W. Akl

Experimental implementation of a cantilevered piezoelectric energy harvester with a dynamic magnifier

Conventional energy harvester consists of a cantilevered composite piezoelectric beam which has a proof mass at its free end while its fixed end is mounted on a vibrating base structure. The resulting relative motion between the proof mass and the base structure produces a mechanical strain in the piezoelectric elements which is converted into electrical power by virtue of the direct piezoelectric effect. In this paper, the harvester is provided with a dynamic magnifier consisting of a spring-mass system which is placed between the fixed end of the piezoelectric beam and the vibrating base structure. The main function of the dynamic magnifier, as the name implies, is to magnify the strain experienced by the piezoelectric elements in order to amplify the electrical power output of the harvester. With proper selection of the design parameters of the magnifier, the harvested power can be significantly enhanced and the effective bandwidth of the harvester can be improved. The theoretical performance of this class of Cantilevered Piezoelectric Energy Harvesters with Dynamic Magnifier (CPEHDM) is developed using ANSYS finite element analysis. The predictions of the model are validated experimentally and comparisons are presented to illustrate the merits of the CPEHDM in comparison with the conventional piezoelectric energy harvesters (CPEH). The obtained results demonstrate the feasibility of the CPEHDM as a simple and effective means for enhancing the magnitude and spectral characteristics of CPEH.

Multi-cell Active Acoustic Metamaterial with Programmable Bulk Modulus

Considerable interest has been devoted to the development of various classes of acoustic metamaterials that can control the propagation of acoustical wave energy through these materials. However, all the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this article, the emphasis is placed on the development of a new class of composite acoustic metamaterials with effective bulk moduli that are programmed to vary according to any prescribed pattern along the volume of the metamaterial. The composite consists of an acoustic cavity, which is coupled with an array of actively controlled Helmholtz resonator to enable the control of the effective bulk modulus distribution along the cavity. The theoretical analysis of this class of multi-cell composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined when the Helmholtz resonators are provided with piezoelectric boundaries. These smart boundaries are used to control the overall bulk modulus of the cavity/resonator assembly through direct acoustic pressure feedback. The interaction between the neighboring cells of the composite metamaterial is modeled using a lumped-parameter approach. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of bulk modulus variation.

Analysis and experimental demonstration of an active acoustic metamaterial cell

Active acoustic metamaterials (AAMM) have been developed to overcome the limited frequency bandwidth characteristics of passive acoustic metamaterials. The AAMM rely in their operation on using piezoelectric active ingredients in a fluid-solid composite structure forming the basic building block of a larger metamaterial periodic arrangement. A prototype of AAMM composite cell is manufactured and active control strategies are implemented on the piezoelectric elements to vary its stiffness in order to control the effective dynamic density of the cell. Acoustic characterization of the developed AAMM cell is carried out by measuring its acoustic impedance and transmission loss and comparing the results with the predictions of a finite element model. The obtained experimental measurements and the predictions of the finite element model are in very good agreement for the considered frequency range. The transfer functions between the reference microphone in the impedance tube and the piezoelectric elements demon...

Quenching of acoustic bandgaps by flow noise

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals.

Experimental characterization of active acoustic metamaterial cell with controllable dynamic density

Controlling wave propagation pattern within acoustic fluid domains has been the motivation for the acoustic metamaterials developments to target applications ranging from acoustic cloaking to passive noise control techniques. Currently, various numerical and analytical approaches exist to predict the fluid domain material properties necessary for specific propagation pattern. Physical attempts to realize such material properties have revealed engineered material constructions that are focused on predefined wave propagation patterns. In the current paper, coupled fluid-structure one-dimensional metamaterial cell, in which piezoelectric active ingredient has been introduced, is manufactured to achieve controllable dynamic density. The density-controllable cell has been manufactured by coupling a water-filled cavity with piezoelectric elements in a cell of 4.5 cm length and 4.1 cm diameter subject to impulse excitation. A finite element model of the cell has been developed and its predictions are validated against the experimental results. The validated model is utilized to predict the changes in the pressure gradient inside the developed cell which is a direct measure of the changes introduced to the dynamic density of the acoustic metamaterial domain. With such predictions, it is demonstrated that densities as high as 3.2 gm/cm3 and as low as 0.72 gm/cm3 can be achieved experimentally for excitation frequencies ranging between 100 Hz and 500 Hz.

Active Acoustic Metamaterial With Simultaneously Programmable Density and Bulk Modulus

Acoustic metamaterials are those structurally engineered materials that are composed of periodic cells designed in such a fashion to yield specific material properties (density and bulk modulus) that would affect the wave propagation pattern within in a specific way. All the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a new class of composite one-dimensional active acoustic metamaterials (CAAMM) with effective densities and bulk moduli that are programmed to vary according to any prescribed patterns along its volume. A cylindrical water-filled cylinder coupled to two piezoelectric elements form a composite cell to act as a base unit for a periodic metamaterial structure. Two different configurations are considered. In the first configuration, a piezoelectric panel is flash-mounted to the face of the cylinder, while the other is side-mounted to the cylinder wall, introducing a variable stiffness along the wave propagation path. In the second configuration, the face-mounted piezoelectric panel remains unchanged, while the side-mounted panel is replaced with an active Helmholtz resonator with piezoelectric base panel. A detailed theoretical lumped-parameter model for the two configurations is present, from which the stiffness of both active elements is controlled via charge feedback control to yield arbitrary homogenized effective bulk modulus and density over a very wide frequency range. Numerical examples are presented to demonstrate the performance characteristics of the proposed. The CAAMM presents a viable approach to the development of effective domains with a controllable wave propagation pattern to suit many applications.

