Discontinuous Galerkin methods for convection-diffusion equations and applications in petroleum engineering

This dissertation contains research in discontinuous Galerkin (DG) methods applying to convection-diffusion equations. It contains both theoretical analysis and applications. Initially, we develop a conservative local discontinuous Galerkin (LDG) method for the coupled system of compressible miscible displacement problem in two space dimensions. The main difficulty is how to deal with the discontinuity of approximations of velocity, u, in the convection term across the cell interfaces. To overcome the problems, we apply the idea of LDG with IMEX time marching using the diffusion term to control the convection term. Optimal error estimates in Linfinity(0, T; L2) norm for the solution and the auxiliary variables will be derived. Then, high-order bound-preserving (BP) discontinuous Galerkin (DG) methods for the coupled system of compressible miscible displacements on triangular meshes will be developed. There are three main difficulties to make the concentration of each component between 0 and 1. Firstly, the concentration of each component did not satisfy a maximum-principle. Secondly, the first-order numerical flux was difficult to construct. Thirdly, the classical slope limiter could not be applied to the concentration of each component. To conquer these three obstacles, we first construct special techniques to preserve two bounds without using the maximum-principle-preserving technique. The time derivative of the pressure was treated as a source of the concentration equation. Next, we apply the flux limiter to obtain high-order accuracy using the second-order flux as the lower order one instead of using the first-order flux. Finally, L2-projection of the porosity and constructed special limiters that are suitable for multi-component fluid mixtures were used. Lastly, a new LDG method for convection-diffusion equations on overlapping mesh introduced in [J. Du, Y. Yang and E. Chung, Stability analysis and error estimates of local discontinuous Galerkin method for convection-diffusion equations on overlapping meshes, BIT Numerical Mathematics (2019)] showed that the convergence rates cannot be improved if the dual mesh is constructed by using the midpoint of the primitive mesh. They provided several ways to gain optimal convergence rates but the reason for accuracy degeneration is still unclear. We will use Fourier analysis to analyze the scheme for linear parabolic equations with periodic boundary conditions in one space dimension. To investigate the reason for the accuracy degeneration, we explicitly write out the error between the numerical and exact solutions. Moreover, some superconvergence points that may depend on the perturbation constant in the construction of the dual mesh were also found out

FINITE VOLUME METHODS FOR LINEAR PARTIAL DIFFERENTIAL EQUATIONS WITH DELTA-SINGULARITIES

In this work, we study hyperbolic conservation law in one space dimension with δ-singularities as the initial data. We use finite volume methods to find the sizes of pollution region. Firstly, we study finite volume method (FVM) with linear weights and weighted essentially non-oscillatory (WENO) scheme and apply both methods to linear partial differential equations without singularities to check the accuracy. Then we use both methods to find the numerical solutions and compute errors of linear equations with δ-singularities. Lastly, we use such results to find the size of pollution region of each method. These results show that the size of the pollution region is approximately of the order O(Δx1/2), where Δx is the spatial mesh size

Clustering Performance Comparison in K-Mean Clustering Variations: A Fraud Detection Study

K-means clustering is a common clustering approach that is based on data partitioning. However, the k-means clustering has significant drawbacks, such as it is sensitive to deciding the initial condition. Several ways to improve the algorithm have been offered. To assess the algorithm's efficiency and correctness, the performance comparison should be evaluated. In this paper, several k-means algorithms, including random k-means, global k-means, and fast global k-means, were evaluated for their efficiency when applied to a fraud detection data set. The accuracy of each method and the Davies-Bouldin index was investigated for each algorithm to compare the clustering performance. The findings demonstrated that when a small number of groups was used, random k-means, global k-means, and fast global k-means gave similar clustering, but fast global k-means offered better errors when a big number of groups was used. Furthermore, global k-means took longer to execute than others

Fourier analysis of local discontinuous Galerkin methods for linear parabolic equations on overlapping meshes

