昆虫是人造微型飞行器设计的重要参照对象。随着人们对非定常空气动力学理解的加深和微加工技术的进步，扑翼飞行器的设计逐渐向着小型化和微型化的方向发展，这使得仿昆飞行器成为可能。除了不断提高飞行器的飞行效率和实现更加灵活的飞行控制，近年来，扑翼飞行时产生的声波（也称为翼音）也逐渐受到了人们的关注。一方面，典型的仿昆微型扑翼飞行器产生的噪声主要集中在人耳可听的频段内。鉴于仿昆微型扑翼飞行器在军事应用中对于低噪声的严格需求，深入研究扑翼翼音的降噪机制变得尤为关键和迫切。另一方面，自然界中的一些昆虫（例如蚊子）能够利用翼音进行性交流、竞争通信以及在复杂环境中的导航。因此，翼音对昆虫生理和进化的基础研究具有重要意义。然而，相比于扑翼飞行高升力机制的研究，昆虫飞行时产生的翼音并未得到流体力学工作者的足够关注。

在本文中，我们采用重叠网格方法和Lighthill声比拟方法对扑翼飞行和发声的机制进行了研究。特别地，我们聚焦于蚊子这一典型的可以使用翼音交流的昆虫。本文的主要创新性工作包括：

1、从空气动力学和飞行力学两个角度对蚊子的扑翼飞行进行了研究。在空气动力学方面，我们给出了蚊子扑翼飞行时的前缘涡机制与快速上仰转动机制的空气动力学特征，并对前缘涡在不同运动模式下的发展情况进行了初步分析。结果表明，相比蚊子的短冲程高频扑翼运动模式，前缘涡在长冲程低频运动模式中会进一步发展。在飞行力学方面，我们对蚊子悬停飞行时，身体的俯仰振动进行了研究。结果表明，蚊子也具有与其它昆虫相似的减振机制。在悬停飞行时，扑翼惯性力与气动力对蚊子身体的作用会相互抵消。除此之外，由于蚊子扑翼细长，扑翼惯性的作用几乎与扑翼的俯仰运动无关。

2、从气动声学的角度对蚊子扑翼飞行时产生的翼音进行了研究。一方面，我们讨论了蚊子扑翼翼音的产生机制以及声场性质。结果表明，远场翼音可以被视为幅值各向同性分布的声场与具有高度指向性的声场的叠加，各向同性分量往往出现在翼音的高频分量中。更进一步，我们给出了一种通过扑翼俯仰角调控基频翼音指向性的策略。在另一方面，我们讨论了蚊子自己产生的翼音在避障过程中的作用，并给出了一种刻画翼音在触角周围诱导的空气运动特征的方法。结果表明，蚊子只有在非常接近地面的距离(大约两个翼长)才能探测到由地面反射的基频翼音。

3、系统地讨论了俯仰角对扑翼飞行与翼音产生的影响。结果表明，不同的俯仰角模式可以适用于不同的实际应用场景。当执行正弦俯仰角模式时，扑翼具有较高的飞行效率和较小的噪声；当执行梯形俯仰角模式时，扑翼可以提供更多的升力，且具有更高的发声效率以进行翼音交流；当执行快速上仰的俯仰角模式时，扑翼可以同时具有较高的飞行效率和较高的发声效率。

Insects serve as crucial reference models for the design of artificial micro-aircraft. With an increasing understanding of unsteady aerodynamics and advancements in microfabrication technology, the design of flapping-wing aircraft is gradually evolving towards miniaturization, making the development of insect-inspired Flapping Wing Micro Aerial Vehicle (FWMAV) a reality.  Alongside efforts to enhance flight efficiency and achieve more agile flight control, attention has also turned to the sound waves generated by the wing flapping (i.e., wing tones). On one hand, the noise generated by typical FWMAV tends to fall within the audible range of human ears. Given the stringent requirements for low noise in military applications of such aircraft, thorough research into the noise reduction mechanisms of wing tones has become particularly crucial and pressing. On the other hand, certain insects in nature, such as mosquitoes, utilize wing tones for sexual communication, competitive signaling, and navigation in complex environments. Therefore, research on wing tones holds significant implications for the physiological and evolutionary studies of insects. However, compared to studies on the high-lift mechanisms of flapping-wing flight, the wing tones have not received sufficient attention from fluid dynamics researchers.

In the present work, the mechanisms of insect flapping flight and sound generation are investigated using the overset grid method and the Lighthill analogy method. Specifically, our focus is on mosquitoes, a typical insect capable of utilizing wing tones for communication. The contributions of the present work include:

1. The flapping flight of the mosquito is investigated from both aerodynamic and flight dynamic perspectives. In terms of aerodynamics, the aerodynamic characteristics of the leading-edge vortex mechanisms and rapid pitch-up rotation mechanisms during mosquito flapping flight are presented. Additionally, the evolutions of the leading-edge vortex under different flapping patterns are compared. The results indicate that compared to the short-stroke high-frequency flapping pattern of mosquitoes, leading-edge vortex can further develop during long-stroke low-frequency flapping pattern. In terms of flight dynamics, we investigated the pitch oscillation of the mosquito's body during hovering flight. The results indicate that mosquitoes can suppress the body pitch oscillation in a way similar to other insects. During hovering flight, the effect of inertial and aerodynamic forces, generated by wing flapping, on the mosquito's body counterbalance each other. Furthermore, due to the elongated shape of mosquito wings, the effect of wing inertia is almost independent of the pitching motion of the wings. Furthermore, due to the elongated shape of mosquito wings, the effect of wing inertia is almost independent of the pitching motion of the wing.

2. The sound waves generated by mosquito wing flapping (i.e., wing tones) are investigated form the perspective of aeroacoustics. On the one hand, the generation mechanism of the wing tones is discussed and the characteristics of the sound field are presented. The results indicate that the far-field of the wing tone can be regarded as the superposition of a sound field with isotropic amplitude distribution and a highly directional sound field. The isotropic component typically appears in the high-frequency wing tone. Furthermore, we proposed a strategy to control the directivity of the wing tone at fundamental frequency by adjusting the pitch angle of the flapping wing. On the other hand, we examined the role of wing tones produced by mosquitoes themselves in obstacle avoidance and introduced a novel method to characterize the air movements around the antennae of mosquitoes, which are induced by the wing tone. The results indicate that mosquitoes can detect the reflected wing tone at wing-beat frequency only when they are extremely close to the ground, approximately two wing lengths away.

3. The influence of pitch angle on flapping flight and sound generation was systematically discussed. The results indicate that different patterns of the pitch angle may be suitable for various practical applications. When performing the sinusoidal pattern of pitch angle, the flapping wing has a higher flight efficiency and lower noise levels. when performing the trapezoidal pattern of pitch angle, the flapping wing can provide more lift and have a higher sound-generation efficiency for communication. when performing the pitch angle pattern named ‘rapid pitch-up’, the flapping wing can simultaneously achieve higher efficiency in both flight and sound generation.