here's a 1-d flame_wave atmosphere run via hse_convergence_general on development with reflecting BCs:

and here's the version with these new one-sided PPM stencils:

Velocity is on the y-axis and height in the atmosphere in on the x-axis.

Notice that with these new stencils, the velocity goes to 0 at the reflecting boundary as it should.

the 2-d axisymmetric Sedov problem NaNs with these new BCs.

this seems to work well for the HSE boundary, but it does not see to work with the -x boundary in axisymmetric coords. This might be because we need to take into account the geometry somehow, like this paper suggested: https://ui.adsabs.harvard.edu/abs/1993ApJS...88..589B/abstract

as an example, Sedov 2-d in axisymmetry crashes after a while.

The way to do this to match what we do with PLM (which uses the Saltzman 1994 approach) is to only use the one-sided stencil for velocity. Then also reflect the interface states at the reflecting boundary -- this will ensure that the flux through the boundary is zero (and we won't have to rely on that hack in the Riemann solvers).

this has been updated to work with pslope