Papers by Author: Abdolreza Ohadi

Abstract: In anechoic chambers, the flat-walled multilayered acoustic lining systems are cheaper and easier to install than conventional wedge type systems. Sequence, material and thickness of layers are all design variables. By choosing the right configuration, the acoustical performance and overall thic- kness of flat-walled multilayered systems can be comparable to conventional wedge systems. In this study, the materials considered include air and poroelastic materials such as polyurethane foams, melamine foams, and glass wool. In order to evaluate acoustical performance of a given configurati- on, the poroelastic materials are modeled using Biot’s formulations instead of a simpler impedance method to get more accurate results. A genetic algorithm implemented within ModeFRONTIER opti- mization software was used to select configurations which have a cut-off frequency of 100 Hz or less. The configuration that met this requirement with the smallest overall thickness was determined optimal. This configuration has an overall thickness of 65.8 cm and is composed of 4 different polyurethane foams. Since a considerable difference was observed between the cut-off frequencies obtained using Biot’s model and the simpler impedance method, this justifies the use of Biot’s model in the optimization.

Abstract: New Reddy-type elements based on Reddy’s higher-order theory are used in the analysis of composite sandwich plates with viscoelastic core, which is achieved by considering independent transverse displacements on two faces and linear variations through depth of plate core. In addition, Young modulus, rotational inertia, and kinetic energy of core are considered and core is assumed as 3D elastic structure with the properties of an orthotropic viscoelastic material. To improve the Reddy’s higher-order theory, a rectangular element with four nodes, which each node has seven degrees of freedom, is proposed. Hamilton’s principle has been used to obtain the equations of motion. A finite element code for structural response analysis of the free vibration is developed. The obtained results are compared with existence researches. The influence of material properties, lamination schemes, and fiber angle on natural frequencies of composite plates are investigated.