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    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... 

    Dynamic Analysis of Torsional Vibration of Ship Shaft

    , M.Sc. Thesis Sharif University of Technology Maleki, Ali (Author) ; Sayyadi, Hasan (Supervisor) ; Mehdigholi, Hamid (Co-Advisor)
    Torsional vibration is a phenomenon that caused shear in rotational shafts by out of center torsion. This issue must be considered in design of shafts. Unfortunately,the rules for designing shafts that suggested by standards causes that producers don’t try to usefurther calculation for designing shafts.
    In this thesis, at first and by use of finite element method and ANSYS, a shaft of a vesselthat designed based on BV rule was modeled in the software, then, the model was analyzed by modal analysis of the software. In the next level, by specifying the defect of designed shaft, optimizing of shaft’s system to minimize the first torsional natural frequency is applied. In this case, by use... 

    A comparison between modal damping ratios identified by NExT-ERA and frequency domain impact test

    , Article Journal of Computational and Applied Research in Mechanical Engineering ; Volume 8, Issue 1 , 2018 , Pages 15-24 ; 22287922 (ISSN) Nouri, A ; Hajirezayi, S ; Sharif University of Technology
    Shahid Rajaee Teacher Tarining University (SRTTU)  2018
    In this research, the modal parameters of a beam in free-free condition are extracted by performing different experiments in the laboratory. For this purpose, two different techniques are employed. The first methodology is considered as a time domain method in Operational Modal Analysis. The other one is frequency domain impact hammer test which is categorized as an Experimental Modal Analysis method and can be regarded as the most common method in modal analysis. Checking the results obtained by the two methods, one can notice a distinct inconsistency in modal damping ratios extracted by each method. However, based on recent publications on the subject, it can be inferred that the time... 

    Multidisciplinary optimisation of a car component under NVH and weight constraints using RSM

    , Article International Journal of Vehicle Noise and Vibration ; Volume 5, Issue 3 , 2009 , Pages 261-270 ; 14791471 (ISSN) Azadi, M ; Azadi, S ; Moradi, M ; Zahedi, F ; Sharif University of Technology
    One of the important challenges in the auto industry is to reduce the mass of the vehicle while meeting structural performance requirements for crashworthiness, noise, vibration and harshness, etc. In this paper, a multidisciplinary optimisation of a car back-bonnet is investigated by using the response surface method. A car body is fully surface modelled in CATIA and meshed in software. Then, modal analysis of the finite element model is performed by NASTRAN software to find natural frequencies. A frequency map of that component is extracted and compared with a reference map to detect defects. Design of experiments methodology is used for a screening of the design space and for the... 

    Modal identification of concrete arch dam by fully automated operational modal identification

    , Article Structures ; Volume 32 , 2021 , Pages 228-236 ; 23520124 (ISSN) Mostafaei, H ; Ghamami, M ; Aghabozorgi, P ; Sharif University of Technology
    Elsevier Ltd  2021
    Modal identification is a type of system identification, which studies on the modal parameters of systems by using modal test. In the case of using operational or ambient modal analysis, there is no need to measure excitation, and the system output data are adequate for identification purposes. These modal parameters of system are of great importance from the engineering point of view particularly in the area of system identification, damage detection, and condition monitoring. In a fully-automated identification approach, the modal parameters are extracted without intervention of a specialized user. In this study, a Fully Automated Operational Modal Identification algorithm is developed to... 

    Nonlinear behavior of a high flexibility wing with long span considering large deflection

