Omar SobhandClaude Opus 5
9be5f4a68f
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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]>
94 lines
3.0 KiB
Rust
94 lines
3.0 KiB
Rust
//! Errors raised when an interface or a transfer cannot be formed.
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use thiserror::Error;
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/// Why a coupling operation was refused.
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#[derive(Debug, Clone, PartialEq, Error)]
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pub enum FsiError {
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/// One side of the interface was empty.
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#[error("empty interface: {side} has no entries")]
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EmptyInterface {
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/// Which side was empty.
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side: &'static str,
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},
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/// Too few structure nodes to reproduce a face centroid exactly.
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///
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/// Linear reproduction in three dimensions imposes four constraints,
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/// so at least four independent nodes are needed. Falling back to a
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/// weaker interpolation would conserve force and silently corrupt
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/// moment, which is worse than refusing.
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#[error("interface needs at least {needed} structure nodes for linear reproduction, got {got}")]
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InsufficientNodes {
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/// Nodes supplied.
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got: usize,
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/// Nodes required.
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needed: usize,
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},
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/// A field had a different length from the mesh it belongs to.
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#[error("{field}: expected {expected} values, got {got}")]
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CountMismatch {
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/// Which field.
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field: &'static str,
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/// Length supplied.
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got: usize,
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/// Length required.
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expected: usize,
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},
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/// A field value was NaN or infinite.
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#[error("non-finite value in {field} at index {index}")]
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NonFinite {
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/// Which field.
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field: &'static str,
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/// Where.
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index: usize,
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},
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/// A parameter given to a coupling scheme was outside its valid
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/// range.
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#[error("invalid coupling parameter {parameter}: {value}")]
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InvalidParameter {
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/// Which parameter.
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parameter: &'static str,
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/// The value supplied.
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value: f64,
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},
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/// The interface iteration ran away.
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///
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/// In partitioned coupling this is almost always the added-mass
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/// effect: once the fluid's added mass exceeds the structural mass the
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/// fixed point becomes repulsive, and a fixed relaxation factor cannot
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/// recover it. Dynamic relaxation can.
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#[error("coupling diverged after {iterations} iterations (residual {residual})")]
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CouplingDiverged {
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/// Iterations completed before divergence was detected.
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iterations: usize,
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/// Residual at that point.
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residual: f64,
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},
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/// The iteration budget ran out before the interface agreed.
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#[error(
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"coupling did not converge in {iterations} iterations (residual {residual}, tolerance {tolerance})"
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)]
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CouplingNotConverged {
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/// Iterations performed.
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iterations: usize,
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/// Residual reached.
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residual: f64,
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/// Tolerance required.
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tolerance: f64,
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},
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/// The interpolation system was singular — the selected nodes are
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/// degenerate, for example all collinear.
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#[error("degenerate node neighbourhood for face {face}: cannot reproduce its centroid")]
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DegenerateNeighbourhood {
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/// Which face.
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face: usize,
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},
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}
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