Adel Elsabbagh

Topology optimization of unconstrained damping treatments for plates

A finite element model for composite plates consisting of an elastic isotropic base layer covered with viscoelastic treatment is presented. The composite plate undergoes bending vibrations in the lateral direction. In contrast to many previous publications, the viscoelastic treatment is not constrained from the top in order to better simulate real cases. The objective is to find the optimum distribution of viscoelastic treatment which maximizes the modal damping ratio (MDR) for a certain volume of treatment. Topology optimization is performed with two strategies: optimizing the whole domain of viscoelastic treatment and optimizing a unit cell of the periodic treatment. Numerical examples show that the presented model is able to increase the MDR by an order of magnitude compared to plain treatments.

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.

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.

Modeling and design of two-dimensional membrane-type active acoustic metamaterials with tunable anisotropic density.

A two-dimensional active acoustic metamaterial with controllable anisotropic density is introduced. The material consists of composite lead-lead zirconate titanate plates clamped to an aluminum structure with air as the background fluid. The effective anisotropic density of the material is controlled, independently for two orthogonal directions, by means of an external static electric voltage signal. The material is used in the construction of a reconfigurable waveguide capable of controlling the direction of the acoustic waves propagating through it. An analytic model based on the acoustic two-port theory, the theory of piezoelectricity, the laminated pre-stressed plate theory, and the S-parameters retrieval method is developed to predict the behavior of the material. The results are verified using the finite element method. Excellent agreement is found between both models for the studied frequency and voltage ranges. The results show that, below 1600 Hz, the density is controllable within orders of magnitude relative to the uncontrolled case. The results also suggest that simple controllers could be used to program the material density toward full control of the directivity and dispersion characteristics of acoustic waves.

Size Optimization of Stiffeners in Bending Plates.

A new approach for the modeling and optimization of bending stiffened plates is presented. Eight node elements are used in the proposed finite element model. The proposed element is hybrid in the sense that it can model both the base plate and the stiffeners, simultaneously. A sizing optimization model aiming at maximizing the natural frequencies of free vibrations at a constant volume of stiffeners is introduced. The method of moving asymptotes is used as the optimization tool. Numerical examples of different plates including an application on wind turbine blades are presented. Results show that the fundamental frequency can be doubled by the proper distribution of stiffeners.

Topology optimization of a plate coupled with acoustic cavity

Optimization of the topology of a plate coupled with an acoustic cavity is presented in an attempt to minimize the fluid-structure interactions at different structural frequencies. A mathematical model is developed to simulate such fluid-structure interactions based on the theory of finite elements. The model is integrated with a topology optimization approach which utilizes the Moving Asymptotes Method. The obtained results demonstrate the effectiveness of the proposed approach in simultaneously attenuating the structural vibration and the sound pressure inside the acoustic domain at several structural frequencies by proper redistribution of the plate material. The presented topology optimization approach can be an invaluable tool in the design of a wide variety of critical structures which must operate quietly when subjected to fluid loading.

Experimental demonstration of one-dimensional active plate-type acoustic metamaterial with adaptive programmable density

A class of active acoustic metamaterials (AMMs) with a fully controllable effective density in real-time is introduced, modeled, and experimentally verified. The density of the developed AMM can be programmed to any value ranging from −100 kg/m3 to 100 kg/m3 passing by near zero density conditions. This is achievable for any frequency between 500 and 1500 Hz. The material consists of clamped piezoelectric diaphragms with air as the background fluid. The dynamics of the diaphragms are controlled by connecting a closed feedback control loop between the piezoelectric layers of the diaphragm. The density of the material is adjustable through an outer adaptive feedback loop that is implemented by the real-time evaluation of the density using the 4-microphone technique. Applications for the new material include programmable active acoustic filters, nonsymmetric acoustic transmission, and programmable acoustic superlens.

A comparison between different finite elements for elastic and aero-elastic analyses

Graphical abstract

Finite Element Modeling of Plates with Arbitrary Oriented Isogrid Stiffeners

The main focus of this paper is the study of grid stiffened plates. The plates are modeled using the finite element approach. A 2D 8-node plate element is proposed. The advantage of the proposed element is that it can model plates with stiffeners within the element. This can yield models with many fewer elements for grid structures. A parametric study is presented to study the effect of the stiffener angle on the element stiffness in an attempt to optimize the stiffener configuration. For validation purposes, three cases are considered; plain plate, plate with straight stiffeners, and plate with isogrid stiffeners. The modal frequencies and modes of the plates are calculated using the proposed model and compared with the predictions of the commercial finite element package ANSYS® as well as experimental measurements taken on three plates manufactured by stereo lithography technique. The comparisons demonstrate the validity of the proposed model and the effectiveness of the isogrid stiffeners compared to straight stiffeners of the same mass in significantly increasing the fundamental frequency. The obtained results emphasize the potential of employing isogrid stiffeners in numerous vibration and noise control applications.

Testing and Validation of a Novel Segmented Wind Turbine Blade

This article presents the structural testing and analyses of a segmented horizontal axis wind turbine blade. The blade’s mass and center of gravity are first determined. Free vibration tests are performed, and the natural frequencies are obtained. Static bending tests in flapwise and edgewise directions are conducted with acoustics emission monitoring. The segmented blade was able to sustain the test loads. However, some superficial defects appeared and were detected by the acoustics emission sensors. Finally, the resonant fatigue test is performed for ten million cycles. The variation of the stiffness against the loading cycles is used to diagnose the initiation of fatigue cracks. Based on the testing results, some modifications are proposed in the design and manufacturing approaches to avoid the noted defects.