Investigation about drag reduction annulus experiment of hydrophobic surface

For the drag reduction application of hydrophobic material, the drag characteristic of typical surface with different roughness or different hydrophobicity is studied by a new annulus experiment. The corresponding torque characteristic and drag reduction rate curve are acquired. The experiment indirectly calculate the surface friction of the annulus by measuring the torque of disk driving annulus and breaks through the limitation of scale in traditional microchannel experiment, avoids the drawbacks of too many influencing factors in water-tunnel experiment, and has important significance in macro application of hydrophobic material. The drag reduction effect of hydrophobic surface is proved at low Reynolds number in macroscale; however, at high Reynolds number, it will be weakened or even changed to drag producing effect, and the rapid increase of pressure drag is the major reason for increasing resistance. Through comparative analysis we find that at low Reynolds number, there will be greater effect of hydrophobicity for drag reduction; where as at high Reynolds number, the roughness will play a greater role, and may even be counterproductive to the increasing resistance.