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Nonlinear vibrations and stability analysis of a rotor on high-static-low-dynamic-stiffness supports using method of multiple scales
, Article Aerospace Science and Technology ; Volume 63 , 2017 , Pages 259-265 ; 12709638 (ISSN) ; Hojjati, M ; Sharif University of Technology
Elsevier Masson SAS
2017
Abstract
This paper presents the vibration and stability analyses of an unbalanced rotor mounted on high-static-low-dynamic-stiffness supports. The stiffness of the supports is modeled as symmetric of cubic order. Then a second-order multiple scales method is used for studying the primary resonance of the system. The types of singular points are investigated and phase-plane of the system is plotted using analytical and numerical methods. The difference between analytical and numerical solutions is less than 2 percent. © 2017 Elsevier Masson SAS
Modeling and Analysis of Power Transmission System of a Gait Training Robot
,
M.Sc. Thesis
Sharif University of Technology
;
Farahmand, Farzam
(Supervisor)
;
Saadat Foomani, Mahmood
(Supervisor)
Abstract
A high percentage of patients with lower extremities disability are consist of paraplegic patients, hemiparetic patients, spinal cord injuries and chronic stroke patients. Currently, a variety of methods are developed and being used for gait rehabilitation of these patients. Recently, beside physiotherapy and different types of exercises, gait trainers are used to help patients to obtain their walking abilities. The main goal of this work is to modeling, analysis and comparison of two different types of power transmission system for a gait training robot. A familiar gearbox and a cable mechanism for power transmission are studied and mechanical impedance and accuracy of systems in...
Optimization of The Engine Mounts to Create the Desired Stiffness to Reduce Noise and Vibration
, M.Sc. Thesis Sharif University of Technology ; Nejat, Hossein (Supervisor) ; Salarieh, Hassan (Supervisor)
Abstract
In order to isolate the engine from the body of the car and chassis to reduce transient vibrations, an engine mount system is used. Using its flexible rubber components, the engine grip keeps the car's engine suspended, allowing it to shift power to the body and chassis. The purpose of this project is to optimize the geometry of the engine mount to achieve the desired static and dynamic stiffness values simultaneously. In this regard, by defining the hyperlastic and viscoelastic properties of the material used in the engine mount, we simulated the static stiffness and dynamic stiffness values of one of the most common engine mounts in the automotive industry. Then we investigate the effect...
Bayesian Identification of Soil-foundation Stiffness Using a Timoshenko Beam Model
, M.Sc. Thesis Sharif University of Technology ; Mahsuli, Mojtaba (Supervisor)
Abstract
A probabilistic method is presented for identifying the dynamic soil-foundation stiffnesses of building structures. It is based on model updating of a Timoshenko beam resting on sway and rocking springs, which respectively represent the superstructure and the soil-foundation system. Unlike those previously employed for this particular problem, the proposed method is a Bayesian one, which accounts for the prevailing uncertainties due to modeling and measurement errors. As such, it yields the probability distribution of the system parameters as opposed to average/deterministic values. In this approach, the joint probability density function of the parameters that control the flexible-base...
Coupled effects of suction and degree of saturation on large strain shear modulus of unsaturated sands
, Article Unsaturated Soils: Research and Applications - Proceedings of the 6th International Conference on Unsaturated Soils, UNSAT 2014 ; Vol. 2, issue , July , 2014 , p. 1559-1564 ; 978-1-138-00150-3 ; Ahmadinezhad, A ; Sadeghi, H ; Sharif University of Technology
Abstract
Predictions of soil behavior in response to dynamic loads require an understanding of dynamic properties of soil. Studies on dynamic properties of unsaturated soils at large-strains are quite limited, while some dynamic loadings, like earthquakes, are in the medium to large-strain ranges. A cyclic simple shear apparatus was developed to measure large-strain shear modulus and damping ratio under different matric suction or degree of saturation. Six cyclic tests, with referring to the soil-water retention curve, were conducted on clean medium sand. Results indicated that shear modulus slightly increases when suction varies from zero up to the air entry value. Approaching the residual value, a...
On dynamic stiffness of spacecraft flexible appendages in deployment phase
, Article Aerospace Science and Technology ; Volume 47 , 2015 , Pages 1-9 ; 12709638 (ISSN) ; Malaek, S. M ; Sharif University of Technology
Abstract
Deployment inertial effects of a spacecraft appendage on its flexible dynamics are investigated. The Euler-Bernoulli beam theory and the actual deployment profile, in which appendage axial motion accelerates from static state and then decelerates to end at zero velocity and acceleration, are employed. The study is concentrated on the arm dynamic stiffness introduced by inertial effects of the arm deployment, and the resultant effects on the arm flexible motions. Lagrange's equations and some appropriate shape functions in the series approximation method are employed to study the arm lateral elastic displacements. Finally a system of ordinary differential equations with time varying...
