Loading...
Search for:
shakouri--ali
0.106 seconds
Total 2517 records
Buckling and Free Vibration Analysis of Joined Conical Shells Using Analytical Methods
, Ph.D. Dissertation Sharif University of Technology ; Kouchakzadeh, Mohammad Ali (Supervisor)
Abstract
In the present study, buckling and free vibration of two joined conical shells made from isotropic and generally laminated composites are presented. The joined conical shells can be considered as the general case that can be used in analysis of single cylindrical and conical shells, joined cylindrical-conical shells, joined cylinder-plates or cone-plates, cylindrical and conical shells with stepped thicknesses, annular plates, laminates with ply drop-off or any case that the stiffness of the laminate changes in the shell. Governing equations are obtained using thin-walled shallow shell theory of Donnell type and Hamilton’s principle. The joining of shells is exerted using various methods and...
On the impulsive formation control of spacecraft under path constraints
, Article IEEE Transactions on Aerospace and Electronic Systems ; Volume 55, Issue 6 , 2019 , Pages 3292-3302 ; 00189251 (ISSN) ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2019
Abstract
This paper deals with the impulsive formation control of spacecraft in the presence of constraints on the position vector and time. Determining a set of path constraints can increase the safety and reliability in an impulsive relative motion of spacecraft. Specially, the feasibility problem of the position norm constraints is considered in this paper. Under assumptions, it is proved that if a position vector be reachable, then the reach time and the corresponding time of impulses are unique. The trajectory boundedness of the spacecraft between adjacent impulses are analyzed using the Gerschgorin and the Rayleigh-Ritz theorems as well as a finite form of the Jensen's inequality. Some...
On the prescribed-time attractivity and frozen-time eigenvalues of linear time-varying systems
, Article Automatica ; Volume 140 , 2022 ; 00051098 (ISSN) ; Sharif University of Technology
Elsevier Ltd
2022
Abstract
A system is called prescribed-time attractive if its solution converges at an arbitrary user-defined finite time. In this note, necessary and sufficient conditions are developed for the prescribed-time attractivity of linear time-varying (LTV) systems. It is proved that the frozen-time eigenvalues of a prescribed-time attractive LTV system have negative real parts when the time is sufficiently close to the convergence moment. This result shows that the ubiquitous singularity problem of prescribed-time attractive LTV systems can be avoided without instability effects by switching to the corresponding frozen-time system at an appropriate time. Consequently, it is proved that the time-varying...
Buckling Analysis of Reinforced Composite Conical Shells under Axial Compressive Load using GDQ Method
, M.Sc. Thesis Sharif University of Technology ; Kouchakzadeh, Mohammad Ali (Supervisor) ; Shakouri, Meysam ($item.subfieldsMap.e) ; Noghabi, Mohammad ($item.subfieldsMap.e)
Abstract
The object of this study is to determine the buckling load of reinforced composite conical shells under axial compression. . Shells are reinforced by stringers and rings and the boundary conditions are assumed to be simply supported. At first the equilibrium equations are obtained using the first order shear deformation theory (FSDT), smeared stiffener technique and principle of minimum potential energy. In the following, the resulting equations which are the system of five variable coefficient partial differential equations in terms of displacement components are investigated by generalized differential quadrature method (GDQM). Finally the standard eigenvalue equation is formed and the...
Fault detection and isolation of satellite gyroscopes using relative positions in formation flying
, Article Aerospace Science and Technology ; Volume 78 , 2018 , Pages 403-417 ; 12709638 (ISSN) ; Assadian, N ; Sharif University of Technology
Elsevier Masson SAS
2018
Abstract
A fault detection and isolation method for satellite rate gyros is proposed based on using the satellite-to-satellite measurements such as relative position beside orbit parameters of the primary satellite. By finding a constant of motion, it is shown that the dynamic states in a relative motion are restricted in such a way that the angular velocity vector of primary satellite lies on a quadratic surface. This constant of motion is then used to detect the gyroscope faults and estimate the corresponding scale factor or bias values of the rate gyros of the primary satellite. The proposed algorithm works even in time variant fault situations as well, and does not impose any additional...
Prescribed-Time control for perturbed euler-lagrange systems with obstacle avoidance
, Article IEEE Transactions on Automatic Control ; Volume 67, Issue 7 , 2022 , Pages 3754-3761 ; 00189286 (ISSN) ; Assadian, N ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2022
Abstract
This article introduces a class of time-varying controllers for Euler-Lagrange systems such that the convergence occurs at an arbitrary finite time, independently of initial conditions, and free of chattering. The proposed controller is based on a mapping technique and is designed in two steps: First, a conventional (obstacle avoidance) asymptotically stable controller is specified for the nominal system; then, by a simple substitution, a prescribed-time (obstacle avoidance) controller is achievable for the perturbed system. It is proved that the proposed scheme is uniformly prescribed-time stable for unperturbed systems and prescribed-time attractive for perturbed systems as it rejects...
