Computational Fluid Dynamics: a Tool to Innovate

By Van Oossanen

Over just the past fifteen years, the use of Computational Fluid Dynamics (CFD) has become mainstream in ship design. CFD is a way to calculate the flow around a ship, allowing to predict accurately the generated wave profile, streamlines and resistance. While CFD has been used for much longer in aerodynamics and single-fluid computations (e.g. pumps), a ship is a lot more complex, because there are two media: air and water. The interface between air and water leads to wave formation (essentially a loss of energy), which for a long time was difficult to compute.

Having used CFD already during the development of the winged keel for Americas Cupper Australia II back in 1983, and with 20 years of experience at MARIN under his belt, hydrodynamicist Piet van Oossanen quickly saw the potential of CFD for optimizing ships. In 2005, his company Van Oossanen Naval Architects became the first commercial user of Fine/Marine CFD software from Numeca, a code developed initially at the university of Nantes, and currently one of the most used CFD softwares packages in ship design. The company nowadays is called Van Oossanen and incorporates three business activities: naval architecture, fluid dynamics and an independent company for a fuel saving ‘sailing wing’ called ‘the Hull Vane®. Van Oossanen Fluid Dynamics still works closely with Numeca to give input and experience-based feedback for new developments of the software.

“This is now used on many superyachts and for example the new patrol boat of the Port of Rotterdam.

“Initially we used CFD to improve our own designs. Studying the hydrodynamic properties of a ship numerically allows you to optimise earlier and more accurately than what we could do before with model testing, where different bulb or transom configurations could be tested, but it wasn’t possible to change the entire hull shape (such as centre of buoyancy, block coefficient, bilge radius, etc). This lead to improved designs, such as our Fast Displacement Hull Form, a hull shape with excellent performance over a very wide speed range. This is now used on many superyachts and for example the new patrol boat of the Port of Rotterdam,” Niels Moerke, managing director of Van Oossanen, said.

CFD looks at ship resistance in a different way than model testing, where only the results can be compared. By making both the frictional and pressure resistance visible for every square metre of the hull, optimisation can be done a lot better. Van Oossanen’s naval architecture department has a leading position in the naval architecture of (super)yachts and other advanced marine vessels.

“Working in the yacht sector allowed us to tune and validate our CFD procedures really well. With five to ten unique vessels launched every year, we have a large database of CFD runs, model tests and full-scale trials which we can compare. We take the approach that more optimisation is always better, because you always choose the result with the lowest resistance.

“We believe that on ships which will sail the world for 25-30 years with often dozens of megawatts of engine power, a little bit more of the America’s Cup or Formula 1 approach would be welcome, especially now that CO2 emissions have to be reduced. On certain ship types, there is still an enormous amount to be gained with minimal costs, as an optimisation doesn’t make the ship more expensive to build. It’s rather the opposite, as engine power is quite expensive and becoming more so with increasing exhaust emission regulations,” Managing director Perry van Oossanen said.

Increasingly, Van Oossanen used CFD to assist other naval architects, shipyards and ship owners with their designs. Their client list includes Damen Shipyards, IHC, Conoship, Jumbo Shipping, Shipyard De Hoop and many others. In some cases, it’s a complete hull optimisation, in other cases it can be focused on a particular thing, such as bulb optimisation or finding a source of vibrations. CFD was also indispensable in the development of the Hull Vane®.

This is an energy saving device invented by Piet van Oossanen which is marketed, designed and built through Hull Vane BV, part of the Van Oossanen group. Here again, it’s the fact that the flow is visible at a distance from the hull which allows to optimise the device much better than what was possible in the towing tank. In spite of the capabilities in CFD, model testing is still very much used by Van Oossanen, for example for independent validation of their results, for R&D work and for seakeeping and manoeuvring trials, which still take up too much computing time to be effective in CFD.

 “We’ve always made our expertise available to our clients, but we found that for small naval architecture offices worldwide, CFD was still too inaccessible due to cost and time constraints. CFD is something which requires a big investment in software, hardware and people. Basically, if you try to take shortcuts, you will get unreliable results. This is why we launched the website www.cfdanalysis.com in 2016. In a very easy interface, it places our own CFD tools – which we have validated and know to be accurate –  in the hands of anyone worldwide, with currently as only limitation that the hull should consist of one single surface (excluding deck and transom). A CFD run is paid by creditcard, and the results are in within 24 hours. This allows naval architects to get an accurate resistance prediction at an early stage or to compare several alternatives of a hull shape,” Niels Moerke said.

Van Oossanen Fluid Dynamics will exhibit at Europort Maritime in booth 8.005. The company will show what is possible with CFD, including new developments such as seakeeping analyses, wind studies, etc.

Note: The opinions, beliefs, and viewpoints expressed in this article do not necessarily reflect the opinions of World Maritime News.

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