Search for: stability-analysis
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    Stability behavior of single-phase natural circulation loop

    , Article 2003 ASME International Mechanical Engineers Congress, Washington, DC, 15 November 2003 through 21 November 2003 ; 2003 , Pages 119-131 ; 0791837157 (ISBN); 9780791837153 (ISBN) Moussavian, S. K ; Misale, M ; D'Auria, F ; Salehi, M ; Sharif University of Technology
    American Society of Mechanical Engineers  2003
    The circulating fluid in natural circulation loops would absorb heat from the hot zone (source) and will transport it to the cold zone (sink) without using of any mechanical pumps or other devices. The fluid density differences in hot and cold zones would lead buoyancy force which is introduced as the body force for the loop. By introducing the one-dimensional conservation equations; mass, momentum and energy in the loop and applying the perturbation method, or linear stability theory, would help us to study the stability of the rectangular natural circulation loop. The conservation equations in transient form were solved using the finite difference method. The results obtained from the... 

    Reliability of slope stability analysis using fuzzy set

    , Article Annual Conference of the Canadian Society for Civil Engineering 2007: Where the Road Ends, Ingenuity Begins, Yellowknife, NT, 6 June 2007 through 9 June 2007 ; Volume 1 , 2007 , Pages 11-20 Bahador, M ; Sharif University of Technology
    In this research, theory of fuzzy sets is used to deal with the uncertain nature of soil parameters and the inaccuracy involved in the analysis simultaneously. Soil parameters are defined using suitable fuzzy sets and the uncertainty inherent in the value of factor of safety is assessed. It is believed that this approach accounts for the uncertainty in soil parameters more realistically compared to the conventional probabilistic approaches reported in the literature. The proposed method is applied to a case study and the results are discussed subsequently  

    Investigation of the transient growth in plane jet by non-modal stability analysis

    , Article Fluid Dynamics Research ; Volume 51, Issue 5 , 2019 ; 01695983 (ISSN) Gohardehi, S ; Afshin, H ; Farhanieh, B ; Sharif University of Technology
    Institute of Physics Publishing  2019
    Linear stability analysis is used to characterize the dynamics of a plane jet by incorporating non-modal stability analysis besides classical global temporal stability analysis. It is explained that similar shapes of different global modes are the result of non-normal characteristics of linearized Navier Stokes equations. Optimal initial disturbances and their eigenfunctions together with transient energy growth are obtained for different time horizons and Reynolds numbers of the jet in the linear unstable configuration. These structures are localized at the upstream of the jet nozzle at the boundary layer. The transient growth of the inlet perturbation in limited time bounds is found in the... 

    Stability analysis in multiwall carbon nanotube bundle interconnects

    , Article Microelectronics Reliability ; Volume 52, Issue 12 , 2012 , Pages 3026-3034 ; 00262714 (ISSN) Haji Nasiri, S ; Faez, R ; Moravvej Farshi, M. K ; Sharif University of Technology
    Based on the transmission line model (TLM), we present an exact and general transfer function formula, useful for both single multiwall carbon nanotube (MWCNT) and MWCNT bundle interconnects. Using the standard parameters for 22-nm technology node we perform the Nyquist stability analysis, to investigate the dependence of the degree of relative stability for both single and bundle interconnects on the number of walls in each MWCNT its geometry and also on the bundle geometry. The numerical results, for 1- to 30-μm long interconnects composed of 3- to 7-wall-CNTs, show that by increasing the length or the outer shell diameter, both single and bundle interconnects become more stable. On the... 

    Viscoelastic dynamics and static responses of a graphene nanoplatelets-reinforced composite cylindrical microshell

    , Article Mechanics Based Design of Structures and Machines ; 2020 Shokrgozar, A ; Ghabussi, A ; Ebrahimi, F ; Habibi, M ; Safarpour, H ; Sharif University of Technology
    Taylor and Francis Inc  2020
    In this study, a cylindrical microshell stability reinforced by graphene nanoplatelets is investigated while an axial load is applied uniformly. In addition, viscoelastic foundation covers the composite nanostructure. Therefore, the impacts of the small scale parameter are studied while nonlocal strain gradient theory (NSGT) is considered. The present research deals for the first time with the consideration of viscoelastic, strain–stress size-dependent parameters along with taking into account of various boundary conditions (BCs), especially C-F ones put into effect on the proposed theory. The governing equations (G.Eqs) and BCs have been obtained utilizing energy method and solved with... 

