284 research outputs found
A mixed finite volume scheme for anisotropic diffusion problems on any grid
We present a new finite volume scheme for anisotropic heterogeneous diffusion
problems on unstructured irregular grids, which simultaneously gives an
approximation of the solution and of its gradient. In the case of simplicial
meshes, the approximate solution is shown to converge to the continuous ones as
the size of the mesh tends to 0, and an error estimate is given. In the general
case, we propose a slightly modified scheme for which we again prove the
convergence, and give an error estimate. An easy implementation method is then
proposed, and the efficiency of the scheme is shown on various types of grids
A unified approach to Mimetic Finite Difference, Hybrid Finite Volume and Mixed Finite Volume methods
We investigate the connections between several recent methods for the
discretization of anisotropic heterogeneous diffusion operators on general
grids. We prove that the Mimetic Finite Difference scheme, the Hybrid Finite
Volume scheme and the Mixed Finite Volume scheme are in fact identical up to
some slight generalizations. As a consequence, some of the mathematical results
obtained for each of the method (such as convergence properties or error
estimates) may be extended to the unified common framework. We then focus on
the relationships between this unified method and nonconforming Finite Element
schemes or Mixed Finite Element schemes, obtaining as a by-product an explicit
lifting operator close to the ones used in some theoretical studies of the
Mimetic Finite Difference scheme. We also show that for isotropic operators, on
particular meshes such as triangular meshes with acute angles, the unified
method boils down to the well-known efficient two-point flux Finite Volume
scheme
A cell-centred finite volume approximation for second order partial derivative operators with full matrix on unstructured meshes in any space dimension
Finite volume methods for problems involving second order operators with full
diffusion matrix can be used thanks to the definition of a discrete gradient
for piecewise constant functions on unstructured meshes satisfying an
orthogonality condition. This discrete gradient is shown to satisfy a strong
convergence property on the interpolation of regular functions, and a weak one
on functions bounded for a discrete norm. To highlight the importance of
both properties, the convergence of the finite volume scheme on a homogeneous
Dirichlet problem with full diffusion matrix is proven, and an error estimate
is provided. Numerical tests show the actual accuracy of the method
Convergence in of weak solutions to perturbed doubly degenerate parabolic equations
We study the behaviour of solutions to a class of nonlinear degenerate
parabolic problems when the data are perturbed. The class includes the Richards
equation, Stefan problem and the parabolic -Laplace equation. We show that,
up to a subsequence, weak solutions of the perturbed problem converge
uniformly-in-time to weak solutions of the original problem as the perturbed
data approach the original data. We do not assume uniqueness or additional
regularity of the solution. However, when uniqueness is known, our result
demonstrates that the weak solution is uniformly temporally stable to
perturbations of the data. Beginning with a proof of temporally-uniform,
spatially-weak convergence, we strengthen the latter by relating the unknown to
an underlying convex structure that emerges naturally from energy estimates on
the solution. The double degeneracy --- shown to be equivalent to a maximal
monotone operator framework --- is handled with techniques inspired by a
classical monotonicity argument and a simple variant of the compensated
compactness phenomenon.Comment: J. Differential Equations, 201
Uniform-in-time convergence of numerical schemes for Richards' and Stefan's models
We prove that all Gradient Schemes - which include Finite Element, Mixed Finite Element, Finite Volume methods - converge uniformly in time when applied to a family of nonlinear parabolic equations which contains Richards and Stefan's models. We also provide numerical results to confirm our theoretical analysis
The gradient discretisation method for linear advection problems
We adapt the Gradient Discretisation Method (GDM), originally designed for
elliptic and parabolic partial differential equations, to the case of a linear
scalar hyperbolic equations. This enables the simultaneous design and
convergence analysis of various numerical schemes, corresponding to the methods
known to be GDMs, such as finite elements (conforming or non-conforming,
standard or mass-lumped), finite volumes on rectangular or simplicial grids,
and other recent methods developed for general polytopal meshes. The scheme is
of centred type, with added linear or non-linear numerical diffusion. We
complement the convergence analysis with numerical tests based on the
mass-lumped P1 conforming and non conforming finite element and on the hybrid
finite volume method
A collocated finite volume scheme to solve free convection for general non-conforming grids
We present a new collocated numerical scheme for the approximation of the
Navier-Stokes and energy equations under the Boussinesq assumption for general
grids, using the velocity-pressure unknowns. This scheme is based on a recent
scheme for the diffusion terms. Stability properties are drawn from particular
choices for the pressure gradient and the non-linear terms. Numerical results
show the accuracy of the scheme on irregular grids
A unified analysis of elliptic problems with various boundary conditions and their approximation
We design an abstract setting for the approximation in Banach spaces of
operators acting in duality. A typical example are the gradient and divergence
operators in Lebesgue--Sobolev spaces on a bounded domain. We apply this
abstract setting to the numerical approximation of Leray-Lions type problems,
which include in particular linear diffusion. The main interest of the abstract
setting is to provide a unified convergence analysis that simultaneously covers
(i) all usual boundary conditions, (ii) several approximation methods. The
considered approximations can be conforming, or not (that is, the approximation
functions can belong to the energy space of the problem, or not), and include
classical as well as recent numerical schemes. Convergence results and error
estimates are given. We finally briefly show how the abstract setting can also
be applied to other models, including flows in fractured medium, elasticity
equations and diffusion equations on manifolds. A by-product of the analysis is
an apparently novel result on the equivalence between general Poincar{\'e}
inequalities and the surjectivity of the divergence operator in appropriate
spaces
Unified convergence analysis of numerical schemes for a miscible displacement problem
This article performs a unified convergence analysis of a variety of
numerical methods for a model of the miscible displacement of one
incompressible fluid by another through a porous medium. The unified analysis
is enabled through the framework of the gradient discretisation method for
diffusion operators on generic grids. We use it to establish a novel
convergence result in of the approximate
concentration using minimal regularity assumptions on the solution to the
continuous problem. The convection term in the concentration equation is
discretised using a centred scheme. We present a variety of numerical tests
from the literature, as well as a novel analytical test case. The performance
of two schemes are compared on these tests; both are poor in the case of
variable viscosity, small diffusion and medium to small time steps. We show
that upstreaming is not a good option to recover stable and accurate solutions,
and we propose a correction to recover stable and accurate schemes for all time
steps and all ranges of diffusion
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