Practical Ship Hydrodynamics

18 Practical Ship Hydrodynamics
modelling of the propeller. They discretize the mean camber surface of the
propeller blade by individual vortex panels. In addition, the free vortices
are modelled by elements of given strength. Other than the BEM described
below, lifting-surface methods do not fulfil exactly the boundary conditions
at the blade’s lower and upper surfaces. However, the resulting errors
are small for thin blades. BEM represent an improvement concerning
the treatment and modelling of the geometry. BEM model both lift and
displacement of the propeller blades by surface panels and/or dipoles.
They can also model the propeller hub. Despite the theoretical superiority,
BEM results were not clearly better than lifting-surface methods results
in benchmark tests. BEM codes for propeller applications often use only
dipole panels which are distributed over hub, blade surfaces, and the
wakes of each blade. Application of viscous flow CFD methods approached
the threshold from pure research to practical applications by the mid-
1990s.
Further, less frequently found applications of CFD in naval architecture
include:
ž Air flow
Only a few CFD applications are known for the computation of the air flow
around the upper hull and superstructure of ships and offshore platforms.
Topics of interest are:
– Wind resistance (especially of fast ships)
For fast ships the wind resistance becomes important. For example, for
one project of a 50 knot SES (surface effect ship D air-cushion catamaran),
the wind resistance constituted ca. 25% of the total resistance.
Hull changes limited to the bow decreased the wind resistance by 40%.
– Wind-over-the-deck conditions for helicopter landing
This application concerns both combatants and offshore platforms.
– Wind loads
Wind loads are important for ships with large superstructures and relatively
small lateral underwater area, e.g. car transporters, car ferries,
container ships, SES, and air-cushion vehicles.
– Tracing of funnel smoke
This is important for passenger vessels (passengers on deck, paintwork)
and for offshore platforms (safety of helicopter operation).
The comparison of CFD, wind-tunnel tests, and full-scale measurements
shows an overall good agreement, even if large discrepancies appear at
some wind directions. The differences between CFD and model-test results
are not generally larger than between full-scale and model-scale results. In
fact, the differences are not much larger than often found when the same
vessel is tested in different wind tunnels. The determination of wind loads
on ships and offshore structures by CFD is a realistic alternative to the
experimental methods. However, due to the time involved in generating the
computational mesh and in computing the solution, CFD was, at least until
the year 2000, not economically competitive to routine wind-tunnel model
testing.
ž Interior flows
Inner flow problems are seldomly treated by naval architects. Exceptions are
research reports on flow calculations for partially filled tanks in a rolling