
ID	0061
Class	BUG
Severity	1
Status	blocked

Models: Cable

Found: Deep review 2026-09-28, worktrees/nethydro/review/deep/other-hydro-forces.md OH A8, at 31210e5

Decision needed: Owner (Karl-Johan), review owner question 4: which constants of the Cd(Re) fit are the intended ones (+0.01646 here and in CableRM, +0.1646 in fhsim_marine_elements TrawlCableConstraintSets.cpp:450)? One shared function?

Evidence

src/cable/Cable.cpp:385: Cd = 1.3 + 8.85 / (pow(Re, 0.85) + 0.01646) for Re < 14, then 0.75 + 2.35 / (pow(Re, 0.38) - 1.67). At Re = 14 the branches give 2.24 and 2.98, a 33 % step. At Re = 0 this fit gives Cd = 539; with +0.1646 it gives 55. Both versions have the step at 14.

Effect

Only at very slow relative flow (Re < 14, below 0.7 mm/s for D = 0.02 m): start-up, slack lines, stagnant points in wakes. The step also makes the drag Jacobian discontinuous there.

Possible fix

One shared, continuous fit with the owner's constants.

Test that would prove it

Cd is continuous at the branch limits.

Risk

Changes results only at very small relative velocity.