Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Ostrich leather less trade restrictions, were able to supply ostrich leather at lower prices. Now there are several ostrich ranches and tanneries in the U.S., but with a 150-year head start Klein Karoo Group is still considered the industry leader, and South Africa the centre of ostrich leather production. Like other skins, ostrich is a small-area skin compared to bovine and horse hides, and is ranked by the follicles per area since they thin out further away from the neck. It is processed consequently in a more particular way to preserve the largest possible area for processing and treatments, and is put