Sharif Digital Repository

This study aims at investigating the vibration analysis and stability of Graphene Nano-Ribbon (GNR) under a magnetic field using continuum mechanics approach and an efficient hybrid modal-molecular dynamics method. The force distribution on the GNR due to the magnetic field is determined by Maxwell's equations, Biot-savart law, magnetic dipoles and Lorentz force law.
Using the continuum mechanics model, the vibration of the GNR in a magnetic field is investigated by some problems and the resonance frequencies, stability boundaries and critical load are studied.
Furthermore, in this present study, an efficient hybrid modal-molecular dynamics method is developed for the vibration... 

In static floating structures such as floating bridges and offshore runways that are in contact with the water surface from the floor and with the open air from the top, three types of interactions occur: the interaction of the structure with air, the interaction of the structure with water and Interaction of air with water. Due to the long length of such structures and approaching their natural frequencies to the standing wave frequencies (seiche frequencies), the analysis of the elastic behavior of the structure alone is not enough, but the structure-air-water interaction must be evaluated simultaneously in a system. Leading research addresses this issue and examines the behavior of an... 

The issue of fluid sloshing in vessels is of paramount importance in aerospace engineering. To suppress sloshing Baffles are used and one kind of baffles is “Perforated Baffle” that is good in weight efficiency. When fluid passes through Perforated Baffle, the energy dissipates and sloshing induces. The purpose of this study is to develop a model for linear sloshing in presence of a Perforated Baffle in the vessel. In this project, the flow is modeled by potential function. The Flow may be rotational in the nearby of baffle due to viscosity when it passes through-out perforations, so Laplace's equation for the velocity potential is useless in this region. To simulate the flow in such zones,... 

In the following research a novel method for vibration analysis of nano-structures has been presented. This method is based on equalizing the lattice stiffness to a continuous model. In this method, an equivalent continuous system is considered for each atomic lattice and for the equivalent system a modified differential equation is presented. The governing equation of the continuous system is modified in such a way that the displacement field of the atomic lattice obtained by solving this differential equation gives similar responses to the displacement field of atomic lattice. This differential equation is derived from the Taylor expansion of the displacement field about the lattice atoms.... 

In this research, the effect of the changes of the factors affecting the stability and vibrations of the rotating shaft with the middle universal joint has been investigated. Using the energy method, the rotor motion equations for two five-degree-of-freedom Jefkat rotors have been obtained. By considering the effect of system vibrations on the angle between the rotors and using the existing constraint relations at the connection point of the rotors and inserting them into the equations, finally the equation of motion of the investigated system has been obtained as seven degrees of freedom. Due to the variable speed of the output shaft, the coefficients of the degrees of freedom are dependent... 

In this study, the effect of the initial bow on vibrations and stability of the rotor with nonlinear supports has been investigated. Using the energy method, the rotor’s equations of motion for the four-degree of freedom Jeffcott rotor and the elastic rotor assuming Tymoshenko beam relations are calculated. It is very important to study the initial bow as one of the common faults in rotating machines that has a great effect on system vibrations. A method was proposed to reduce rotor vibrations and balancing systems. Choosing a way to eliminate machine defects is one of the points considered by rotor dynamic engineers. Many factors can cause a bow in the rotor, some of the most important of... 

The present research aims to achieve two major objectives in the Rotordynamic phenomenon. The first one is to study the linear and nonlinear dynamic characteristics of tilting pad journal bearings and calculating their dynamic coefficients like damping and stiffness, and the second one, studies the linear and nonlinear behavior of rotors standing on nonlinear pedestal, say nonlinear tilting pad journal bearing. By use of Partial Derivative method along with Perturbation method the linear dynamic coefficients are calculated for three different model of tilting pad journal bearing and these results are validated with the results presented by previous researches. The presented procedure is... 

Morphing concepts can potentially improve the performance and fuel consumption of modern aircrafts. Smart materials can reduce the weight and complexity, and also are of the best choices to improve efficiency and reliability for realizing morphing wings. Developing of a simple model and optimization of smart piezocomposite actuators position of a thin wing with variable camber is considered. The goal is to gain desired properties in incompressible subsonic regime. Lifting surfaces of a composite wing are simplified using shell model. Keeping the problem simple, a piezocomposite patch is used to change the shell curvature. Basic modes are extracted from FEM software and behavior of bimorph... 

A series of algorithms developed for performing aeroelastic topology optimization applying stress constraint on a three dimentional structure of a supersonic wing. The structure modeled using finite element method by three dimentional solid hexahedron elements. Bi-directional evolutionary structural optimization method used for developing optimization algorithms. In order to apply supersonic aerodynamic loading and developing aeroelastic model algorithms, the piston aerodynamic theory utilized. In order to apply static pressure loading, the maximum angle of attack of the wing performance is proposed. Using developed MATLAB code by modeling and meshing wing structure in ABAQUS a software... 