    Stability analysis of descriptor systems with multiple commensurate time-delays

    , Article Journal of the Franklin Institute ; Volume 356, Issue 15 , 2019 , Pages 8690-8705 ; 00160032 (ISSN) Zahedi, F ; Haeri, M ; Sharif University of Technology
    Elsevier Ltd  2019
    A new method is proposed to analyze stability of descriptor systems with multiple commensurate time-delays. In this method, purely imaginary roots of the system are calculated, asymptotic behavior of root path at these roots is studied, and unstable regions of the system in the space of delays are determined. Moreover, the number of roots in each unstable region is calculated. Also necessary and sufficient conditions for τ-stabilizability of these systems are presented. Examples are provided to illustrate the effectiveness and validity of the proposed method. © 2019 The Franklin Institute  

    Numerical investigation of stability of deep excavations supported by soil-nailing method

    , Article Geomechanics and Geoengineering ; Volume 16, Issue 6 , 2021 , Pages 434-451 ; 17486025 (ISSN) Pak, A ; Maleki, J ; Aghakhani, N ; Yousefi, M ; Sharif University of Technology
    Taylor and Francis Ltd  2021
    Deep excavation in urban areas can cause instability problems due to significant settlement at the ground surface and large movements at the excavation facing walls. One of the most popular methods used to stabilise these excavations is utilising soil-nailing method. This method has also been widely used to stabilise natural slopes and earth retaining structures. Because of the complexity involved in the mechanism of this stabilising system due to interacting effects of the soil, nails, grout and shotcrete, numerical modelling with high accuracy should be used to analyse the behaviour of the soil-nailed walls. Considering all aspects of soil-structure interaction in the present research, a... 

    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) Navazi, H. M ; Hojjati, M ; Sharif University of Technology
    Elsevier Masson SAS  2017
    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  

    Robust tuning of transient droop gains based on kharitonov's stability theorem in droopcontrolled microgrids

    , Article IET Generation, Transmission and Distribution ; Volume 12, Issue 14 , 2018 , Pages 3495-3501 ; 17518687 (ISSN) Mahdian Dehkordi, N ; Sadati, N ; Hamzeh, M ; Sharif University of Technology
    This study addresses the robust stability analysis for an islanded microgrid with droop-controlled inverter-based distributed generators (DGs). Owing to large load changes, microgrid structure reconfiguration, and higher-power demands, the low-frequency (LF) dominant modes of a microgrid stir toward unstable zone and make the system more oscillatory or even unstable. In this study, a robust two-degree-of-freedom (2DOF) decentralised droop controller, which is the combination of the conventional droop with a robust transient droop function, is utilised for each inverter-based DG unit. Unlike conventional tuning of 2DOF droop controllers, a new design procedure is proposed to robustly... 

    Stability analysis of distributed-order nonlinear dynamic systems

    , Article International Journal of Systems Science ; Volume 49, Issue 3 , 2018 , Pages 523-536 ; 00207721 (ISSN) Taghavian, H ; Tavazoei, M. S ; Sharif University of Technology
    Taylor and Francis Ltd  2018
    The problem of asymptotic stability analysis of equilibrium points in nonlinear distributed-order dynamic systems with non-negative weight functions is considered in this paper. The Lyapunov direct method is extended to be used for this stability analysis. To this end, at first, a discretisation scheme with convergence property is introduced for distributed-order dynamic systems. Then, on the basis of this tool, Lyapunov theorems are proved for asymptotic stability analysis of equilibrium points in distributed-order systems. As the order weight function assumed for the distributed-order systems is general enough, the results are applicable to a wide range of nonlinear distributed-order... 

