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
Nappa leather Nappa leather Nappa leather is a leather, typically dyed, made from goat-, cow-, calf-, lamb- or other skin by tanning. It is noted for its soft hand. It is a generic term in the leather field and has no distinct test for characterization. Because of this ambiguity, the term is often used in advertising to imply that a leather is particularly supple. Among other uses, Nappa leather is often used in leather products such as furniture, clothing, handbags, and shoes. As the term is used today, Nappa leather may be either natural grain, or more likely, corrected grain. Nappa leather