8 research outputs found
A subcell-enriched Galerkin method for advection problems
In this work, we introduce a generalization of the enriched Galerkin (EG)
method. The key feature of our scheme is an adaptive two-mesh approach that, in
addition to the standard enrichment of a conforming finite element
discretization via discontinuous degrees of freedom, allows to subdivide
selected (e.g. troubled) mesh cells in a non-conforming fashion and to use
further discontinuous enrichment on this finer submesh. We prove stability and
sharp a priori error estimates for a linear advection equation by using a
specially tailored projection and conducting some parts of a standard
convergence analysis for both meshes. By allowing an arbitrary degree of
enrichment on both, the coarse and the fine mesh (also including the case of no
enrichment), our analysis technique is very general in the sense that our
results cover the range from the standard continuous finite element method to
the standard discontinuous Galerkin (DG) method with (or without) local subcell
enrichment. Numerical experiments confirm our analytical results and indicate
good robustness of the proposed method
The parallel finite element system M++ with integrated multilevel preconditioning and multilevel Monte Carlo methods
We present a parallel data structure for the discretization of partial differential equations which is based on distributed point objects and which enables the flexible, transparent, and efficient realization of conforming, nonconforming, and mixed finite elements. This concepts is realized for elliptic, parabolic and hyperbolic model problems, and sample applications are provided by a tutorial complementing a lecture on scientific computing.
The corresponding open-source software is based on this parallel data structure, and it supports multilevel methods on nested meshes and 2D and 3D as well as in space-time. Here, we present generic results on porous media applications including multilevel preconditioning and multilevel Monte Carlo methods for uncertainty quantification
Postprocessing of Non-Conservative Flux for Compatibility with Transport in Heterogeneous Media
A conservative flux postprocessing algorithm is presented for both
steady-state and dynamic flow models. The postprocessed flux is shown to have
the same convergence order as the original flux. An arbitrary flux
approximation is projected into a conservative subspace by adding a piecewise
constant correction that is minimized in a weighted norm. The application
of a weighted norm appears to yield better results for heterogeneous media than
the standard norm which has been considered in earlier works. We also
study the effect of different flux calculations on the domain boundary. In
particular we consider the continuous Galerkin finite element method for
solving Darcy flow and couple it with a discontinuous Galerkin finite element
method for an advective transport problem.Comment: 34 pages, 17 figures, 11 table
Vers des méthodes immerées generalisées:une approche Shifted Boundary P1 avec des flux d'ordre pour les équations de Darcy
In this paper, we propose to extend the recent embedded boundary method known as "shifted boundary method" to the Darcy flow problems. The aim is to provide an improved formulation that would give, using linear approximation, at least second order accuracy on bothflux and pressure variables, for any kind of boundary condition, considering embedded simulations.The strategy adopted here is to enrich the approximation of the pressure using Taylor expansionsalong the edges. The objective of this enrichment is to give a quadratic shape to the pressure. The resulted scheme provides high order accuracy on both variables for embedded simulations with an overall second order accuracy, that is bumped to third order for the pressure when only Dirichletboundaries are embedded
Enriched mixed finite element models for dynamic analysis of continuous and fractured porous media
The final publication is available at Elsevier via https://dx.doi.org/10.1016/j.cma.2018.08.011 © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/Enriched Finite Element Models are presented to more accurately investigate the transient and wave propagation responses of continuous and fractured porous media based on mixture theory. Firstly, the Generalized Finite Element Method (GFEM) trigonometric enrichments are introduced to suppress the spurious oscillations that may appear in dynamic analysis with the regular Finite Element Method (FEM) due to numerical dispersion/Gibbs phenomenon. Secondly, the Phantom Node Method (PNM) is employed to model multiple arbitrary fractures independently of the mesh topology. Thirdly, frictional contact behavior is simulated using an Augmented Lagrange Multiplier technique. Mixed Lagrangian interpolants, bi-quadratic for displacements and bi-linear for pore pressure, are used for the underlying FEM basis functions. Transient (non-wave propagation) response of fractured porous media is effectively modeled using the PNM. Wave propagation in continuous porous media is effectively modeled using the mixed GFEM. Wave propagation in fractured porous media is simulated using a mixed GFEM-enriched Phantom Node Method (PNM-GFEM-M). The developed mixed GFEM portion of the model is verified through a transient consolidation problem. Subsequently, the ability of the enriched FEM models to capture the dynamic response of fractured fully-saturated porous media under mechanical and hydraulic stimulations is illustrated. The superior ability of the PNM-GFEM-M in inhibiting spurious oscillations is shown in comparison against the regular finite element solutions of some impact problems. It is demonstrated that by embedding appropriate enrichment basis functions in both displacement and pore pressure fields the results obtained are more accurate than those obtained using standard finite element approximations or approximations in which only the displacement is enriched.Natural Sciences and Engineering Research Council of Canad
Simulation of Hydraulic Stimulation: Acoustic Wave Emission in Fractured Porous Media Using Local and Global Partition-of-Unity Finite Element
Hydraulic Fracturing (HF) is an effective stimulation process for extracting oil and gas from
unconventional low-permeable reservoirs. The process is conducted by injecting high-pressure fluids into the ground to generate fracture networks in rock masses and stimulate natural fractures
to increase the permeability of formation and extract oil and gas. Due to the multiple and
coupled-physics involved, hydraulic fracturing is a complex engineering process.
