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Author	SHA1	Message	Date
Omar SobhandClaude Opus 5	b5814a304f	rtx-cfd: manufactured solution finds the diffusion conductances were 1/h too ... large Applies MMS to the SIMPLE solver. It found a major discretisation error on the first run, which is the point of the method. The diffusion conductances read `mu / dx` and `mu / dy`. Finite volume requires `Gamma * A / delta` — the face area over the distance between the nodes it separates — so they should be `mu * dy / dx` and `mu * dx / dy`. The face area was missing entirely, making viscosity too large by a factor of `1/h`: sixty-five times on a 65x65 mesh. Every other term in the equation was already a force (`dp * dy` for pressure, `rho u dy` for the convective flux), so the mismatch was confined to diffusion. The consequence was that the solver ran at an effective Reynolds number far below the one requested. Before the fix the manufactured-solution error did not reduce under refinement at all — observed order about -0.05, because the spurious viscosity grows with the mesh. After it, the error falls monotonically. This also explains an apparent regression that is really a correction. The cavity vortex position moved from y = 0.484 to y = 0.391 against Ghia's 0.4531, which reads as worse agreement. It is not: a strongly over-diffusive cavity approaches Stokes flow, whose vortex sits near mid-height, so the old number was closer to the reference than the scheme deserved. Correcting the viscosity exposed the discretisation's own error. The test now states that disagreement plainly rather than asserting a band around the reference. What MMS reports now, and it is not yet good enough: n = 16 L2 velocity error = 2.586104e-1 order - n = 32 L2 velocity error = 1.797373e-1 order 0.52 n = 64 L2 velocity error = 1.277188e-1 order 0.49 First-order upwind should give 1. It gives about 0.5, and the u component is markedly further from exact than v on the same mesh. Both say there is at least one more defect in the discretisation or its boundary treatment, and the asymmetry between the two momentum equations is the clue. The test asserts only monotone error reduction — what is established — and records the shortfall, because asserting a rate the solver does not achieve would either redden the suite or invite someone to weaken it later. This changes the plan: raising the observed order to 1 is now a precondition for the second-order convection work rather than a consequence of it. There is no value in adding a higher-order scheme to a discretisation that has not demonstrated first order. Supporting changes: - `SimpleSolver::set_momentum_source` applies a volumetric body force, which is what lets a manufactured solution be imposed at all. - Divergence is now detected by growth, not only by NaN. The 8x8 case at Reynolds 10^6 reached 1e149 before anything caught it, because `is_finite` stays true right up until it does not. - `test_simple_solver_workflow` specified water properties on a unit domain, which is Reynolds 10^6 on ten cells: no steady laminar solution exists and the solver diverges on it, correctly. It passed only while the excess diffusion stabilised it. Now set to Reynolds 100. 561 tests across the three crates, 0 failing. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>	2026-08-19 12:25:52 -07:00