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    Vibration Modeling and Analysis of a Wind Turbine Blade based on Third-Order Structural Nonliearities

    , Ph.D. Dissertation Sharif University of Technology Rezaei, Mohammad Mahdi (Author) ; Behzad, Mehdi (Supervisor) ; Haddadpour, Hassan (Co-Advisor) ; Moradi, Hamed (Co-Advisor)
    Abstract
    Aiming to improve the extraction performance of wind energy has led to the noticeable increase of the structural dimensions in the modern wind turbines. The larger blade with more flexibility experiences large structural deformation even under nominal operational loading, so the nonlinear modeling and analysis of these structures have become as important subject of the recent wind turbine researches. In this dissertation, the geometrical exact model of the rotating wind turbine blade under the effects of the tower tip's motion, and also the operational loading comprising the aerodynamic and gravitational loadings is presented. In this way, the geometrical exact beam formulation is developed... 

    Analytical Nonlinear Solution of a Structurally Integrated Piezoelectric Trapizoidal Tapered Beam for Energy Harvesting

    , M.Sc. Thesis Sharif University of Technology Ahsan Esfahani, Nafiseh Sadat (Author) ; Hosseini Kordkheili, Ali (Supervisor)
    Abstract
    Tapering the beam along its length may lead to an increase in the voltage density of piezoelectric energy harvester. In this study, the energy harvester voltage with linear variable cross section is studied by developing an analytical method in both linear and nonlinear states. In linear mode, the results are compared with the experimental results. For the development of linear and nonlinear analytic relations, the Euler–Bernoulli beam and linear voltage variations in the thickness direction are used, and for the separation of variables, the mass normalized mode shapes has been used. The results of the linear analytical method are compared with the results of past research and experimental... 

    Mechanics and morphology of single-walled carbon nanotubes: From graphene to the elastica

    , Article Philosophical Magazine ; Volume 93, Issue 17 , 2013 , Pages 2057-2088 ; 14786435 (ISSN) Delfani, M. R ; Shodja, H. M ; Ojaghnezhad, F ; Sharif University of Technology
    2013
    Abstract
    The elastica is referred to the shape of the curve into which the centreline of a flexible lamina is bent. Hence, single-walled carbon nanotubes (SWCNTs) are treated as the elastica obtained from bending of graphene. The corresponding large deformation accompanies both the material and geometrical non-linearities. The morphology of the free-standing SWCNTs such as the natural angle of twist, bond lengths, tube radius and wall thickness are determined. Moreover, it is shown that the induced self-equilibriated strain field has a remarkable impact on the mechanical behaviour of the nanotube. Utilization of an appropriate non-linear continuum constitutive relation for graphene leads to exact... 

    Modeling geometric non-linearities in the free vibration of a planar beam flexure with a tip mass

    , Article Proceedings of the ASME Design Engineering Technical Conference, 12 August 2012 through 12 August 2012 ; Volume 4, Issue PARTS A AND B , August , 2012 , Pages 363-371 ; 9780791845035 (ISBN) Moeenfard, H ; Awtar, S ; Sharif University of Technology
    2012
    Abstract
    The objective of this work is to create an analytical framework to study the non-linear dynamics of beam flexures with a tip mass undergoing large deflections. Hamilton's principal is utilized to derive the equations governing the nonlinear vibrations of the cantilever beam and the associated boundary conditions. Then, using a single mode approximation, these non-linear partial differential equations are reduced to two coupled non-linear ordinary differential equations. These equations are solved analytically using combination of the method of multiple time scales and homotopy perturbation analysis. Closed-form, parametric analytical expressions are presented for the time domain response of... 

    Dynamic finite element modeling of electrostatical actuated micro structures considering squeeze film damping effect

    , Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 1096665X (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) Ahmadian, M. T ; Borhan, H ; Moghimi Zand, M ; Sharif University of Technology
    American Society of Mechanical Engineers (ASME)  2006
    Abstract
    Developing a transient fully-meshed model of coupled-domain microsystems is of paramount importance not only for accurate simulation and design but also for creating more accurate low-order or macro dynamic models. So in this paper, a complete nonlinear finite element model for coupled-domain MEMS devices considering electrostatic and squeeze film effects is presented. For this purpose, we use the Galerkin weighted-residual technique for developing the finite element model that capture the original microsystem's nonlinear behaviors, such as the structural dynamics, the squeeze-film damping, the electrostatic actuation and the geometric nonlinearity caused by inherent residual stresses. In... 

