Successful proof of drag reduction through “artificial dolphin skin”
Dolphins are believed to maintain low-drag laminar flow along a large portion of their body, which qualifies them as an inspiration to marine engineers. Dolphin skin is thick, flexible and able to interact with the surrounding flow so as to delay laminar-turbulent transition and reduce frictional drag. Such kind of drag reduction is desirable for ship hulls, too. HSVA and three partner institutions therefore conducted the research project FLIPPER with the aim to develop a drag-reducing bionic hull coating. Calculations were initially carried out in order to identify suitable viscoelastic coating parameters. To this end, HSVA used a numerical model of dolphin skin and combined it with a prediction tool for laminar-turbulent transition. The theory revealed that compliant coatings were indeed able to delay transition to turbulence: for a six-metre hull model of a SAR vessel, for instance, the coating shifted the laminar-turbulent transition front by some two metres in the downstream direction (cf. HSVA Newswave 1-16, pp. 6-8).
The FLIPPER project finally culminated in HYKAT experiments for proof of concept. A wooden hull model of the SAR vessel was built and equipped with an exchangeable bow segment (see figure). Several such segments were made and laminated with different polymeric coatings. A load cell was used to measure the drag force on the bow segments. The best-performing “artificial dolphin skin” led to a drag reduction of 6% compared to a standard-paint reference bow. For fully laminar flow at low speed, the compliant coatings did not have any effect while the drag reduction improved for increasing water-tunnel speed, suggesting that laminar-turbulent transition delay was indeed the physical mechanism at play. In conclusion, the FLIPPER project successfully demonstrated the viability of the compliant-coating concept for maritime applications.
Further reading (in German): http://www.ifam.fraunhofer.de/de/Presse/Vorbild_Delfinhaut.html