Sharif Digital Repository

In the current study, stability and nonlinear vibration of bending-torsional extensible beams conveying internal pulsatile and uniform external flow are studied. The internal and external fluid flows are assumed to be incompressible and frictionless. Applying the extensibility to the pipe along its length and using the nonlinear Euler-Bernoulli beam model and including some appropriate additional assumptions, the 3D nonlinear equations of motion of a cantilevered composite pipe is obtained. In order to derive the equations governing the fluid-structure interaction, the internal flow effect is modeled as a distributed load along the pipe which contains the inertia, Coriolis and centrifugal... 

Nowadays Aeroelasticity is one of most important branch of aerospace science. The Aeroelastic analysis and get on aeroelastic safety margin, is one of the major steps that must be taken in flying crafts design. Diverse software have been developed for this purpose. Due to the increasing the demands of use of composite materials in aircraft structures, need to examine the effect of various parameters such as angles and stacking sequence on the aeroelastic instability (flutter and divergence) composite aircraft structures is determined. The analysis of wind turbines as a means of extracting energy from the wind and having a large diameter blades, causing the interference effects of aerodynamic... 

In this thesis, the dynamic behavior of a pinned-pinned fractional viscoelastic pipe conveying fluid is examined in the external cross flow. The Galerkin method is employed to discretize the nonlinear coupled equations of motion for viscoelastic pipe conveying fluid in the external cross flow. Consequently, four modes of system are obtained. In addition, direct perturbation method of multiple scale is used to solve the governing nonlinear coupled equations of motion for the fractional viscoelastic pipe conveying pulsating fluid in the external cross flow. Moreover, time response diagrams are drawn in order to investigate under effects of the internal fluid velocity, external fluid reduced... 

In this thesis, the effect of elastodynamic behaviour of an interceptor on it’s trajectory in space and accuracy in desired positioning and situation is investigated. The governing equations of motion are derived and linearized by using the basic equations of motion in the body frame axes and the mode summation method for elastic deformations. In order to control the interceptor, the closed-loop control on the yaw and pitch angles, is considered. The governing equations of motion are solved to determine the time response to investigate the structural effects on the stability, position and situation of interceptor. Finally, the effect of elasticity and vibrations on the interceptor’s... 

In the present investigation, graphene sheets are simulated as Nano resonators using Kirchhoff plate theory and nonlocal continuum theory. External force and interlayer forces are simulated as harmonic and Winkler-like elastic foundation, respectively. Equations are solved analytically and numerically and corresponding results are presented. Considering a two layer resonator, natural frequencies and oscillation amplitude are investigated by free vibration and forced vibration analysis. Also, the effects of parameters such as small scale, sheets dimensions and inciting frequency on the vibration of the resonator, are displayed  

Thin Walled Beams are widely used in engineering applications with the minimum weight design criteria, ranging from civil to aerospace and many other industrial fields. The weight reduction of the beam leads to importance of the dynamic behavior of the structure. Frequently used thin walled beams have low torsional stiffness and their torsional deformations may be of such magnitudes that it is not adequate to treat the angles of cross section rotation as small. When the vibration amplitudes are moderate or large, the geometric nonlinearity must be included and some new phenomena which do not exist in linear torsional dynamic come into play. Unfortunately, the equations of motion of nonlinear... 

In the present work, dynamic behavior of submerged slender bodies under external hydrodynamic forces with internal sloshing tanks is studied. The main application of this thesis is in submarines. For this purpose slender body theory is used for hydrodyanamics of body and unsteady hydrodynamics is used for the vehicle control fins. In submarines there are some water tanks that control the buoyancy, so that sloshing can be importatnt. An equivalent mechanical model is developed for simulating the dynamics of sloshing tanks. Assuming that a submarine is a slender body, elastic deformation of submarine body can affect dynamics of the whole system. Elastic behavior of the main body is simulated... 

In this study, high frequency vibration analysis of two connected plates by a beam interface is carried out using Energy Finite Element Analysis (EFEA). In structural vibration analysis at low frequencies, classical methods such as Finite Element Method or Boundary Element Method present accurate results. But, using these methods at high frequencies requires expense of high calculation cost and time. Therefore, using of statistical methods that are based on average estimates of vibrations, at high frequencies is most popular. EFEA is a method that gives average response of structure at a certain point and frequency band. This method is developed on the basis of finite element analysis of... 

In this thesis, the high frequency random vibrations analysis of a reinforced plate with a cutout is carried out using Energy Finite Element Analysis (EFEA) and the final results are validated with experimental results. EFEA method uses the average energy density as the primary variable to form the governing differential equations of energy flow in the structure. In this method the equations of space and time averaged energy density are derived in the Far Field and the energy intensity is considered as continues variable over the structure. In this work, at first for simple beam and plate an EFEA code is developed. Then the relationships for joints are derived using structural dynamics. Some... 

The present thesis aims at investigating the dynamic interaction of slosh and aeroelasticity in the fuel contained aero vehicles. The main approach in this research is to develop reduced order models for description of the coupled system. In this regard, boundary element method is used to develop slosh dynamic model. Axisymmetric boundary element method with non-symmetric boundary conditions is used to develop slosh dynamic model for axisymmetric containers. Zoning method is used to develop slosh dynamic model for the multi-baffled tanks and based on it, slosh equivalent mechanical model is developed for multi-baffled tanks. Finite element method along with modal technique is used to develop... 

