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Author	SHA1	Message	Date
Omar SobhandClaude Opus 5	bfd9f4dfd2	rtx-cfd: repair the pressure-velocity coupling, LBM walls and mesh quality ... Clears the rest of the quarantine. All three crates now run 558 tests with 0 failures and no `#[ignore]` markers. SIMPLE could not converge, and the reason was not slow convergence but wrong physics. The pressure correction equation used a bare Laplacian, 1/dx^2 and 1/dy^2, while the velocity correction divided by a_p = rho dx dy / dt. SIMPLE requires these to be each other's inverse: substituting the corrected velocities into continuity must reproduce the pressure equation, which fixes a_E = rho d dy/dx with d = dV/a_p. The two disagreed by roughly 1/(h^2 dt) -- about 2e4 on a 16x16 cavity -- so the pressure correction was that many times too weak to enforce continuity. The consequence was visible and specific. A lid-driven cavity at Re=100 produced a monotonic profile rising from 0 at the floor to 1 at the lid: Couette flow, with no recirculation anywhere, and a peak pressure of 1.6e-4 against the rho U^2 scale of 1. The return flow in a cavity is driven entirely by the pressure gradient, so with the pressure pinned near zero there was nothing to turn the flow around. With the coefficients made consistent the profile recirculates, the peak pressure is 2.9, and the solver converges. Also in SIMPLE: - `p'` was never reset between outer iterations. It is a correction that `pressure_update_step` folds into `p`, so carrying it forward applied the same correction twice. - The convergence measure was the inner Gauss-Seidel residual, which goes to zero whether or not the flow satisfies continuity. Now the mass imbalance. - The velocity correction used only the transient part of a_p, `rho dV/dt`, rather than the diagonal the momentum equation was actually solved with. - All four convective face fluxes were computed from a single cell-centred velocity, so `fe` and `fw` were the same number, as were `fn` and `fs`. Upwinding then picked the same direction on opposite faces of the control volume. Now interpolated per face on the staggered grid. Not claimed: agreement with Ghia, Ghia & Shin (1982). The vortex centre moves toward their y = 0.4531 under refinement (0.400 at 16^2, 0.419 at 32^2, 0.460 at 64^2) but the minimum centreline velocity reaches only -0.130 against their -0.2109, and the converged field still depends slightly on the pseudo-time step, which a true steady state cannot. The cavity test therefore asserts what is established -- convergence, recirculation, vortex position, and an O(1) pressure field -- and the remaining gap is recorded in omni-cortex/docs/solver_status.md rather than papered over with a loose tolerance. LBM bounce-back was doing neither of the things its name claims. It was written as assignment (`f[2] = f[4]`) rather than a swap, discarding the population being reflected -- bounce-back is a permutation and conserves mass exactly, so the domain leaked 0.013% of its mass every 100 steps and would have kept draining. And the pairs used were 5<->8 and 6<->7, which reverse only the wall-normal component: that is specular reflection, a free-slip wall, so the no-slip condition the walls were supposed to impose never held. Mesh quality: - Quadrilateral aspect ratio included the diagonals in the maximum but not the minimum, so it could never return 1: a unit square reported sqrt(2) and a 2:1 rectangle sqrt(5). - Triangle aspect ratio used longest-over-shortest edge, which does not detect the failure mode that matters. A sliver with vertices (0,0), (10,0), (5,0.1) scores 2.0 -- indistinguishable from a healthy 2:1 triangle -- while its area is a twentieth of what its edges suggest. Now the radius ratio R/2r, which is 1 for equilateral and 1250 for that sliver, and which also fixes the quality histogram. - StructuredMesh aspect ratio took bounding-box extents and guarded the z-extent with `.max(1e-10)`. On a 2-D mesh the depth is exactly zero, so the guard became the minimum and a unit square reported 2e10. Mesh refinement produced meshes that failed their own validation. `subdivide_triangle` reserved midpoint ids as `next_node_id + k`, then advanced the counter by 3, after which `refine_cells` called `add_node` and advanced it three more -- so every refined cell referenced vertices three ids away from the ones actually created. Separately, the position lookup selected by slot rather than by id ("This is simplified, should look up correct midpoint"), so three of four sub-triangles had their areas computed from the wrong points; the quadrilateral version mapped every new id to the cell centre. Fixtures corrected rather than tolerances loosened: a structured mesh test asserted 0.16 for the average cell volume while the comment beside it computed 0.25 from the node-count convention the code actually uses; the Zou-He pressure test built a *velocity* boundary at u = 1.2, far above the lattice speed of sound, making the density negative; and the cavity-setup test required the lid to influence the domain centre 16 rows away in 10 steps, which exceeds the lattice propagation speed. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>	2026-08-19 08:40:09 -07:00
redclawsystems	4d88dc0584	Initial commit	2026-03-04 00:08:42 +00:00