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 (15%). 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. (C) 2011 American Institute of Physics. [doi:10.1063/1.3639133]