Department of Mechanical Engineering, School of Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan.
Yamashina Institute for Ornithology, 115 Konoyama, Abiko, Chiba, 270-1145, Japan.
Bioinspir Biomim. 2022 Aug 18;17(5). doi: 10.1088/1748-3190/ac7f71.
The bodies of penguins, which swim underwater to forage, are densely covered with feathers, in which the barbs are oriented in the longitudinal direction. We hypothesize that these barbs act as riblets and reduce friction drag during swimming. Considering various real-world swim conditions, the drag reduction effect is expected to be robust against changes in the flow speed and yaw angle relative to the flow. To test this hypothesis, we created trapezoidal riblets based on the morphology of these barbs and measured the drag of flat plates with these fabricated riblets in a water tunnel. The spacing, width, and height of the barbs were found to be approximately 100, 60, and 30 m, respectively. This spacing resulted in a nondimensional spacingof 5.5 for a typical penguin swimming speed of 1.4 m s. We fabricated four types of riblets on polyimide films by ultraviolet laser ablation. The first was a small-scale riblet for which the spacing was decreased to 41 m to simulate the surface flow condition of the usual and slower swim behaviors in our water tunnel. The other three were manufactured to the actual scale of real barbs (spacing of 100 m) with three different rib ridge widths: 10, 25, and 50 m. Yaw angles of 0°, 15°, 30°, and 45° were also tested with the actual-scale riblets. The drag reduction rate of the small-scale riblet was maximized to 1.97% by the smallestof 1.59. For all three actual-scale riblets, increasing the yaw angle from zero to 15° enhanced the drag reduction rate for the full range ofup to 13.5. The narrow-ridge riblet reduced drag at an even higher yaw angle of 45°, but the drag increased with zero yaw angle. Overall, the medium-ridge riblet, which was representative of the barbs, was well-balanced.
企鹅的身体被密集的羽毛覆盖，这些羽毛的羽枝沿纵向排列。我们假设这些羽枝起到了扰流片的作用，从而减少了游泳时的摩擦阻力。考虑到各种实际的游泳条件，这种减阻效果应该能够抵抗相对于水流的速度和偏转角的变化。为了验证这一假设，我们根据这些羽枝的形态制作了梯形扰流片，并在水洞中测量了带有这些制作扰流片的平板的阻力。我们发现羽枝的间距、宽度和高度分别约为 100、60 和 30μm。对于典型的企鹅游泳速度 1.4m/s，这个间距导致无量纲间距为 5.5。我们通过紫外激光烧蚀在聚酰亚胺薄膜上制作了四种扰流片。第一种是小尺度扰流片，其间距减小到 41μm，以模拟我们水洞中通常和较慢游泳行为的表面流条件。另外三种扰流片的实际尺寸与真实羽枝的尺寸相同（间距为 100μm），但肋脊宽度不同：10、25 和 50μm。还测试了实际尺寸扰流片的 0°、15°、30°和 45°偏转角。小尺度扰流片的减阻率最大可达 1.97%，减阻率最小值为 1.59%。对于所有三种实际尺寸的扰流片，当偏转角从 0°增加到 15°时，减阻率在整个范围内增加了 13.5%。在偏转角为 45°的情况下，窄肋扰流片甚至可以降低阻力，但在偏转角为 0°时阻力会增加。总体而言，具有代表性的中等肋脊扰流片平衡性能良好。