    Robust stability analysis of uncertain multiorder fractional systems: young and Jensen inequalities approach

    , Article International Journal of Robust and Nonlinear Control ; Volume 28, Issue 4 , March , 2018 , Pages 1127-1144 ; 10498923 (ISSN) Taghavian, H ; Tavazoei, M. S ; Sharif University of Technology
    John Wiley and Sons Ltd  2018
    Robust stability analysis of multiorder fractional linear time-invariant systems is studied in this paper. In the present study, first, conservative stability boundaries with respect to the eigenvalues of a dynamic matrix for this kind of systems are found by using Young and Jensen inequalities. Then, considering uncertainty on the dynamic matrix, fractional orders, and fractional derivative coefficients, some sufficient conditions are derived for the stability analysis of uncertain multiorder fractional systems. Numerical examples are presented to confirm the obtained analytical results. Copyright © 2017 John Wiley & Sons, Ltd  

    Stability Analysis of Fractional Time Delayed Linear Time Invariant Systems with Infinitesimal Fractions

    , M.Sc. Thesis Sharif University of Technology Nasiri, Hamid Reza (Author) ; Haeri, Mohammad (Supervisor)
    Time delay generates exponential transcendental terms in characteristic equations. Subsequently, the applied methods are narrow to assess the stability map of delayed fractional-order systems with vanishing fractions. In this case, it is convenient to approximate these equations with its integer-order counterparts to design controllers or investigating features of the systems. But, delay can cause enormous differences between characteristic of these two equations. This study offers a method to survey analytically the stability of fractional-order time delayed linear time invariant systems with infinitesimal fractions. In this method, equations are transferred to an explicit expression which... 

    On properties of a particular class of directed graphs used in stability analysis of flocking algorithms

    , Article Proceedings of the IEEE International Conference on Control Applications, 3 October 2012 through 5 October 2012 ; 2012 , Pages 605-608 ; 1085-1992 (ISSN) ; 9781467345033 (ISBN) Atrianfar, H ; Haeri, M ; Sharif University of Technology
    In this paper, we present sufficient conditions to address a larger class of digraphs, including balanced ones, whose members' Laplacian (L) makes L 1L + LTL1 to be positive semi-definite, where L1 is the Laplacian associated with a fully connected equally-edged weighted graphs. This property can be later utilized to introduce an appropriate energy function for stability analysis of flocking algorithms in a larger class of networks with switching directed information flow. Also, some of their properties are investigated in the line of matrix theory and graph theory  

    Linear active disturbance rejection control from the practical aspects

    , Article IEEE/ASME Transactions on Mechatronics ; 2018 ; 10834435 (ISSN) Ahi, B ; Haeri, M ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc  2018
    In this paper, important issues that could come up in practice of Active Disturbance Rejection Control (ADRC) due to a common assumption and some inevitable practical restrictions are investigated. The main idea behind ADRC is modeling effects of both internal uncertainties and external disturbances as an extra state called the total disturbance which is timely estimated. In the majority of recent works, it is assumed that time derivative of total disturbance is bounded. First we prove that, beside the system characteristics, the validity of this assumption strongly depends on the tuning parameters, even in the case of simple classes of linear second order systems. Thus, one should be... 

    Influence of Darcy number on the onset of convection in a porous layer with a uniform heat source

    , Article International Journal of Thermal Sciences ; Volume 47, Issue 8 , August , 2008 , Pages 1020-1025 ; 12900729 (ISSN) Nouri Borujerdi, A ; Noghrehabadi, A. R ; Rees, D. A. S ; Sharif University of Technology
    This note considers the effect of different Darcy numbers on the onset of natural convection in a horizontal, fluid-saturated porous layer with uniform internal heating. It is assumed that the two bounding surfaces are maintained at constant but equal temperatures and that the fluid and porous matrix are in local thermal equilibrium. Linear stability theory is applied to the problem, and numerical solutions obtained using compact fourth order finite differences are presented for all Darcy numbers between Da = 0 (Darcian porous medium) and Da → ∞ (the clear fluid limit). The numerical work is supplemented by an asymptotic analysis for small values Da. © 2007 Elsevier Masson SAS. All rights... 

