Shanghai Jiao Tong University and Chiba University International Cooperative Research Center (SJTU-CU ICRC), 800 Dongchuan Road, Minhang District, Shanghai 200240, People's Republic of China. Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
Bioinspir Biomim. 2017 Jul 4;12(4):046008. doi: 10.1088/1748-3190/aa7013.
Owls are widely known for silent flight, achieving remarkably low noise gliding and flapping flights owing to their unique wing morphologies, which are normally characterized by leading-edge serrations, trailing-edge fringes and velvet-like surfaces. How these morphological features affect aerodynamic force production and sound suppression or noise reduction, however, is still not well known. Here we address an integrated study of owl-inspired single feather wing models with and without leading-edge serrations by combining large-eddy simulations (LES) with particle-image velocimetry (PIV) and force measurements in a low-speed wind tunnel. With velocity and pressure spectra analysis, we demonstrate that leading-edge serrations can passively control the laminar-turbulent transition over the upper wing surface, i.e. the suction surface at all angles of attack (0° < AoA < 20°), and hence play a crucial role in aerodynamic force and sound production. We find that there exists a tradeoff between force production and sound suppression: serrated leading-edges reduce aerodynamic performance at lower AoAs < 15° compared to clean leading-edges but are capable of achieving both noise reduction and aerodynamic performance at higher AoAs > 15° where owl wings often reach in flight. Our results indicate that the owl-inspired leading-edge serrations may be a useful device for aero-acoustic control in biomimetic rotor designs for wind turbines, aircrafts, multi-rotor drones as well as other fluid machinery.
猫头鹰以无声飞行而闻名，由于其独特的翅膀形态，能够实现非常低噪音的滑翔和拍打飞行，这些形态通常具有前缘锯齿、后缘边缘和天鹅绒般的表面。然而，这些形态特征如何影响空气动力产生和声音抑制或降噪，仍然不是很清楚。在这里，我们通过结合大涡模拟（LES）、粒子图像测速（PIV）和低速风洞中的力测量，对具有和不具有前缘锯齿的受猫头鹰启发的单根羽毛翼模型进行了综合研究。通过速度和压力谱分析，我们证明前缘锯齿可以被动控制上翼表面（即所有攻角 0°<AoA<20°的吸力面）的层流-湍流过渡，因此在空气动力产生和声音产生中起着关键作用。我们发现，在较低攻角 AoAs <15°时，锯齿前缘会降低空气动力性能，但在较高攻角 AoAs >15°时，锯齿前缘可以实现降噪和空气动力性能的平衡，而猫头鹰翅膀在飞行中通常会达到这个攻角。我们的结果表明，受猫头鹰启发的前缘锯齿可能是仿生转子设计中用于风力涡轮机、飞机、多旋翼无人机以及其他流体机械的声学风洞控制的有用装置。
