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rustytorch/crates/specialized/rtx-fsi/Cargo.toml
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Omar SobhandClaude Opus 5 9be5f4a68f
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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

19 lines
571 B
TOML

[package]
name = "rtx-fsi"
version = "1.0.0"
edition.workspace = true
rust-version = "1.92"
authors = ["RustyTorch Team"]
license = "MIT OR Apache-2.0"
description = "Partitioned fluid-structure interaction coupling: conservative load and motion transfer across non-matching interfaces"
keywords = ["fsi", "fluid-structure", "coupling", "aeroelasticity", "simulation"]
categories = ["science", "simulation"]
repository = "https://github.com/rustytorch/rustytorch"
[dependencies]
nalgebra = { workspace = true }
thiserror = { workspace = true }
[lints]
workspace = true