A study of spatial and time discretizations for discontinuous Galerkin methods.

摘要

This dissertation consists of two different research efforts. In the first one, a new approach to the treatment of viscous flux in the context of discontinuous Galerkin spatial discretization is addressed. In the second part of the dissertation, an approach to constructing high-order W-methods is discussed.In the first part of the dissertation, a study of boundary and interface conditions for discontinuous Galerkin approximations of fluid flow equations is undertaken. While the interface flux for the inviscid case is usually computed by approximate Riemann solvers, most discretizations of the Navier-Stokes equations use an average of the viscous fluxes from neighboring elements. A methodology for constructing a set of stable boundary/interface conditions that can be thought of as "viscous" Riemann solvers and are compatible with the inviscid limit is presented.In the second part, we turn our attention to temporal discretizations. Implicit methods are the natural choice for solving stiff systems of ODEs. Rosenbrock methods are a class of linear implicit methods for solving such stiff systems of ODEs. In the Rosenbrock methods the exact Jacobian must be evaluated at every step. These evaluations can make the computations costly. By contrast, W-methods use only occasional calculations of the Jacobian matrix. This makes the W-methods popular among the class of linear implicit methods for numerical solution of stiff ODEs. However the price one has to pay is the large amount of work needed to find the coefficients of the W-methods. As the order of the W-methods increases, the number of order conditions of the W-methods increases very fast. This makes the design of high-order W-methods difficult. In the second part, an approach to constructing high-order W-methods is given.