The current study is dedicated to free vibration analysis of a thin cylindrical shell with an oblique end. To this end, governing equations of motion and the corresponding boundary conditions are derived using Hamiltonian formulations. The differential form of equations is obtained by applying by part integration to the integral form of equations of motion. Equations of motions have been solved by Galerkin method for two different kinds of boundary conditions. Convergence process for different kinds of conditions has been done and results compared with papers. The effect of different parameters such as, length of cylinder, oblique angle, thickness etc. on the fundamental frequencies has been... 

Because the structural stability is directly related with structural damage, it is considered one of the most important issues in the industry. One of the applied cases in the stability issue discuss about the stability of the beam under follower loads. Follower loads obtained from aerodynamic pressure, rocket’s thrust, dry friction of the rotating disk, drilling and etc. Because the follower loads are always perpendicular to the beam cross section, thus with changing the angle of their location, their directions are changed. Spatial dependence makes a non-conservative and dynamic problem. So these loads causes dynamic instability that say flutter. In this study, the stability of a... 

The current study is dedicated to free vibration and buckling analysis of thick FGM conical shells conveying hot flows with the consideration of initial geometric imperfections. To this end, the higher order governing equations of motion and the corresponding boundary conditions are derived using Hamiltonian formulations. Due to the solution procedure of Frobenius series expansion, the differential form of equations is obtained by applying by part integration to the integral form of equations of motion. Radial and longitudinal temperature distributions are considered while pressure distribution and geometric imperfection variations are found to be in longitudinal direction, only. The final... 

The present study aims at investigating the stability of flexible spinning cylinders conveying flow in an external fluid Medium. Using the linearized Navier-Stokes equations for the flow, a two-dimensional model is developed governing the fluid motion. The resultant force exerted on the flexible cylinder wall due to the fluid interactions is calculated as a function of the lateral acceleration. Applying the Hamilton principle, the governing equations of flexural vibration of the rotary flexible cylinder mounted on simply supported axles are derived. Having the forces due to the conveying fluid calculated and substituting into the governing equations, a coupled field governing equations of... 

The purpose of this study is to develop the reduced order nonlinear modeling of single cell closed section thin walled composite beams. In this way, global behavior of one dimensional beam under axial load, bending and torsional moment is produced. This model is based on the classical lamination theory, and the nonlinear model is developedby usingthe von-karman strains. In this process the effects of material anisotropy and axial warping are considered. Numerical results are obtained for thin-walledcomposites box beams, addressing the effects of fiber angle and laminate stacking sequence. The nonlinear model is compared with theoretical results of homogeneous beams and the natural... 

The current study is dedicated to free vibration and Aeroelastic Stability Analysis of Truncated Conical Panels in Supersonic Flows. Governing equations of motion and the corresponding boundary conditions are derived using Hamiltonian formulations. The aeroelastic stability problem is formulated based on first-order shear deformation theory as well as classical shell theory with the linearized first-order piston theory for aerodynamic loading and solved using Galerkin method. The flutter boundaries are obtained for truncated conical shells with different semi-vertex cone angles, different subtended angles, and different thickness  

Here the differential equations of a Graphene sheet deformation is determined by making use of the most general form of first Strain Gradient Elasticity (SGE) theory. In order to derive the equations of in-plane and out-of-plane deformations, the matrix representation of Strain Gradient constitutive law is used. Vibration of the nano-plate is studied by modeling the plate based on Kirchhoff theory and application of variation of energy, therefor the in-plane and out-of-plane vibration of the nano-plate is found through Navier analytical solution. After all natural frequencies are obtained based on SGE parameters. SGE parameters are determined by calculating the potential of a representative... 

In this study, an analytical investigation of the hydrodynamic damping due to vertical baffles in circular cylindrical storage tanks is investigated. A lot of research about damping characteristics of different types of baffles has been done but because of specific applications of vertical baffles, studies are limited in this case. For analytical approach the average rate of energy dissipation is obtained with using the solution of Laplace’s equation by estimation the rate of the work done by baffles drag force to the oscillatory flow. The analytical solution validate by a series of experiments that conducted at the Langley Research Center of the National Aeronautics and Space... 

This thesis deals with developing and applying reduced order modeling techniques for fluid and structure interaction problems. First, the basis and formulations of reduced order modeling technique in the function space are reviewed. Then the governing equations of structural dynamics as well as incompressible flow are reviewed and some simplifications are applied. Based on the modal analysis technique along with the finite element model of the structure and the boundary element model for flow field, some reduced order models are represented. The represented models are developed for liquid sloshing in moving and elastic tanks, fluid structure interaction in flexible shells conveying flow,... 

In this project, the forced vibrations of a pair of strips carrying electric current inside the fluid have been analyzed. Euler-Bernoulli theory is used to model the structure and von-Karman strain is included in the structural equations. In terms of the degrees of freedom of the structure, the bending motion along with the twisting is defined for the current-carrying strips. In this way, the system has 4 degrees of freedom, whose equations are coupled to each other due to the mentioned assumptions. In order to model hydrodynamics and the forces corresponding to it, with the help of numerical simulation of the movement of the strip section inside the fluid (CFD analysis), the corresponding...