Fast preconditioned iterative methods for finite volume discretization of steady-state space-fractional diffusion equations

A potential and stream function analysis of two-dimensional steady-state convective diffusion equations involving Laplace fields

A unified analytical solution of the steady-state atmospheric diffusion equation

An alternative general solution of the steady-state heat diffusion equation

Fractional differential equations have attracted considerable interest because of their ability to model anomalous transport phenomena. Space fractional diffusion equations with a nonlinear reaction term have been presented and used to model many problems of practical interest. In this paper, a two-dimensional Riesz space fractional diffusion equation with a nonlinear reaction term (2D-RSFDE-NRT) is considered. A novel alternating direction implicit method for the 2D-RSFDE-NRT with homogeneous Dirichlet boundary conditions is proposed. The stability and convergence of the alternating direction implicit method are discussed. These numerical techniques are used for simulating a two-dimensional Riesz space fractional Fitzhugh-Nagumo model. Finally, a numerical example of a two-dimensional Riesz space fractional diffusion equation with an exact solution is given. The numerical results demonstrate the effectiveness of the methods. These methods and techniques can be extended in a straightforward method to three spatial dimensions, which will be the topic of our future research.

Numerical analysis and fast implementation of a fourth-order difference scheme for two-dimensional space-fractional diffusion equations

TT-M FE method for a 2D nonlinear time distributed-order and space fractional diffusion equation

Preconditioners are popularly used for speeding up Krylov subspace iterative method for solving linear systems from discretization of fractional differential equations (FDEs) on rectangular domains. Though some recent works have been developed for FDEs in general convex domains, it still has room for improvement in the design of preconditioner. For this sake, in this paper, we theoretically study the preconditioner problem for two-dimensional conservative space-fractional diffusion equations with variable coefficients and propose a robust preconditioner with penalty term which can deal with any convex domains significantly. We further prove that the proposed preconditioner equals to the coefficient matrix plus a low rank matrix and a matrix with small norm under certain conditions. It implies that the new preconditioner can effectively accelerate the convergence rate of Krylov subspace method. Experimental results show the good performance of the robust preconditioner.

PARCS code multi-group neutron diffusion constants generation using Monte Carlo method

Measurements of effective sizes and diffusivities of nano-colloids and micro-particles

Problems involving the dynamics of heating or cooling of physical systems submitted to certain boundary conditions are of great importance in a wide range of situations. Since simple situations involving cooling of food where the temperature of each part of the system needs to be controlled and, in general, does not involve the presence of heat sources, even more complex situations like nuclear reactors, where there is a heat source which is dependent on position and the dependence of the reactor reactivity on the temperature requires a good knowledge of the dynamics of this quantity, both for safety and efficiency. Problems of the steady-state heat diffusion equation for a series of physical systems are easy to solve and can be found in a vast literature on the subject. To find the time evolution of temperature T (x, t) of a system submitted to a heat source, however, is not a simple task and this type of problem is usually treated in an approximate way. In this work, for one-dimensional and homogeneous system, equilibrium T(x) and transient T (x, t) solutions are presented for cases of the existence of sources permeating throughout the medium. Each system considered is supposed to have constant temperature in the edges. Calculations are presented for two cases: one is the case of a source non-dependent on position and time and the other is the case of a source with sinusoidal spatial dependence, a situation close to what would occur in a one-dimensional nuclear reactor. The extension to a three-dimensional case, homogeneous sphere with constant temperature on its surface will also be presented. The case involving the contact of two subsystems of different materials will also be analyzed.

Proton radiography is an important diagnostic method for laser plasma experiments and is particularly important in the analysis of magnetized plasmas. The theory of radiographic image analysis has heretofore only permitted somewhat limited analysis of the radiographs of such plasmas. We furnish here a theory that remedies this deficiency. We show that to linear order in magnetic field gradients, proton radiographs are projection images of the MHD current along the proton trajectories. We demonstrate that in the linear regime (i.e., the small image contrast regime), the full structure of the projected perpendicular magnetic field can be reconstructed by solving a steady-state inhomogeneous 2-dimensional diffusion equation sourced by the radiograph fluence contrast data. We explore the validity of the scheme with increasing image contrast, as well as limitations of the inversion method due to the Poisson noise, discretization errors, radiograph edge effects, and obstruction by laser target structures. We also provide a separate analysis that is suited to the inference of isotropic-homogeneous magnetic turbulence spectra. Finally, we discuss extension of these results to the nonlinear regime (i.e., the order unity image contrast regime).

In this paper we describe, analyse and implement a parallel iterative method for the solution of the steady-state drift diffusion equations governing the behaviour of a semiconductor device in two space dimensions. The unknowns in our model are the electrostatic potential and the electron and hole quasi-Fermi potentials. Our discretisation consists of a finite element method with mass lumping for the electrostatic potential equation and a hybrid finite element with local current conservation properties for the continuity equations. A version of Gummel's decoupling algorithm which only requires the solution of positive definite symmetric linear systems is used to solve the resulting nonlinear equations. We show that this method has an overall rate of convergence which only degrades logarithmically as the mesh is refined. Indeed the (inner) nonlinear solves of the electrostatic potential equation converge quadratically, with a mesh independent asymptotic constant. We also describe an implementation on a MasPar MP-1 data parallel machine, where the required linear systems are solved by the preconditioned conjugate gradient method. Domain decomposition methods are used to parallelise the required matrix-vector multiplications and to build preconditioners for these very poorly-conditioned systems. Our preconditioned linear solves also have a rate of convergence which degrades logarithmically as the grid is refined relative to subdomain size, and their performance is resilient to the severe layers which arise in the coefficients of the underlying elliptic operators. Parallel experiments are given.

The mathematical model proposed by Chapman and Antano (Electrochimica Acta, 56 (2010), 128–132) for the catalytic electrochemical–chemical (EC’) processes in an irreversible second-order homogeneous reaction in a microelectrode is discussed. The mass-transfer boundary layer neighbouring an electrode can contribute to the electrode’s measured AC impedance. This model can be used to analyse membrane-transport studies and other instances of ionic transport in semiconductors and other materials. Two efficient and easily accessible analytical techniques, AGM and DTM, were used to solve the steady-state non-linear diffusion equation’s infinite layers. Herein, we present the generalized approximate analytical solution for the solute, product, and reactant concentrations and current for the small experimental values of kinetic and diffusion parameters. Using the Matlab/Scilab program, we also derive the numerical solution to this problem. The comparison of the analytical and numerical/computational results reveals a satisfactory level of agreement.

Accurate thermoplasmonic simulation of metallic nanoparticles

Often in reactor dynamics, higher eigenfunctions of the multigroup diffusion equation must be determined. An algorithm to calculate higher eigenfunctions (modes) of the [lambda]-eigenvalue problem corresponding to the steady-state two-group neutron diffusion equation is presented. The method is based on a special type of subspace iteration for large sparse nonsymmetric eigenvalue problems. Having been tested using an International Atomic Energy Agency benchmark problem and also applied to a VVER-1000 pressurized water reactor assembly, the algorithm was found to work very effectively and reliably. In its application, the algorithm presented is not restricted to the [lambda]-eigenvalue problem only but is also generally applicable to large sparse nonsymmetric eigenvalue problems even with multiple and complex eigenvalues.