CelerisLab/docs/research/moving_boundary_force_spikes.md

Moving-boundary force spikes: evidence and source index

Updated 2026-10-31.

This note indexes supplied audited observations and primary-source extractions. It does not report a newly measured cure or implement an alternative method. Paper details below were supplied by the parent investigation; no new source retrieval or independent paper verification was performed for this note.

Current implementation and interpretation

• Celeris M1 reconstructs a fresh node as feq(rho_bar, u_wall) + donor NEQ.
• In exact arithmetic, the donor non-equilibrium component has zero mass and momentum when formed against the donor's own equilibrium moments.
• Re-projecting that same reconstruction onto the same wall velocity therefore does not, by itself, change its mass or momentum or supply a cure for spikes.
• Floating-point residuals should be measured separately from this algebraic fact.
• Peng's alternative differs upstream: extrapolate PDFs from three donors, then replace selected moments. It is not simply another projection of current M1.
• PPO interval-force observations and native per-step FSI right-hand-side forces are distinct signals; comparisons must identify which signal is being used.

Audited run observations supplied to this note

• D40 revision: e9213dd2cc9cfed0e8487a3cf4ebdd94d23dc64d.
• Window: first 97.303 D/U of physical time.
• Maximum displacement: max |y|/D = 0.304893.
• Maximum native force-coefficient increment: max |ΔCy| = 0.66688.
• D20 over the same physical-time window: max |ΔCy| = 1.80735.
• The largest D40 event involved 30 nodes.
• These are supplied audited observations, not measurements made for this note.
• Time steps and trajectories differ between runs. This comparison establishes neither spatial convergence nor a unique cause for the force spikes.

Evidence locations supplied by the investigation:

• Horsea: /tmp/celeris-results-pinball-resolution.
• Mac: validation-evidence/pinball-force-diagnosis.

These locations index run evidence, not local copies of the cited papers.

Tao, Hu, and Guo (2016): time-step decoupling

Computers & Fluids 133, 1–14. DOI: 10.1016/j.compfluid.2016.04.009. Publisher abstract.

• Abstract-level support only: non-equilibrium refill reduces oscillations.
• With local iteration, the compared refill methods give similar oscillation magnitudes.
• Time-step decoupling reduces oscillations further in the reported sedimentation, Couette, and Poiseuille examples.
• The exact decoupling schedule and time-step ratios have not been verified.
• This abstract does not establish equivalence to an arbitrary force filter.
• Do not implement a claimed reproduction until the actual methods are available.

Marson et al. (2021): local iterations

Physical Review E 103, 053308. DOI: 10.1103/PhysRevE.103.053308. Open repository record.

• §VI, p. 18 discusses pressure waves associated with fresh nodes.
• Appendix E.3, p. 25 and Fig. 18, p. 24 describe the local procedure.
• Copy a local sublattice, perform local collision and streaming, write the fresh nodes, and repeat before the global collision and streaming operation.
• One to three iterations performed best in the reported tests; this is not a universal iteration-count recommendation.
• The reported smoothed torque-noise assessment is qualitative here.
• Its thin-shell setting differs from the current moving-body configuration.

Peng (2016): extrapolation and moment replacement

Computers & Mathematics with Applications 72, 349–374. DOI: 10.1016/j.camwa.2015.08.027.

• §5.1.2, Eq. (50), p. 368: extrapolate PDFs along an outward normal direction: f_new(x) = 3 f(x + e) - 3 f(x + 2e) + f(x + 3e).
• §5.1.3, Eq. (51): wall equilibrium plus donor non-equilibrium is the family containing the current M1 reconstruction.
• §5.1.4, Eqs. (52)–(54): in MRT space, replace momentum with j = rho0 u_wall, preserve the other moments, then apply the inverse transform.
• A compressible Celeris adaptation must choose the appropriate density; copying the paper's rho0 blindly is not justified.
• Three valid donors require new eligibility checks and fallback behavior beyond the current one-link shared-body clearance rule.
• §§5.2–5.3 retain nonzero fluctuations in specific configurations; these results do not promise elimination of force spikes in the present problem.

Bounded next investigation

1. Retain current M1 as the baseline and compare matched prescribed trajectories over matched physical-time windows.
2. Record raw force RHS, node crossings, donor moments, mass and momentum changes, impulse, spectra, and trajectory response together.
3. Evaluate local iterations, Peng-style reconstruction, and force filtering as separate interventions, with explicit definitions and diagnostics.
4. Obtain the actual time-step-decoupling methods before implementing that method.

The current task indexes evidence only; it makes no alternative-method change.