A novel approach to modelling of flow in fractured porous medium

summary:There are many problems of groundwater flow in a disrupted rock massifs that should be modelled using numerical models. It can be done via “standard approaches” such as increase of the permeability of the porous medium to account the fracture system (or double-porosity models), or discrete stochastic fracture network models. Both of these approaches appear to have their constraints and limitations, which make them unsuitable for the large- scale long-time hydrogeological calculations. In the article, a new approach to the modelling of groudwater flow in fractured porous medium, which combines the above-mentioned models, is described. This article presents the mathematical formulation and demonstration of numerical results obtained by this new approach. The approach considers three substantial types of objects within a structure of modelled massif important for the groudwater flow – small stochastic fractures, large deterministic fractures, and lines of intersection of the large fractures. The systems of stochastic fractures are represented by blocks of porous medium with suitably set hydraulic conductivity. The large fractures are represented as polygons placed in 3D space and their intersections are represented by lines. Thus flow in 3D porous medium, flow in 2D and 1D fracture systems, and communication among these three systems are modelled together

Mixed-Hybrid Formulation of Multidimensional Fracture Flow

We shall study Darcy flow on the heterogeneous system of 3D, 2D, and 1D domains and we present four models for coupling of the flow. For one of these models, we describe in detail its mixed-hybrid formulation. Finally, we show that Schur complements are suitable for solution of the linear system resulting form the lowest order approximation of the mixed-hybrid formulation

A novel block non-symmetric preconditioner for mixed-hybrid finite-element-based flow simulations

In this work we propose a novel block preconditioner, labelled Explicit Decoupling Factor Approximation (EDFA), to accelerate the convergence of Krylov subspace solvers used to address the sequence of non-symmetric systems of linear equations originating from flow simulations in porous media. The flow model is discretized blending the Mixed Hybrid Finite Element (MHFE) method for Darcy's equation with the Finite Volume (FV) scheme for the mass conservation. The EDFA preconditioner is characterized by two features: the exploitation of the system matrix decoupling factors to recast the Schur complement and their inexact fully-parallel computation by means of restriction operators. We introduce two adaptive techniques aimed at building the restriction operators according to the properties of the system at hand. The proposed block preconditioner has been tested through an extensive experimentation on both synthetic and real-case applications, pointing out its robustness and computational efficiency

Dual variable methods for mixed-hybrid finite element approximation of the potential fluid flow problem in porous media

Mixed-hybrid finite element discretization of Darcy's law and the continuity equation that describe the potential fluid flow problem in porous media leads to symmetric indefinite saddle-point problems. In this paper we consider solution techniques based on the computation of a null-space basis of the whole or of a part of the left lower off-diagonal block in the system matrix and on the subsequent iterative solution of a projected system. This approach is mainly motivated by the need to solve a sequence of such systems with the same mesh but different material properties. A fundamental cycle null-space basis of the whole off-diagonal block is constructed using the spanning tree of an associated graph. It is shown that such a basis may be theoretically rather ill-conditioned. Alternatively, the orthogonal null-space basis of the sub-block used to enforce continuity over faces can be easily constructed. In the former case, the resulting projected system is symmetric positive definite and so the conjugate gradient method can be applied. The projected system in the latter case remains indefinite and the preconditioned minimal residual method (or the smoothed conjugate gradient method) should be used. The theoretical rate of convergence for both algorithms is discussed and their efficiency is compared in numerical experiments. Copyright © 2006, Kent State University

Transport processes in fractured porous media

112 stranThis habilitation thesis summarizes author's theoretical work related to development of the Flow123d simulator. This includes especially methods and algorithms for solving Darcy ow problems in saturated and unsaturated fractured porous media. A model with semi-discrete fractures called mixed dimension model is derived at the beginning. Then the abstract model for advection-di usion equation is applied to the Darcy ow. The mixed-hybrid formulation of the Darcy ow mixed dimension problem is presented followed by its discretization using Raviart-Thomas nite elements. An analytical solution to a test single fracture problem is supplied which allows veri cation of the model's implementation. Finally, the BDDC method is applied to obtain a scalable solver of the linear systems arising from the problem's discretization. Subsequently, new developments for the non-conforming mixed meshes are presented. Four methods with common strategy are used to introduce a coupling between equations living on the intersecting nite element meshes of di erent dimension. Further a family of e cient algorithms for computing mesh intersections is presented. Final chapter is devoted to the Richards' equation and modi cation of the mixed-hybrid scheme in order to satisfy discrete maximum principle. This is of particular importance for the Richards' equation where short time steps are often necessary which leads to strong oscillations for the schemes that violate DMP