Shark Skin Separation Control Mechanisms

Drag reduction by marine organisms has undergone millions of years of natural selection, and from these organisms biomimetic studies can derive new technologies. The shortfin mako (Isurus oxyrinchus), considered to be one of the fastest and most agile marine predators, is known to have highly flexible scales on certain locations of its body. This scale flexibility is theorized to provide a passive, flow-actuated mechanism for controlling flow separation and thereby decreasing drag. Recent biological observations have found that the shortfin mako has highly flexible scales, bristling to angles in excess of 50 degrees, particularly on the sides of the body downstream of the gills. High "contragility," which is explicitly defined here as the ability to change or move in a new or opposing direction while already in a turn, would occur if form drag were minimized. This would thus indicate the potential control of flow separation on body regions aft of the point of maximum girth or in regions of adverse pressure gradient. Thus results are consistent with the hypothesis that scale bristling controls flow separation. This scale flexibility appears to be a result of a reduction in the relative size of the base of the scales as well as a reorganization of the base shape as evidenced by histological examination of the skin and scales. Probable mechanisms leading to separation control are discussed.