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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	c0666bf22a	rtx-fsi: C3 — FSI3 opened: the added-mass regime, its impulse artefact, and the coupler hygiene it demanded ... The march is extracted from the FSI2 test into a shared tests/fsi2_harness/march.rs (MarchConfig from env / MarchResult / window statistics), the harness parameterised by BenchmarkCase (FSI2, FSI3: inflow, solid density, modulus, rigid-flag drag reference), and turek_hron_fsi3.rs written on top (Re 200, density ratio 1, E 5.6e6). The committed FSI2 default is bit-identical through all of it — every new knob defaults to FSI2's behaviour; re-verified twice to every printed digit (uy 3.7732 / 3.7920 mm). FSI3's first contact, traced pass by pass (RTX_FSI3_TRACE), exposed the added-mass instability in its purest form and, one measured mechanism at a time, what a partitioned coupling at unit density ratio needs: - C^1 interface motion (MarchConfig::c1_interface): a constant per-step interface velocity is a velocity JUMP at each step start, and the incompressible fluid answers with an impulsive added-mass load ~ rho L dv / dt_fluid (8x the physical reaction under subcycling): 1,600 N at release, 48,000 N and a 59 mm response one step later, the flag's Newton dead the pass after. Constant acceleration from the previous end velocity to 2 dd/dt - v_n removes the impulse (loads 1,700-2,400 N). - IQN first-pass relaxation as a knob (initial_relaxation, FSI3 0.05): |1 - omega (1 + g)| must contract; 0.5 diverges past gain 3. - The divergence verdict waits for the secant (IqnIls): the exploratory second pass on a high-gain map legitimately overshoots 10x before the first secant column exists. Pinned by a gain-40 model test. - Kinematic predictor (predictor: "kinematic", velocity only): the structure-alone predictor ignores an added mass comparable to the flag's and overshoots 2-5x, drawing 5-6x loads every first pass; and NOT with the acceleration — Newmark average acceleration carries an inconsistent initial acceleration as a sign-alternating mode (d + dt v + dt^2 a / 2 predicted 22 mm at release; converged 0.14). - Quiescent release (quiescent_release): the structure-alone "consistent" initial acceleration M^-1 F ignores the added mass. - Coupler hygiene (IqnIls): a stalled or diverged step's secant columns are no longer retained (a bistable mask flip's columns extrapolated a 30 mm interface jump on the next step); two-window stagnation detection reports a plateau early instead of bouncing to the budget (a single-window test misjudged a slowly converging step and is recorded as such); trust region tightened to 10x the residual. A noise-column filter at the tolerance was measured to HURT (stalled a converging step at 5.5e-4) and is disabled (threshold 0). - The floor measured, not borrowed (fsi2_interface_noise.rs gains RTX_NOISE_CASE=fsi3): flip jumps 3.6e-4 (12x FSI2's), the subcycle-8 release map stalling near 1e-6, the subcycle-2 map converging to 9e-10; and through the release transient (flag at ~0.3 m/s) the subcycle-8 floor rides up to ~1e-3 with the motion — which moved the FSI3 default to subcycle 2 at a 3e-5 floor. FSI3 committed default (ny 62, release t = 4, t_end 4.2, subcycle 2): 581 steps, 3.8 subit/step (max 8), 0 stalls, 0 retries, conservation 4.3e-13; uy 10.1 +- 27.5 mm and ux -3.1 +- 3.2 mm over the first 0.2 s (reference cycle 1.45 +- 34.90, -2.86 +- 2.70). Machinery invariants pinned; physics bands await the study record. Rigid-flag drag 426.9 vs CFD3's 439.45 (-2.9%). The FSI2 mode-2 study pin (IQN / subcycle 2, t_end >= 16) was measured with the pre-hygiene coupler; re-verify on the next s = 2 study run. 48 lib tests green, clippy clean. Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq	2026-08-23 19:43:02 -05:00
Omar SobhandClaude Fable 5	140310b223	rtx-fsi: the noise floor interrogated — smoothing refuted, IQN-ILS lands, tight coupling reopened ... The tenth session ended on "lower the interface noise floor". This builds the levers and measures them, and the measurements overturn the diagnosis: - smooth_tractions: arclength moving average over the wetted surface, area-weighted, smooth normal-similarity factor so corners do not mix and the smoothed load stays continuous in the geometry. Nine unit tests. MEASURED NEGATIVE RESULT: the flip-scan floor is unchanged to 0.2% at radii 1-3h — the flip's load jump is coherent through the fluid field (mask rebuild shifts the pressure around the flipped cell), and a surface average preserves coherent shifts. Default off; the probe pins the attribution so nobody re-reaches for this lever. - IqnIls: interface quasi-Newton with inverse least squares (Degroote 2009) — filtered MGS least squares over secant columns (filter RELATIVE to column norm), cross-step history reuse, per-step set_tolerance. Model-map tests: exact on anisotropic linear maps within dim+2 passes (scalar Aitken provably cannot be), scale invariant, history reuse shortens the next step, stalls at the noise scale instead of diverging (fixture lesson: per-pass noise, not state-dependent noise — the latter has a genuine fixed point). - tests/fsi2_harness/: the FSI2 machinery extracted shared; verified pure code motion (committed release response reproduced to every printed digit). March gains RTX_FSI2_SMOOTH / RTX_FSI2_COUPLER=iqn / RTX_FSI2_REUSE knobs; pinned bands guard the default configuration. - tests/fsi2_interface_noise.rs: the probe. Flip-scan floor at subcycle 8: 3.05e-5 (pinned); smoothing attribution pinned; the cross-subcycle scan recorded but unpinned (the fixed geometry increment's wall-velocity trend, increment/dt_c, swamps the flip signal at small dt_c — a dt_c^2 scaling hypothesis died in that operationalization). THE OPERATIONAL FLOOR — the real release step subiterated at tolerance 1e-9 with residuals traced — converges DEEP at both subcycles: s8 aitken 3.4e-9 / iqn 1.6e-9, s2 both ~6.4e-10 in 5-6 passes. The flip jumps are events at specific geometries, not a floor under every step: the tenth session's subcycle-2 blowup was tolerance mis-budgeting (2e-4 held fixed while dt_c shrank), not an impassable floor. Probe bug found and fixed on the way: stale shared geometry leaked a 4.5e-5 phantom first residual into the first stall run; every measurement now resets the geometry on entry. All 924+17 tests green: lib 44 (was 27), piston 2, curved edge 1, FSI1, the committed FSI2 march (release response identical), the probe. Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq	2026-08-21 06:42:05 -07:00