    , Article Collect. of Pap. - 45th AIAA/ASME/ASCE/AHS/ASC Struct., Struct. Dyn. and Mater. Conf.; 12th AIAA/ASME/AHS Adapt. Struct. Conf.; 6th AIAA Non-Deterministic Approaches Forum; 5th AIAA Gossamer Spacecraft Forum, Palm Springs, CA, 19 April 2004 through 22 April 2004 ; Volume 6 , 2004 , Pages 4549-4559 ; 02734508 (ISSN) Lahidjani, M. H. S ; Haddadpour, H ; Shams, Sh ; Sharif University of Technology
    This paper presents a method for nonlinear aeroelastic analysis of a Human Powered Aircraft (HPA) wings. In these types of aircrafts there is a long wing with high flexibility. Wing flexibility coupled with the long span leads to the possibility of the large deflections during normal flight operation, so, nonlinear modal analysis technique is used for structural modeling. Unsteady linear aerodynamic theory is used for determination of aerodynamic loading on the wing (linear aerodynamics and nonlinear structure). Combining these two types of formulation yields a nonlinear aeroelastic model. Mode summation techniques along with Galerkin's method are used to determine the governing equations of... 

    Modal Analysis of Space-Time Periodic Diffraction Gratings

    , M.Sc. Thesis Sharif University of Technology Ansari Dogaheh, Amir Reza (Author) ; Mehrany, Khashayar (Supervisor) ; Khavasi, Amin (Supervisor)
    Over the past few years, time-periodic and space-time-periodic media have garnered a considerable amount of attention, thanks to their exotic and unique interactions with electromagnetic phenomena. These media do not generally comport with Lorentz's reciprocity theorem, thus can be utilized in order to implement non-reciprocal amplification and frequency transformation. One class of such media, that have been the subject of less discussion and interest, are space-time periodic diffraction gratings.In this thesis, we aim to develop a series of semi-analytical methods in order to analyze such structures. Our focus will be on a subset of methods which make use of the modal expansion of fields... 

    Design a new spherical super element to static and modal analysis of spherical structures

    , Article Proceedings of the ASME Design Engineering Technical Conference, 28 August 2011 through 31 August 2011 ; Volume 1, Issue PARTS A AND B , August , 2011 , Pages 493-497 ; 9780791854785 (ISBN) Faghani, H ; Ahmadian, M. T ; Sharif University of Technology
    Hollow Spherical Structures such as spherical pressure vessels are widely used in industries. Using faster numerical methods to solve these spherical problems has been one of the research interests in the last decade. Super elements are elements capable of producing the same accurate result in comparison with several conventional elements. In this paper, a new spherical super element is designed and implemented to study the static deformation and modal analysis of a spherical structure. In the static analysis mean of the inner radial displacements of the hollow structure under the internal pressure is evaluated. also performing the modal analysis, natural frequencies of the structure under... 

    Study of chatter stability in boring process using operating deflection shape

    , Article ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE), 12 November 2010 through 18 November 2010 ; Volume 3, Issue PARTS A AND B , 2010 , Pages 947-953 ; 9780791844274 (ISBN) Mohammadi, Z ; Movahhedy, M. R ; Sharif University of Technology
    Chatter is a limiting factor in machining systems with adverse effects for workpiece surface, tool and machine life and material removal rate. Proper modelling of this phenomenon requires that the dynamic characteristics of the machining system is known. It is common to obtain the dynamic of the system by modal tests in stationary condition. However, the vibration characteristics of a machine change between its operating and resting mode. The goal of this work is to study vibrations in boring using the vibrational characteristic in operational conditions using the operating deflection shape method. In this paper, vibration characteristics of the in the boring bar is obtained by both modal... 

    Modal parameter identification of rotary systems based on power spectral density transmissibility functions

    , Article 2018 SAE World Congress Experience, WCX 2018, 10 April 2018 through 12 April 2018 ; Volume 2018-April , 2018 ; 01487191 (ISSN) Khodaygan, S ; Sharif University of Technology
    SAE International  2018
    Operational modal analysis based on power spectral density transmissibility functions (PSDT) is a powerful tool to identify the modal parameters with low sensitivity to excitations. The rotor systems may have the asymmetric damping or stiffness matrices which can lead to increase the difficulties of the identification procedure. In this paper, a new method is proposed to identify the modal parameters of the asymmetric rotary systems by the operational modal analysis based on the power spectral density transmissibility functions. For pole extraction from the PSDT function, a proper parametric identification method such as the Poly-reference Least Squares Complex Frequency-domain method... 

