Investigation of the Role of Body Shape on an Air Lubrication System Inspired by Penguins

Bubble-assisted drag reduction is a promising strategy for improving the energy efficiency of underwater bodies, but its effectiveness depends not only on air injection but also on the ability of the body geometry to retain bubbles close to the surface. Drawing inspiration from the air retention and bubble release mechanisms of penguin plumage, this study examines whether a simplified penguin-inspired geometry can enhance bubble coverage compared to a conventional axisymmetric body. Prototypes featuring a penguin-inspired shape and a torpedo shape, each equipped with an air diffuser and injection holes, were designed and evaluated in a dedicated towing tank. At low air pressure, the penguin-inspired body demonstrated a 31.5% reduction in drag coefficient compared to the condition without bubbles. In contrast, the torpedo-shaped body did not exhibit a reduction in drag coefficient under identical air-injection conditions. Both geometries showed diminished performance at higher pressures, likely due to bubble growth, coalescence, and decreased near-wall retention. These findings indicate that the effectiveness of bubble-assisted drag reduction is highly dependent on body geometry and that bioinspired body morphology may play a critical role in sustaining a layer rich in air bubbles near the surface. This study offers a preliminary experimental assessment of penguin-inspired air lubrication and identifies bubble size, air-flow control, and quantification of surface coverage as key areas for future research.