Stability analysis of active acoustic metamaterial with programmable bulk modulus

Acoustic metamaterials (AMMs) have been considered as an effective means of controlling the propagation of acoustical wave energy through metamaterials. However, most of the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a new class of one-dimensional acoustic metamaterials with effective bulk moduli that are programmed to vary according to any prescribed pattern along the volume of the metamaterial. Acoustic cavities coupled with either actively controlled Helmholtz or flush-mounted resonators are introduced to develop two possible configurations for obtaining active AMMs (AAMMs) with programmable bulk modulus capabilities. The resonators are provided with piezoelectric boundaries to enable control of the overall bulk modulus of the acoustic cavity through direct acoustic pressure feedback. Theoretical analyses of these two configurations of AAMMs are presented using the lumped-parameter modeling approach. The presented analyses are utilized to study the stability characteristics of the two configurations in an attempt to define their stable regions of operation. Numerical examples are presented to demonstrate the performance characteristics of the proposed AAMM configurations and their potential for generating prescribed spatial and spectral patterns of bulk modulus variation.

Acoustic metamaterials with circular sector cavities and programmable densities

Considerable interest has been devoted to the development of various classes of acoustic metamaterials that can control the propagation of acoustical wave energy throughout fluid domains. However, all the currently exerted efforts are focused on studying passive metamaterials with fixed material properties. In this paper, the emphasis is placed on the development of a class of composite one-dimensional acoustic metamaterials with effective densities that are programmed to adapt to any prescribed pattern along the metamaterial. The proposed acoustic metamaterial is composed of a periodic arrangement of cell structures, in which each cell consists of a circular sector cavity bounded by actively controlled flexible panels to provide the capability for manipulating the overall effective dynamic density. The theoretical analysis of this class of multilayered composite active acoustic metamaterials (CAAMM) is presented and the theoretical predictions are determined for a cascading array of fluid cavities coupled to flexible piezoelectric active boundaries forming the metamaterial domain with programmable dynamic density. The stiffness of the piezoelectric boundaries is electrically manipulated to control the overall density of the individual cells utilizing the strong coupling with the fluid domain and using direct acoustic pressure feedback. The interaction between the neighboring cells of the composite metamaterial is modeled using a lumped-parameter approach. Numerical examples are presented to demonstrate the performance characteristics of the proposed CAAMM and its potential for generating prescribed spatial and spectral patterns of density variation.

Active Vibration and Noise Control using Smart Foam

A new class of smart foam is developed and attached to a flexible aluminum plate coupled with a rigid acoustic cavity in an attempt to evaluate its effectiveness in controlling the structural acoustics of the plate/cavity system. The proposed foam consists of a passive foam layer bonded to one surface of an active piezoelectric composite whose other surface is bonded to the surface of the vibrating plate. In this manner, the active piezoelectric composite can control from one side the porosity and the acoustic absorption characteristics of the foam and from the other side can suppress the vibration of the flexible plate. With such capabilities, the proposed smart foam can simultaneously control structural and acoustic cavity modes over a broad frequency range. A finite element model is presented for the behavior of the smart foam when coupled with a plate/cavity system. The predictions of the model are validated experimentally using vibration and/or sound pressure level feedback control strategies. The obtained experimental results are found to be in good agreement with the predictions of the finite element model. Furthermore, it is found that vibration and noise attenuations of about 90% are obtained with control voltages of less than 180 volts for both vibration and sound pressure feedback.

Efficient virtual reality design of quiet underwater shells

Efficient computational tools are developed to model, visualize, and feel the structural-acoustics of shells in a virtual reality environment. These tools aim at building the structural-acoustic models of shells from an array of basic building blocks including: beams, shells, and stiffeners. The concepts of finite element analysis, sub-structuring, model reduction, meta-modeling, and parallel computations form the main steps to be followed for building simplified computational models of complex shell systems. The resulting models are particularly suitable for the efficient application of multi-criteria optimization techniques in order to select the optimal design parameters of these complex shell systems. The developed integrated analysis tools enable the engineers to design complex systems in a cost effective and a timely manner. Furthermore, engineers will be immersed in an audio-visually coupled tele-operated environment whereby direct interaction and control of the design process can be achieved. In this manner, the behavior of synthetic models of shells can be monitored by literally walking through the shell and adjusting its design parameters as needed to ensure optimal performance while satisfying design and operational requirements. For example, engineers can move electronic wands to vary the number, size, type, and location of stiffeners in the shell, monitor the resulting structural-acoustic visually or by haptic feedback and simultaneously listen to the radiated sound pressure field. Such manipulations of the virtual shells in the scene are carried out while the engineer is navigating through and around the shell to ensure that the vibration and sound levels, at any critical locations, are within the acceptable limits. The developed integrated approach also serves as a means for virtual training of students and engineers on designing and operating complex smart structures on the site as well as through collaborative efforts with other virtual reality sites. Such unique capability will enable engineers to design prototypes of expensive vehicles without building them. Examples of these vehicles include aircraft, submersibles, torpedoes, and others that can share this virtual experience and can be profoundly impacted upon by the proposed approach. The presented optimal design approach is implemented in the Virtual Reality CAVE Laboratory at the University of Maryland that is controlled by an eight parallel processor Silicon Graphics Infinite Reality (ONYX2) computer.