0116 — Auv has its own axial and cross-flow drag outside the environment laws (ADR 0002)

ID	0116
Class	ARCHITECTURE
Severity	1
Status	blocked

Models: — (annotated on the fhsim_fishery library group)

Found: 2026-10-20, hydrodynamics overview section 7 (doc/hydrodynamics-overview.md in fhsim_workspace), feature m2-hydro-doc-gaps

Decision needed: Owner: migrate the Auv drag to the environment cylinder laws, or keep it? Auv has axial and cross-flow drag of an AUV hull and fins, which may be a different model (not a circular cylinder); if kept, say why in the header.

Evidence

src/auv/Auv.cpp:461-500 has its own axial drag (Auv::AxialDragWrench, axialDragCoeff on frontalArea) and its own cross-flow section drag (sectionDrag * crossFlowSpeed * crossFlowY). fhsim_environment ADR 0002 says one copy of each law lives in the environment.

Effect

The AUV drag cannot share fixes or the sourced coefficients of the env laws. It may be deliberate: the hull is not a circular cylinder in every part and the model has its own fitted coefficients.

Possible fix

Compare the formulas with cylinder_hydro::DragForce (and the env body laws). If they are the same law with other coefficients, call the env law and pass the coefficients. If not, record the reason in the Auv.h header comment and close this issue.

Test that would prove it

The existing Auv tests, byte compared; for a migration, a case where both formulas give the same load.

Risk

Medium for a migration; none for keeping.