Bow segment with coated surface in water tunnel.
HSVA GmbH
Dolphins provide an excellent model for elegant and economical movement in water. These mammals can achieve amazing swimming feats due to their body shape and elastic skin. Their special skin properties cause a significant reduction in drag by maintaining a laminar flow along their surface. Drag-reducing hull coatings are of interest to the shipping industry since reducing ship’s drag significantly lowers fuel consumption. As part of the FLIPPER project funded by German Federal Ministry for Economic Affairs and Energy, researchers at Fraunhofer IFAM, HSVA and two other research partners have therefore developed a surface coating that, like dolphin skin, measurably reduces the drag in water.
Based on flow calculations for an almost six-metre-long ship’s model, Fraunhofer IFAM has developed a gel-like elastic polymer that is applied in layers a few millimetres thick on the bow area of the ship’s model. A thin, robust film forms the stable outer skin in the end. Field tests have shown a reduction in drag of up to six percent with this coating compared to a painted reference bow. The theoretical prediction that drag along the artificial dolphin skin is reduced with increasing layer thickness has been confirmed. Furthermore, this positive effect is shown to rise with growing speed, i.e. with increasingly turbulent flow. This clearly indicates that the compliant coating actually does work by delaying the transition from laminar flow to turbulence.
The next aim is to make this technology usable for industrial applications. The potential savings can be maximised by combining the compliant coating with a grooved coating ("shark skin") at the ship’s stern. With this combined coating, artificial dolphin skin, which controls the laminar flow in the bow, would merge into artificial shark skin, which influences the turbulent flow in the middle and stern of the ship. Such functional coatings are attractive because they work without having to expend extra energy, making them potentially more cost-effective than active systems for controlling flow.