An Extended HLLD Riemann Solver for the Numerical Simulation of Magneto-Hydrodynamics

By revisiting the derivation of multi-state HLL approximate Riemann solver for the ideal magneto-hydrodynamics, an extended HLLD Riemann solver is constructed based on the assumption that the normal velocity is constant over the Riemann fan, which is bounded by two fast waves, and separated by two compound waves and a middle contact wave. Compared with the HLLD solver, the slow waves are allowed to persist inside the Riemann fan, so that the two Alfvén waves are replaced by the two compound waves that are the merging product of the Alfvén and slow waves. Conseq uently, the corresponding wave speeds are chosen to be an interpolation between the Alfvén and slow waves for simplicity. The numerical tests showed that the extended HLLD solver (called HLLD-P) has better performance for the capture of slow waves than the HLLD solver, and exhibits overall better accuracy in some situations where the slow waves exist. However, the new solver does not capture the Alfvén wave as well as the HLLD solver once the estimated speeds of compound waves deviate from the Alfvén wave speeds. Overall, the HLLD-P solver is fully compatible with the HLLD solver as long as the compound waves degenerate to the Alfvén waves inside the Riemann fan. It is indicated that the HLLD-P solver can be used for the various applications of MHD simulation, especially for those cases where the slow waves are expected to be generated.