[petsc-users] PetscFV without ghost boundary conditions

Jed Brown jed at jedbrown.org
Mon Jan 17 11:46:59 CST 2022

Matthew Knepley <knepley at gmail.com> writes:

> What you suggest (flux boundary conditions) would require short-circuiting
> the Riemann solver loop to stick in the flux. It is doable, but we opted
> against it in the initial
> design because it seemed more complex than ghost cells and did not seem to
> have any advantages. For multi-domain things, wouldn't you require that the
> cell average of
> the adjacent cell be used as the ghost value for consistency? We would need
> a new interface to mark existing cells as ghosts, but that is quite simple
> compared to a flux BC
> addition.

FV methods usually need a ghost for slope reconstruction. FE methods (CG or DG) don't have a reconstruction and thus can use a flux boundary condition by solving a degenerate Riemann problem. IMO, this is simpler than creating a ghost cell and solving a full Riemann problem. It's what we use for subsonic inflow in libceed-fluids, for example.

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