    Dynamic response of geometrically nonlinear, elastic rectangular plates under a moving mass loading by inclusion of all inertial components

    , Article Journal of Sound and Vibration ; Volume 394 , 2017 , Pages 497-514 ; 0022460X (ISSN) Rahimzadeh Rofooei, F ; Enshaeian, A ; Nikkhoo, A ; Sharif University of Technology
    Academic Press  2017
    Abstract
    Dynamic deformations of beams and plates under moving objects have extensively been studied in the past. In this work, the dynamic response of geometrically nonlinear rectangular elastic plates subjected to moving mass loading is numerically investigated. A rectangular von Karman plate with various boundary conditions is modeled using specifically developed geometrically nonlinear plate elements. In the available finite element (FE) codes the only way to distinguish between moving masses from moving loads is to model the moving mass as a separate entity. However, these procedures still do not guarantee the inclusion of all inertial effects associated with the moving mass. In a prepared... 

    Parametric Evaluation of the Y- Shaped Bracing Frame and Introducing a New Method for Design

    , M.Sc. Thesis Sharif University of Technology Sedaghati, Parshan (Author) ; Mofid, Masoud (Supervisor)
    Abstract
    This paper investigates the behavior of the Y-shaped bracing system. Because of the special type of eccentricity in this system, the behavior of the frame is strongly dependent on the geometrical changes of the braces. So that even in the elastic region, it shows the geometrically nonlinear behavior. The amount of eccentricity is very effective on the nonlinear behavior of the frame. Also the ratio of cross sections of the bracing members has a very important impact on the expected performance of the frame. The results show that this system tolerates large displacements, and can act like a base isolation system. Firstly, using a two-dimensional accurate modeling related to the geometrical... 

    Nonlinear Modelling of Fluid and Solid Interaction in Shells Conveying Fluid Using Combination of FEM and BEM

    , M.Sc. Thesis Sharif University of Technology Khalil Arjomandi, Bahram (Author) ; Haddadpour, Hassan (Supervisor) ; Kord kheili, Ali (Supervisor) ; Dehghani Firouzabadi, Rohollah (Co-Advisor)
    Abstract
    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... 

    Geometrically Nonlinear Vibration of Concrete Funicular Shells Under Impulse Loads

    , M.Sc. Thesis Sharif University of Technology Daneshmand, Niloofar (Author) ; Mofid, Massoud (Supervisor)
    Abstract
    Reinforced concrete shells are widely used to cover the small to large area with more aesthetics at minimum cost. Shell structures carry load through their shape rather than material strength. Funicular shells are special type of shells that their shape is obtained so that stresses be compressive under a special load (for shell, this load is its dead weight). This study deals with geometrically nonlinear vibration of funicular shells on a rectangular ground plan under impulse loads using nonlinear shallow shells theory. The boundary conditions are considered as clamped edges. Displacement components are product of position and time functions. The analysis is based on the expansion of... 

    Geometrically Nonlinear Random Vibration of Structures Using Finite Element Method

    , M.Sc. Thesis Sharif University of Technology Dabouee Moshkabadi, Mehdi (Author) ; Hosseini Kordkheili, Ali (Supervisor)
    Abstract
    Indeterminate behavior of some forces in the aerospace industry due to flight at high speeds, gust, combustion, etc., has led to the exposure of structures to dynamic loads with random behavior in the nonlinear manner. To analyze problems in which the loading is random or the system parameters are random, the only possible way is to describe the system response in statistical values.Since most modern structures have complex geometry and the number of degrees of freedom is very high, advanced numerical solution methods are used to obtain the system response. In this study, the geometric nonlinear vibrations of structures under random loading are investigated by the finite element... 

