Search for: natural-frequencies
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    Free vibration analysis of joined cross-ply laminated conical shells

    , Article International Journal of Mechanical Sciences ; Vol. 78, issue , 2014 , pp. 118-125 ; ISSN: 00207403 Kouchakzadeh, M. A ; Shakouri, M ; Sharif University of Technology
    The present study deals with vibrational behavior of two joined cross-ply laminated conical shells. The natural frequencies and mode shapes are investigated. The joined conical shells can be considered as the general case for joined cylindrical-conical shells, joined cylinder-plates or cone-plates, cylindrical and conical shells with stepped thicknesses and also annular plates. Governing equations are obtained using thin-walled shallow shell theory of Donnell type and Hamilton's principle. The appropriate expressions among stress resultants and deformations are extracted as continuity conditions at the joining section of the cones. The equations are solved assuming trigonometric response in... 

    Analytical and Experimental Study of Transverse Vibration of Ulerr-Bernoulli Beam with Dry Friction Boundary Condition

    , M.Sc. Thesis Sharif University of Technology Kavyanpoor, Mobin (Author) ; Dehghani Firouzabadi, Roohollah (Supervisor)
    The dry friction and its effect on the dynamics and vibrations of structures have always been one of the remarkable engineering issues. In the structures of engineering, dry friction mostly exists at the joints of structures and substrates and dry friction can lead to the nonlinearity and even chaos in the vibration of structures. There are different theories for the friction modeling and the most famous theory is the relation between the proportional ratio of the force of friction and perpendicular force to the surface. The main characteristic of friction as a damper force is the change of direction along with change in the direction of velocity. In the present study, at first, the nature... 

    Natural frequencies of C60, C70, and C80 fullerenes

    , Article Applied Physics Letters ; Volume 96, Issue 2 , 2010 ; 00036951 (ISSN) Sakhaee Pour, A ; Vafai, A ; Sharif University of Technology
    This letter adopts an atomistic modeling approach to study free vibrational characteristics of C60, C70, and C80 fullerenes. In this regard, we use the molecular structural mechanics consisting of equivalent structural beams to calculate the nonzero natural frequencies. The simulation results indicate that the first natural frequency of the fullerene is in the order terahertz and decreases nonlinearly with respect to the number of the carbon atoms  

    Comparison of the existing methods in determination of the characteristic polynomial

    , Article Wec 05: Fourth World Enformatika Conference, Istanbul, 24 June 2005 through 26 June 2005 ; Volume 6 , 2005 , Pages 130-133 ; 9759845857 (ISBN); 9789759845858 (ISBN) Tavazoei, M. S ; Haeri, M ; Sharif University of Technology
    This paper presents comparison among methods of determination of the characteristic polynomial coefficients. First, the resultant systems from the methods are compared based on frequency criteria such as the closed loop bandwidth, gain and phase margins. Then the step responses of the resultant systems are compared on the basis of the transient behavior criteria including overshoot, rise time, settling time and error (via IAE, ITAE, ISE and ITSE integral indices). Also relative stability of the systems is compared together. Finally the best choices in regards to the above diverse criteria are presented. COPYRIGHT © ENFORMATIKA  

    Force and moment measurements on a fighter model in heaving motion

    , Article 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 10 January 2005 through 13 January 2005 ; 2005 , Pages 6197-6206 Davari, A. R ; Soltani, M. R ; Sharif University of Technology
    American Institute of Aeronautics and Astronautics Inc  2005
    Extensive wind tunnel tests have been conducted to study the unsteady aerodynamic behavior of a standard dynamics model, SDM, oscillating in plunge mode. The unsteady aerodynamic behavior of various missile and aircraft configurations under pitching motion has been studied so far. However up to now, there is little or no result on the plunging behavior of an aircraft or missile as a whole and the present experiments can be considered as one of the first attempts to study the compressible flowfield over an aircraft undergoing plunging motion. The experiments involved measuring the normal force and pitching moment of the model oscillating in plunge at Mach numbers of 0.4, 0.6 and 1.5 and... 

