Relativistic MHD with central-type schemes. Axisymmetric simulations of Pulsar Wind Nebulae

A third order shock-capturing numerical scheme for three-dimensional special relativistic magnetohydrodynamics (3-D RMHD) is presented and validated against several numerical tests of astrophysical interest. The scheme avoids completely spectral decomposition into characteristic waves, computationally expensive and subject to many degenerate cases in the magnetic case, while it makes use of a two-speed Riemann solver that just require the knowledge of the two local fast magnetosonic velocities. Moreover, the onset of spurious magnetic monopoles, which is a typical problem for multi-dimensional MHD upwind codes, is prevented by properly taking into account the solenoidal constraint and the specific antisymmetric nature of the induction equation. The extension to generalized orthogonal curvilinear coordinate systems is included, thus the scheme is ready to incorporate general relativistic (GRMHD) effects. Finally, the code is parallelized under MPI directives and is able to run on all supercomputing platforms with excellent scalability.