boundary_conditions Module

Provides apply_bcs(state), which must be called at the start of every residual evaluation. It reads the current solution array and the BC type strings from @p state, then writes the appropriate ghost values directly into the halo cells of state%ub.

Phase B rank-awareness: apply_bcs is a no-op on inner ranks. Only rank 0 writes the left-boundary halo, and only rank N-1 writes the right-boundary halo. Halo exchange (called from compute_resid) fills the interior halo cells of inner ranks with neighbour data. At -np 1 a single rank is both my_rank == 0 and my_rank == n_ranks - 1, so both edges are written and behaviour matches the serial code.

Supported BC types (set via bc_left / bc_right in the &initial_condition namelist group):

'dirichlet' — fixed prescribed state (halo cells preset by the driver, no update here) 'inflow' — same as Dirichlet; use for a driven supersonic inlet 'outflow' — linear extrapolation: ghost = 2boundary - penultimate cell (applied to every halo cell with the same formula) 'reflecting' — solid wall: density and energy mirrored, momentum negated 'periodic' — at -np > 1 handled by halo_exchange (left/right neighbours wrap around for periodic decomp); at -np 1 apply_bcs performs the in-process periodic copy directly 'nonreflecting' — characteristic (LODI/Thompson) non-reflecting outflow. Two algorithm variants are selected by nrbc_mode: 'pressure' (default) — isentropic pressure-relaxation; zeroes the incoming acoustic wave by constructing the ghost via isentropic relation and dp = rhocdu. 'characteristic' — full LODI characteristic targeting (Poinsot & Lele 1992, Sec. 3); decomposes the boundary state into f1/f2/f3 characteristic amplitudes and relaxes each independently. 'supersonic_inlet' — fully prescribed supersonic inflow (Ma > 1). Halo cells preset by the driver and never updated. 'supersonic_outlet' — all characteristics exit; zero-gradient ghost. 'subsonic_inlet' — characteristic-based subsonic inflow (0 < Ma < 1). 'subsonic_outlet' — characteristic-based subsonic outflow (0 < Ma < 1). 'neumann' — homogeneous Neumann BC: ghost = boundary cell. 'neumann_gradient' — prescribed-gradient Neumann: ghost = q_wall + (dq/dn)dx*k.

For the multi-cell halo (halo_width > 1), the "constant ghost" BCs (dirichlet, inflow, supersonic_inlet, supersonic_outlet, neumann, the nonreflecting and subsonic variants) fill every halo cell with the same ghost vector — equivalent to zero-gradient extrapolation past the first halo cell. 'outflow' applies the linear extrapolation to every halo cell using the same penultimate-cell baseline. 'reflecting' mirrors halo cell k about the boundary onto interior cell k (so halo cell 1-k = mirror of interior cell k).

'outflow' uses first-order linear extrapolation (LeVeque 2002, Sec. 7.3). A positivity guard falls back to zero-gradient if extrapolation produces a non-physical ghost state (negative density or pressure).

'nonreflecting' follows Thompson (1987) / Poinsot & Lele (1992). At supersonic outflow all waves leave and the ghost degenerates to zero-gradient. sigma_nrbc in [0,1] blends between fully non-reflecting (0) and a soft Dirichlet target (1) for both modes.