فهرست مطالب roohollah talebitooti

Metamaterials are manmade materials used due to their different properties originating from their geometry. Sound attenuation is a property, which depends on structure geometry; the small structures could reduce sound propagation within a mid-high frequency. To change the range of sound propagated in metamaterial from high-frequency to low-frequency, however, internal resonators could be used due to their rotational vibration having highly effect on sound attenuation. In this paper, an acoustical analysis is done on a hexachiral lattice structure with an internal resonator showing the ability to decrease sound wave propagation among the structure in the low-frequency range and causes corresponding bandgaps in this frequency range (1-4500 Hz). Geometry parameters that can affect the width and range of bandgaps are studied including the radius of the resonator, the thickness of the ligament, and the distance between each node. However, the radius of the resonator has a positive impact on the ability of attenuation, but the distance between each node has a more negative impact on the bandgap. Optimization is done on the geometric parameters considering the weight of the structure, which leads us to the construction of light structures capable of reducing sound propagation.

In this research, the frequency characteristic equation for some non-uniform and homogeneous beams based on Euler-Bernoulli beam theory is presented in closed analytical form. At both ends, the beam carries objects with deviations from the center, mass moment of inertia and linear and rotational elastic constraints. Therefore, the closed analytical characteristic equation has the ability to provide frequency parameters for a wide range of non-classical boundary conditions. Solving the governing differential equation and applying boundary conditions leads to the solution of an eigenvalue problem. Since the beam is non-uniform, the exact solution of the governing differential equation depends on finding a closed analytical solution for the beam deflection. Therefore, a limited type of non-uniform beams can be accurately solved. In order to verify the validity and accuracy of the presented relationships, the results of the method presented in this research have been compared with the existing results for uniform beams. Also, the mode shape of the deflection are given for a beam sample.

In this article, the free vibrations of non-uniform and non-homogeneous beams with the general non-classical boundary conditions including end objects with mass moment of inertia, eccentricity, and rotational and translational flexible constraints are investigated. A review of related beam sources shows that unlike many studies and various methods used to investigate the vibrations of non-uniform and non-homogeneous beams, the presented method often does not lead to a unique solution for the beam with the general boundary conditions. Hence presented solution cannot be used for the other special cases. Therefore, the purpose of this thesis is to present a new method based on the application of the Cauchy’s formula for iterated integrals which according to the best of author’s knowledge has not been considered so far. By using this method and its related equations, it is possible to analyze the vibrations of different types of homogeneous uniform, homogeneous non-uniform, functionally graded uniform, and functionally graded none-uniform beams. The validation of the obtained results for some of the studied configurations have been compared with the available data reported in related studies and good agreement observed. Also parametric study has been presented in order to investigate the effect of aspect ratio for wedge and tapered beams. To carry out convergence test and other calculations throughout this article a code in Mathematica software has been written.

In this paper, the effects of higher-order terms in aerodynamic force model have been investigated on the response of galloping piezoelectric energy harvesters. The system comprised a PZT beam with a bluff body and was exposed to a fluid force. First, the dimensionless governing electromechanical equations were provided. To model the aerodynamic force, the 3rd and 7th order galloping models have been employed by adopting the quasi-steady assumption. Then, the dynamic response based on the 3rd and 7th order aerodynamic force models has been studied using a numerical integration method. Besides, an approximateanalytical solution based on the multiple scales method (MSM) has been provided. Next, the mechanical and electrical responses of the system are obtained using the MSM solutions. Finally, the optimum electrical power and the corresponding dimensionless load resistance have been obtained. The results reveal that considering higher-order terms in aerodynamic force expansion is necessary for accurate characterization of the mechanical and electrical responses.

Vibroacoustic behavior of graphene reinforced composite plate (GRCP) is studied to investigate the sound transmission loss (STL), Due to the importance of the behavior of the structure against sound waves. Many materials have been used to increase the stiffness and improve the vibroacoustic behavior of sandwich structures. which in recent years, Graphene has prompted many researchers to use it in sandwich structures. Governing equations are derived based on three-dimensional elasticity and state vector method is used to solve equations of motion. To evaluate the validity of the results from previous studies, two studies that have examined the sound transmission loss of plate have been used. To find the effective parameters on the amount of plate STL, changes in graphene weight fraction, plate thickness, incidence angle, thickness of graphene nanoplatelets (GPLs) and width of GPLs are investigated. Finally, numerical results show that adding a small amount of GPLs to the composite matrix increases its stiffness and thus improves the behavior of the structure against the pressure of sound waves. Also, Increasing the plate thickness has a good effect on the STL, and increasing width of GPLs with constant length has no effect on the vibroacoustic behavior of the structure.