In this study the interaction of sloshing and structural vibrations equations in tanks are investigated and developed. For this purpose the governing equations of structural dynamics and potential flow are used and a reduce order model for fluid is derived. Three methods are used for external forces calculation of structural dynamics model which are the sloshing fluid, quiescent fluid and lumped mass. Then free vibrations of coupled system for launch vehicle with two tanks is investigated. The results of this study are compared and validated with existent results in the literature and finally the effects of fluid height in tank, tank thickness and parameters of launch vehicle like... 

Vibrations with high amplitudes in continuous systems and models with multiple degrees of freedom, in specific conditions, is concomitant with a phenomenon called internal resonance. In the presence of this phenomenon, energy is transferred from one directly excited mode to the other vibrating modes of the structure results in response of the structure to be combination of the excited modes. In that situation occurrence of the internal resonance may decrease the level of undesired vibrations of the structure. This behavior is of interest especially in sandwich structures with viscoelastic cores that are designated to improve the damping characteristics of the structure and the property may... 

In this project, the landing process is simulated by using of multi-body dynamics commercial software. Various factors, including landing situations, aircraft structures and climate are used in this simulation. The purpose of this project is to determine the forces exerted on the aircraft landing gears in landing process in various landing conditions. For this purpose, the ADAMS multi-body dynamics software is used. At first, different scenarios based on FAR-25, including level landing, tail-down landing, crab landing are simulated. Results of dynamic simulation software with landing load factor obtained from the analytical solution are compared. Then for the level landing, different... 

The present study investigates the vibrational behavior of a special type of nano-motor based on a thin cylindrical shell model and the nonlocal elasticity theory of Eringen. The presumed nano-motor is comprised of three single walled carbon nanotubes (CNT), where two of them are cantilevered and the third one is located in the middle. The CNT in the middle will suspend due to the Van-der-Waals forces generated from the interaction by the cantilevered CNT pair. Some proper chirality arrangements can conduct electricity and make the middle CNT rotate in extremely high speeds. In fact, the middle CNT rotates on a frictionless elastic foundation. In the present study, the nano-motor is modeled... 

In the present work the stability analysis for the coupled aeroelasticity and fuel sloshing in supersonic wings is represented. The wing and store structure is modeled using the finite element method. The supersonic wing aerodynamic is modeled by the linear piston theory and store aerodynamic is modeled by the semi-stable slender body piston theory. Fuel sloshing in the store with desired geometry is modeled by the boundary element method. The coupled structure, aerodynamic and sloshing governing equations are drawn by using the Galerkin method and reduce order modeling teqnique for them and an effective numerical model was developed that is capable to analyze the stability for... 

In this thesis, vibration analysis of multi-walled carbon nanoconesstudied by molecular mechanics approach. In this simulation, atoms of carbon and bondings of them modeled by concentrated mass and structural elements, respectively and then mode shapes and natural frequencies of these structures calculated and effects of height, apex angle, boundary conditions and number of layers on natural frequencies of carbon nanocones studied. Also, critical buckling load of multi-walled carbon nanocones due to axial and in-plane loads obtained and effects of height, apex angle and number of layers on critical buckling loads of carbon nanocones studied. These results validated by molecular dynamics and... 

Different aspects of oscillatory behaviors of sandwich structures with viscoelastic cores are investigated in the present study in four parts. In the first part, an analytical study is conducted to determine the variation of linear frequencies and damping ratio of different families of modes with system parameters. The participation of different families of modes such as the so-called pumping or thickness-shear modes along with the overall bending modes in the transverse response of the structure to transverse wide-band excitation is also investigated in this part. The formulation of the problem is established by assuming quadratic variation of displacement components of the core through the... 

Carbo nanocones are one of the nanostructures that are investigated and takes a large deal of attention in nanotechnology. In the present study the governing equilibrium equations of motion of carbon nanotubes under external pressure are derived using a nonlocal shell model and first order shear deformation theory. The natural frequency and buckling load are extracted using modal method along with Galerkin technique.Afterthis using both Winkler and Pasternak elastic foundation models, the governing equations of motion of nanocones embedded in the elastic medium extracted using a novel approach with the nonlocal shell model along with Hamilton’s principle. These equations solved using... 

This study presents a developed successive boundary element method to determine the sloshing natural frequencies, mode shapes and equivalent mechanical models for multi baffled 2D and axisymmetric containers with arbitrary geometries. The developed fluid model is based on the Laplace equation and Green's theorem. The governing equations of fluid dynamic and free surface boundary condition are also applied to proposed model. A zoning method is presented to model arbitrary arrangement of baffles in multi baffled axisymmetric tanks. The influence of each zone on neighboring zones is applied by introducing interface influence matrix which correlates the velocity potential of interfaces to their... 

In this thesis a mutliScale model based on the Cauchy-Born hypothesis and via usage of XFEM is proposed for crack modeling. By solving an example, the important of surface effects in the surface stresses region is shown. Considering not being able to model the surface effects with the Cauchy-Born method, the boundary Cauchy-Born method for modeling crack effects is used. Moreover, three Molecular Dynamics method for modeling crack will be proposed. According to the obtained results from these methods, it was deduced that for calculating the correct surface stresses in Molecular Dynamics the mutual interaction of upper and lower atoms of crack should be omitted. Finally, the validation of...