A framework for prescribed-time control design via time-scale transformation
, Article IEEE Control Systems Letters ; 2021 ; 24751456 (ISSN) ; Assadian, N ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2021
Abstract
This letter presents a unified framework for the design of prescribed-time controllers under time-varying input and state constraints for normal-form unknown nonlinear systems with uncertain input gain. The proposed approach is based on a time-domain mapping method by which any infinite-time system can be corresponded to a prescribed-time system and vice versa. It is shown that the design of a constrained nonasymptotic prescribed-time controller can be reduced to the asymptotic control design for an associated constrained infinite-time system. Faà di Bruno’s formula and Bell polynomials are used for a constructive representation of the associated infinite-time system. The presented results...
Prescribed-time control for perturbed euler-lagrange systems with obstacle avoidance
, Article IEEE Transactions on Automatic Control ; 2021 ; 00189286 (ISSN) ; Assadian, N ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2021
Abstract
This paper introduces a class of time-varying controllers for Euler-Lagrange systems such that the convergence occurs at an arbitrary finite time, independently of initial conditions, and free of chattering. The proposed controller is based on a mapping technique and is designed in two steps: First, a conventional (obstacle avoidance) asymptotically stable controller is specified for the nominal system; then, by a simple substitution, a prescribed-time (obstacle avoidance) controller is achievable for the perturbed system. It is proved that the proposed scheme is uniformly prescribed-time stable for unperturbed systems and prescribed-time attractive for perturbed systems as it rejects...
Prescribed-Time control with linear decay for nonlinear systems
, Article IEEE Control Systems Letters ; Volume 6 , 2022 , Pages 313-318 ; 24751456 (ISSN) ; Assadian, N ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2022
Abstract
In this letter, a new notion of stability is introduced, which is called triangular stability. A system is called triangularly stable if the norm of its state vector is bounded by a decreasing linear function of time such that its intersection point with the time axis can be arbitrarily commanded by the user. Triangular stability implies prescribed-time stability, which means that the nonlinear system is converged to zero equilibrium at an arbitrary finite time. A prescribed-time controller with guaranteed triangular stability is developed for normal form nonlinear systems with uncertain input gain, which is able to reject the disturbances and unmodeled dynamics. Numerical simulations are...
A framework for prescribed-time control design via time-scale transformation
, Article IEEE Control Systems Letters ; Volume 6 , 2022 , Pages 1976-1981 ; 24751456 (ISSN) ; Assadian, N ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2022
Abstract
This letter presents a unified framework for the design of prescribed-time controllers under time-varying input and state constraints for normal-form unknown nonlinear systems with uncertain input gain. The proposed approach is based on a time-domain mapping method by which any infinite-time system can be corresponded to a prescribed-time system and vice versa. It is shown that the design of a constrained nonasymptotic prescribed-time controller can be reduced to the asymptotic control design for an associated constrained infinite-time system. Faà di Bruno's formula and Bell polynomials are used for a constructive representation of the associated infinite-time system. The presented results...
Free vibration analysis of joined conical shells: Analytical and experimental study
, Article Thin-Walled Structures ; Vol. 85, issue , December , 2014 , p. 350-358 ; Kouchakzadeh, M. A ; Sharif University of Technology
2014
Abstract
Natural frequencies and mode shapes of two joined isotropic conical shells are presented in this study. The joined conical shells can be considered as the general case for joined cylindrical-conical shells, joined cylinder-plates or cone-plates, conical and cylindrical shells with stepped thicknesses and also annular plates. Governing equations are obtained using thin-walled shallow shell theory of Donnell and Hamilton's principle. The continuity conditions at the joining section of the cones are appropriate expressions among stress resultants and deformations. The equations are solved assuming trigonometric response in circumferential and series solution in meridional directions and all...
Stability analysis of joined isotropic conical shells under axial compression
, Article Thin-Walled Structures ; Volume 72 , 2013 , Pages 20-27 ; 02638231 (ISSN) ; Kouchakzadeh, M. A ; Sharif University of Technology
2013
Abstract
In this study, the buckling of two joined isotropic conical shells under axial compression and simply supported boundary conditions is investigated. The governing equations are obtained using thin-walled shallow shell theory of Donnell-type and theorem of minimum potential energy. The continuity conditions at the joining section of the cones are appropriate expressions among stress resultants and deformations. The equations are solved by assuming trigonometric response in circumferential and series solution in meridional direction. The results are validated in comparison with the available results in the literature. Effects of semi-vertex angles and meridional lengths on the buckling load...
Analytical solution for vibration of generally laminated conical and cylindrical shells
, Article International Journal of Mechanical Sciences ; Volume 131-132 , 2017 , Pages 414-425 ; 00207403 (ISSN) ; Kouchakzadeh, M. A ; Sharif University of Technology
2017
Abstract
In the present study, a simple analytical method is introduced for determination of natural frequencies of generally laminated conical and cylindrical shells with arbitrary boundary conditions. The governing equations of motion employed are those of thin-walled shell theory of Donnell. The free-vibration equations are solved using state space method and series solution in meridional direction. The results are compared and validated with the available especial results in the literature. The effects of bending-stretching coupling, semi-vertex angle, meridional length, shell thickness, fiber directions of composite plies, and lamination sequences on the natural frequency of conical and...
