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
Leather carving enhance its appearance. These cuts may be made in parts of the design that have already been stamped, which is why it is necessary to leave this step until last. Leather carving Leather carving is the process of giving a three-dimensional appearance to leather craft objects or works of art by a process of cutting and stamping the surface. The only type of leather suitable for carving is vegetable tanned, full grain leather. This is because the vegetable tanning process allows the leather to absorb water, which is used to soften the leather before the carving process, and the grain