CelerisLab/docs/validation/leontini_viv_predeclare.md

Single-cylinder free transverse VIV — PREDECLARATION (agreed at 1abfe954)

Stage goal (Yanqi via TRON): prescribed Dütsch case + one VIV case both "reasonably close" to a predeclared reference = development-stage success, then cleanup. The frozen Dütsch settling failure (0/28 pairs at 31.25 T) stays on record. Practical closeness is not a formal validation claim. Agreed by Claude, Grok and Astra at 1abfe954; implemented in research/viv_re150/.

Primary case (one case)

• Reference: Qin et al., arXiv:2003.12186, §4.5 (maximum amplitudes at U_red = 3…8: .075, .551, .520, .459, .370, .084 D in the text, as quoted by Grok/Astra) for the target; setup from Kolahdouz et al., arXiv:2003.12046, §4.1.1 (read 2026-10-30 via ar5iv): U* sweep 3–8 with amplitudes only in Fig. 3(b) (no text values), domain [−30, 45] D × [−30, 30] D with the cylinder "initially at rest and centered at the origin", uniform inflow on the left, zero normal traction and tangential velocity at the outlet, zero normal velocity and tangential traction on y = ±30 D; no initial perturbation and no total simulated time stated.
• Re = 150, U* = U/(f_n D) = 5, m/(rho D²) = 2 (m* = rho_s/rho_f = 8/pi), zeta = 0, f_n = sqrt(K/M)/(2 pi) in vacuum. Transverse DOF only.
• Target: A_max = .520 D (peak amplitude, Qin §4.5). Multi-code reference (Kolahdouz ILE-NS, Qin II-LBM). Reference uncertainty: not stated; the spread between codes is not quantified here.

Our reproduction (declared differences)

• D = 40, MRT, dense double buffer; U_lat = .04, nu = U D / 150 = .010667, tau = .532.
• Box [0, 75 D] × [0, 60 D] (3000 × 2400) with the cylinder centre at (30 D, 30 D): 30 D from the inlet, 45 D from the outlet, centred in y (Kolahdouz §4.1.1). Qin's own box ([0, 40 D] × [0, 20 D], centre (10 D, 10 D)) is the target paper's, not ours.
• Boundaries: uniform Zou–He inlet, NEQ outlet (rho = 1, backflow clamp), free-slip y walls. Kolahdouz uses a traction outlet: declared mismatch.
• Qin's own VIV box (40 D × 20 D) is not used; a smaller box would be a declared cost cut with more blockage, not "the same case".
• Allowed y offset ±1.5 D; peak transverse speed ≈ .026 cells/step (bound .5).

Structure, coupling and time levels

• M = 2 rho D² (lattice rho = 1), f_n = U/(5 D) = 2e-4 per step, K = M (2 pi f_n)², zeta = 0. Equation of motion uses the structural mass only.
• Per-step sequence (binding): known (y_n, v_n) and the last completed force F_last → candidate (y_{n+1}, v_{n+1}) by the update below → set_body with offset y_{n+1} and wall velocity (y_{n+1} − y_n)/dt with lattice dt = 1 (interval average; a different quantity from the endpoint v_{n+1}) → run(1) → only on success: commit y_n ← y_{n+1}, v_n ← v_{n+1}, F_last ← force read for that step (failure semantics: see the window section). Whether the measured force represents the interval's start, midpoint or end is not asserted; the one-step lag is declared, not corrected.
• Initial state: fluid initialized by Simulation.initialize() to the uniform inlet velocity U everywhere (the code's actual initial condition; the source gives no fluid initial condition), body at rest at the declared centre, y_0 = v_0 = 0, F_last = 0.
• Structural update over one step with constant force F = F_last (exact harmonic solution, zeta = 0): y_e = F/K, w = 2 pi f_n, y^{n+1} = y_e + (y^n − y_e) cos w + (v^n / w) sin w, v^{n+1} = −(y^n − y_e) w sin w + v^n cos w.
• Primary RHS (single coupled run): CME + Wen (theory-motivated stage choice: Galilean-invariant link correction; G4 could not separate it from CME). Raw CME and the node terms are logged as diagnostics of that trajectory; the node impulse is not added. A run with CME-only as RHS is a separate, labelled sensitivity case, decided separately. Neither RHS is claimed proven.

Window, failure semantics, acceptance (engineering, development stage)

Clocks: lattice dt = 1; T_n = 1/f_n = 5,000 steps. Response cycles are counted from the measured signal; T_n is used only for the seed check.

