Department of Aerospace Engineering, The University of Alabama, Tuscaloosa, AL 35487, United States of America.
Bioinspir Biomim. 2024 Aug 6;19(5). doi: 10.1088/1748-3190/ad679c.
This smooth flat experimental study investigates the capability of mako shark scales to control flow separation when placed downstream of the onset of turbulent boundary layer separation and within the reattachment region. The objective of the study is to validate the hypothesis that the shark scales' bristling and recoiling would prevent the flow separation on the flank region (the fastest flow region) of the shark. A rotating cylinder was used to induce an adverse pressure gradient over a flat plate to produce a region of separated flow where the shark skin specimen was mounted. Two types of mako shark scales (flank (B2) and between flank and dorsal fin (B1)) were positioned in the preferred flow direction on a flat plate. The B2 scales are slender, 200m tall, and can bristle up to 50°. In contrast, B1 scales are wider, shorter, and can bristle at 30°. The bristling angle and shape are the main mechanisms by which the scales act to inhibit flow from moving upstream near the wall. Thus, the difference in the bristling angles and structures of the scales is attributed to the fact that the B2 scales function in a thicker boundary layer (behind the shark's gills) where they must bristle sufficiently high into the boundary layer to control the flow separation, and because the adverse pressure gradient in this region is higher where flow separation is more likely. The scales are placed in the reattachment region to elucidate their ability to control and reattach an already separated turbulent flow. The results show that B2 scales placed in the reattachment region reduce the size of the turbulent separation bubble and decrease the turbulent kinetic energy, while B1 scales have the opposite effect.
本平滑平面实验研究旨在探究灰鲭鲨鳞片在湍流边界层分离起始点下游和再附区域内控制流动分离的能力。该研究的目的是验证鲨鱼鳞片的刺状结构和回弹会阻止流在鲨鱼侧翼（最快流区）发生分离的假设。旋转圆柱被用于在平板上产生逆压梯度，以产生分离流区域，在该区域安装鲨鱼皮样本。两种灰鲭鲨鳞片（侧翼（B2）和侧翼与背鳍之间（B1））以优选的流动方向安装在平板上。B2 鳞片细长，高 200m，可竖起高达 50°。相比之下，B1 鳞片更宽、更短，可竖起 30°。竖起角度和形状是鳞片抑制壁面附近上游流动的主要机制。因此，鳞片的竖起角度和结构的差异归因于以下事实：B2 鳞片在更厚的边界层（鲨鱼鳃后面）中起作用，它们必须将竖起角度足够高进入边界层以控制流动分离，并且由于该区域的逆压梯度更高，更有可能发生流动分离。将鳞片放置在再附区域，以阐明其控制和再附已分离的湍流的能力。结果表明，放置在再附区域的 B2 鳞片减小了湍流分离泡的尺寸并降低了湍流动能，而 B1 鳞片则产生相反的效果。
