Gluing two pieces of shark skin together to produce a flexible foil, Oeffner repeated the swimming experiment, and this time the denticles had a dramatic effect. The intact skin foil swam 12.3% faster than the sanded skin. The shark's rough surface improved the swimming performance spectacularly.
However, when the duo tested the swimming performance of two shark skin mimics a sharp-edged riblet design and the famous Speedo Fastskin FS II fabric they were in for a shock. Although the riblet surface improved the flexible foil's swimming speed by 7.2%, the dented surface of the Speedo fabric had no effect at all. However, Lauder points out that figure-hugging Fastskin swimming costumes probably enhance the swimmer's performance in other ways.
After proving that the denticles on shark skin significantly improve the fish's propulsion, Lauder and Oeffner were keen to find out how they affect fluid flows around the body. Returning the flexible shark skin foil to the swim tunnel, Oeffner and Lauder captured the water's swirling motion with laser light and realised that in addition to reducing drag, the skin was actively generating thrust.
'That's the number one surprise. It's not just the drag-reducing properties, but the denticles alter the structure of flow near the shark skin in a way that enhances thrust', explains Lauder. He is now keen to design physical models to see how altered denticle arrangements affect fluid flows over the skin and to build a computational model to tease apart the beneficial effects of the skin's thrust and drag reduction.
|Contact: Kathryn Knight|
The Company of Biologists