
inequalities. The method bypasses (pre-)computations on the full domain and
naturally generalizes to time dependent problems, it also provides a natural
solution to the inverse problem of recovering the solution of a divergence form
elliptic equation from a finite number of point measurements.Comment: ESAIM: Mathematical Modelling and Numerical Analysis. Special issue
(2013
A Multiscale Finite Element Method for an Elliptic Distributed Optimal Control Problem with Rough Coefficients and Control Constraints
We construct and analyze a multiscale finite element method for an elliptic
distributed optimal control problem with pointwise control constraints, where
the state equation has rough coefficients. We show that the performance of the
multiscale finite element method is similar to the performance of standard
finite element methods for smooth problems and present corroborating numerical
results.Comment: 26 page
Error Estimate of Multiscale Finite Element Method for Periodic Media Revisited
We derive the optimal energy error estimate for multiscale finite element
method with oversampling technique applying to elliptic system with rapidly
oscillating periodic coefficients under the assumption that the coefficients
are bounded and measurable, which may admit rough microstructures. As a
by-product of the energy estimate, we derive the rate of convergence in
Lnorm
Flux norm approach to finite dimensional homogenization approximations with non-separated scales and high contrast
We consider divergence-form scalar elliptic equations and vectorial equations
for elasticity with rough (, )
coefficients that, in particular, model media with non-separated scales
and high contrast in material properties. We define the flux norm as the
norm of the potential part of the fluxes of solutions, which is equivalent to
the usual -norm. We show that in the flux norm, the error associated with
approximating, in a properly defined finite-dimensional space, the set of
solutions of the aforementioned PDEs with rough coefficients is equal to the
error associated with approximating the set of solutions of the same type of
PDEs with smooth coefficients in a standard space (e.g., piecewise polynomial).
We refer to this property as the {\it transfer property}.
A simple application of this property is the construction of finite
dimensional approximation spaces with errors independent of the regularity and
contrast of the coefficients and with optimal and explicit convergence rates.
This transfer property also provides an alternative to the global harmonic
change of coordinates for the homogenization of elliptic operators that can be
extended to elasticity equations. The proofs of these homogenization results
are based on a new class of elliptic inequalities which play the same role in
our approach as the div-curl lemma in classical homogenization.Comment: Accepted for publication in Archives for Rational Mechanics and
Analysi
A multiscale method for heterogeneous bulk-surface coupling
In this paper, we construct and analyze a multiscale (finite element) method
for parabolic problems with heterogeneous dynamic boundary conditions. As
origin, we consider a reformulation of the system in order to decouple the
discretization of bulk and surface dynamics. This allows us to combine
multiscale methods on the boundary with standard Lagrangian schemes in the
interior. We prove convergence and quantify explicit rates for low-regularity
solutions, independent of the oscillatory behavior of the heterogeneities. As a
result, coarse discretization parameters, which do not resolve the fine scales,
can be considered. The theoretical findings are justified by a number of
numerical experiments including dynamic boundary conditions with random
diffusion coefficients
A two-level enriched finite element method for a mixed problem
The simplest pair of spaces is made inf-sup stable for the mixed form of the Darcy equation. The key ingredient is to enhance the finite element spaces inside a Petrov-Galerkin framework with functions satisfying element-wise local Darcy problems with right hand sides depending on the residuals over elements and edges. The enriched method is symmetric, locally mass conservative and keeps the degrees of freedom of the original interpolation spaces. First, we assume local enrichments exactly computed and we prove uniqueness and optimal error estimates in natural norms. Then, a low cost two-level finite element method is proposed to effectively obtain enhancing basis functions. The approach lays on a two-scale numerical analysis and shows that well-posedness and optimality is kept, despite the second level numerical approximation. Several numerical experiments validate the theoretical results and compares (favourably in some cases) our results with the classical Raviart-Thomas elemen
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