SUPERYACHT #9
Summer 2006
Article selected from our quarterly magazine dedicated to the largest
and most luxurious boats with information, interviews, technical
articles, images and yachting news
Summary
Subscription
Yachting catalogue
Navigation tests
Used boats
Boatshow
Video Nautica
Article by
Mario Felli
|
|
Optimisation and experimentation:
new frontiers of ship design
Development of a ship design involves careful evaluation of
technical solutions aimed at maximising vessel performance.
Setting out from design-related inputs, typically linked to
dimensions, volume, displacement and maximum working speed, the
choice of type and form of the ship's bottom - together with
propulsive system and appendages - there is often a clash between
aspects concerning hydrodynamic performances and requisites of
manoeuvrability and safety. For example, hydrodynamic design has
essentially regarded aspects linked to both reduction of
resistance to progress and to wave resistance (see A. Sinisi's article
"The Rounded Ship's Bottom" within this issue), employing remedies
to reduce the rough surface of the quickwork (flush welds,
sandblasting, low friction coefficient paints and air lubrication
systems, which have resulted in a friction resistance of around
80% in comparison with vessels built in the early post-war period)
and investigation into hull conformations such as to minimise
energy dissipated due to the formation of wave motion.
Noise sources and levels typical of a Superyacht. The
problem of vibration and noise reduction in passengers' quarters
(comfort and sound fatigue) and with regard to the external
environment (sound pollution) is one of the main aims of modern
design approaches.
In recent years however the growing need to increase comfort
aboard, reducing noise emission and the extent of induced
vibrations and hull oscillation, has led to increased attention
being paid to such aspects, especially with regard to the
interaction with the propulsive system and the profiling of hull
appendages. Incorporation of these aspects into the design
specifications brings a number of advantages, mainly linked to the
possibility of modulating comfort requirements with the typical
vessel performance parameters (minimum resistance, propulsive
efficiency, manoeuvrability), intervening with possible solutions
within the optimisation process. In this way you avoid a
posteriori remedies to "rehabilitate" any failures of the finished
vessel in corresponding to design specifications, remedies which
are wholly empirical, dictated by the designer's experience and
often resulting in higher building costs, not to mention delays in
delivery time.
In this scenario, design increasingly becomes a process of
integration and mediation between operative needs and solutions
that are often contradictory and where it is necessary to
simultaneously keep in mind all the problems regarding the vessel-
system. This itinerary is a critical point in the design of a
vessel in which control of the said problems isn't often possible
from a practical viewpoint. Structural problems cannot be
separated from those of hydrodynamic seakeeping and resistance to
vessel progress, just as those of operational management of the
vessel cannot be separated from aspects linked to comfort and
performance.
Measurement of the wave profile on the initial
configuration and optimised by the bow-bulb. The optimised bottom
has reduced resistance (less pressures on the bulb) and a more
contained wave formation.
In the past, achievement of design requirements was based solely
on the designer's experience and intuition, with an interactive
"trial and error" approach which, however, with regard to negative
fallout on costs and development times and analysis of the various
alternatives, did not guarantee an optimal solution. This
situation laid the foundations for development of a new design
approach in which the support of mathematical and calculation
tools meant that the problem could be tackled automatically,
probing all alternative possibilities until an "optimal" solution
was achieved. The design problem was thus transferred to the
isolation of a grouping of variables that characterise the
phenomenon, to the choice and implementation of an optimisation
algorithm as well as analysis of performances aimed at
verification of correspondence with design requirements. This
phase is a complex and subdivided process in which the designer's
experience cannot be separated from the synergic backup of Test
Tank experimentation and the aid of specific numerical codes.
Seakeeping tests carried out at the INSEAN rectilinear
basin. The study of the impact of wave systems on the hull is
highly important for increasing safety and comfort levels on board.
Modern design approaches are based on this synergy, approaches
inherited from the aerospace sector and today widespread
especially in niche situations of marine engineering (racing
boats, military vessels) where the optimisation process based on
the use of numerical codes is integrated with the backup of
advanced experimental techniques for their validation and
performance verification. The optimisation algorithms used in the
past were prevalently concentrated on the solution of isolated
problems, without considering overall improvement of the vessel-
system or any effects deriving from "conflicting" solutions. The
weak point of that approach was therefore a partial vision of the
actions and effects brought to bear on the vessel-system,
neglecting possible interactions between "conflicting" solutions.