    Effect of centrifugal force on natural frequency of lateral vibration of rotating shafts

    , Article Journal of Sound and Vibration ; Volume 274, Issue 3-5 , 2004 , Pages 985-995 ; 0022460X (ISSN) Behzad, M ; Bastami, A. R ; Sharif University of Technology
    Academic Press  2004
    This paper investigates the effect of shaft rotation on its natural frequency. Apart from gyroscopic effect, the axial force originated from centrifugal force and the Poisson effect results in change of shaft natural frequency. D'Alembert principle for shaft in cylindrical co-ordinate system, along with the stress-strain relation, gives the non-homogenous linear differential equation, which can be used to calculate axial stress in the shaft. Numerical results of this study show that axial stress produced by shaft rotation has a major effect on the natural frequency of long high-speed shafts, while shaft diameter has no influence on the results. In addition, change in lateral natural... 

    Nonlinear free vibrations of a Timoshenko beam using multiple scales method

    , Article Proceedings of the 7th Biennial Conference on Engineering Systems Design and Analysis - 2004, Manchester, 19 July 2004 through 22 July 2004 ; Volume 2 , 2004 , Pages 101-106 ; 0791841731 (ISBN); 9780791841730 (ISBN) Ramezani, A ; Ghorashi, M ; Sharif University of Technology
    American Society of Mechanical Engineers  2004
    In this paper, the large amplitude free vibration of a cantilever Timoshenko beam is considered. To this end, first Hamilton's principle is used in deriving the partial differential equation of the beam response under the mentioned conditions. Then, implementing the Galerkin's method the partial differential equation is converted to an ordinary nonlinear differential equation. Finally, the method of multiple scales is used to determine a second order perturbation solution for the obtained ODE. The results show that nonlinearity acts in the direction of increasing the natural frequency of the thick-cantilevered beam  

    Saturation and its application in the vibration control of nonlinear systems

    , Article Proceedings of the 7th Biennial Conference on Engineering Systems Design and Analysis - 2004, Manchester, 19 July 2004 through 22 July 2004 ; Volume 2 , 2004 , Pages 93-100 ; 0791841731 (ISBN); 9780791841730 (ISBN) Shoeybi, M ; Ghorashi, M ; Sharif University of Technology
    American Society of Mechanical Engineers  2004
    An investigation on the nonlinear vibrations of a system with two degrees of freedom when subjected to saturation is presented. The method is especially applied to a system that consists of a DC motor with a nonlinear controller and subjected to a harmonic forcing voltage. Approximate solutions are sought using the method of multiple scales. Also, singular point analysis is used to study stability of solutions. These findings are then compared with the corresponding numerical results. Good agreement between the two is observed  

    On self-similar behaviour of the hysteresis loops in pitching motions

    , Article Aeronautical Journal ; Volume 108, Issue 1086 , 2004 , Pages 427-434 ; 00019240 (ISSN) Soltani, M. R ; Davari, A. R ; Sharif University of Technology
    Royal Aeronautical Society  2004
    A new similarity parameter has been used for analyzing the unsteady aerodynamic, behaviour of vehicles undergoing sinusoidal pitching motion. If this parameter is identical for two unsteady manoeuvres with different reduced frequencies and oscillation amplitudes, the corresponding hysteresis loops of the force and moment collapse on each other, To support and verify this, extensive unsteady wind tunnel tests have been conducted on a standard model, which is a well known fighter type configuration. The acquired data were used to train a certain type of neural network, called the Generalised Regression Neural Network (GRNN), to reduce the number of wind tunnel runs. The scheme, once proved to... 

    Design and implementation of a new spherical super element in structural analysis

    , Article Applied Mathematics and Computation ; Volume 218, Issue 14 , March , 2012 , Pages 7546-7561 ; 00963003 (ISSN) Sarvi, M. N ; Ahmadian, M. T ; Sharif University of Technology
    Finite Element Method (FEM) has proved to be a powerful tool for the analysis of mechanical problems such as finding natural frequency and deformation of structures. Design and implementation of super elements with a purpose in reduction of computational time along with accuracy is one of the challenges facing engineers in the past decade. In this study a newly spherical super element is designed and implemented to some problems. This element is generated in such a way that the user can select as many numbers of nodes as desired. Proper formulation is presented to generate the shape function for each node in this element. Some examples of static and vibration analysis using this element are... 

