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. 2021 Sep 17;16(6). doi: 10.1088/1748-3190/ac1309.
There is an increasing need in industry for noise reduction in fans. Inspired by owls' silent flight, we propose four owl-inspired blade designs for a mixed-flow fan to examine whether leading-edge (LE) and/or trailing-edge (TE) serrations can resolve the tradeoff between sound suppression and aerodynamic performance. We investigate the blades' aeroacoustic characteristics through various experimental methods and large-eddy simulation (LES)-based numerical analyses. Experimental results suggest that 'slotted', simply-fabricated LE serrations can achieve a lowering of the noise level while sustaining the aerodynamic performance of the fan, whereas TE serrations fail. In addition, the inclination angle can improve LE serration performance in aeroacoustic and aerodynamic performance with a reduction in the specific noise level by around 1.4 dB. LES results and noise spectral analysis indicate that the LE serrations can suppress flow separation, reducing the broadband noise at low-to-middle frequencies (40-4k Hz). This passive-flow-control mechanism, likely due to local higher incidence angles associated with LE serrations, is capable of alleviating the intensive pressure gradient while suppressing wall-pressure fluctuations over the LE region, hence weakening the Kelvin-Helmholtz instability. The tonal noise also shows a marked reduction at the highest peak frequency associated with fan-vane interaction. Moreover, we find that the high-frequency noise by-product radiates mainly from the LE serrations andsurroundings, due to the small eddies broken up when the vortical flows pass through the LE serrations. Our results demonstrate that the biomimetic design of the LE serrations can facilitate the break-up of LE vortices passively and effectively without negatively impacting aerodynamic performance, which can be utilized as an effective device to improve the aeroacoustic performance of fan blades.
在工业界，人们对风扇的降噪需求日益增长。受猫头鹰无声飞行的启发，我们提出了四种基于猫头鹰翼型的混合流风扇叶片设计，以检验前缘（LE）和/或后缘（TE）锯齿是否可以解决降噪和气动性能之间的权衡问题。我们通过各种实验方法和基于大涡模拟（LES）的数值分析来研究叶片的空气动力声学特性。实验结果表明，简单制造的 LE 锯齿开槽可以在保持风扇气动性能的同时降低噪声水平，而 TE 锯齿则不行。此外，倾斜角可以改善 LE 锯齿的空气动力声学和气动性能，使比噪声级降低约 1.4dB。LES 结果和噪声频谱分析表明，LE 锯齿可以抑制流动分离，从而降低中低频（40-4k Hz）的宽带噪声。这种被动流动控制机制可能是由于 LE 锯齿处的局部入射角较高，能够缓解前缘区域的强压力梯度并抑制壁压脉动，从而削弱了开尔文-亥姆霍兹不稳定性。与风扇叶片相互作用相关的最高峰值频率处的音调噪声也明显降低。此外，我们发现高频噪声副产物主要由 LE 锯齿及其周围辐射产生，这是由于涡旋流通过 LE 锯齿时分裂出的小涡流所致。我们的结果表明，LE 锯齿的仿生设计可以被动有效地促进 LE 涡的破裂，而不会对气动性能产生负面影响，因此可以用作改善风扇叶片空气动力声学性能的有效装置。
