Turbulent drag reduction on seal fur surfaces

Abstract

The pressure losses for three kinds of fur surfaces (those of ribbon, spotted and harp seals) were measured in a rectangular water channel flow using the water and a glycerol-water mixture. The data were compared with those of smooth and riblet surfaces. The drag reduction could be obtained for all kinds of seal fur surfaces; the maximum reductions were estimated to be about 12% on ribbon seal, 8.5% on spotted seal and 3.5% on harp seal for glycerol-water. Compared to the riblet surface, the range of Reynolds number where the drag reduction ratio was almost the maximum was wider for ribbon and spotted seal fur surfaces with hydrophilic property, while it was not for harp seal fur surface with hydrophobic property. It was found that for the ribbon seal, unlike the riblet, a drag increase due to the effect of surface roughness did not appear even at the highest Roynolds number tested. The measurements of seal fur surfaces using a 3D laser microscope revealed that the roughness of three kinds of seal fur surfaces was comprised of various wavelengths.