A new local discontinuous Galerkin method for convection–diffusion equations on overlapping mesh was introduced in Du et al. (BIT Numer Math 1–24, 2019). In the new method, the primary variable u and auxiliary variable p = ux are solved on different meshes. The stability and suboptimal error estimates for problems with periodic boundary conditions were derived. Numerical experiments demonstrated that the convergence rates cannot be improved if the dual mesh is constructed by using the midpoint of the primitive mesh. Several alternatives to gain optimal convergence rates were demonstrated in Du et al. (2019). However, the reason for accuracy degeneration is still unclear. In this paper, we will use Fourier analysis to analyze the scheme for linear parabolic equations with periodic boundary conditions in one space dimension.We explicitly write out the error between the numerical and exact solutions, and investigate the reason for the accuracy degeneration. Moreover, we also find out some superconvergence points that may depend on the perturbation constant in the construction of the dual mesh. Since the current work is based on Fourier analysis, we only consider uniform meshes. Numerical experiments will be given to verify the theoretical analysis

High-order bound-preserving discontinuous Galerkin methods for compressible miscible displacements in porous media on triangular meshes

© 2018 Elsevier Inc. In this paper, we develop high-order bound-preserving (BP) discontinuous Galerkin (DG) methods for the coupled system of compressible miscible displacements on triangular meshes. We consider the problem with multi-component fluid mixture and the (volumetric) concentration of the jth component, cj, should be between 0 and 1. There are three main difficulties. Firstly, cj does not satisfy a maximum-principle. Therefore, the numerical techniques introduced in Zhang and Shu (2010) [44] cannot be applied directly. The main idea is to apply the positivity-preserving techniques to all cj′s and enforce ∑jcj=1 simultaneously to obtain physically relevant approximations. By doing so, we have to treat the time derivative of the pressure dp/dt as a source in the concentration equation and choose suitable fluxes in the pressure and concentration equations. Secondly, it is not easy to construct first-order numerical fluxes for interior penalty DG methods on triangular meshes. One of the key points in the high-order BP technique applied in this paper is the combination of high-order and lower-order numerical fluxes. We will construct second-order BP schemes and use the second-order numerical fluxes as the lower-order one. Finally, the classical slope limiter cannot be applied to cj. To construct the BP technique, we will not approximate cj directly. Therefore, a new limiter will be introduced. Numerical experiments will be given to demonstrate the high-order accuracy and good performance of the numerical technique

Maximum-principle-preserving high-order discontinuous Galerkin methods for incompressible Euler equations on overlapping meshes

In this paper, we construct a new local discontinuous Galerkin (LDG) algorithm to solve the incompressible Euler equation in two space dimensions on overlapping meshes. This method solves the vorticity, velocity field and potential function on different meshes. Different from the traditional LDG method, the overlapping meshes used in this paper make the velocity to be continuous along the interfaces of the primitive meshes. Therefore, the upwind fluxes can be applied. We derive two sufficient conditions to obtain the maximum principle of vorticity. The first one is the divergence-free numerical approximation of the velocity field. This condition further grants that the scheme of the vorticity equation keeps constant solutions. The second one is to preserve the positivity of the numerical vorticity. We select suitable time step sizes to construct positive numerical cell averages of the vorticity provided the vorticity in the previous time step is positive. Then a slope limiter can be applied to enforce the positivity of the numerical approximation of the vorticity. Thanks to the above two conditions, we can arbitrarily add constants to the vorticity function and construct high-order MPP LDG methods on overlapping meshes for the two-dimensional incompressible Euler equation in the vorticity stream function formulation. Numerical tests will be given to demonstrate the good performance of the proposed method

Conservative Local Discontinuous Galerkin Method for Compressible Miscible Displacements in Porous Media

© 2017, Springer Science+Business Media, LLC. In Guo et al. (Appl Math Comput 259:88–105, 2015), a nonconservative local discontinuous Galerkin (LDG) method for both flow and transport equations was introduced for the one-dimensional coupled system of compressible miscible displacement problem. In this paper, we will continue our effort and develop a conservative LDG method for the problem in two space dimensions. Optimal error estimates in L∞(0, T; L2) norm for not only the solution itself but also the auxiliary variables will be derived. The main difficulty is how to treat the inter-element discontinuities of two independent solution variables (one from the flow equation and the other from the transport equation) at cell interfaces. Numerical experiments will be given to confirm the accuracy and efficiency of the scheme