    Nonlinear Analysis of Vibration of Tapered Piezo Electromagnetic Beam in Presence of External Permanent Magnet in Order to Energy Harvesting Improvement

    , M.Sc. Thesis Sharif University of Technology Babaee Nikoo, Mohammad Javad (Author) ; Hosseini Kordkheili, Ali (Supervisor)
    Abstract
    With the increasing development in the field of electronics and information technology, the required dimensions and power of electronic devices and sensors have decreased. The range of applications of these electronic devices is very large, which is the product of technological progress development. There is a problem in the development and application of such devices, the supply of electrical power used in them, especially in the set of wireless sensors, this problem is more significant.The use of piezoelectric materials is one of the methods of converting mechanical energy into electrical energy. The voltage generated by the piezoelectric material can be used to charge the capacitor or... 

    A size-dependent nonlinear Timoshenko microbeam model based on the strain gradient theory

    , Article Acta Mechanica ; Volume 223, Issue 6 , 2012 , Pages 1233-1249 ; 00015970 (ISSN) Asghari, M ; Kahrobaiyan, M. H ; Nikfar, M ; Ahmadian, M. T ; Sharif University of Technology
    2012
    Abstract
    The geometrically nonlinear governing differential equations of motion and the corresponding boundary conditions are derived for the mechanical analysis of Timoshenko microbeams with large deflections, based on the strain gradient theory. The variational approach is employed to achieve the formulation. Hinged-hinged beams are considered as an important practical case, and their nonlinear static and free-vibration behaviors are investigated based on the derived formulation  

    Effects of imperfection shapes on buckling of conical shells under compression

    , Article Structural Engineering and Mechanics ; Volume 60, Issue 3 , 2016 , Pages 365-386 ; 12254568 (ISSN) Shakouri, M ; Spagnoli, A ; Kouchakzadeh, M. A ; Sharif University of Technology
    Techno Press 
    Abstract
    This paper describes a systematic numerical investigation into the nonlinear elastic behavior of conical shells, with various types of initial imperfections, subject to a uniformly distributed axial compression. Three different patterns of imperfections, including first axisymmetric linear bifurcation mode, first non-axisymmetric linear bifurcation mode, and weld depression are studied using geometrically nonlinear finite element analysis. Effects of each imperfection shape and tapering angle on imperfection sensitivity curves are investigated and the lower bound curve is determined. Finally, an empirical lower bound relation is proposed for hand calculation in the buckling design of conical... 

    Parametric study and design approach of off-center bracing systems

    , Article Structural Design of Tall and Special Buildings ; Volume 26, Issue 3 , 2017 ; 15417794 (ISSN) Sedaghati, P ; Lotfollahi, M ; Mofid, M ; Sharif University of Technology
    Abstract
    This paper presents an effective approach for the seismic design of off-center bracing systems (OBSs). The nonlinear behavior of an OBS can be specified by evaluation of two yielding stages representing tensile yielding of different bracings. This can be achieved when stiffness of the corner brace member is deliberately considered less enough to act as a fuse-like component. An accurate two-dimensional finite element modeling for the geometric and material nonlinearity of such systems considering buckling behavior of the brace members is developed. Through an extensive parametric study, the optimal ratios of the influential parameters of OBS are obtained, and their effects on the nonlinear... 

    Corotational nonlinear finite element formulation and modeling of electrostatically actuated microbeams

    , Article JVC/Journal of Vibration and Control ; Volume 15, Issue 4 , 2009 , Pages 617-640 ; 10775463 (ISSN) Ahmadian, M. T ; Borhan, H ; Esmailzadeh, E ; Sharif University of Technology
    2009
    Abstract
    A complete nonlinear finite element model for coupled-domain microelectromechanical system devices with electrostatic actuation and squeeze film effect was developed. for this purpose, a corotational finite element formulation for the dynamic analysis of planer Euler-Bernoulli microbeams considering geometrical nonlinearities due to both large structural deformation and electrostatic actuation is developed. In this method, the internal forces due to deformation and residual stresses, the elemental inertias, and the damping effect of the squeeze-film are systematically derived by consistent linearization of the fully geometrically nonlinear beam theory using d'Alembert and the virtual work... 

