Drag reduction of a hairy disk

We investigate experimentally the hydrodynamics of a hairy disk immersed in a two-dimensional flowing soap film. Drag force is measured as a function of hair length, density and coating area. An optimum combination of these parameters yields a drag reduction of 17\%, which confirms previous numerical predictions of 15\% by Favier et al (2009). Flow visualization indicates the primary mechanism for drag reduction is the bending, adhesion and reinforcement of hairs trailing the disk, which reduces wake width and traps ``dead water.'' Thus, the use of hairy coatings can substantially reduce an object's drag while negligibly increasing its weight.