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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	b82f307cae	rtx-fsi + rtx-cfd + rtx-fea: FSI1 — the coupled cylinder and flag ... The summit rung C1: all the verified pieces joined into the first coupled Turek–Hron computation (rtx-fsi/tests/turek_hron_fsi1.rs). The embedded fluid computes tractions on the DEFORMED flag surface (EmbeddedMask::traction_at, factored from surface_force); the flag's wetted boundary is a polygon whose vertex list sits behind a lock, so the moving-body mask rebuild picks up every shape update (EmbeddedBody::polygon + pub polygon_signed_distance); WettedSurface — rebuilt on the deformed interface every subiteration — carries the loads to the flag's boundary nodes (NonlinearStaticAnalysis::set_nodal_forces); Subiterated::aitken drives the exchange, each pass marching the fluid to flag-load stagnation so the coupling map is a function of geometry, not of the fluid's transient. Result (ny = 62, 6 Aitken passes, 420 s): coupled drag 15.360 (+7.5%, the rigid CFD1 band at this grid), lift 0.7977 (+4.4%), ux(A) 2.647e-5 vs 2.270e-5 (+16.6%; +6.1% at ny = 82), uy(A) 3.90e-4 vs 8.21e-4 at h = 6.6 mm and 1.124e-3 (+37%) at h = 5 mm — the resolutions BRACKET the reference through the flag's 3 -> 4-cell thickness transition, like the rigid-flag lift; conservation 7.4e-12 every pass. Bands asserted are the measured ones; RTX_FSI1_NY runs studies. Two real rtx-fsi defects found by this rung (15th and 16th of the campaign), both regression-tested (tests/transfer_curved_edge.rs): 1. solve_weights built its constraint Gram from RAW coordinates: the condition number grows as (position/spacing)^2 — ~1e4 for a flag edge at x ~ 0.26 with 5 mm spacing — and the 4x4 SVD pseudo-inverse lost enough accuracy that the (correctly strict) partition-of-unity / reproduction verification rejected healthy neighbourhoods: the operator's behaviour depended on WHERE the interface sat. Now centred on the face and scaled by the neighbourhood radius — identical constraints, O(1) conditioning, translation-invariant. 2. A NEARLY collinear neighbourhood (the nearest nodes of a face on a smoothly deformed edge: y is almost linear in x, off by the curvature sagitta) cannot satisfy exact centroid reproduction with bounded weights — the offending singular value is too large to truncate and too small to invert. The recruitment now widens (8 -> 16 -> 32 -> all) until the verified constraints hold; for a thin structure that pulls in the opposite face, exactly the transverse spread the system needs. Findings measured before believed: the transfer is faithful (a strictly local two-node split of the same tractions moved the tip by 2%); the uy error is the sampled lift PROFILE on a 3-cell flag (a uniform distribution of the same net lift bends 4x more), confirmed by the resolution study; TVD limiter chatter (+-0.5% steady load — limited schemes stall short of machine steady state) defeats steady fixed-point coupling, so steady coupled cases run upwind while the time-marched FSI2/FSI3 keep TVD; and the mask never chattered at FSI1's sub-cell amplitude (fluid-cell count constant through every pass). rtx-fsi 29 -> 31 green (lib 27, piston 2, curved-edge 1, FSI1 1). Co-Authored-By: Claude Fable 5 <[email protected]>	2026-08-20 17:22:27 -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