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Viennese Whirl
Posted Mon February 09, 2004 @05:41PM
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Application Almost 80 years ago Viennese inventor Ernst Schneider had a very good idea; an idea that had the potential to make ships more manoeuvrable than had ever been possible with the conventional screw propeller.

An e4engineering article describes Schneider's system that combined propulsion and steering in a single unit and allowed thrust of varying magnitude to be generated in any direction.

Until recently all knowledge of the hydrodynamic behaviour of the propeller was based on extensive model testing. However, the adoption of CFD software by propeller manufacturer Voith has enabled its design team to improve its behaviour substantially.

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Led by mechanical engineer Michael Palm, the designers used Comet, a CFD (Computational Fluid Dynamics) tool from CD Adapco, to gain better understanding of the complex flow patterns around the propeller. With CFD they could predict the hydrodynamic loads on the blades and improve the efficiency and strength of the propeller. This was brought about primarily by altering both the blade profile and the oscillatory motion of the blade.

As well as the calculations for the propeller itself, the software was also used to model the flow around ship structures and significantly improve the interaction between the propeller and the hull. For instance, through visualising the computed flow field around ships, Palm's team could ascertain that certain combinations of hull shape and the Voith-Schneider Propeller led to massive vortex generation at both the bow and the stern of the ship hull. This would clearly have a serious impact on performance.

Armed with this knowledge, the team can now make slight changes in hull shape to improve the performance of the ship considerably.

While the overall efficiency of the propeller has been improved in many different ways, perhaps the most significant enhancement, said Palm, is a six per cent improvement in bollard pull. This is a fairly abstract design criterion traditionally used in the selection of propellers for tugboats. It is a measurement of the theoretical thrust achieved at zero speed and full engine RPM.

Although this state cannot actually be achieved (mainly because propellers accelerate water as they spin so they never really see water at zero speed), bollard pull is nevertheless a neat way of comparing the towing abilities of different propellers.

Palm said that the improvements made possible through the use of CFD have made the Voith-Schneider Propeller competitive with conventional propellers over a wider range of applications. However, he admitted that there is still plenty of scope for improvement and hopes to continue using CFD to improve the propeller's behaviour. Future projects, he said, are likely to concentrate on improving hull-propeller interaction using optimisation algorithms.

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