Cross-section of the evolution of the mildly relativistic shock-wave across the grid shown in Fig. 8. The run was performed using the GFORCE¹ scheme at a CFL of 0.95, and using the van Leer slope-limiter. The green + signs are the numerical solution on the overlapping grid, the red circles are from an equivalent simulation on a single 2D Cartesian grid, and the blue line is the exact solution for the problem. The two numerical solutions have been interpolated from along the line x = y.