Kazemi M, Mani M
Aerospace Engineering Department, Amirkabir University of Technology, Tehran, Iran.
Sci Rep. 2025 Jul 2;15(1):23261. doi: 10.1038/s41598-025-06309-x.
The investigation of low Reynolds number flows is crucial, particularly for applications such as wind turbines and small-scale UAVs. This study compares the owl airfoil with the NACA0012 and hawk airfoils through wind tunnel testing, utilizing pressure sensors and force balance to examine the aerodynamic noise sources and aerodynamic performance of the airfoils. A total of six airfoils were investigated at various Reynolds numbers from 44 × 10 to 160 × 10, considering the glide flight envelop for various owl species. Wind tunnel test results showed higher Cl and L/D ratio for the owl airfoil, outperforming the NACA0012 and hawk airfoils by up to 6.7% and 44.1%, respectively. This is attributed to the optimal camber of the owl airfoil compared to the two other airfoils, and its lower relative thickness too. This helps flight with this airfoil at lower AOA, which reduces noise. In addition, the stall angle for owl airfoil was ranging from 8° to 15° higher than NACA0012 airfoil, which stalled at 10°-11°, and higher than 6° to 12° hawk stall angle. This feature allowed owls to perform efficient flights in glide phase at lower AOA that minimized the main aerodynamic noise sources such as the separations and pressure fluctuations. Pressure measurements represented the initiation of LSB for the owl airfoil at around AOA = 6° to 10° at different Reynolds numbers, while the hawk airfoil shown the presence of LSB starting from AOA = 0°. A detailed analysis of the pressure fluctuations showed that the owl airfoil had fewer sources of aerodynamic noise, such as LSB, stall phenomena, and separated shear layers, on both its upper and lower surfaces, compared to other types of airfoils. Additionally, an analysis in the frequency domain showed that the amplitude of FFT for NACA0012 and hawk airfoil is generally higher compared with the owl airfoil. These findings shed light on the aerodynamic characteristics and noise generation mechanisms of owl airfoil for future research and design considerations.
低雷诺数流动的研究至关重要，特别是对于风力涡轮机和小型无人机等应用。本研究通过风洞试验，将猫头鹰翼型与NACA0012和鹰翼型进行比较，利用压力传感器和力平衡来研究翼型的气动噪声源和气动性能。考虑到各种猫头鹰物种的滑翔飞行包线，在44×10至160×10的不同雷诺数下共研究了六种翼型。风洞试验结果表明，猫头鹰翼型具有更高的升力系数（Cl）和升阻比（L/D），分别比NACA0012和鹰翼型高出6.7%和44.1%。这归因于与其他两种翼型相比，猫头鹰翼型具有最佳的弯度，并且其相对厚度也较低。这有助于该翼型在较低攻角下飞行，从而降低噪声。此外，猫头鹰翼型的失速角比在10° - 11°失速的NACA0012翼型高8°至15°，比鹰翼型的失速角高6°至12°。这一特性使猫头鹰能够在较低攻角的滑翔阶段进行高效飞行，从而将诸如分离和压力波动等主要气动噪声源降至最低。压力测量表明，在不同雷诺数下，猫头鹰翼型在攻角约为6°至10°时开始出现低频宽带噪声（LSB），而鹰翼型在攻角为0°时就已出现LSB。对压力波动的详细分析表明，与其他类型的翼型相比，猫头鹰翼型在其上下表面的气动噪声源（如LSB、失速现象和分离剪切层）较少。此外，频域分析表明，与猫头鹰翼型相比，NACA0012和鹰翼型的快速傅里叶变换（FFT）幅度通常更高。这些发现为未来的研究和设计考虑揭示了猫头鹰翼型的气动特性和噪声产生机制。
