Practical Ship Hydrodynamics

Ship manoeuvring 169
the difference between symmetrical and asymmetrical flow is linearized.
The asymmetrical flow is then determined by a lifting body method with an
additional source distribution above the free surface.
ž Field methods
In spite of the importance of viscosity for manoeuvring, viscous hull force
calculations appeared in the 1990s only as research applications and were
mostly limited to steady flow computations around a ship with a constant
yaw angle. Difficulties in RANSE computations for manoeuvring are:
– The number of computational cells is much higher than for resistance
computations, because both port and starboard sides must be discretized
and because vortices are shed over nearly the full ship length.
– The large-scale flow separation makes wall functions (e.g. in the standard
k-ε turbulence model) dubious. But avoiding wall functions increases the
necessary cell numbers further and deteriorates the convergence of the
numerical solution methods.
State of the art computations for ship hulls at model scale Reynolds numbers
were capable of predicting transverse forces and moments reasonably well
for steady flow cases with moderately constant yaw angle, but predicted
the longitudinal force (resistance) with large relative errors. Flow details
such as the wake in the aftbody were usually captured only qualitatively.
Either insufficient grid resolutions or turbulence models were blamed for the
differences with model tests. By the late 1990s, RANSE results with freesurface
deformation (waves) were also presented, but with the exception
of Japanese research groups, none of the computations included dynamic
trim and sinkage, although for shallow water these play an important role
in manoeuvring.
Despite these shortcomings, RANSE computations including free-surface
effects will grow in importance and eventually also drift into practical
applications. They are expected to substantially improve the accuracy of
manoeuvring force predictions over the next decade.
5.3 Experimental approaches
5.3.1 Manoeuvring tests for full-scale ships in sea trials
The main manoeuvring characteristics as listed in the introduction to manoeuvring
are quantified in sea trials with the full-scale ship. Usually the design
speed is chosen as initial speed in the manoeuvre. Trial conditions should
feature deep water (water depth > 2.5 ship draft), little wind (less than Beaufort
4) and ‘calm’ water to ensure comparability to other ships. Trim influences
the initial turning ability and yaw stability more than draft. For comparison
with other ships, the results are made non-dimensional with ship length and
ship length travel time (L/V).
The main manoeuvres used in sea trials follow recommendations of the
Manoeuvring Trial Code of ITTC (1975) and the IMO circular MSC 389
(1985). IMO also specifies the display of some of the results in bridge posters
and a manoeuvring booklet on board ships in the IMO resolution A.601(15)
(1987) (Provision and display of manoeuvring information on board ships).
These can also be found in Brix (1993).