e x t r a m e g a y a c h t s
High-Performance Simulations
for High-Performance Vessels
by Prof. Dr Volker Bertram
and Axel Köhimoos,
Germanischer Lloyd
. . I f
•
i
CFD simulation of smoke dispersion. • • •
Introduction
Ship design has always been primarily an experience-based provide a fast method to investigate numerous parameters skill. While this is still true to some extent today, we increas- (wave length, wave direction, ship speed, etc.). Studying ingly rely on "virtual experience" drawn from computer sim- extreme motion patterns, particularly those involving slam-ulation. The rapid development of fast and unconventional ming, usually requires non-linear computational fluid dynam-ships since the 1980s would have been impossible without Ics (CFD) simulations, which place high demands on comput-advanced simulation technology. Simulation is especially
cru-cial in megayacht design where innovative and unusual
con-er resources and are limited to relatively short time pcon-eriods (seconds to minutes). Combining intelligently linear frequen-cepts requested by customers often involve additional risks, cy-domain methods with non-linear time-domain simulations
Megayachts have grown beyond 200 metres in length; this is takes advantage of the strengths of both approaches. This just one example of how designers push the limits. Whenever detailed analyses enables designs with considerably improved we leave the "comfort zone" of our experience and venture
Into new designs, simulations ensure that these designs are efficient and not only viable, but also safe.
Simulation has become a powerful tool to support better business processes within the megayacht industry by:
ensuring comfort;
increasing freedom of design;
achieving better designs in less time and
enabling fast and efficient trouble-shooting.
Ensuring Comfort
Comfort, a key requirement for a megayacht, comprises many aspects. The most important ones are discussed below from an engineer's perspective.
seakeeping behaviour and passenger comfort.
Aerodynamics affect noise and smoke propagation as well as local flow conditions on helicopter decks, sunbathing decks and similar locations. While wind tunnel tests are stil widely used, CFD offers the advantage of overcoming scale effects, which can be significant when thermodynamic pro-cesses are involved. CFD can be combined with formal
opti-misation, e.g. to minimise smoke dispersion on decks.
Ventilation system designs are usually based on simple estimates, which often result in over-dimensioned and ener-gy-inefficient systems and sometimes poor performance. CFD, on the other hand, delivers detailed insight, leading the way to a smarter layout.
Noise and vibration limits are standard elements of build-ing specifications for megayachts. High comfort expectations Seakeeping is vital for passenger comfort. For many sea- must be accounted for during the initial design stages. The keeping issues, linear analyses (assuming small wave height) simulation tools to support low-noise design are currently
"TURANOR PLANETSOLAR". The obtainable speed in
waves was predicted at the conceptual design stage.
being expanded to also include underwater noise, an aspect receiving growing attention in the megayacht industry.
Increasing Freedom of Design
Several projects illustrate how advanced simulation-based design methods increase the freedom of design for shipyards and owners. The lightweight design of the record-breaking trimaran "Earthrace" was only made possible by using
so-phisticated CFD simulations for the load and subsequent finite-element analyses of the composite hull structure. In
another pioneering project, the solar catamaran "Turanor PlanetSolar", the obtainable speed in waves - crucial for the
first circumnavigation of the world by a solar-powered ves-sel - was predicted as early as during the conceptual design stage. Yet another example is the largest megayacht of its time, which set new standards in terms of size and speed. In all these cases, highly innovative designs were implemented
based on high-performance simulations of both hydrody-namic and structural behaviour.
Composite materials are increasingly used in high-per-formance vessels, including megayachts. It is the combi-nation of low weight, high strength and mouldability that makes these materials attractive for designers. The choices in designing a particular component to suit its purpose are
often only limited by the development budget or the >
STERN OF A MEGAYACHT.
Original design (left)
and modified design for reduced slamming impact (right); slamming pressures were reduced by almost
30 per cent.
