The particular difficulty of the ship resistance problem is this: wave resistance depends on the Froude number, frictional resistance on the Reynolds number. In a scaled model the two cannot be matched simultaneously. The model test is run at Froude similarity and the frictional component is scaled separately to full scale by a procedure.
CFD can close that gap, because it can be solved directly at full scale. The price is that the boundary layer has to be resolved at the full-scale Reynolds number — which sets the mesh requirement directly.
The ITTC framework
The International Towing Tank Conference publishes recommended procedures for ship CFD applications; these documents place domain definition, mesh, solution and verification steps on a common footing [1]. Following that framework is not a preference but a practical necessity if results are to be comparable.
Free surface and trim
A ship problem is by definition a free-surface problem, and it brings two additional difficulties:
Resolving the wave system. The Kelvin wave pattern radiates away from the hull. The mesh has to resolve the wavelength with enough cells; at insufficient resolution the waves are damped numerically and the wave resistance comes out low. Unless a deliberate damping zone is left at the outflow side of the domain, waves reflect off the boundary and corrupt the solution.
Trim and sinkage freedom. A ship is not fixed; as speed rises it pitches and sinks. Locking these degrees of freedom gives the wrong resistance, particularly at high Froude numbers. Released, the problem becomes a two-way coupled moving-boundary problem: the flow sets the force, the force sets the attitude.
The propeller: open water, or behind the hull?
For an open-water curve a single-passage periodic model in steady state is sufficient. But a propeller works in the wake of the hull, and the flow arriving there is not uniform. Blade loading fluctuates through the revolution, and cavitation begins at the peak of that fluctuation.
The efficiency question is therefore cheap and the cavitation and vibration question expensive — the latter requires the hull-propeller interaction to be solved transiently. See the rotating machinery note and the cavitation note.
References
- International Towing Tank Conference. ITTC Recommended Procedures and Guidelines 7.5-03-02-03: Practical Guidelines for Ship CFD Applications. ITTC.
- Celik, I. B., Ghia, U., Roache, P. J., Freitas, C. J., Coleman, H. & Raad, P. E. (2008). Procedure for estimation and reporting of uncertainty due to discretization in CFD applications. Journal of Fluids Engineering, 130(7), 078001. DOI: 10.1115/1.2960953