    A method for system identification in the presence of unknown harmonic excitations based on operational modal analysis

    , Article 2019 SAE Automotive Technical Papers, WONLYAUTO 2019, 1 January 2019 through 1 January 2019 ; Volume 2019-January, Issue January , 2019 ; 01487191 (ISSN) Khodaygan, S ; Sharif University of Technology
    SAE International  2019
    Operational modal analysis techniques classically have been developed based on the assumption that the input to the system is a stationary white noise. While, in many practical cases, the systems are excited by combination of white noise and colored noises (harmonic excitations). Consequently, in conditions where non-white noises are present, the existing OMA methods cannot completely distinguish between the system poles and the induced poles due to colored noises. In order to overcome this weakness of OMA methods, some researches have been conducted in the field. In this paper, a new method is proposed for identifying the modal parameters of the system under the unknown colored noises,... 

    Performing building vibration assessments by acoustic measurements

    , Article Building Acoustics ; Volume 27, Issue 1 , December , 2020 , Pages 21-33 Isavand, J ; Peplow, A ; Kasaei, A ; Sharif University of Technology
    SAGE Publications Inc  2020
    This article presents an innovative application of the frequency domain decomposition method based on an acoustic and vibration response. Frequency domain decomposition method has been frequently used for operational modal analysis testing in the last decade to identify modal parameters for in-situ case studies. For these studies, the outputs of the vibration response through accelerometers have been employed in the analysis. In this article, the frequency domain decomposition method is employed, for the first time, to analyze both acoustic and vibration response of the building which is a novel application in building vibration response. As a case study, a cylindrical shaped seven-story... 

    Multi-modal distance metric learning: A bayesian non-parametric approach

    , Article Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 6 September 2014 through 12 September 2014 ; Volume 8927 , September , 2015 , Pages 63-77 ; 03029743 (ISSN) ; 9783319161983 (ISBN) Babagholami Mohamadabadi, B ; Roostaiyan, S. M ; Zarghami, A ; Baghshah, M. S ; Rother, C ; Agapito, L ; Bronstein, M. M ; Sharif University of Technology
    Springer Verlag  2015
    In many real-world applications (e.g. social media application), data usually consists of diverse input modalities that originates from various heterogeneous sources. Learning a similarity measure for such data is of great importance for vast number of applications such as classification, clustering, retrieval, etc. Defining an appropriate distance metric between data points with multiple modalities is a key challenge that has a great impact on the performance of many multimedia applications. Existing approaches for multi-modal distance metric learning only offer point estimation of the distance matrix and/or latent features, and can therefore be unreliable when the number of training... 

    A new active method in damping possible resonances in active filters

    , Article Iranian Journal of Electrical and Computer Engineering ; Volume 7, Issue 1 , 2008 , Pages 61-67 ; 16820053 (ISSN) Rahimi, M ; Mokhtari, H ; Zafarabadi, Gh ; Sharif University of Technology
    This paper investigates resonance phenomenon in active filters. The paper studies the effect of system/filter parameters on resonance conditions. It also proposes an active technique for damping such resonances. The study is based on modal analysis in which unstable modes or modes with slow damping are first extracted. Then, using a new technique, such modes are considerably damped. PSCAD/EMTDC software is also employed to verify the performance of the proposed technique. © 2008 ACECR  

    A new cylindrical element formulation and its application to structural analysis of laminated hollow cylinders

    , Article Finite Elements in Analysis and Design ; Volume 44, Issue 9-10 , 2008 , Pages 617-630 ; 0168874X (ISSN) Ahmadian, M. T ; Bonakdar, M ; Sharif University of Technology
    Design and application of superelements in efficient prediction of the structural behavior in a short time has been one of the research interests in the last decade. Superelements are those elements which are capable of producing the same accurate result instead of several combined simple elements. In this paper a new 16-node cylindrical superelement is presented. Static and modal analyses of laminated hollow cylinders subjected to various kinds of loadings and boundary conditions are performed using this element. Some numerical examples are given to illustrate and validate the developed method. The accuracy of the results is evident from comparison of the findings with exact solution method... 