The extent of the induced fractures and stimulated volume and reactivation of natural faults
and fractures are some of the practical issues associated with hydraulic fracturing. Acoustic
Emission (AE) monitoring and analysis are used to probe the behaviour of solid materials in
such applications. The process of elastic wave propagation induced by an abrupt local release
of stored strain energy is known as acoustic, microseismic, and seismic emission (depending on
the context and the magnitude of the event). These emissions can be triggered by material
bifurcation-instabilities like slope slipping, fault-reactivation, pore collapsing, and cracking -
processes that are all categorized as localization phenomena.
The microseismic monitoring industry attempts to relate acoustic emissions measured by
geophones to the nature of the stimulated volume created during hydraulic fracturing. This
process is full of uncertainties and researchers have not yet focused on both explicitly modeling
the process of fracture reactivation and the accurate simulation of acoustic wave propagations
resulting from the localization. The biggest gap in the modeling literature is that most of the
previous works fail to accurately simulate the process of transient acoustic wave propagation
through the fractured porous media following the elastic energy release. Instead of explicitly modeling fracturing and acoustic emission, most previous studies have aimed to relate energy
release to seismic moment.
To overcome some of the existing shortcomings in the numerical modeling of the coupled
problem of interface localization-acoustic emission, this thesis is focused on developing new computational
methods and programs for the simulation of microseismic wave emissions induced by
interface slip instability in fractured porous media. As a coupled nonlinear mixed multi-physics
problem, simulation of hydraulic stimulation involves several mathematical and computational
complexities and difficulties in terms of modeling, stability, and convergence, such as the inf-sup
stability problems that arise from mixed formulations due to the hydro-mechanical couplings
and contact conditions. In AE modeling, due to the high-frequency transient nature of the
problem, additional numerical problems emerging from the Gibbs phenomenon and artificial
period elongation and amplitude decay are also involved.
The thesis has three main objectives. The first objective is to develop a numerical model
for simulation of wave propagation in discontinuous media, which is fulfilled in Chapter 2 of
the thesis. In this chapter a new enriched finite element method is developed for simulation
of wave propagation in fractured media. The method combines the advantages of the global
Partition-of-Unity Method (PUM) with harmonic enrichment functions via the Generalized Finite
Element Method (GFEM) with the local PUM via the Phantom Node Method (PNM).
The GFEM enrichments suppress the spurious oscillations that can appear in regular Finite
Element Method (FEM) analysis of dynamic/wave propagations due to numerical dispersions
and Gibbs phenomenon. The PNM models arbitrary fractures independently of the original
mesh. Through several numerical examples it has been demonstrated that the spurious oscillations
that appear in propagation pattern of high-frequency waves in PNM simulations can
be effectively suppressed by employing the enriched model. This is observed to be especially
important in fractured media where both primary waves and the secondary reflected waves are
present.
The second objective of the thesis is to develop a mixed numerical model for simulation
of wave propagation in discontinuous porous media and interface modeling. This objective
is realized in Chapter 3 of the thesis. In this chapter, a new enriched mixed finite element
model is introduced for simulation of wave propagation in fractured porous media, based on
an extension of the developed numerical method in Chapter 2. Moreover, frictional contact at
interfaces is modeled and realized using an augmented Lagrange multiplier scheme. Through
various numerical examples, the effectiveness of the developed enriched FE model over conventional
approaches is demonstrated. Moreover, it is shown that the most accurate wave results
with the least amount of spurious oscillations are achieved when both the displacement and
pore pressure fields are enriched with appropriate trigonometric functions.
The third objective of the thesis is to develop computational models for the simulation of
acoustic emissions induced by fracture reactivation and shear slip. This objective is realized in
Chapter 4 of the thesis. In this chapter, an enriched mixed finite element model (introduced
in Chapter 3) is developed to simulate the interface slip instability and the associated induced
acoustic wave propagation processes, concurrently. Acoustic events are triggered through a
sudden release of strain energy at the fracture interfaces due to shear slip instability. The
shear slip is induced via hydraulic stimulation that switches the interface behaviour from a
stick to slip condition. The superior capability of the proposed enriched mixed finite element
model (i.e., PNM-GFEM-M) in comparison with regular finite element models in inhibiting
the spurious oscillations and numerical dispersions of acoustic signals in both velocity and
pore pressure fields is demonstrated through several numerical studies. Moreover, the effects
of different characteristics of the system, such as permeability, viscous damping, and friction
coefficient at the interface are investigated in various examples