fsi2_flag_modes_dump writes the march's structure operators (TL SVK tangent at u = 0, plane strain; consistent mass; root-clamped free DoFs) per mesh for an outside eigen-solve; fsi2_flag_free_vibration releases the march's NonlinearDynamicStepper from a mode shape at a given Newmark gamma and records the probe's uy(t). Nothing in the march changes. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>