NSCBC 1/7: characteristic ghost-cell boundary conditions, 1-D-normal LODI limit - #1058
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This was referenced Aug 28, 2026
…LODI limit The base characteristic treatment as a ghost-cell fill: one wave decomposition in an outward-normal frame, two boundary types. Outgoing invariants extrapolate from the interior on a minmod-limited slope; the incoming acoustic is modelled as a Poinsot-Lele relaxation written as a gradient increment (a mesh-independent rate, so literature coefficients transfer), sigma at outflows, relax_u/relax_t at inflows, with a stateless feed-forward slot (Target::dudt) for problems that inject signals. Supersonic faces are exact. Flow reversal at an outflow runs the same closure with the material slopes upwinded off across a narrow Mach band -- one code path on both sides of u_out = 0. The fill is a pure function of pre-launch interior data: idempotent, order-independent, bit-reproducible across restarts and decompositions, GPU-resident (POD captures, counted fallbacks instead of device aborts). Every fallback increments a counter and the counters report by default -- a silent fallback is indistinguishable from a healthy boundary. Problems opt in per boundary POINT through a bcnormal_nscbc() hook returning a Target; a face may mix inflow, outflow and wall. EB requires eb_zero_body_state (fatal otherwise, the fill detects covered cells by non-positive density); an AMR fine face touching a characteristic boundary warns. This PR is the 1-D-normal LODI limit on purpose: transverse terms and the reacting-boundary closures arrive as the next two PRs in this stack, each an additive, default-off correction to the modelled incoming wave (some comments here forward-reference them). Verification driver and regression cases follow in the stack; headline number from the case PR: an acoustic pulse leaves through this boundary with 0.8% reflection against 97% for a hard-pressure fill. Also removes the dead nscbc_adv/nscbc_diff keys of the deleted Fortran implementation (aborting, not ignoring, when an inputs file still sets them) and retires the 1 K default for eb_boundary_T in favour of 300 K. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01XU8M23nucKsu1do2WxXFeq
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Part 1 of the NSCBC stack tracked in #1057. Merges first; every later part is stacked on this one.
The base characteristic treatment as a ghost-cell fill: one wave
decomposition in an outward-normal frame, two boundary types. Outgoing
invariants extrapolate from the interior on a minmod-limited slope; the
incoming acoustic is modelled as a Poinsot-Lele relaxation written as a
gradient increment (a mesh-independent rate, so literature coefficients
transfer), sigma at outflows, relax_u/relax_t at inflows, with a
stateless feed-forward slot (Target::dudt) for problems that inject
signals. Supersonic faces are exact. Flow reversal at an outflow runs
the same closure with the material slopes upwinded off across a narrow
Mach band -- one code path on both sides of u_out = 0.
The fill is a pure function of pre-launch interior data: idempotent,
order-independent, bit-reproducible across restarts and decompositions,
GPU-resident (POD captures, counted fallbacks instead of device
aborts). Every fallback increments a counter and the counters report by
default -- a silent fallback is indistinguishable from a healthy
boundary. Problems opt in per boundary POINT through a
bcnormal_nscbc() hook returning a Target; a face may mix inflow,
outflow and wall. EB requires eb_zero_body_state (fatal otherwise, the
fill detects covered cells by non-positive density); an AMR fine face
touching a characteristic boundary warns.
This PR is the 1-D-normal LODI limit on purpose: transverse terms and
the reacting-boundary closures arrive as the next two PRs in this
stack, each an additive, default-off correction to the modelled
incoming wave (some comments here forward-reference them). Verification
driver and regression cases follow in the stack; headline number from
the case PR: an acoustic pulse leaves through this boundary with 0.8%
reflection against 97% for a hard-pressure fill.
Also removes the dead nscbc_adv/nscbc_diff keys of the deleted Fortran
implementation (aborting, not ignoring, when an inputs file still sets
them) and retires the 1 K default for eb_boundary_T in favour of 300 K.
🤖 Generated with Claude Code
https://claude.ai/code/session_01XU8M23nucKsu1do2WxXFeq