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feat: opt-in MRT collision for blocked/refined stepping

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Additive compile-time COLLISION_MODEL == 2 branches in BlockStep,
BlockChannelStep and patch_collide repeat the uniform collide_dispatch
MRT sequence (zero forcing, omega clamp, collide_mrt_no_force). SRT
statements, including the shifted retained-delta/direct-momentum path,
are unchanged under the alternate branch. Guards accept SRT or MRT only;
TRT, LES, non-FP32, non-double-buffer and shifted dense/mixed refinement
stay rejected. Transfers, subcycling, BFL, raw ledgers, channel closures,
shifted-Jet SRT guard and PeriodicBlockStepper are untouched. Existing
MRT operator, shifted representation and fixed kinetic rates are reused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Claude
2026-10-18 10:26:55 +00:00
co-authored by Claude Opus 5.5
parent 9805383f9c
commit 00d7a54829
8 changed files with 168 additions and 10 deletions
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docs/development/refinement_mrt.md
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# MRT reuse in blocked/refined stepping (candidate)
Base `9805383`. Opt-in by `LBMConfig(collision="MRT")`; SRT remains the default and its
kernel statements, including the shifted retained-delta/direct-momentum path, are unchanged
under the alternate compile branch.
## What changes
- `blocks.cu` (`BlockStep`), `step/block_channel.cu` (`BlockChannelStep`) and `refinement.cu`
(`patch_collide`) gain a compile-time `COLLISION_MODEL == 2` branch that repeats the uniform
`collide_dispatch` sequence with zero forcing: `compute_rho_u`, omega clamp to
`[OMEGA_COLLISION_MIN, OMEGA_COLLISION_MAX]`, `collide_mrt_no_force`.
- Kernel and host guards accept SRT or MRT only. TRT, LES, non-FP32 storage, non-double-buffer
streaming, and shifted dense/mixed refinement remain rejected.
- Transfers, subcycling, BFL, raw exchange ledgers and channel closures are untouched. The
existing pressure-NEQ outlet selects its MRT branch (unknowns-only NEQ extrapolation) from
`COLLISION_MODEL`, so MRT channel parity must use uniform MRT as reference.
## Contracts (existing operator, not redefined here)
- Shifted storage uses the existing MRT representation (equilibrium moments from `rho - 1`),
not the SRT retained-delta/direct-momentum precision. Mass and momentum are conserved
moments (relaxation rate 0).
- Kinetic rates are the operator's fixed `s_e = s_eps = s_q = 1.2` on every level; the shear
rate is the existing per-level omega. Under acoustic 2:1 scaling the physical bulk viscosity
therefore halves on each finer level. No rate rescaling is applied.
- No LES, no external forcing (`d_params` force fields stay zero).
- Shifted Jets stay SRT-only: `body/jets.py` keeps its existing guard. This extension does not
provide shifted MRT Jets.
- `PeriodicBlockStepper` (uniform blocked stepper) remains SRT-only.
## Evidence status
CPU: `tests/unit/test_refinement_mrt.py` (host guards, source contract). GPU:
`tests/validation/multigrid/test_refinement_mrt_gpu.py` (uniform-MRT channel parity, AMR MRT
smoke) and unchanged SRT GPU tests. Strict parity may be subject to cross-kernel FMA
contraction differences already documented for SRT. Not a DFG/benchmark validation.