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Author	SHA1	Message	Date
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
Omar SobhandClaude Fable 5	4bd98b5264	rtx-cfd + rtx-fsi: the added-mass piston — partitioned FSI on the real ALE fluid ... The first coupled fluid-structure computation in the workspace, verified against a closed form, and the first time rtx-fsi's added-mass claims run against a real discretised fluid rather than a linear model map. ALE extensions: per-side boundaries (Velocity / SlipWall / PressureOutlet) and moving boundary lines. A moving Velocity side is a material wall whose prescribed normal velocity must equal the line's own motion; a pressure outlet takes Dirichlet p' = 0 in the projection (replacing the Neumann anchor) with a zero-gradient predictor on its faces. Fluid half verified alone (tests/ale_piston_channel.rs): prescribed piston motion, slip walls, outlet. The incompressible rigid column is exact DISCRETELY - continuity forces every u to the wall's discrete velocity (8e-12) and the projected pressure is exactly linear with gradient rho times the wall's backward-difference acceleration (2.5e-9). Coupled benchmark (rtx-fsi/tests/piston_added_mass.rs): elastic piston (Newmark average acceleration) against added mass rho*L*H at mass ratio 6.25, rtx-fsi's Subiterated driving a real fluid/structure pass per step: - plain staggered diverges in 7 subiterations (Causin-Gerbeau-Nobile on a real solver); - Aitken converges at 3.0 subiterations/step onto T = 1.07009 vs the closed form 1.06999 - 9.8e-5 relative, halving with dt; - outlet flux matches the piston sweep to ~1e-9 every step. Discrete-analysis finding: Newmark beta scales the staggered added-mass threshold - the iteration gain is beta*m_a/(M + K*beta*dt^2), so the continuous ratio 2.5 CONVERGES at beta = 1/4 (gain 0.625, measured ~17 passes/step) and the benchmark needs ratio 6.25 (gain 1.56). Two real defects found and fixed, twelfth and thirteenth of the campaign: 1. rtx-cfd ale::advance re-stamped boundary faces at t_old from the current boundary function, which in a coupling loop carries the NEW interval's wall velocity - the predictor's old state had interior u = w0 but wall face u = w1, leaving an O(dt) pressure artifact confined to the wall-adjacent cells (p exact to 6e-11 everywhere except the wall cell at 4.7e-5). The start-of-step boundary faces are whatever the previous step's end-of-step application left there. 2. rtx-fsi aitken_factor guarded its denominator - a SQUARED residual- difference norm - against a bare f64::EPSILON, silently disabling Aitken below residual ~1e-8 and degrading to unit relaxation exactly in the well-converged regime; the repulsive fixed point then amplified 1e-9 residuals back up and the coupling diverged. Third instance of the absolute-threshold species (NNLS, ECSW). The guard is relative now; aitken_is_scale_invariant pins it at initial residual 1e-9. rtx-cfd 293 green (+1), rtx-fsi 29 green (+3). rtx-fsi's lib gains only the relative guard; the coupling layer still depends on no solver (rtx-cfd is a dev-dependency of its tests). Co-Authored-By: Claude Fable 5 <[email protected]>	2026-08-20 06:10:13 -07:00
Omar SobhandClaude Opus 5	9be5f4a68f	rtx-fsi: partitioned fluid-structure coupling ... rtx-cfd (18,715 lines) and rtx-fea (36,576 lines) both exist and nothing connects them -- rtx-fea is commented out of rtx-cfd's dependencies. This is the coupling layer, and it is the piece Prof. Charbel Farhat's 2026 Guggenheim Medal citation is actually about. It depends on NEITHER solver. The properties that make a partitioned coupling correct -- conservation of force, moment and interface work -- are statements about the transfer operators alone, so they can be validated now, on solvers whose canonical-benchmark validation is still outstanding. Adapters to the concrete solvers belong above this. TRANSFER (transfer.rs). Weights satisfy two constraints: sum(w_i) = 1 partition of unity -> force conserved sum(w_i x_i) = x_face linear reproduction -> MOMENT conserved The second is the one that gets skipped. Inverse-distance weighting satisfies the first and generally violates the second, conserving force while corrupting moment -- which shows up as slow spurious rotation rather than as an obvious error. Underdetermined for >4 nodes, so it takes the minimum-norm solution w = A^T (A A^T)^+ b. That is a PSEUDO-inverse, and not for defensiveness. A wetted surface is a surface, so its nodes are usually planar, and for a planar patch the z constraint row is an affine multiple of the ones row -- A A^T is genuinely rank-deficient. The constraint is redundant there, not unsatisfiable. An ordinary inverse rejects the most ordinary interface there is; I found this because my first test fixture was collinear and the code correctly refused it. Constraints are then verified against the weights actually obtained, since a pseudo-inverse returns a least-squares answer whether or not the system was consistent. Motion transfer uses the TRANSPOSE of the load operator, which makes interface work conserved identically: (Hf).v = f.(H^T v). Any other pairing leaks energy every step, and the leak looks like physics until it destabilises. COUPLING (coupling.rs). Staggered and Aitken-relaxed subiteration. The decisive tests reproduce the added-mass effect: at a gain of 2.5 the fixed-relaxation scheme DIVERGES and is reported as CouplingDiverged rather than as an exhausted budget, and Aitken recovers the same case. A partitioned coupling that cannot reproduce its own classic failure mode is not being tested hard enough. Aitken is exact for a linear fixed point, so convergence is asserted at <=4 iterations -- pinning that this is the real delta-squared formula and not an under-relaxation that happens to work. SCOPE, stated up front in the crate docs: small-displacement transpiration coupling on a fixed mesh. Deliberately not ALE and not embedded-boundary, so the Discrete Geometric Conservation Law does not yet apply -- the mesh does not move. Large motion needs an embedded boundary treatment; that is the next phase, not an oversight. External comparator named at entry: Turek-Hron FSI2/FSI3, not yet reached. 26 tests written red-first; cargo test/fmt/clippy -D warnings clean. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>	2026-08-19 06:49:36 -07:00