Department of Mechanical & Aerospace Engineering, Seoul National University, Seoul, Korea.
PLoS One. 2013 Dec 2;8(12):e81323. doi: 10.1371/journal.pone.0081323. eCollection 2013.
The sailfish and swordfish are known as the fastest sea animals, reaching their maximum speeds of around 100 km/h. In the present study, we investigate the hydrodynamic characteristics of these fishes in their cruise speeds of about 1 body length per second. We install a taxidermy specimen of each fish in a wind tunnel, and measure the drag on its body and boundary-layer velocity above its body surface at the Reynolds number corresponding to its cruising condition. The drag coefficients of the sailfish and swordfish based on the free-stream velocity and their wetted areas are measured to be 0.0075 and 0.0091, respectively, at their cruising conditions. These drag coefficients are very low and comparable to those of tuna and pike and smaller than those of dogfish and small-size trout. On the other hand, the long bill is one of the most distinguished features of these fishes from other fishes, and we study its role on the ability of drag modification. The drag on the fish without the bill or with an artificially-made shorter one is slightly smaller than that with the original bill, indicating that the bill itself does not contribute to any drag reduction at its cruise speed. From the velocity measurement near the body surface, we find that at the cruise speed flow separation does not occur over the whole body even without the bill, and the boundary layer flow is affected only at the anterior part of the body by the bill.
旗鱼和剑鱼是众所周知的最快的海洋动物，达到其最大速度约为 100 公里/小时。在本研究中，我们研究了这些鱼类在其巡航速度约为 1 个体长/秒的流体动力学特性。我们在风洞中安装了每一种鱼类的标本，并测量了在其巡游条件下相应的雷诺数时其身体的阻力和表面上方边界层的速度。基于自由流速度和湿表面积，测量了旗鱼和剑鱼的阻力系数，分别为 0.0075 和 0.0091，在其巡航条件下。这些阻力系数非常低，与金枪鱼和梭子鱼的阻力系数相当，小于狗鱼和小鱼的阻力系数。另一方面，长吻是这些鱼类与其他鱼类最显著的特征之一，我们研究了它在阻力修正能力方面的作用。没有长吻或人工制造的短吻的鱼的阻力略小于具有原始长吻的鱼的阻力，这表明长吻本身在其巡航速度下不会减少任何阻力。从身体表面附近的速度测量中，我们发现即使没有长吻，在整个身体上也不会发生流动分离，边界层流动仅在身体的前部受到长吻的影响。
