The Noise-Reduction Characteristics of Microstructure of Dragonfly Wing Leading Vein

Dragonfly wings have many excellent functions, such as superhydrophobic, fatigue resistance, anti-reflection, etc. However, there are few reports on the low noise flight of dragonfly wings. For this reason, the microgeometry of dragonfly wings was studied in this paper to reveal the mechanism of low-noise flight of dragonfly leading veins. The micromorphology of dragonfly wings was observed by scanning electron microscopy. It was found that the leading-edge veins of dragonfly wings have a triangular prism-like serrated structure, which has been proven to have the effect of improving aeroacoustics. According to the principle of scale law of flying organisms, a bionic model with the leading-edge microstructure of dragonfly’s front wing was established, and computational fluid dynamics (CFD) analysis of serration bionic microstructure was carried out. The effects of geometric parameters, such as height, width and overall amplification factor of microstructure on aeroacoustics were obtained. The distribution of pressure fluctuation on the surface of the bionic wing was also analyzed in this paper. It was found that the serrated microstructure can significantly suppress the noise generation in the mid-frequency band. Finally, wind tunnel tests were simulated using a designed low-noise rotating test platform. The test results confirmed that the serration microstructure has certain noise-reduction characteristics.

Study on flow noise characteristics of Bionic cylinder based on acoustic analogy

The drag and noise reduction of the flow around a cylinder is one of the important topics in hydrodynamics and acoustics. In this paper, three typical bionic cylinders are designed based on the serrated structure on the surface of shark skin. Using Large eddy turbulence model and Lighthill’s acoustic analogy method, the flow noise characteristics of smooth cylinder and three kinds of bionic cylinders at different Reynolds numbers were compared, and the structure of cylinder surface was optimized. The results show that the main source of the flow noise around a cylinder is dipole noise, which is caused by the periodic fluctuating pressure on the cylinder surface.The bionic cylinder can reduce the amplitude of the fluctuating pressure, improve the wake flow field and reduce the wake vorticity, so as to reduce the noise. Among the three kinds of bionic cylinder, V-shaped bionic cylinder has the best noise reduction effect, and the critical value of S/H of V-shaped cylinder is about 2.5. When s / h > 2.5, V-shaped bionic cylinder has no effect of noise reduction.

Improvement on a 2 × 2 Elements High-Gain Circularly Polarized Antenna Array

A novel antipodal Vivaldi antenna with tapering serrated structure at the edges is proposed. Compared with traditional Vivaldi antennas without serrated structure, the gain of the designed antenna is significantly improved in the desired frequency band (4.5–7.5 GHz). In addition, a 2 × 2 Vivaldi antenna array with an orthorhombic structure is designed and fabricated to achieve a circular polarization (CP) characteristic. With this configuration, the 3 dB axial ratio bandwidth of the array reaches about 42% with respect to the center frequency of 6 GHz and a high gain is achieved as well. The novel Vivaldi antenna and CP antenna array both have ultrawide band (UWB) and high-gain characteristics, which may be applied to the field of commercial communication, remote sensing, and so forth.