Nonstationary Convection-diffusion Equation Research Articles

A strong positivity‐preserving finite volume scheme for convection–diffusion equations on tetrahedral meshes

AbstractIn this paper, we present a nonlinear strong positivity‐preserving finite volume (FV) scheme on tetrahedral meshes for nonstationary convection–diffusion equations with general right‐hand term, whose exact solution is assumed to be non‐negative, while the right‐hand term is allowed to be negative. A nonlinearization technique, which can be regarded as a correction of the linear scheme, is used to the discretization of both the diffusion and convection terms. Moreover, we impose nonlinear correction for the right‐hand term if it is a sink. The resulting scheme is cell‐centered and locally conservative, and has a fixed stencil. And in the construction of the scheme, it is unnecessary to assume that auxiliary unknowns are non‐negative. Besides, the existence of a solution and the strong positivity‐preserving property for the nonlinear system are proved. Finally, the numerical examples are presented to show the positivity of the new scheme and the second‐order convergence rate for the solution.

ЧИСЛЕННОЕ РЕШЕНИЕ ЗАДАЧИ ДВУХФАЗНОЙ ФИЛЬТРАЦИИ С НЕОДНОРОДНЫМИ КОЭФФИЦИЕНТАМИ МЕТОДОМ КОНЕЧНЫХ ЭЛЕМЕНТОВ

Рассматривается процесс фильтрации двухфазной жидкости в пористой неоднородной среде. Данный процесс описывается связанной системой уравнений для насыщенности, скорости фильтрации и порового давления. Рассмотрены математические модели с учетом и без учета капиллярных сил, при наличии которого для насыщенности имеем нестационарное уравнение конвекции-диффузии. Поскольку данный процесс характеризуется существенным преобладанием конвективного слагаемого в уравнении для насыщенности, используются противопотоковые аппроксимации посредством добавления неоднородной искусственной диффузии. Скорость и давление аппроксимируются с использованием смешанного метода конечных элементов. Представлены результаты численных расчетов для двумерного случая с сильно неоднородными коэффициентами проницаемости пористой среды. Рассмотрены несколько случаев, связанных с линейными и нелинейными коэффициентами относительной проницаемости флюида и наличием капиллярных сил. We consider the process of filtration of a two-phase fluid in a porous, heterogeneous medium. This process is described by a coupled system of equations for saturation, filtration rate, and pore pressure. We consider mathematical models with and without capillary forces, in the presence of which, for saturation, we have a nonstationary convection-diffusion equation. Since this process is characterized by a significant predominance of the convective term in the equation for saturation, countercurrent approximations are used by adding non-uniform artificial diffusion. Speed and pressure are approximated using a mixed finite element method. The results of numerical calculations for a two-dimensional case with strongly heterogeneous permeability coefficients of a porous medium are presented. Several cases of relative fluid permeability associated with linear and nonlinear coefficients and the presence of capillary forces are considered.

Difference Schemes on Nonuniform Grids for the Two-Dimensional Convection–Diffusion Equation

New second-order accurate monotone difference schemes on nonuniform spatial grids for two-dimensional stationary and nonstationary convection–diffusion equations are proposed. The monotonicity and stability of the solutions of the computational methods with respect to the boundary conditions, the initial condition, and the right-hand side are proved. Two-sided and corresponding a priori estimates are obtained in the grid norm of C. The convergence of the proposed algorithms to the solution of the original differential problem with the second order is proved.

Stability of Difference Splitting Schemes for Convection Diffusion Equation

We consider numerical modeling of the propagation of pollution in the air on the basis of geometrical splitting method for three-dimensional nonstationary convection diffusion equations. Splitting difference schemes in the form of explicit computing schemes are proposed to solve the obtained one-dimensional problems. The approximation, monotonicity, and stability of the proposed difference schemes are investigated.

