Structurally embedded reflectors and mirrors for elastic wave focusing and energy harvesting

The harvesting of elastic waves propagating in structures can be enhanced dramatically via spatial focusing using mirror and lens concepts. Recent efforts on the mirror design have employed cylindrical stubs that form elliptical and parabolic geometries to focus structure-borne waves originating from a point source and incident plane waves, respectively. In those first-generation heuristic efforts, bulky cylindrical attachments served as effective reflectors while drastically altering thin host structures. Enabling structurally embedded mirror (SEM) configurations that avoid substantial modification of the host system requires a thorough understanding of the elastic mirror dynamics. This work presents a detailed investigation of SEM design, analysis, and experimental validation for enhanced elastic wave energy harvesting, among other applications that can benefit from spatially focused wave intensity. The SEM concept proposed in this effort uses metallic spheres (e.g., tungsten, lead, and steel) inserted into blind holes in a flat aluminum plate domain. The results show that, while SEM performance improves with property mismatch between the inclusion and the host materials, transmission resonances of the inclusions are detrimental to reflection performance and have to be avoided. A relationship between elastic mirror geometry and wavelength is unveiled to minimize the energy concentration in the side lobes around the intended focus, enabling small-sized and simple harvester design. These basic concepts and principles, demonstrated through finite-element simulations, are validated experimentally over a range of frequencies. Dramatic enhancement of the harvested power (by an order of magnitude) is also demonstrated using an elliptical SEM made from spherical tungsten inclusions in an aluminum plate. The case of a perforated mirror is also addressed briefly.