    On the effect of large deflection on nonlinear behavior of an eccentric bracing system

    , Article Structural Design of Tall and Special Buildings ; Volume 17, Issue 4 , 2008 , Pages 795-808 ; 15417794 (ISSN) Rasekh, A ; Mofid, M ; Khezrzadeh, H ; Sharif University of Technology
    2008
    Abstract
    In this article, a particular off-center bracing system is introduced. In this system, the tensile diagonal strut is not straight. When the load is applied, the original geometry changes and thus causes the system to harden. Based on this behavior, a step-by-step analytical method is developed. Using this method, a Geometrically Nonlinear Analysis Program (GNAP) is designed to analyze the elastic force-displacement relation. Extension of this program to dynamic analysis led to a Dynamic Nonlinear Analysis Program (DNAP). The object of this article is to show that such bracing systems will lead to less seismic force on such structures compared to classic concentric bracing systems. On the... 

    Geometrically nonlinear micro-plate formulation based on the modified couple stress theory

    , Article International Journal of Engineering Science ; Volume 51 , 2012 , Pages 292-309 ; 00207225 (ISSN) Asghari, M ; Sharif University of Technology
    2012
    Abstract
    The couple stress theory is a non-classical continuum theory which is capable to capture size effects in small-scale structures. This property makes it appropriate for modeling the structures in micron and sub-micron scales. The purpose of this paper is the derivation of the governing motion equations and boundary conditions for the geometrically nonlinear micro-plates with arbitrary shapes based on the modified version of the couple stress theory. The consistent boundary conditions are provided at smooth parts of the plate periphery and also at the sharp corners of the periphery using variational approach  

    Analysis of large amplitude free vibrations of unsymmetrically laminated composite beams on a nonlinear elastic foundation

    , Article Acta Mechanica ; Volume 219, Issue 1-2 , January , 2011 , Pages 65-75 ; 00015970 (ISSN) Jafari Talookolaei, R. A ; Salarieh, H ; Kargarnovin, M. H ; Sharif University of Technology
    2011
    Abstract
    The large amplitude free vibration of an unsymmetrically laminated composite beam (LCB) on a nonlinear elastic foundation subjected to axial load has been studied. The equation of motion for the axial and transverse deformations of a geometrically nonlinear LCB is derived. Using the Ritz method, the governing equation is reduced to a time-dependent Duffing equation with quadratic and cubic nonlinearities. The homotopy analysis method (HAM) is used to obtain exact expressions for the dynamic response of the LCB. This study shows that the third-order approximation of the HAM leads to highly accurate solutions that are valid for a wide range of vibration amplitudes. The effects of different... 

    Nonlinear analysis of 2D flexible flapping wings

    , Article Nonlinear Dynamics ; Volume 81, Issue 1-2 , July , 2015 , Pages 299-310 ; 0924090X (ISSN) Abedinnasab, M. H ; Zohoor, H ; Yoon, Yong Jin ; Sharif University of Technology
    Kluwer Academic Publishers  2015
    Abstract
    Natural flyers have flexible wings, which deform significantly under the combined inertial and aerodynamic forces. In this study, we focus on the role of chord wise flexibility in 2D pitch and plunge motions. We derive the exact nonlinear 2D equations of motion for a flexible flapping wing with flying support. In achieving the closed-form equations, we use the exact strain field concerning considerable elastic deformations. After numerically solving the novel equations, we validate them in simulations with highly deformable wings. By coupling the derived equations of motion with fluid flow, we study the aerodynamic performance of the geometrically nonlinear flexible flapping wing. Through... 

    A geometrically nonlinear beam model based on the second strain gradient theory

    , Article International Journal of Engineering Science ; Volume 91 , June , 2015 , Pages 63-75 ; 00207225 (ISSN) Karparvarfard, S. M. H ; Asghari, M ; Vatankhah, R ; Sharif University of Technology
    Elsevier Ltd  2015
    Abstract
    The geometrically nonlinear governing differential equation of motion and corresponding boundary conditions of small-scale Euler-Bernoulli beams are achieved using the second strain gradient theory. This theory is a non-classical continuum theory capable of capturing the size effects. The appearance of many higher-order material constants in the formulation can certify that it appropriately assesses the behavior of extremely small-scale structures. A hinged-hinged beam is chosen as an example to lay out the nonlinear size-dependent static bending and free vibration behaviors of the derived formulation. The results of the new model are compared with the previously obtained results based on...