On this matter the question of anti-roll appendages and
stabilising fins may be taken as an example of how important it is
to tackle all design and operational aspects of the vessel-system
globally: the sizing of these appendages should be done
reasonably, not only looking at optimisation of vessel performance
in terms of stability but also, for example, at hydrodynamic
effects (linked to increased resistance to progress) and
propulsive effects (any interactions between vortexes released by
the appendages and the stabilising fins with the propulsion).
Another typical example of conflicting interaction between design
requisites is the phenomenon known as "whipping", which refers to
a bothersome resonant phenomenon determined by small but continual
impacts of the stern with the water surface. The "whipping" effect
is especially evident when the vessel is stationary: in this
situation the active control systems are completely inefficacious
and only an efficient stern design can reduce the phenomenon.
Optimisation however must mainly take account of the vessel's
performance under way, so the two opposite tendencies of raising
the position of the transom to reduce vibrations or lowering it to
increase performance must be tackled with a global design
philosophy aimed at optimisation. In the same way we should point
out cases in which a hydrodynamically optimised hull, with a
minimised resistance to progress and vertical motion, has
nonetheless demonstrated a net reduction in propulsive efficiency
with a negative fallout in terms of performance.
In recent years researchers at INSEAN (National Institute for
Studies and Experiences in Marine Architecture) have developed
sophisticated automatic calculation instruments for hydrodynamic
optimisation of vessels, by considering the vessel-system as an
overall grouping of linked hydrodynamic sub-problems which can
globally and efficiently model the complexity of interactions
between the marine environment and the vessel.
With this approach the characteristics required for a new design,
defined in terms of objectives (e.g. reduction of friction and
form resistance, increase in propulsion efficiency, improvement of
performance in manoeuvre and/or in a heavy sea, diminution of
accelerations, reduction of broad amplitude motions) and
operational restrictions of a geometrical
(e.g. vessel dimensions, displacement, payload volumes)
or functional (e.g. maximum height of wave profile, minimum distance between
the whirling structures of the stabilising fins of the propulsion shaft)
type are described through a mathematical expression known as objective
function. In this way you can automatically analyse all the best
configurations that satisfy design aims in respect of operational
limitations and set out from an initial geometrical configuration
of the hull. Of course the number of solutions to the optimisation
problem will depend on the number and type of the objectives and
limitations set. In some cases, in particular, the problem of
optimisation will be without solution and it will be necessary to
pass on down to solutions of compromise, partially remodelling the
objectives or limitations which nevertheless must be formalised
and evaluated in the virtual reality of the CFD simulation, and
therefore with a negligible fallout on design time and costs. A
more detailed overview of aspects concerning the design of a
vessel and the related experimental verification will appear in
the next number of "Superyacht" with a description of the main
problems and methodologies for tackling development of an
optimisation algorithm and of the most advanced experimental techniques.
Reduction of the total resistance of an optimised hull
(progress in blue) with respect to the original design (progress in red).
GLOSSARY
Laser Doppler Anemometry. Experimental technique for measuring
field of velocity in a fluid medium. The Laser Doppler anemometer
is an instrument that precisely measures the velocity field in a
fluid with high accuracy and without perturbing the field of motion.
CFD. (Computational Fluid Dynamics) is a discipline of applied
physics that came into being around the 1960's. As the name
suggests, it is a reference to the study, by computer, of the
dynamics of a fluid in which there may be physical phenomena such
as heat exchange, acoustic radiation, vibration transmission,
cavitation etc.
Validation. Validation of CFD models is the experimental
verification of results obtained, carried out through numerical
simulation of an assigned engineering problem. This process of
"certification" of simulation quality is generally done by
comparing the values of the numerical model output variables with
those measured experimentally, under the same operational
conditions and at corresponding points in the field.
Whipping. Hydroelastic phenomenon due to the impact of the stern
on the water surface which brings about a resonant vibration in
the hull. This phenomenon causes longitudinal flexions of the hull
(Figure below) which have a negative influence on comfort aboard.
Deformation of a hull following the "whipping"
phenomenon (deformation has been increased to better describe the phenomenon).
|
