Document Type : Invited by Abdolreza Ohadi
Authors
1
PhD student, Acoustics Research Laboratory, Mechanical Engineering Department, Amirkabir University of Technology (Tehran Polytechnic), Tehran, IRAN.
2
Assistant Professor, Vehicle Technology Research Institute, Amirkabir University of Technology (Tehran Polytechnic), Tehran, IRAN.
3
Professor, Acoustics Research Laboratory, Mechanical Engineering Department, Amirkabir University of Technology (Tehran Polytechnic), Tehran, IRAN.
10.22064/tava.2025.2061787.1265
Abstract
This paper presents the design and simulation of acoustic metamaterial lenses that focus elastic waves at predetermined focal points. The modified Luneburg refractive index profile is used in the design process to define focal point locations, a capability not previously explored in elastic wave research. This new approach is important because it enables more precise spatial control of waves, resulting in enhanced resolution for elastic wave focusing applications. Three lenses, each targeting specific focal points, are designed by proposing hexagonal unit cells containing blind holes with varying diameters. Dispersion curves are calculated by finite element simulations to determine wave properties of unit cells, including refractive indices. These unit cells provide a wide range of refractive indices (1.0314-1.4959) at the design frequency of 50 kHz, which is suitable for constructing Luneburg lenses. Unit cells are then arranged according to the discretized refractive index profiles to form the lenses. Numerical simulations validate effective wave focusing at the intended focal points (F=R, 1.5R, 2R) with three lenses. The highest amplification of waves and the narrowest focal zone are for the lens with F=R. As the focal point shifts toward 2R, wave distribution becomes scattered along the focal axis. Decay length analysis of F=1.5R and 2R lenses indicates their suitability for long distribution of high-velocity regions. Frequency-dependent simulations across 46–52 kHz reveal that all lenses maintain efficient focusing between 49–51 kHz. At more distant off-design frequencies, amplifications result from refractive index shifts that misalign the focal point.
Highlights
- Designed 3 acoustic metamaterial Luneburg lenses to focus waves at a preset focal point.
- The hexagonal unit cells with blind holes provide a wide refractive index range.
- Achieved high amplification up to 5.96 times the reference velocity and FWHM of 0.71λ.
- Focal shift scatter wave along the focal axis and decay lengths reach up to 15.63λ.
- Frequency-dependent study revealed efficient focusing within a 49–51 kHz band.
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