Focusing Behavior of Plate Wave by using Gradient-Index Phononic Crystal
Date Issued
2011
Date
2011
Author(s)
Chen, Yan-Ting
Abstract
Similar to the photonic crystals, band gap and negative refraction also exist in the cases of acoustic waves in periodically arranged composites called phononic crystals and can potentially be applied to the filter and flat lens.
In this thesis, we demonstrate the focusing behavior of a gradient-index phononic crystal numerically by finite element (FE) method and experimentally by ultrasonic experiments. In order to understand the propagation behavior of different modes in a plate, we analyzed the band structure, and further to determine the mode which is suitable for wave focusing by the equal frequency contour (EFC) analysis. Based on the study, the lowest anti-symmetric Lamb mode was employed due to its nearly isotropic EFC. Based on the concept of Gradient-Index optics, we designed the gradient index and calculated the relationship between the filling fraction and refraction index at various frequencies. We also discussed the effect of thickness on the focusing efficiency.
Finally, we demonstrate the feasibility of using GRIN PC as a beam width compressor. A plate waveguide with periodic stubbed surface is adopted to achieve a band gap with frequency around the operating frequency of the GRIN PC. The simulated results show good performance on compressing plate waves into the entrance of a narrow waveguide.
Subjects
Phononic crystal
Lamb wave
Finite element method
Band structure
Equal frequency surface
Type
thesis
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