Fluid drag reduction by penguin-mimetic laser-ablated riblets with yaw angles

Abstract

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 μm, respectively. This spacing resulted in a nondimensional spacing s+ of 5.5 for a typical penguin swimming speed of 1.4 m/s. We fabricated four types of riblets on polyimide films by UV laser ablation. The first was a small-scale riblet for which the spacing was decreased to 41 μm 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 μm) with three different rib ridge widths: 10, 25, and 50 μm. 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 smallest s+ of 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 of s+ up 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.

UV laser-processed microstructure for building biohybrid actuators with anisotropic movement

“…UV laser-scanning ablation (OLMUV-355-7W-K; OPI, Saitama Japan) was performed on PI films on which MGs were designed by computer-aided design (CAD) software (Rhinoceros 6) with straight lines at equal intervals (20, 30, or 40 µm) according to the parameter (1% power, 50 kHz frequency, 200 mm s −1 processing speed, one scan, 0 mm distance from focus, and pulse width <25 ns). The UV-laser system can achieve a theoretical diameter of less than 15 µm for the focusing spot using a diode-pumped solid-state nanosecond laser (Nd:YVO4; 355 nm, 7 W; AONano Compact 355-7-50-V; Advanced Optowave, Ronkonkoma, NY, USA) utilizing two-axis galvanometer mirrors (GM-1015; Canon, Tokyo, Japan) [40]. Additionally, the UV-laser ablation was performed on a PI film with circular or curved lines or lines combining several patterns at equal intervals (20 µm) (figures 1(b)-(d) and S1).…”

“…Microgrooved PI films were prepared by the UVlaser ablation according to the design. PI films are reportedly thermally stable and capable of processing microstructures with laser ablation systems [40,44,45]. Processing conditions that could consistently produce MGs suitable for cell alignment were investigated, aiming for a depth of less than 5 µm and width of less than 20 µm, based on previous reports [25,28,46].…”