    Dynamic behavior and modal control of beams under moving mass

    , Article Journal of Sound and Vibration ; Volume 306, Issue 3-5 , 2007 , Pages 712-724 ; 0022460X (ISSN) Nikkhoo, A ; Rahimzadeh Rofooei, F ; Shadnam, M. R ; Sharif University of Technology
    Academic Press  2007
    The constitutive equation of an Euler-Bernoulli beam under the excitation of moving mass is considered. The dynamics of the uncontrolled system is governed by a linear, self-adjoint partial differential equation. A Dirac-delta function is used to describe the position of the moving mass along the beam and its inertial effects. An approximate formulation to the problem is obtained by limiting the inertial effect of the moving mass merely to the vertical component of acceleration. Having defined a "critical velocity" in terms of the fundamental period and span of the beam, it is shown that for smaller velocities, the approximate and exact approaches to the problem almost coincide. Since, the... 

    Estimation of modal parameters using bilinear joint time-frequency distributions

    , Article Mechanical Systems and Signal Processing ; Volume 21, Issue 5 , 2007 , Pages 2125-2136 ; 08883270 (ISSN) Roshan Ghias, A ; Shamsollahi, M. B ; Mobed, M ; Behzad, M ; Sharif University of Technology
    In this paper, a new method is proposed for modal parameter estimation using time-frequency representations. Smoothed Pseudo Wigner-Ville distribution which is a member of the Cohen's class distributions is used to decouple vibration modes completely in order to study each mode separately. This distribution reduces cross-terms which are troublesome in Wigner-Ville distribution and retains the resolution as well. The method was applied to highly damped systems, and results were superior to those obtained via other conventional methods. © 2006 Elsevier Ltd. All rights reserved  

    Modal analysis of metallic nanocantilevers with FCC lattice using atomic approximation method

    , 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) Amiri, F ; Pishkenari, H. N ; Mahboobi, S. H ; Meghdari, A ; Sharif University of Technology
    American Society of Mechanical Engineers (ASME)  2006
    In this research, we will use an accelerator for MD simulations, called Atomic Approximation Method, which leads to more efficient simulations without harming the physical properties in simulations. The main idea of this method is to define a virtual material with lower number of atoms, which has the same physical properties as the original material. To demonstrate the validity of the proposed technique the modal analysis results are compared with the ones of the conventional molecular dynamics method. It is shown that this method accelerates conventional MD simulations significantly. Copyright © 2006 by ASME  

    Analytical prediction of panel flutter using unsteady potential flow

    , Article Journal of Aircraft ; Volume 40, Issue 4 , 2003 , Pages 805-807 ; 00218669 (ISSN) Soltani, N ; Esfahanian, V ; Haddadpour, H ; Sharif University of Technology
    American Inst. Aeronautics and Astronautics Inc  2003
    An analytical procedure for supersonic flutter analysis of two-dimensional panels was developed using unsteady potential flow aerodynamic theory. The local spatial influence was considered as an integral over the plate area. The U-g method was used for flutter prediction. Results indicate a stabilizing effect of the unsteady potential flow aerodynamics theory compared to the quasi-steady first-order piston theory  

    Enhanced stabilization diagram for automated modal parameter identification based on power spectral density transmissibility functions

    , Article Structural Control and Health Monitoring ; Volume 26, Issue 7 , 2019 ; 15452255 (ISSN) Afshar, M ; Khodaygan, S ; Sharif University of Technology
    John Wiley and Sons Ltd  2019
    Operational modal analysis based on power spectral density transmissibility (PSDT) functions is a useful tool to identify the modal parameters with low sensitivity to excitations. For pole extraction from the PSDT function, a proper parametric identification method such as the polyreference least squares complex frequency-domain method or poly-Max method can be used. Then, the poles are selected from a stabilization diagram (SD) with overestimating the system model order. Therefore, spurious modes can be identified that must be distinguished and removed from the system poles. To reach this aim, many techniques have been proposed and applied. In this paper, a new algorithm is proposed to...