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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	4e90177aa9	rtx-fsi: C2 — FSI2's coupled march, and the wrong attractor measured ... The unsteady coupling on the piston pattern with the real solvers: per TIME STEP, Aitken subiterations of (8x-subcycled embedded TVD/multigrid fluid <-> flag nonlinear-Newmark step), the fluid re-runnable through snapshot/restore + a field clone, the flag re-runnable because the stepper commits nothing, the moving polygon carrying the flag's actual interface velocity. Conservation 8.3e-12 over 9,263 coupled steps; the rigid phase reproduces this solver's own CFD2 values at both grids before anything couples. What the 30 s studies measured (ny = 62 AND 82, three protocols): the coupled system self-excites into a wake-forced cycle at 3.729/3.728 Hz with uy(A) +-17.3 mm at both grids - grid-converged, protocol- independent, kinematically genuine (ux locks 2x uy) - and NOT the benchmark's mode-2 resonant cycle (1.93 Hz / 81.6 mm; the flag's vacuum mode 2 is 1.9245 Hz). Diagnosis measured stepwise: the staggered phase lag starves the resonant channel (a subcycle=2 probe redirected growth to 1.9 Hz as predicted), and tighter dt_c is blocked because wall- velocity noise = displacement-tolerance / dt_c while the needed tolerance sits below the discrete interface noise floor (~1.3e-4 at full inflow, mask flips through beta dt^2/m). The route to the benchmark cycle is lowering that floor, not iterating against it. Robustness, both measured: rare wild tractions (19 samples in 2.4M) clamped at 20x the sample median - clamped, not dropped: a hard drop makes the pass discontinuous and the subiteration bounces at step scale; and Aitken "divergence" verdicts within 5x tolerance accepted as noise bounces over well-predicted steps (counted, bounded). The committed default (t_end = 7, ~9.5 min) pins the deterministic release response (uy 3.773 +- 3.792 mm, band +-35%); study horizons >= 25 s pin the measured 3.73 Hz attractor so any material change is loud. Full study record in the module docs and omni-cortex solver_status.md. Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq	2026-08-21 02:45:43 -07:00