    A new wave equation for MHD instabilities in aluminum reduction cells

    , Article TMS 2007 Annual Meeting and Exhibition, Orlando, FL, 25 February 2007 through 1 March 2007 ; 2007 , Pages 345-349 ; 01470809 (ISSN) ; 0873396596 (ISBN); 9780873396592 (ISBN) Kadkhodabeigi, M ; Saboohi, Y ; Sharif University of Technology
    To overcome the problem of magneto hydrodynamic instabilities at the aluminum-cryolite interface in aluminum reduction cells, the use of a simple and efficient model has been suggested. In this paper a new model for evaluation of magneto hydrodynamic instabilities in aluminum reduction cells is proposed. This model is in the form of a wave equation for interfacial oscillations and is constructed based on Shallow Water approximation and linear stability analysis. In this model two terms for linear friction in each molten layer of aluminum and cryolite has been considered and also Lorentz forces in cryolite layer have not been neglected. By accounting of these additional terms in our model,... 

    Temperature integrated load sharing of paralleled modules

    , Article 2006 1st IEEE Conference on Industrial Electronics and Applications, ICIEA 2006, Singapore, 24 May 2006 through 26 May 2006 ; 2006 ; 078039514X (ISBN); 9780780395145 (ISBN) Barnette, J. L ; Zolghadri, M. R ; Walters, M ; Homaifar, A ; Sharif University of Technology
    Paralleling power modules is designed to share system loads (stresses) equally to improve system reliability. Due to variations in the parameters in the power converter system, temperature mismatches may occur. These mismatches may lead to unequal life expectancy of individual converters in the total system. It is believed that equalizing the operating temperature of the semiconductor devices may improve total system reliability. In this paper a new thermal-based load-sharing scheme for parallel converters is proposed. In this scheme, the load current is redistributed among parallel converters based on the temperature of the converters. The main goal is to minimize the relative temperature... 

    The Stability Analysis of Distributed Amplifier

    , M.Sc. Thesis Sharif University of Technology Pasdar, Abbas (Author) ; Medi, Ali (Supervisor)
    Advancement in the field of electronics can often be observed from the improvement in performance of active circuits and systems. The amplifier, because it is a highly versatile functional circuit, has always been one of the first to benefit from new developments. Of the many characteristic of an amplifier, perhaps the most notable are the gain and the frequency response. In many applications, both the gain and the bandwidth need to be optimized. One of the most significant structures which have proper gain and bandwidth, are distributed amplifiers. But these structures are nonsymmetrical and have many active loops. As a result, the stability analysis of distributed amplifier is quite... 

    Stability Analysis and Stabilization of Descriptor Systems with Time-Delay

    , M.Sc. Thesis Sharif University of Technology Zahedi, Fatemeh (Author) ; Haeri, Mohammad (Supervisor)
    In this thesis, the general form of descriptor linear systems with time-delay is studied. First, an approach is presented to analyze the stability of descriptor linear systems with commensurate time-delay. Through this approach, not only stable and unstable regions of the system can be determined, but also the number of unstable roots in each unstable region can be calculated. Additionally, a decomposition approach is proposed, which transforms the descriptor systems with time-delay to normal time-delay systems with a lower order. As a result, a large number of methods which are only available for normal systems with time-delay can be utilized for descriptor systems with time-delay. Also,... 

    Assessment of Performance Based Behavior of of the Rock Mass Abutment Anchorage System during Construction of Karun IV Bridge by Numerical Methods

    , M.Sc. Thesis Sharif University of Technology Ghorbanzadeh, Alireza (Author) ; Sadaghiani, Mohammad Hossein (Supervisor)
    Three dimensional analyses have been carried out for predicting the behavior of the jointed rock slope of the abutments of the bridge which is proposed to be constructed across the river Karun4 in Iran using 3DEC. The rock overall slope angle is approximately 60 to 70 degrees, composed of highly jointed rock mass and the joint spacing and orientation are varying at different locations. Since 3DEC is a three-dimensional numerical code, utilizes a Lagrangian calculation scheme to model large movements and deformations of a blocky system, allows for modeling of large movements and rotations, and including complete detachment of rigid or deformable discrete blocks has been utilized for the...