    Accurate calculation of the natural frequencies of reticulated and solid cylindrical composite shells

    , Article Applied Mechanics and Materials, 29 July 2011 through 31 July 2011, Bangkok ; Volume 110-116 , July , 2012 , Pages 4598-4606 ; 16609336 (ISSN) ; 9783037852620 (ISBN) Hashemian, A. H ; Kargarnovin, M. H ; Jam, J. E ; Sharif University of Technology
    There are hundreds models of reticulated structures including the squared reticulated cylindrical shells. It is considered as comprising of a number of circumferential and longitudinal rods. Analytical governing equation for natural frequencies has been derived for this type of structures and to verify the validity of solutions, Finite Element Method (FEM) is used. The comparison of results demonstrate close agreement between analytical and FE solutions. Also a comparison is preformed between a reticulated and equivalent solid hollow cylinder shell. The equivalent solid hollow cylinder has equal weight, length and outer diameter with the squared reticulated cylindrical shell. This comparison... 

    Free and forced vibration analysis of FG beam considering temperature dependency of material properties

    , Article Journal of Mechanical Science and Technology ; Volume 25, Issue 1 , 2011 , Pages 69-80 ; 1738494X (ISSN) Azadi, M ; Sharif University of Technology
    This paper presents a finite element method (FEM) free and forced lateral vibration analysis of beams made of functionally graded materials (FGMs). The temperature dependency of material properties along with damping had not previously been taken into account in vibration analysis. In the present study, the material properties were assumed to be temperature-dependent, and were graded in the thickness direction according to a simple power law distribution of the volume fractions of the constituents. The natural frequencies were obtained for functionally graded (FG) beams with various boundary conditions. First, an FG beam was assumed to be isotropic (metal rich) and the results were compared... 

    Theoretical and experimental analysis of the free vibrations of a shell made of n cone segments joined together

    , Article Thin-Walled Structures ; Volume 108 , 2016 , Pages 416-427 ; 02638231 (ISSN) Sarkheil, S ; Saadat Foumani, M ; Navazi, H. M ; Sharif University of Technology
    Elsevier Ltd 
    This paper investigates the free vibrations of a shell made of n cone segments joined together. The governing equations of the conical shell were obtained by applying the Sanders shell theory and the Hamilton principle. Then, these governing equations are solved by using the power series method and considering a displacement field which is harmonic function about the time and the circumferential coordinate. Using the boundary conditions of the two ends of the shell and the continuity conditions at the interface section of shell segments, and solving the eigenvalue problem, the natural frequencies and the mode shapes are obtained. Very good agreements exist between the analytical results of... 

    Bridged single-walled carbon nanotube-based atomic-scale mass sensors

    , Article Applied Physics A: Materials Science and Processing ; Volume 122, Issue 8 , Volume 122, Issue 8 , 2016 ; 09478396 (ISSN) Ali Akbari, H. R ; Shaat, M ; Abdelkefi, A ; Sharif University of Technology
    Springer Verlag 
    The potentials of carbon nanotubes (CNTs) as mechanical resonators for atomic-scale mass sensing are presented. To this aim, a nonlocal continuum-based model is proposed to study the dynamic behavior of bridged single-walled carbon nanotube-based mass nanosensors. The carbon nanotube (CNT) is considered as an elastic Euler–Bernoulli beam with von Kármán type geometric nonlinearity. Eringen’s nonlocal elastic field theory is utilized to model the interatomic long-range interactions within the structure of the CNT. This developed model accounts for the arbitrary position of the deposited atomic-mass. The natural frequencies and associated mode shapes are determined based on an eigenvalue... 