In this paper, the Three-dimensional (3-D) theory of elasticity has been used for sound transmission loss (TL) of a thin-walled cylindrical shell with infinite length. This structure is excited by an obliquely plane wave. Governing equations of the thin shell have been derived in radial, axial, and circumferential directions. Then, Helmholtz decomposition is applied to solve the equations. Therefore, the displacement field is considered in terms of Lame potential functions This method describes the vibrational behavior of the shells more accurately than other theories, including the classical theory and the first and third order of shear, due to the consideration of the effects of rotation and shear. A comparison of the present method results with those obtained from classical shell theory (CST) indicates an excellent agreement. However, at the high frequencies the classical encounter insufficient accuracies as a result of increasing the rotational terms as well as shear wave effects. The results show that with increasing shell thickness, due to increased flexural stiffness of the shell, the sound transmission loss has increased. By increasing the Mach number, due to the negative stiffness, reduces the sound transmission loss in the stiffness-controlled region and conversely increases the sound transmission loss in the mass-controlled region due to damping in the structure. Finally, it is shown that the critical and coincidence frequencies increase with the growing up Mach number.

The main goal of this research is to provide a more detailed investigation of the size-dependent response of magneto-electro-thermo-elastic (METE) nanobeams subjected to propagating wave, employing sinusoidal shear deformation beam theory (SSDBT). With the aim to consider the size influences of the structure, the nonlocal strain gradient theory (NSGT) is utilized. Hamilton’s principle within constitutive relations of METE materials is incorporated to derive thegoverning equations. Utilizing Maxwell’s relation and magnet-electric boundary conditions, proper distributions for magnetic and electric potentials along the nanobeam are obtained. Thereafter an exact analysis is used to obtain the axial and flexural dispersion relations of METE nanobeams. In numerical results, detailed investigations of wave dispersion behavior related to three modes are addressed. In addition, a relation is introduced to determine the cut-off frequency of the system. Moreover, the effectiveness of various parametersincluding length scale and nonlocal parameters, nanobeam thickness, and theloadings due to imposed thermo-electro-magnetic field on the response ofpropagating wave in METE nanobeams are examined.

In this paper, an analytical solution is presented to study acoustic transmission through a laminated composite cylindrical shell. Therefore, a new and exact model is employed to solve the three-dimensional sound transmission loss in a composite cylindrical shell based on three-dimensional linear theory of elasticity. The model presented in this paper is a composite cylindrical shell with arbitrary thickness and infinite length and immersed in a fluid. Also an oblique plane wave impinges on the external sidewall of the shell. The state space method is used to investigate laminate approximated model along with transfer matrix approach to analysis the sound transmission loss though the composite cylindrical shells. The results obtained from presents study have been compared with those of other researchers. This comparison shows an excellent agreement between the results. Finally, the parameters affecting on the sound transmission loss have been studied. The results indicate that the laminated composite cylindrical shells are more advantageous rather than other materials as a result of enhancing the sound transmission loss with change the stacking sequence of laminated composite. This characteristic can be used to increase the amount of sound transmission loss in composite shells.

In this paper, sound transmission loss through double-walled orthotropic cylindrical shells based on three-dimensional elasticity theory is investigated. Hence, the purpose of this paper is to analyze the effect of the acoustic wave incidence under two different angels on sound transmission loss through the shell. The present model is a double-walled orthotropic cylindrical shell immersed in a fluid with an infinite length, whereas the acoustic plane incident waves impinge upon the shell with two different angels of θ and δ. The state space method is used to investigate the laminate approximated model along with transfer matrix approach for modeling both walls of cylindrical shell. In order to consider the two different angles of θ and δ, the corresponding wave equations have been modified according to the wave numbers. Comparing the results obtained from presents study with those of other researchers shows an excellent agreement between the results. Moreover, the effects of different parameters on sound transmission loss through the shell have been evaluated. The results show an enhancement of sound transmission loss in double-walled cylindrical shells rather than single-walled cylindrical shells particularly in high frequency range. Also, the results indicate the dependency of sound transmission loss on both of two θ and δ angels. In other word, the variety of two incident angles may cause the significant variations in sound transmission loss.

In this thesis for improving in samand behavior in besides collision، internal part of B-pillar of SAMAND modeling by use of laminated composite made up of carbon – epoxy and simulation beside collision test based on FMVSS-214 standard. This survey is performance on finite element analys and show the condition and quality of stacking sequence of difrent layers. Since change in orientation layers cause of change in structure resistance affected by collision، using of Genetic Algurithm and neural lattice get better result for minimum transfiguration of B-pillar. Also to achive the steady stacking sequence use the result of 49 Taguchi’s studys and simulation for 8 layers in -90 to 90 degree and 120 Hamersly’s tests for 8 layers in 0 to 90 degree، and 120 Hamersly test for 4 layers in 0 ti 90 degree. Based on this result and gained data from optimization algoritm and complet survey of test result، the best collision behavior of structure occure in 40 to 50 degree stacking sequence. Stracture we offer in this study own minimum transfiguration and displacement. And System in this manner، exist maximum safty and security for passengers that is the most important chalengein besides couission.

In this paper, classical thin shell theory is used to analyze vibration and critical speed of simply supported rotating orthotropic cylindrical shell. The effects of centrifugal and Coriolis forces due to the rotation are considered in the present for mulation.. In addition, axial load is applied on cylinder as a ratio of critical buckling load. Finally the effects of orthotropic ratio, material and geometry of the shell as well as axial loads on bifurcation of natural frequency are investigated.