Sharif Digital Repository

In Mechanical systems there is always possibility of statical and dynamical following force. These forces are generated due to aerodynamic pressure, temperature gradient, and jet thrusters. In unstabel systems, if the value of this force exceeds a certaim amount the system becomes unstable. This value is called as critical force. In order to analyse these systems, fisrt of all, the governing dynamic equations are obtained by using the Galerkin-Ritz method. Then, by utilising the modal analysis, these equation are uncoupled. And then, the Ito set of equations are derived. By utilising the Lyapanov method, Ito equations are transfered to another state. and using the fokker-planc equation the... 

In this project, the acoustoelastic simulation of baffled cavities using boundary element method is investigated. At first step, the acoustic field for an enclosed rigid cavity with baffle is formulated and then solved by dual reciprocity boundary element method. In order to derive the governing equation, the acoustic domain is considered without mean flow and the fluid is assumed to be compressible and inviscid. Using the developed model and applying the zoning method, behavior and response of baffled cavities are investigated. In order to study the structure_acoustic interaction, modal reduced order model is used to explain the vibration of structure in combination to acoustic field. 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... 

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... 

The aeroelastic behavior of wings made of functionally graded materials in supersonic flow is studied. For this purpose the flutter speed is calculated and in the next step the post flutter behavior is investigated. The structural model is based on the classical plate theory and material properties are assumed to be graded in the thickness direction according the power law distribution in terms of the volume fraction of material constituents. The linear piston theory which is modified to include thickness effects is used for supersonic aerodynamic modeling. The nonlinear structural model based on the Von-Karman strains is considered for prediction of the post-flutter behavior of the wing.... 

This M.Sc thesis aims at studding system of fluid and solid interaction. Based on the potential flow assumption along with using the perturbation technique, the governing kinematic and dynamic boundary conditions of the first- and second-order velocity potential are derived. Then, a boundary element model is developed for the sloshing dynamics which is formulated in terms of the velocity potential of the liquid free-surface. The boundary element model is used to determine the natural mode shapes of sloshing and their corresponding frequencies. Using the modal analysis technique, a nonlinear model is represented for calculation of the first- and second-order potential which can be used to... 

In this M.Sc. thesis dynamics of a system of 3-D fluid (internal) and solid interaction hase been studied. Structure (shell) filled with dense fluid (liquid) and it is under large amplitude vibrations excitations. Shell is analized using geometrical nonlinear general shells formulations. The nonlinear analysis includes large displacements, large rotations and large strains without considering material nonlinearity. Based on the potential flow assumption the governing equations of fluid are derived. And using boundary elements formulations fluid hydrodynamic pressure in each step is drived. In order to simultaneous and mutual influence of fluid and structure, numerical model of combined... 

Static and dynamic instability of truncated conical shells made up of functionally graded materials (FGMs) conveying hot fluid is presented in this paper. Temperature-dependent material properties are considered and Material properties of the truncated FGM conical shells are functionally graded in the thickness direction according to a volume fraction power-law distribution. Governing equation of FGM conical shells is written based on First-Order Shear Deformation Theory (FSDT). The treatment is developed within the theory of linear elasticity, when materials are assumed to be isotropic and inhomogeneous through the thickness direction. The two-constituent functionally graded shell consists... 

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... 

This study considers the flutter analysis rotating blade of a turbo machinery made of magneto rheological elastomer core. The Smith theory is used for modeling the compressible subsonic flow. The classical Hamilton’s principle along with the assume mode method is used to set up the equations of motion. The validity of the derived formulation is confirmed through comparison with those obtained from the finite element software and the available results in the literature. Various parametric studies including the effects of applied magnetic field, core layer thicknesses, rotating speed, setting angle are examined for considered configuration of the rotating blade. The results show that the first... 

In this research, the effect of application of viscoelastic material with fractional derivatives pattern in aeroelastic stability analysis for a sandwich wing, has been examined. In order to study the wing’s aeroelastic behavior and the effect of inner layer on aeroelastic instability boundaries (fluttering), the wing is modeled as a cantilever sandwich beam with a fractional viscoelastic core. The Wagner approximation is used for applying the unstable aeroelastic loads. To obtain the characteristics and behaviors of the structure some assumptions including non-classical effects, are considered. These non-classical effects are the inner layer shear stress, the axial movements, the wing’s... 

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... 

The present thesis follows two aims in the phenomenon of vibration and control of cable of cable-stayed bridge under rain-wind induced vibration (RWIV). The first one is to study the response of the cable and consequently the second one aims to control these vibrations that occurred in the cable. The assumed continuous cable has lateral, in plane bending (IP), out of plane bending (OOP) and torsion mode. The effect of rainfall on the cable has been modeled with mobile upper rivulet on the cable surface and the effect and shape of lower rivulet have been neglected. RWIV is not vortex-induced vibration (VIV), nor wake galloping, because the dominant frequency of RWIV is lower than VIV and... 

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... 

Purpose of this thesis is to discover the governing equation of Flying Wing aircrafts. In Flying Wing aircrafts crew, payload, fuel, and equipment are typically housed inside the main wing structure. A clean flying wing is sometimes presented as theoretically the most aerodynamically efficient (lowest drag) design configuration for a fixed wing aircraft. We assume Flying wing as plate. Achieve structural formulation needs to use modal deflections and put them in Lagrangian equations. In the right had side, pressure on flying wing can be found. To obtain air pressure on flying wing in this thesis, applying Quasi steady aerodynamic and Howe distribution is necessary. Applying this method on ... 

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... 

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... 

In this Thesis hydroelastic analysis of 3D hydrofoil is investigated while partial sheet cavitation effects are considered. To this aim, first steady cavity boundary must be recognized.This region is calculated by means of conventional iterative procedure, stood upon potential flow theorem. In the second step, the reduced order fluid dynamics equations are derived based on potential flow theorem along with finite element method. This procedure is done in a way that real time system of equations are derived and iterative algorithm of conventional methods is omitted. To this aim, it is assumed that amplitude and frequency of the body oscillations are altered so that the cavity length in... 

In the present project an analytical model is proposed for the instability of spinning composite cylindrical shells partially filled with fluid. For this purpose, using the linearized Navier-Stokes equation for the incompressible flow, a 2-D model is developed for fluid motion at each section of the cylinder. The resultant pressure exerted on the cylinder wall as the result of the fluid motion, are calculated in terms of elastic displacements of the cylinder. Applying the Hamilton principle, the governing equations of motion of the cylinder are derived and then combined with the equations describing the fluid pressure to obtain the coupled-field equations of the structural-fluid motion.... 

Various public transportation types, e.g., Trains, Buses, and Airplanes, are susceptible to damages made by the impacts of the external objects, which are typically classified as low to medium velocity impacts. This problem reveals the significance of investigating the effects of impact on thin-walled structures, which are the main components of these vehicles' bodies. Owing to the controllable rheological properties of the Magnetorheological fluid with respect to the magnetic field, it can be utilized to control the structure exposed to impact adaptively and minimize the damages. Due to this purpose, sandwich structures such as sandwich beams and plates, thanks to their extended response...