'outflow' ghost: first-order linear extrapolation with a positivity guard.
ghost = 2*boundary - penultimate (LeVeque 2002, Sec. 7.3). Falls back to zero-gradient (q_wall) when the extrapolation yields a non-physical ghost state (negative density or pressure).
| Type | Intent | Optional | Attributes | Name | ||
|---|---|---|---|---|---|---|
| type(solver_state_t), | intent(in) | :: | state | |||
| real(kind=wp), | intent(in) | :: | q_wall(3) | |||
| real(kind=wp), | intent(in) | :: | q_penult(3) | |||
| real(kind=wp), | intent(out) | :: | q_ghost(3) |