COMPOSITES. Structural analyses
of trimaran "LAMU II
> designer's creative imagination. Classical, "by-the-book" approaches to structural design do not work for high-per-formance, light-weight concepts. Prescriptive rules are often too inflexible, especially for advanced composite elements.
In addition, it is necessary today to give due attention to the recycling properties of composite materials, besides their strength aspects.
Achieving Better Designs in Less Time
Using CFD simulation to develop the hull lines of high-per-formance yachts has been standard procedure for years. For-mal optimisation, however, is a relatively new field of ap-plication. CFD simulation may improve the required power (and associated weight). Alternatively, the gain in efficiency may be used to increase the speed of the boat at the giv-en installed power. Optimisation requires considerable
com-3 8
puter resources due to the complex flow patterns and the enormous number of design alternatives to be investigated.
However, in our experience the effort is justified by the good results obtained, and CFD simulation is quickly becoming a standard tool in high-performance yacht design.
Simulations are similarly useful when designing prope -Iers, water jets, rudders and other appendages. On fast ships especially, these components are frequently affected by cavi-tation issues. Unfortunately, problems often remain unde-tected until the ship undergoes sea trials or has been in op-eration for some time. Even then, simulation can guide effi-cient troubleshooting and redesign, but avoiding cavitation-related problems in the first place by performing appropriate checks during the design stage is clearly preferable. Tradi-tionally, naval architects have resorted to standard baseline designs, statistical regression models and systematic series data as a basis for their design. Most of these data series.
however, are hopelessly outdated. Today, the FRIENDSHIP-Framework software offers an environnnent for compiling custom data series for numerical simulation.
To develop a parametric design, a family of design vari-ants can be created by systematically modifying the param-eters. The key benefit of this method is that it allows the designer to build a knowledge base ahead of an anticipated project. Once this has been done, the designer can quickly extract data from the numerical model series during the con-ceptual design stage.
Enabling Fast and Efficient
Trouble-Shooting
The first indication of a design problem, such as noise or vi-bration, is often encountered during sea trials. In some cases, however, it can take months or even years of operation, or a rare off-design situation, for a design flaw to become
appar-ent (e.g. fatigue cracks or cavitation erosion on rudders). In some cases, the diagnosis of the problem is straight-forward,
in others a more extensive analysis is required to pinpoint the root cause of a problem. Once this is known,
redesign-ing the affected structure or operational procedure is com-paratively easy. Simulation techniques are employed in both the diagnosis of the problem and the effective assessment of the redesign options. Vibration, including vortex-induced
vi-bration, is an area where simulation has advanced our proc-esses considerably.
Highly Skilled Experts
Make the Difference
Time and time again our advanced design projects have shown that simulation, when used skilfully, saves time and money.
Ideally, the parties involved should agree on the scope and
procedure for simulations at the earliest time possible. >
FLOW. Simulation for a fast twin-screw vessel. FRIENDSHIP F R A M E W O R K . Tailored design series for
megayachts. • r »
Vibration Level in Owner's Cabin
Ï t I t I 1 1 i mm/s ! "1 I-Vortex shedding-induced excitation depending on speed through water
m * • , . p Resonancejconditioh T t ^ E . I I ^ . . 1 .
r
i I i "1 • ^I)
t .L l 1 Natural frequency at 3.6 Hz : Machinery-induced excitationdepending on speed through water
8 10
Vibration Level in Owner's Cabin - After Modification
mm/s1.1
-Hz Hz
> Unless the shipowner (as the main benefactor) spe-cifically asks for simulations for design optimisation in his
project specifications, time and budget constraints will of-ten prevent them from being carried out. Furthermore, the mere use of advanced simulation software does not guaran-tee that true optimisation will be achieved. More than ever
before, engineering involves the art of modelling and finding the right balance between the level of detail and the aval -able resources (time, manpower). This requires significant ex-perience. The true value offered by engineering services thus lies not in software licenses or hardware but in highly skilled staff putting these resources to best use.
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