In recent years, acoustic metamaterial becomes an attractive research of material in the field of acoustics. Acoustic metamaterial is a kind of composite structure, not material. It can realize the function of negative refraction, sound focusing, super lens and stealth in the frequency range of negative equivalent parameters. However, acoustic metamaterials are rarely applied underwater. In this paper, an acoustic metamaterial structure with size far smaller than wavelength is designed and applied to airfoil surface. The effect of sub-wavelength structures distributing on the acoustic scattering of airfoil are investigated. The results show that the airfoil with sub-wavelength structures can reduce the target strength and change the directivity.
Acoustic metamaterials first developed from the electromagnetic metamaterials. It is a kind of structures made artificially, and its scale is far less than the wavelength of sound wave, so it is called the "sub wavelength" structure. The propagation of sound waves in metamaterials is influenced not only by the constituent materials, but also has a great relationship with their geometric structure. Periodic or other forms of microstructural units can modulate the transmission sound wave to show a variety of special physical phenomena, such as negative refraction (Valentine, Zhang, 2008), self-collimation (Moreau, Centeno, 2011), hyper-lens (Chiang, Wu, 2011) and so on. The noteworthy feature of metamaterials is the regulation of wave propagation through the material's specific structure. Some of the peculiar wave propagation properties are usually related to the key physical scales in the structure. Therefore, metamaterials can achieve different properties by being designed in different geometric sizes (Johnson, Khokhar, 2006). metamaterials have become a hot topic for decades and has made a lot of progress in this field. However, there is a lack of research on the application of acoustic metamaterials to underwater structures.
The earliest research on acoustic metamaterials is based on the local resonant phononic crystal structure (Liu, Zhang, 2000). Then the metamaterials are realized by the periodic arrangement of some resonant structures (Li, 2004; Fang, 2006; Ding, 2007). By introducing double resonance elements into liquid, Zhang, Yin, and Fang (2009) realize the negative correlation between mass density and body modulus, and achieve negative refraction imaging, and further realize the acoustic stealth by using this structure (Zhang, Xia, and Fang, 2011). Ding (2011) arranges the hollow ball on the sponge substrate and found that the absorption peak is related to the changes in the aperture of the ball. The numerical design of directional stealth cape is studied (Colombi, Roux, 2015) and the best directional cloaking is obtained when the resonator's length decreases from the central to the outer ring. Zhu, Chen, and Wang (2016) realize strong anisotropic single-phase elastic metamaterial through adjustable locally resonant motions. About the study of acoustic superconducting materials without resonance was first seen in 2002 (Cervera, Sanchis, 2002). A scheme is given (Martin, Theodore, 2010) to realize the anisotropy of mass density in the acoustic frequency range in the air. A cylindrical plane wave lens is designed (Titovich, Norris, 2014) by the conformal transformation of unit circle. Viard, Gallardo, and Xu (2015) optimize a subwavelength metamaterial consisting of a hollow cylindrical array in a soft elastic matrix, and greatly improve the wave absorbing capability. The use of a periodic array of brass cylindrical tubes to form a metamaterial layer adds to the design of the slab sound absorbing material, reducing the reflectance (Ouahabi, Krylov, and O'Boy, 2015). by respectively dealing with the single and multiple panels in the thickness direction, Park, Lee, and Kim (2016) evaluate the performance of anisotropic acoustic metamaterials.
