Add a solids4foam solid participant to turek-hron-fsi3

[philipcardiff]

Important

Draft: blocked on precice/openfoam-adapter#417 ("Support reading Stress in solid participants"). This case cannot run until that adapter change is released, and the reference-results tarball cannot be generated until it reaches the system-test images. Marking ready for review once #417 is merged.

Summary

Adds a solids4foam solid participant to turek-hron-fsi3, adapted from the HronTurekFsi3 benchmark that solids4foam ships. This is a natural fit: solids4foam maintains its own version of this benchmark, so the tutorial gains a cross-check against the deal.II and Nutils participants.

• Neo-Hookean, nonlinear-geometry total-Lagrangian solid model, PETSc SNES, alpha stabilisation.
• Material properties match the deal.II case: E = 5.6 MPa, nu = 0.4, rho = 1000 kg/m³.
• 630-cell beam (105 x 6), patches renamed to interface / fixed / frontAndBack to match the repo's conventions.
• The interface patch uses the solids4foam solidTraction boundary condition, whose tractionField option consumes the solidTraction volVectorField that the adapter fills.

Results

Full 15 s run (15000 time windows), fluid on 20 MPI ranks, solid serial. Flap-tip y-displacement over the settled limit cycle (t = 8 to 15 s), against the Turek-Hron FSI3 benchmark value of uy = 1.48 +/- 34.38 mm:

	mean [mm]	amplitude [mm]	amplitude error
benchmark	1.48	34.38	—
this case (polynomialOrder 3)	1.47	34.50	+0.35%

Choice of polynomial order

Since the high-order (polynomial reconstruction) discretisation is a cost/accuracy knob, I ran the full 15 s simulation three times, changing only the solid discretisation order and holding everything else fixed:

solid discretisation	amplitude [mm]	amplitude error	mean error	solid solver cost	end-to-end runtime
p = 1, default scheme (no highOrderCoeffs)	32.59	-5.2%	-2.2%	1.0x	1.00x
p = 2, polynomialOrder 2	33.34	-3.0%	-2.2%	1.8x	0.96x
p = 3, polynomialOrder 3 (used here)	34.50	+0.3%	-0.4%	3.2x	1.03x

The first row is the default scheme rather than highOrderCoeffs with polynomialOrder 1. Both are first order and give essentially the same answer, differing only through the stencil size, so only the default one is listed.

Accuracy improves monotonically with order. The solid solver is genuinely 3.2x more expensive at p=3 (solid CPU time 5023 s -> 15913 s), but the end-to-end runtime is essentially unchanged, because in this tutorial the fluid participant dominates and the extra solid work hides behind the coupling. Wall-clock times were 28594 s, 27315 s and 29345 s respectively — within the noise of other jobs sharing the node.

I therefore propose polynomialOrder 3, but the trade-off is documented in the README and switching is a one-line edit, so please say if you would rather ship a cheaper solid. Note that polynomialOrder 2 is both cheaper and more accurate than no high-order reconstruction at all, so the plain low-order option is dominated.

One observation independent of the solid settings: all three variants give an oscillation frequency of 5.50-5.64 Hz against the benchmark's 5.3 Hz. Since polynomial order does not move it, that offset comes from the fluid side or the coupling rather than the solid discretisation.

Constitutive law

Flagging a discrepancy that predates this PR and is worth a decision. This case uses a compressible neo-Hookean law (neoHookeanElastic), which matches the deal.II participant (set Model = neo-Hookean) and the HronTurekFsi3 case that solids4foam ships. However, the original Turek-Hron benchmark specifies a St. Venant-Kirchhoff solid, and that is what the Nutils participant implements (S_ij = lambda E_kk delta_ij + 2 mu E_ij). So the three solid participants in this tutorial are not currently consistent with each other on the material model.

At the strain levels reached in FSI3 the two laws are very close, and the p = 3 result above lands within 0.35% of the benchmark amplitude, so this does not look like a source of error here. But if you would prefer the solids4foam case to follow the benchmark definition, StVenantKirchhoffElastic is a drop-in replacement in constant/mechanicalProperties with the same E/nu entries, and I am happy to re-run the 15 s case with it and post the comparison. Say the word and I will do that rather than leave it as a note in the README.

Tested with OpenFOAM v2606, preCICE 3.4.1, solids4foam (development) and the OpenFOAM adapter from precice/openfoam-adapter#417.

Notes

• Requires solids4foam v2.4 or later built with PETSc, and an OpenFOAM-preCICE adapter with support for reading Stress in solid participants.
• The reference-results tarball still needs to be generated; the tests/tests.yaml entry already points at the expected path.
• This PR and Add a solids4foam solid participant to elastic-tube-3d #914 / the multiple-perpendicular-flaps PR each add an entry to the solids4foam test suite in tests/tests.yaml, so whichever merges last will need a trivial rebase there.

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