Schematic representation of the method for injecting SPH particles. The horizontal axes represent the time since ejection, or interchangeably the distance from the boundary. The numbered dots represent particle shells, numbered by their index. Older shells have a lower index. Top: before the hydro timestep the shell with index 12 lies below the injection boundary, and is therefore part of the handled shells, and has imposed physical properties. Bottom: after the hydro timestep, the shell with index 12 has moved beyond the injection boundary, and no longer has imposed physical properties.