    Analysis of in-plane vibration and critical speeds of the functionally graded rotating disks

    , Article International Journal of Applied Mechanics ; Volume 11, Issue 2 , 2019 ; 17588251 (ISSN) Bagheri, E ; Jahangiri, M ; Sharif University of Technology
    World Scientific Publishing Co. Pte Ltd  2019
    In this paper, the in-plane free vibration analysis of the functionally graded rotating disks with variable thickness is presented utilizing DQM. It is assumed that the rotational velocity of the disk is constant and the thickness and material properties including modulus of elasticity and density vary along the radial coordinate. The distribution of the forward and backward traveling waves versus the angular velocity is demonstrated for several modal circles and nodal diameters with respect to the fixed and rotating coordinate systems. After presenting the accuracy and convergence of the numerical method, the derived formulation and the solution method are validated by comparing the results... 

    Effects of magnetoelastic loads on free vibration characteristics of the magnetorheological-based sandwich beam

    , Article Journal of Intelligent Material Systems and Structures ; Volume 31, Issue 7 , 2020 , Pages 1015-1028 Rokn Abadi, M. R ; Shahali, P ; Haddadpour, H ; Sharif University of Technology
    SAGE Publications Ltd  2020
    In this contribution, we have investigated the effects of magnetoelastic loads on free vibration characteristics of the magnetorheological-based sandwich beam. The considered sandwich beam consists of a magnetorheological core with elastic top and base layers. For these means, the structural governing equations are derived using the Hamilton principle and solved by the finite element method. The results are validated in comparison with the existing results in the literature. The effects of variation in the parameters such as magnetic field intensity and the thickness of the core and top layers on the deviation of the first natural frequency and the corresponding loss factor are studied as... 

    Analysis of Vibrations and Dynamics of Sandwich Panels Under Dynamically Moving Loads

    , M.Sc. Thesis Sharif University of Technology Niknami, Ali (Author) ; Ahmadian, Mohammad Taghi (Supervisor)
    The purpose of this project is the analysis of free and forced vibrations of sandwich panels under moving loads. In order to analyze the problem, sandwich panels are modeled as composite square plates. By using third order shear deformation theory, equations of motion are derived. Equations of motion with six different boundary conditions are solved utilizing generalized differential quadrature method. Natural frequencies are obtained for simple cases reported in the literature.In addition, a sandwich panel under moving load with simply supported boundary conditions is analyzed and time series response of the middle point is obtained. The time series response is also studied parametrically... 

    Vibration Control of Micro-Satellite Shelves Using Shpae Memory Alloy

    , M.Sc. Thesis Sharif University of Technology Hedayati Dezfuli, Farshad (Author) ; Abedian, Ali (Supervisor)
    Dynamic properties such as the natural frequency of a structure have a significant role in the design process. The importance of this consideration is because of resonance. In this case, the amplitude of the vibration is magnified to levels that may lead to a catastrophic event. While the usual design process depends on the collected experiences and statistical data, a developing trend is to implement smart technologies to develop smart structures that are capable of self-monitoring, diagnostics and repair. Smart material components such as shape memory alloys (SMAs) and piezoelectric ceramics are increasingly being used by vibration control practitioners. Shape memory alloys are applied as... 

    Vibration and Stability Analysis of Multiwall Carbon Nanocones on Elastic Foundation Using Nonlocal Theorem

    , Ph.D. Dissertation Sharif University of Technology Fotouhi, Mohammad Mostafa (Author) ; Haddadpour, Hassan (Supervisor) ; Dehghani Firoozabadi, Rouhollah (Supervisor)
    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... 

    Detection and extraction of periodic noises in audio and biomedical signals using Kalman filter

    , Article Signal Processing ; Volume 88, Issue 8 , August , 2008 , Pages 2114-2121 ; 01651684 (ISSN) Kazemi, R ; Farsi, A ; Ghaed, M. H ; Karimi Ghartemani, M ; Sharif University of Technology
    This paper studies the subject of adaptive noise cancelation using the Kalman filtering technique to achieve high precision and fast convergence. It is shown that the Kalman filter can successfully be designed to detect and extract periodic noises which may be constituted of different sinusoidal components with possibly unknown and/or time-varying frequencies. This highlights the feature of Kalman filter in synthesizing periodic noises in the time-domain which is not possible using Fourier-based methods such as DFT. Usefulness of the method is discussed in the context of two examples: active cancelation of periodic noises from audio waveforms and filtering of electrocardiogram measurements....