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
Iranian leather industry encourages the industry to process high added value products of leather goods, leather shoes and leather clothing so as to increase the industry's profits". 170 leather companies are still active in Iran with the annual production of 4 million and 334 thousand square feet of leather annually. Most of the leather companies belong to the private sector, yet most these factories just work with 40% of their capacity despite the high investments in leather industry in the country. Despite the industry's long history and skin hide proficiency, Iranian products have not achieved a significant place in the international markets. Iranian