Amir Darabi

Continuous profile flexural GRIN lens: Focusing and harvesting flexural waves

A significant challenge in flexural wave energy harvesting is the design of an aberration-free lens capable of finely focusing waves over a broad frequency range. To date, flexural lenses have been created using discrete inclusions, voids, or stubs, often in a periodic arrangement, to focus waves via scattering. These structures are narrowband either because scattering is efficient over a small frequency range or the arrangements exploit Bragg scattering bandgaps, which themselves are narrowband. In addition, current lens designs are based on a single frequency and approximate the necessary refractive index profile discretely, introducing aberrations and frequency-dependent focal points. Here, we design a flexural GRIN lens in a thin plate by smoothly varying the plates rigidity and thus its refractive index. Our lens (i) is broadband since the design does not depend on frequency and does not require bandgaps, (ii) has a fixed focal point over a wide range of frequencies, and (iii) is theoretically capable of zero-aberration focusing. We numerically explore our Continuous Profile GRIN lens (CP-GRIN lens) and then experimentally validate an implemented design. Furthermore, we use a piezoelectric energy harvester disk, located at the first focus of the CP-GRIN, to document improvements in power gain.A significant challenge in flexural wave energy harvesting is the design of an aberration-free lens capable of finely focusing waves over a broad frequency range. To date, flexural lenses have been created using discrete inclusions, voids, or stubs, often in a periodic arrangement, to focus waves via scattering. These structures are narrowband either because scattering is efficient over a small frequency range or the arrangements exploit Bragg scattering bandgaps, which themselves are narrowband. In addition, current lens designs are based on a single frequency and approximate the necessary refractive index profile discretely, introducing aberrations and frequency-dependent focal points. Here, we design a flexural GRIN lens in a thin plate by smoothly varying the plates rigidity and thus its refractive index. Our lens (i) is broadband since the design does not depend on frequency and does not require bandgaps, (ii) has a fixed focal point over a wide range of frequencies, and (iii) is theoretically capab...

Broadband Bending of Flexural Waves: Acoustic Shapes and Patterns

Directing and controlling flexural waves in thin plates along a curved trajectory over a broad frequency range is a significant challenge that has various applications in imaging, cloaking, wave focusing, and wireless power transfer circumventing obstacles. To date, all studies appeared controlling elastic waves in structures using periodic arrays of inclusions where these structures are narrowband either because scattering is efficient over a small frequency range, or the arrangements exploit Bragg scattering bandgaps, which themselves are narrowband. Here, we design and experimentally test a wave-bending structure in a thin plate by smoothly varying the plate’s rigidity (and thus its phase velocity). The proposed structures are (i) broadband, since the approach is frequency-independent and does not require bandgaps, and (ii) capable of bending elastic waves along convex trajectories with an arbitrary curvature.

Closed-Form Solutions for Electroacoustic Wave Energy Harvesting in a Coupled Plate-Harvester System

This paper introduces an analytical framework for predicting wave energy harvested by a circular piezoelectric layer from a harmonic point source excitation. The explored acoustic system analyzes a circular piezoelectric disk attached to an infinite host domain. An harmonic point source is located away from the piezoelectric disk on the infinite host layer. The analysis approach decomposes the system into two subdomains, the piezoelectric disk and an infinite plate, which are then separately analyzed. In contrast to traditional analysis methods for such systems (a sandwich of layers with different dimensions), this technique uses internal interaction forces between the different subdomains to find a close-form solution for the vibration of propagating waves over the entire field. In addition, the voltage generated by the harvester is calculated by using coupled electromechanical equations. The analysis is validated by comparing response quantities and frequency response functions at different points on the piezoelectric circular layer and host layers to those predicted using COMSOL simulations, which document good agreement. Analysis of this system is an important stepping stone to the next goal, which is optimization of energy captured from the propagating wave by designing boundary walls (reflector) on the host layer which focus the energy of the wave onto the piezoelectric domain. This focused energy can then be transferred to the electrical power by via a piezoelectric layer through an electrical circuit.Copyright