Self-collimated In Sonic Crystals And Double Negative Acoustic Metamateirals

In recent years, there has been growing interest in the artificial periodic composite(known as Sonic Crystal) for their ability to manipulate acoustic/elastic waves. Many novel acoustic devices have been reported by utilizing Sonic Crystals. With the in-depth study of Sonic Crystal, much attention has been focused on the research of the double negative materials, known as metamaterials. Elastic waves will propagate in unique ways in the metamaterials due to their abnormal responses caused by their sub-wavelength building blocks. This dissertation applies finite elements software to conduct numerical simulations, mainly designing a kind of acoustic self-collimated splitter and two special acoustic metamaterials.The main works involved in this dissertation are as follows:Firstly, in the second chapter, the finite element method is used to investigate self-collimation of acoustic beams in an Sonic Crystal(SC) of hexagonal lattice. It is shown that 60 ° and 120 ° bending of self-collimated acoustic waves can be simultaneously realized by simply truncating the two-dimensional hexagonal SC.Bended imaging for a point source with a subwavelength resolution of 0.38 can also be realized by truncating the SC structure. In addition, a scheme for 60 ° and 120 °splitting of self-collimated acoustic waves is also proposed by introducing line-defects into the hexagonal SC. It is demonstrated that an incoming self-collimated beam can be split into a 60 °(or 120 °) bended one and a transmitted one, with the power ratio adjusted by the value of defect size. We believe that this hexagonal-SC-based bending and splitting mechanism will offer more flexibilities to the beam control in the design of acoustic devices and will be useful in integrated acoustic applications.Secondly, in the third chapter, a water-immersed epoxy plate injected with periodical steel columns is designed to achieve negative-dynamic-mass response, which arises from the collective excitation of surface modes in the plate. A slited hollow steel cylinder immersed in water is designed as a Helmholtz resonator to achieve negative bulk modulus. By combining these two designed structures together, an acoustic metamaterial simultaneously possessing a negative mass density and bulk modulus is realized. Compared to previous realized three-dimensional double negative metamaterials, this two-dimensional double negative metamaterial is much easy to be fabricated.Finally, in the fourth chapter, we achieved an abnormal shear model in certain two-dimensional elastic phononic crystals(EPCs). By finite elements method, we analyzed the eigen modes of the model. We show that the interaction between the scatterer and the hosts capture the essence of the physics of the abnormal shear model.We also find that the material parameters of the scatter can regulation the shear modes.By adjusting the radius of the scatter, it allows us to engineer an EPC that exhibits an unusual Dirac-like cone at the Brillouin center. Further investigation revealed that the observed Dirac-like cone is formed by the coupling between the rotational and dipolar modes.