Dynamic Stability of Parallel Kinematic Machine Tools
, M.Sc. Thesis Sharif University of Technology ; Zohoor, Hassan (Supervisor) ; Movahhedy, Mohammad Reza (Supervisor)
Abstract
Parallel kinematics machine tools (PKMs) are being received in recent years and their capability for machining has been widely studied. One of the disadvantages of these kinds of machine tools is dependency of their structural properties on their configuration. In this thesis, the effect of this dependency on machine stability for the case of a hexapod and a hexaglide machine tool is studied. The process of modeling and analysis are implemented for the hexaglide and hexapod machine tools. In the modeling, the legs are modeled using rod and beam elements whereas platform is assumed to be rigid. Both experimental and analytical analysis show that dominant frequencies of PKMs at tool tip are in...
Investigation of Dynamic Response of Foundations on Sandy Soils to Two-Way Horizontal Harmonic Loading by Physical Model Tests
, M.Sc. Thesis Sharif University of Technology ; Jafarzadeh, Fardin (Supervisor)
Abstract
Increasing requirements of industries and research institutes to analytical results of interaction soil-foundation related systems, reveals the importance of the dynamic impedance functions than ever before. The dynamic impedance function relations are presented for massless rigid foundations which is possible to obtain dynamic response of foundations for different frequencies and masses accordingly. In this study, the dynamic impedance functions were investigated using physical modeling tests on sandy soil with finite thickness soil stratum over bedrock. The tests were carried out inside a steel container of dimensions 1*1*0.8m in length, width and height respectively which was filled into...
Investigation of Dynamic Response of Shallow Foundations on Layered Soils to Horizontal Harmonic Loading by Physical Model Tests
, M.Sc. Thesis Sharif University of Technology ; Jafarzadeh, Fardin (Supervisor)
Abstract
Increasing requirements of industries and research institutes to analytical results of interaction soil-foundation related systems, reveals the importance of the dynamic impedance functions than ever before. The dynamic impedance function relations are presented for massless rigid foundations which is possible to obtain dynamic response of foundations for different frequencies and masses accordingly. In this study, the dynamic impedance functions were investigated using physical modeling tests on sandy soil with finite thickness soil stratum over bedrock. The tests were carried out inside a steel container of dimensions 110.8m in length, width and height respectively which was filled into...
Chatter instability analysis of spinning micro-end mill with process damping effect via semi-discretization approach
, Article Acta Mechanica ; Vol. 225, issue. 3 , 2014 , pp. 715-734 ; ISSN: 00015970 ; Movahhedy, M. R ; Akbari, J ; Sharif University of Technology
Abstract
In this paper, the stability of delay differential equations (DDEs), describing self-excited vibrations in a micro-milling process, is investigated based on semi-discretization (SD) method. Due to the stubby geometry of micro-tools, the shear deformation and rotary inertia effects are considered for modeling the structure. The extended Hamilton's principle is used to derive a detailed dynamical model of the spinning micro-tool with the support of misalignment in which the gyroscopic effects cause coupling of equations. Considering the actual geometry of the micro-end mill, exact dynamic stiffness (DS) formulations are developed to investigate the tool's free vibration characteristics. The...
A concurrent multi-scale modeling for dynamic behavior of nano-crystalline structures
, Article Computational Materials Science ; Volume 79 , 2013 , Pages 841-856 ; 09270256 (ISSN) ; Aramoon, A ; Jahanbakhshi, F ; Dormohammadi, H ; Sharif University of Technology
2013
Abstract
In this paper, a new multi-scale technique is developed for concurrent coupling of atomistic-continuum domains in modeling nano-mechanical behavior of atomic structures. A Lagrange multiplier method is employed over an overlapping domain to coupling the continuum nodal velocities with atomic lattice velocities. The Hamiltonian method is applied to combine the continuum and molecular Hamiltonians with the same weight in the overlapping domain. The mass and stiffness matrices of the continuum domain are obtained using a linear bridging map of the atomic lattice displacement laid underneath the continuum grid to the element displacements. Numerical examples are performed by presenting the...
Size dependent vibrations of micro-end mill incorporating strain gradient elasticity theory
, Article Journal of Sound and Vibration ; Volume 332, Issue 15 , 2013 , Pages 3922-3944 ; 0022460X (ISSN) ; Movahhedy, M. R ; Akbari, J ; Sharif University of Technology
2013
Abstract
In this paper, a size-dependent formulation is presented for vibration analysis of micro-end mill tool. The formulation is developed based on the strain gradient elasticity theory in order to enhance the modeling capability of micro-size structures. Due to stubby geometry of micro-tool, the shear deformation and rotary inertia effects are considered in the derivation of equations. Hence, based on the strain gradient Timoshenko beam theory, the extended Hamilton's principle is used to formulate a detailed dynamical model of the rotating micro-tool. The dynamical model includes a set of partial differential equations with gyroscopic coupling produced due to the spindle rotation. The governing...
