An Unstaggered, High‐Resolution Constrained Transport Method for Magnetohydrodynamic Flows

Abstract

The ideal magnetohydrodynamic (MHD) equations are important in modeling phenomena in a wide range of applications, including space weather, solar physics, laboratory plasmas, and astrophysical fluid flows. Numerical methods for the MHD equations must confront the challenge of producing approximate solutions that remain accurate near shock waves and that satisfy a divergence-free constraint on the magnetic field. Failure to accomplish this often leads to unphysical solutions. In this paper, a high-resolution wave propagation method is developed that utilizes a novel constrained transport technique to keep the magnetic field divergence-free. This approach is based on directly solving the magnetic potential equation in conjunction with a new limiting strategy to obtain a nonoscillatory magnetic field. It is demonstrated in this work that an unstaggered definition of the divergence is the correct one to use in the case of wave propagation methods. Therefore, we solve the magnetic potential equation on the same grid as the MHD equations; hence the usual grid staggering that is found in constrained transport methods is eliminated. We demonstrate through truncation error analysis and direct numerical simulation that the resulting method is second order accurate in space and time for smooth solutions and nonoscillatory near shocks and other discontinuities. The resulting numerical method has been implemented as an extension to the clawpack software package and can be freely downloaded from the Web.