Alternating triangular schemes for convection–diffusion problems

Explicit–implicit approximations are used to approximate nonstationary convection–diffusion equations in time. In unconditionally stable two-level schemes, diffusion is taken from the upper time level, while convection, from the lower layer. In the case of three time levels, the resulting explicit–implicit schemes are second-order accurate in time. Explicit alternating triangular (asymmetric) schemes are used for parabolic problems with a self-adjoint elliptic operator. These schemes are unconditionally stable, but conditionally convergent. Three-level modifications of alternating triangular schemes with better approximating properties were proposed earlier. In this work, two- and three-level alternating triangular schemes for solving boundary value problems for nonstationary convection–diffusion equations are constructed. Numerical results are presented for a two-dimensional test problem on triangular meshes, such as Delaunay triangulations and Voronoi diagrams.

Robusta posteriorierror estimates for stabilized finite element methods

There is a wide range of stabilized finite element methods for stationary and non-stationary convection-diffusion equations such as streamline diffusion methods, local projection schemes, subgrid-scale techniques, and continuous interior penalty methods to name only a few. We show that all these schemes give rise to the same robust a posteriori error estimates, i.e. the multiplicative constants in the upper and lower bounds for the error are independent of the size of the convection or reaction relative to the diffusion. Thus, the same error indicator can be used modulo higher order terms caused by data approximation.

Unconditionally stable schemes for convection-diffusion problems

Convection-diffusion problems are basic ones in continuum mechanics. The main features of these problems are connected with the fact that their operators may have an indefinite sign. In this paper we study the stability of difference schemes with weights for convection-diffusion problems where the convective transport operator may have various forms. We construct unconditionally stable schemes for nonstationary convection-diffusion equations based on the use of new variables. Similar schemes are also used for parabolic equations where the operator represents the sum of diffusion operators.

Homogenization of a nonstationary convection-diffusion equation in a thin rod and in a layer

The paper deals with the homogenization of a non-stationary convection-diffusion equation defined in a thin rod or in a layer with Dirichlet boundary condition. Under the assumption that the convection term is large, we describe the evolution of the solution’s profile and determine the rate of its decay. The main feature of our analysis is that we make no assumption on the support of the initial data which may touch the domain’s boundary. This requires the construction of boundary layer correctors in the homogenization process which, surprisingly, play a crucial role in the definition of the leading order term at the limit. Therefore we have to restrict our attention to simple geometries like a rod or a layer for which the definition of boundary layers is easy and explicit.

Homogenization and concentration for a diffusion equation with large convection in a bounded domain

We consider the homogenization of a non-stationary convection–diffusion equation posed in a bounded domain with periodically oscillating coefficients and homogeneous Dirichlet boundary conditions. Assuming that the convection term is large, we give the asymptotic profile of the solution and determine its rate of decay. In particular, it allows us to characterize the “hot spot”, i.e., the precise asymptotic location of the solution maximum which lies close to the domain boundary and is also the point of concentration. Due to the competition between convection and diffusion, the position of the “hot spot” is not always intuitive as exemplified in some numerical tests.

Homogenization of convection-diffusion equation in infinite cylinder

The paper deals with a periodic homogenization problem for a non-stationary convection-diffusion equation stated in a thin infinite cylindrical domain with homogeneous Neumann boundary condition on ...

Robust A Posteriori Error Estimates for Nonstationary Convection-Diffusion Equations

We consider discretizations of convection dominated nonstationary convection-diffusion equations by A-stable $\theta$-schemes in time and conforming finite elements in space on locally refined, isotropic meshes. For these discretizations we derive a residual a posteriori error estimator. The estimator yields upper bounds on the error which are global in space and time and lower bounds that are global in space and local in time. The error estimates are fully robust in the sense that the ratio between upper and lower bounds is uniformly bounded in time, does not depend on any step-size in space or time nor on any relation between these both, and is uniformly bounded with respect to the size of the convection. Moreover, the estimates are uniform with respect to the size of the zero-order reaction term and also hold for the limit case of vanishing reaction.

Analysis of least-squares mixed finite element methods for nonlinear nonstationary convection-diffusion problems

Some least-squares mixed finite element methods for convection-diffusion problems, steady or nonstationary, are formulated, and convergence of these schemes is analyzed. The main results are that a new optimal a priori L 2 error estimate of a least-squares mixed finite element method for a steady convection-diffusion problem is developed and that four fully-discrete least-squares mixed finite element schemes for an initial-boundary value problem of a nonlinear nonstationary convection-diffusion equation are formulated. Also, some systematic theories on convergence of these schemes are established.