System Identification of Soil-Structure Systems through Seismic Signal Processing
, Ph.D. Dissertation Sharif University of Technology ; Ghannad, Mohammad Ali (Supervisor)
Abstract
Dynamic characteristics of structures—viz., natural frequencies, damping ratios, and mode shapes—are central to earthquake resistant design (model validation) and health monitoring. For civil structures like buildings and bridges, these characteristics can be highly influenced by surrounding soil in addition to the super-structure’s properties and cannot be correctly estimated using numerical modeling. For example, soil’s flexibility decreases the natural frequencies of the system; and in most cases, soil provides additional damping due to material hysteresis and radiation. On account of this fact, system identification techniques must be applied to extract such information from field data,...
Investigation of the effects of process damping on chatter instability in micro end milling
, Article Procedia CIRP ; Volume 1, Issue 1 , 2012 , Pages 156-161 ; 22128271 (ISSN) ; Movahhedy, M. R ; Akbari, J ; Sharif University of Technology
2012
Abstract
In this paper, chatter instability of micro end mill tools is studied by taking into account the process damping effect. The actual geometry of the micro tool including shank, taper part and fluted section is considered in the analysis. Timoshenko beam theory is utilized to consider the shear deformation and rotary inertia effects due to short and thick beam-type structures of each parts of the micro tool. The extended Hamilton's Principle is used to formulate a detailed dynamical model of the rotating micro end mill. The governing equations are solved by assumed mode model expansion. An exact dynamic stiffness method is developed to investigate modal characteristics of the tool including...
Experimental study on dynamic properties of sand with emphasis on the degree of saturation
, Article Soil Dynamics and Earthquake Engineering ; Volume 32, Issue 1 , 2012 , Pages 26-41 ; 02677261 (ISSN) ; Sadeghi, H ; Sharif University of Technology
Abstract
Cyclic drained and undrained tests were performed in a modified simple shear device in order to specifically investigate the effect of saturation ratio on shear modulus and damping parameters of sand. In addition to the degree of saturation, the effects of number of cycles, vertical consolidation stress, shear strain amplitude and relative density on dynamic properties of sand were investigated. Dynamic stiffness and damping characteristic were found to be substantially independent of saturation ratio in the range of 25-75%. However, by approaching the full saturation state, the values of modulus fall sharply and damping of loose samples increases dramatically from corresponding values of...
On the coupled dynamics of small spacecraft and elastic deployable appendages
, Article Multibody System Dynamics ; 2016 , Pages 1-26 ; 13845640 (ISSN) ; Malaek, S. M ; Abedian, A ; Sharif University of Technology
Springer Netherlands
2016
Abstract
A thorough investigation of the dynamics of finite-mass satellites with a deployable elastic arm is presented. This work is focused on the interaction between spacecraft rigid body motion and its flexible arm dynamics during the deployment process. The classical Newton–Euler formulation and the Lagrangian approach are applied to the study of the dynamics of spacecraft and its deploying arm. Utilizing a non-Newtonian floating frame to define the arm elastic deformation field, the interactions between the spacecraft and its moving arm have been simulated. Complete equations of motion show that the spacecraft motion induces dynamical stiffness on the arm; in addition, axial and lateral motions...
On the coupled dynamics of small spacecraft and elastic deployable appendages
, Article Multibody System Dynamics ; Volume 40, Issue 3 , 2017 , Pages 235-260 ; 13845640 (ISSN) ; Malaek, S. M ; Abedian, A ; Sharif University of Technology
Springer Netherlands
2017
Abstract
A thorough investigation of the dynamics of finite-mass satellites with a deployable elastic arm is presented. This work is focused on the interaction between spacecraft rigid body motion and its flexible arm dynamics during the deployment process. The classical Newton–Euler formulation and the Lagrangian approach are applied to the study of the dynamics of spacecraft and its deploying arm. Utilizing a non-Newtonian floating frame to define the arm elastic deformation field, the interactions between the spacecraft and its moving arm have been simulated. Complete equations of motion show that the spacecraft motion induces dynamical stiffness on the arm; in addition, axial and lateral motions...
Bayesian identification of soil-foundation stiffness of building structures
, Article Structural Control and Health Monitoring ; Volume 25, Issue 3 , 2018 ; 15452255 (ISSN) ; Mahsuli, M ; Ghahari, F ; Taciroglu, E ; Sharif University of Technology
John Wiley and Sons Ltd
2018
Abstract
A probabilistic method is presented for identifying the dynamic soil-foundation stiffnesses of building structures. It is based on model updating of a Timoshenko beam resting on sway and rocking springs, which respectively represent the superstructure and the soil-foundation system. Unlike those previously employed for this particular problem, the proposed method is a Bayesian one, which accounts for the prevailing uncertainties due to modeling and measurement errors. As such, it yields the probability distribution of the system parameters as opposed to average/deterministic values. In this approach, the joint probability density function of the parameters that control the flexible-base...