Meeting Abstract
The shortfin mako shark Isurus oxyrinchus is a fast swimming ocean predator that preys on some of the fastest swimmers in the ocean. The mako’s fast swimming speeds and its ability to migrate over distances up to thousands of kilometers suggest that drag reduction is crucial for this species. Mako skin is embedded with a layer of often flexibly attached, toothlike scales. Past studies have attributed drag reducing effects of shark skin to the presence of scale ridges oriented parallel to the swimming direction. More recent studies have examined the drag reduction role of scale bristling in which scales move rapidly up and down, which may reduce backflow in reversing, turbulent boundary layer flows. Scale bristling is difficult to observe, however, due to the small spatial and short temporal scales involved. We used ultra high-speed video (50,400 frames per second) to record turbulent boundary layer flow over mako skin from the flank region of the body, a region of high flow separation and the region where scales are most flexible. We recorded videos of mako scales bristling rapidly up and down over approximately two milliseconds. Scale bristling occurred under flow conditions representative of cruise swimming and were associated with two flow features. The first was a downward and backward (toward the anterior of the shark) flow associated with flow reversal that pushed up the scale. The second was an eddy just anterior to the scale that produced a low pressure region that pulled up the scale. This study was the first to directly observe flow induced scale bristling.
