ペンギンの体表を模倣したリブレットの3次元胴体モデルにおける抗力低減効果

Abstract

Penguins have adapted to underwater swimming in the course of evolution. Previous study showed that the riblets mimicking the body feathers of the penguins reduced drag on a flat plate by up to 2.0%. In this study, we measured drag reduction rate of the penguin-mimetic riblets on a 3D body model. Based on a sideview image of a swimming penguin, a 3D spindle model of which cross section was a regular 24-gon shape was created. Riblets were fabricated on flexible polyimide films by laser scanning ablation so that the riblet films were attached to the curved surfaces. Two different cross sections of the riblets were compared: one was the penguin-mimetic trapezoidal cross section (medium-ridge riblet), and the other was a near-rectangular cross section (wide-ridge riblet). Drag of the 3D bodies covered with films with or without riblets was measured in a water channel. As a result, the maximum drag reduction rate of the medium-ridge riblet for the 3D body was 3.5% when s+ was 7.29, which was higher than the drag reduction rate for the flat plate with similar s+ (0.58% with s+ of 7.23) or the maximum value (0.60% with s+ of 10.4). Moreover, the medium-ridge riblet showed higher drag reduction ratio than the wide-ridge riblet for full range of flow speed in this study. The results suggest that the penguin-feather-mimetic riblet reduces not only frictional drag but also pressure drag for the 3D body.