docs: Dütsch Re100/KC5 prescribed-oscillation protocol draft

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation/duetsch_re100_kc5_protocol.md

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+# Prescribed in-line oscillation: Dütsch Re 100 / KC 5 — PROTOCOL DRAFT (not frozen)
+
+Production under test: `claude/moving-bodies-s3a` at `653fd997` (unchanged).
+Primary source: Dütsch, Durst, Becker & Lienhart, J. Fluid Mech. 360, 249–271
+(1998), doi:10.1017/S002211209800860X. Items marked ⟨verify⟩ must be taken from
+the paper itself (Grok) before freezing; nothing here is read from plots.
+
+## Criteria (kept separate)
+
+- Same-grid frame consistency (G4, frozen 2%): already evaluated; not reused here.
+- Cross-resolution criterion: every reported observable's error against the
+  reference, and the fine-pair difference, must decrease monotonically over the
+  resolution ladder. No numeric grid threshold and no order label is added.
+- Reference agreement: only against values with provenance (section 6).
+
+## 1. Kinematics
+
+- In-line only: `x(t) = -A sin(2 pi t / T)`, `u(t) = -U_max cos(2 pi t / T)`
+  ⟨verify sign and phase origin⟩; `KC = U_max T / D = 2 pi A / D = 5`,
+  `Re = U_max D / nu = 100`, so `A = 5 D / (2 pi)`.
+- Prescribed per step through `set_body(offset, vx)` + `run(1)`, offset and
+  velocity evaluated analytically at step n (no differencing).
+- Start: the paper's motion from rest ⟨verify Dütsch start⟩. Measured window:
+  whole cycles 4–6, fixed now, never shortened afterwards. A one-cycle smooth
+  ramp is a separately named variant that only checks start independence.
+
+## 2. Resolution ladder (uniform only, fixed Mach)
+
+| D (cells) | U_max | nu_lat | tau | steps / period |
+|---|---|---|---|---|
+| 20 | .04 | .008 | .524 | 2,500 |
+| 40 | .04 | .016 | .548 | 5,000 |
+| 80 | .04 | .032 | .596 | 10,000 |
+
+- MRT collision (SRT at tau ≈ .524 is the recorded unstable regime).
+- Mach check: U_max .02 vs .04 at D = 40 (separate, labelled).
+
+## 3. Domain and boundaries
+
+- Square still-fluid box, default 40 D; domain check 20 D vs 40 D at D = 40
+  ⟨verify Dütsch domain extent⟩.
+- Boundary family declared by the runner owner before running (x-periodic /
+  y free-slip or closed walls, whatever the dense runner supports without
+  production change); swept band ±A + R + 2 clear of any seam.
+- Per-step displacement ≤ .04 cells (bound .5).
+
+## 4. Observables (raw, per step, preserved)
+
+- `obs` Fx, Fy (CME) and the diagnostics columns (`MovingTables.DIAG`):
+  CME, CME + Wen, CME + cover − uncover reported separately; crossings,
+  eq_only, no_donor.
+- Velocity u, v on lines at the paper's x-stations relative to the mean
+  cylinder position and at its phase angles ⟨verify stations and phases⟩;
+  cell-centred values, documented bilinear interpolation in physical units.
+
+## 5. Metrics
+
+- Morison C_D, C_M by least squares over the measured cycles with the paper's
+  force definition and sign ⟨verify formula⟩; per-cycle RMS force; force
+  phase history; phase-resolved velocity profiles.
+- Unit map for each candidate stated with `k = rho dx^3 / dt^2` per span.
+
+## 6. Reference data provenance
+
+- Accepted: values from the paper's tables, or digitized figure curves with
+  figure number, tool and uncertainty recorded. Curves without provenance are
+  qualitative only. Values quoted by later papers are secondary until checked.
+
+## 7. Order
+
+Protocol review → test driver (no production change) → runs → physical
+validation verdict → only then single-cylinder VIV. No AMR, no pinball.