Mesh Convergence Issues

Abstract

(U) This document is written to describe issues that impact on the ability of codes to converge to a unique answer. Several issues that could be discussed in the area of mesh convergence and resolution the topics include the resolution of geometry, stress gradients, shock propagation, material softening, and localization of deformation. After a quick discussion of what mesh convergence means, this paper presents a review of some of the criteria for mesh convergence. With an introductory level understanding of these criteria, two problems are presented to demonstrate convergence and non-­convergence. The first example is a simple convergent hemisphere of gas, known as the Noh problem. With this problem convergence is shown. The Noh problem is also used to investigate the impact of mesh stiffening, or viscosity. The second problem is that of an explosively driven hemisphere. The hemisphere made of U6Nb with an inner radius of 40 mm and an outer radius of 41 mm was filled with PBX9501 and tested. Test results included surface temperature and information on the breakup, such as distribution of fines. Convergence computational methods are discussed along with some proposals for obtaining mesh convergence for a wide range of applications.