- tests/fsi2_embedded3_impulse.rs (new, ignored, cuda): the added-mass timing instrument — the R8-a fluid
with a prescribed centreline velocity step (a one-step acceleration delta), baseline and impulse passes
from one device snapshot, the load difference per step. Pitch shape on the slab ny 62: generalized added
mass 39.11 / 39.09 / 39.07 kg/m at dt x 1 / 0.75 / 0.5, the step after carries 1.7 / 1.3 / 0.9 % above
the steady tail — the cut-cell fluid answers in the same step (the lagged-added-mass hypothesis refuted).
- fsi2_embedded3.rs: `RTX_E3FSI_LINE_CSV` (per-step centreline + mid/bottom/top y per station) showed the
growing 2dt mode is a through-thickness mode — the edge nodes move against the mid-plane nodes (~4:1):
the fluid reads the mid-plane, the loads land on the edges, so the added mass acts on that mode with the
wrong sign (a negative effective added mass; the scalar Newmark model is unstable for it at omega dt
below ~1-2 with gamma 0.7, stable at larger dt or gamma 0.9 — R8-a's observed pattern).
`RTX_E3FSI_KIN=surface` (default off, byte-identical when unset): the fluid's centreline = the mean of
the bottom and top edge nodes (the nodes the loads land on) — dt x 0.75 / 0.5 / 0.25 stable at gamma 0.7.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