Experimental and numerical survey on tensile fracture of polycrystalline graphite using design of experiments
, Article Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications ; Volume 230, Issue 1 , 2016 , Pages 151-159 ; 14644207 (ISSN) ; Khodadad, A ; Rezaeibana, R ; Sharif University of Technology
SAGE Publications Ltd
2016
Abstract
A new test specimen for a tensile test of industrial graphite is introduced using experiment and finite element results. The most influencing factors on the tensile strength of the test specimens are considered and the number of tests, which are necessary to optimize the tensile strengths are reduced based on the design of experiment using the Taguchi method and the effects of all factors on strength of test samples are studied. Comparison of the predicted strengths based on the Taguchi approach with the measured experimental results and finite element analysis shows a good correlation between them. In addition, results show that the new introduced specimen makes a 33% increase in the...
Design and operation of a simple beam shaping system
, Article Journal of Applied Sciences ; Volume 9, Issue 18 , 2009 , Pages 3350-3356 ; 18125654 (ISSN) ; Golnabi, H ; Shakouri, M ; Sharif University of Technology
2009
Abstract
Design and performance of a beam shaping device based on a simple flexible plastic fiber-bundle stripe and a prism duct is described in this study. Such a system offers practical means to modify and change the output beam shape and also provides quantitative information concerning the transmitted power. It is possible to measure transmitted power signal by using a precise photodetector and also analyze beam images taken by a digital camera. The photograph picture of the illuminating LED beam just at its output point shows a circular shape with a radius of about 4 mm and the fiber-bundle output beam is rectangular shape with a dimension of 22.5×2 mm. A regular duct is tested in this study and...
Seismic Performance of Slit Steel Dampers
, M.Sc. Thesis Sharif University of Technology ; Kazemi, Mohammad Taghi (Supervisor)
Abstract
In this thesis the performance of anew energy dissipative device, slit steel damper (SSD) is investigated. The slit steel damper is manufactured with I-shaped, wide flange H-shaped profiles with a number of slits cut from the web. The device is installed between beam and chevron brace and fastened by four bolts on each side, which enables the replacement of the device due to strong earthquake.A numerical study is conducted to studythe nonlinear behavior of the SSD systems by using a nonlinear FE program. It is found that the numerical modeling results have a good agreement with the existing test results.In the next step a parametric study is performed to study the effects of the number of...
Re-interpreting simultaneous buckling modes of axially compressed isotropic conical shells
, Article Thin-Walled Structures ; Vol. 84 , November , 2014 , pp. 360-368 ; ISSN: 02638231 ; Spagnoli, A ; Kouchakzadeh, M. A ; Sharif University of Technology
2014
Abstract
Elastic stability of shell structures under certain loading conditions is characterized by a dramatically unstable postbuckling behavior. The presence of simultaneous 'competing' buckling modes (corresponding to the same critical buckling load) is understood to be largely responsible for such behavior. In this paper, within the framework of linear bifurcation eigenvalue analysis and Donnell shallow shell theory, the presence of simultaneous buckling modes in axially compressed isotropic cones is determined using the semi-analytical method of Galerkin. The results are presented in the plane of the dimensionless reciprocal meridional and circumferential buckling half wavelengths, and are...
Buckling analysis of cross-ply laminated conical panels using GDQ method
, Article Composites Part B: Engineering ; Volume 55 , 2013 , Pages 440-446 ; 13598368 (ISSN) ; Kouchakzadeh, M. A ; Shakouri, M ; Sharif University of Technology
2013
Abstract
The buckling analysis of cross-ply laminated conical shell panels with simply supported boundary conditions at all edges and subjected to axial compression is studied. The conical shell panel is a very interesting problem as it can be considered as the general case for conical shells when the subtended angle is set to 2π and also cylindrical panels and shells when the semi-vertex angle is equal to zero. Equations were derived using classical shell theory of Donnell type and solved using generalized differential quadrature method. The results are compared and validated with the known results in the literature. The effects of subtended angle, semi-vertex angle, length, thickness and radius of...
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) ; Spagnoli, A ; Kouchakzadeh, M. A ; Sharif University of Technology
Techno Press
2016
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...
Torsional buckling of generally laminated conical shell
, Article Meccanica ; Volume 52, Issue 4-5 , 2017 , Pages 1051-1061 ; 00256455 (ISSN) ; Sharghi, H ; Kouchakzadeh, M. A ; Sharif University of Technology
Springer Netherlands
2017
Abstract
Buckling of generally laminated conical shells under uniform torsion with simply-supported boundary conditions is investigated. The Donnel type strain displacement relations are used to obtain potential strain energy of the shell and membrane stability equation is applied to acquire the external work done by torsion. The Ritz method is used to solve the governing equations and critical buckling loads are obtained. The accuracy of the results is validated in comparison of with other investigations and finite element method. The effects of lamination sequence, semi-vertex angle and length to radius ratio of the cone are evaluated and mode shapes are presented for two types of lamination...