• Start: body at rest at the declared centre (the only initial condition the source states), y_0 = v_0 = 0, F_last = 0; fluid uniform at U (as initialized by the code; our choice, not the source's).
• Seed (conditional, one time, declared): at step n_seed = 200,000 (t = 40 T_n), if max |y_t| over states t in [175,000, 200,000) is below .01 D, then in that step, after the check and before the candidate update, set v_n += 2 pi f_n (.01 D) (F_last unchanged) and proceed normally; the geometry moves continuously through the update. seed_applied, step and size are logged; never repeated or changed.
• Cycle boundary (discrete, fixed threshold): step n closes a cycle iff y[n−1] < 0 and y[n] ≥ 0 (upward crossing of the equilibrium y = 0); y_0 = 0 is not a crossing; no boundary fewer than 2 samples after the previous one.
• Per cycle: positive peak max(y), negative peak min(y), mean offset (time-mean of y), amplitude a = (max − min)/2 (the only amplitude used for settling).
• Eligibility: a completed cycle is counted (toward the settling proof and the 150 budget) only if a ≥ .01 D; ineligible cycles are logged, never counted, so near-zero motion cannot settle or pass.
• Seed reset: if the seed is applied, the incomplete cycle at n_seed is discarded and the settling proof starts empty; the 150 budget and the native cap keep running from t = 0.
• Windows: adjacent non-overlapping blocks of 10 counted cycles; W_k = mean amplitude of block k (W_k ≥ .01 D by eligibility, so no zero denominator).
• Block rule R(k), k ≥ 2: |W_k − W_{k−1}| / W_{k−1} ≤ .02 and, for the last 5 cycles of block k, max_i |a_i − ā_5| / ā_5 ≤ .01 with ā_5 their mean.
• Settled at the end of block k when R(k) holds. Scoring = the next two blocks k+1, k+2 (20 counted cycles); both must satisfy R. If one fails, that block becomes the new reference: the settling proof restarts from it and continues within the caps (deterministic; no other retry). A pass requires one complete stable 20-cycle scoring window before a cap; success stops after its 20th cycle.
• Caps (both always armed; the first hit wins and is reported): 150 counted response cycles including the 20 scoring cycles (so settling must complete by counted cycle 130) → timeout_response_cycles; 1,000,000 native steps (200 T_n) → timeout_native_steps.
• Durations reported: total steps, t U/D, counted and uncounted cycles, measured T_resp (mean boundary interval); N_resp / f_n only as a labelled secondary clock.
• Growth is not a failure: initial VIV growth is physical. Checked every step (FAIL-CLOSED): non-finite y, v or force; |y| > 1.0 D (the tighter gate; about twice the largest reference peak, .551 D) or the ±1.5 D offset box reached; no_donor > 0; a failed upload/step/sync.
• On any failure the driver stops and reports; it never retries, sub-iterates, retunes the RHS or parameters, or starts a second case. Device-side failures (upload/step/sync, no_donor) mark the Simulation failed through the existing lifecycle; input rejection by set_body (offset box, speed, step bound) and timeouts end the driver run without that mark.
• Target metric over the 20 scoring cycles: A_max/D = max over those cycles of max(|positive peak|, |negative peak|) about y = 0 (Qin reports "maximum amplitude"); also reported: mean amplitude, positive and negative peaks, mean offset, f/f_n (no target).
• Acceptance: |A_max − .520 D| ≤ .052 D (±10 %), stated together with the boundary mismatch and the absence of a quoted reference uncertainty. A pass is "reasonably close" for the stage: not grid convergence, not force-RHS validation, and it does not touch the Dütsch settling record.
• Driver reads force and the small diagnostics per step only; field reads only at a few predeclared snapshots. No runtime claim: there is no measured timing on this grid yet (the D40/60D receipt is a different, sampled case).

Plumbing smoke (not the benchmark)

run.py --smoke --D 20 --steps 32 runs exactly 32 steps of the same BC package and coupling loop (no seed, settle or score; never scored); status smoke_ok iff every step is finite, no_donor stays 0 and the body stays inside its box. The physical case is only the default invocation (--D 40, no --smoke).

Queued, not in this stage

Leontini et al. JFS 22 (2006) Re = 200, m* = 10, zeta = 0.01, U* = 5.0 (needs the owners' c_crit convention and a Fig. 1 digitization). D80, literature claims, pinball, AMR, BC redesign, threshold changes.