Since penguins spend long time in the sea, their body surface presumably evolved to reduce fluid friction drag. The barbs of the body feathers are oriented in the longitudinal direction and form a group of microgrooves, so we hypothesized that they work as riblets to reduce friction drag. In addition, considering various maneuvers of penguins, the drag reduction effect of the barbs is desired to be robust to changes in the yaw angle to the flow direction. To test those hypotheses, we fabricated trapezoidal riblets based on the morphology of the barbs, followed by drag measurement of flat plates with the riblets in a water tunnel. We measured dimensions of the real barbs of penguins and designed the riblets based on the spacing and cross-sectional shape of them. We fabricated two types of riblets on polyimide films by laser scanning ablation. One was an actual-scale riblet with yaw angles of 0° 15°, 30°, or 45° which was same size as the barbs. The other was a small-scale riblet to reproduce the surface flow condition of a swimming penguin in our water tunnel. As a result, the actual-scale riblet showed the largest drag reduction ratio of 1.26% at a yaw angle of 15°. This indicates that the drag reduction effect is robust to yaw angle up to 15°. The small-scale riblet showed the largest drag reduction of 1.97% when s+ was 1.59. The drag reduction effect was also observed in all s+ ranges, indicating that the barbs of penguins have a drag reduction effect during swimming. These results indicate that penguin barbs have the drag reduction effect in the all range of swimming speeds, and that the drag reduction effect is greatest when the yaw angle is 15°.
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