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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	d8a30db155	rtx-cfd: Ghia Re=400 as a quantitative claim — and the stopping-tolerance trap ... The Re=400 lid-driven cavity, 128^2 TVD on an exactly-unit domain, sampled on the staggered faces that lie exactly on the centrelines, against Ghia, Ghia & Shin (1982) — reference values sourced from two independent transcriptions that agree digit for digit (Mramor/Vertnik/Sarler CMC 2013 Table 1, and the ivan-pi benchmark collection): u_min -0.32667 at y = 0.2852 (Ghia: -0.32726 at 0.2813 — 0.18%) v_min -0.45024 at x = 0.8633 (Ghia: -0.44993 at 0.8594 — 0.07%) v_max 0.30044 at x = 0.2305 (Ghia: 0.30203 at 0.2266 — 0.53%) max |u - Ghia| over 15 profile stations: 0.0051 Ghia's own values carry ~0.3% discretisation error (Sahin & Owens 2003 put u_min at -0.32838), so this is at the reference's own accuracy. The finding worth the commit message: the first run used the Re=100 test's residual tolerance of 1e-4 and read u_min = -0.31987 — "converged", 2.3% shy — and refining to 192^2 made it WORSE (-0.30879, profile deviation doubled from 0.034 to 0.074). The residual had dropped below tolerance while the field was still developing, and the effect grows with mesh size because SIMPLE's per-iteration contraction weakens as h -> 0: at fixed residual tolerance the finer mesh stops at an EARLIER stage of convergence. Tightening the stop (3e-5, then 1.5e-5, until the movement per halving fell below the reference's own error) was the fix, and the test's bands are set so the premature-stop state fails all of them. "The residual converged" must never stand in for "the answer stopped moving". Co-Authored-By: Claude Fable 5 <[email protected]>	2026-08-19 23:23:32 -07:00