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Total 197 records

    New sufficient conditions for robust stability analysis of interval matrices

    , Article Systems and Control Letters ; Volume 61, Issue 12 , 2012 , Pages 1117-1123 ; 01676911 (ISSN) Firouzbahrami, M ; Babazadeh, M ; Karimi, H ; Nobakhti, A ; Sharif University of Technology
    2012
    Abstract
    This letter presents new sufficient conditions for robust Hurwitz stability of interval matrices. The proposed conditions are based on two approaches: (i) finding a common Lyapunov matrix for the interval family and (ii) converting the robust stability problem into a robust non-singularity problem using Kronecker operations. The main contribution of the letter is to derive accurate and computationally simple optimal estimates of the robustness margin and spectral bound of general interval matrices. The evaluation of the condition relies on the solutions of linear matrix inequalities (LMIs) and eigenvalue problems, both of which are solved very efficiently. The improvements gained by using... 

    Graetz problem extended to mixed electroosmotically and pressure-driven flow

    , Article Journal of Thermophysics and Heat Transfer ; Volume 26, Issue 1 , 2012 , Pages 123-133 ; 08878722 (ISSN) Sadeghi, A ; Veisi, H ; Saidi, M. H ; Chakraborty, S ; Sharif University of Technology
    2012
    Abstract
    Thermally developing mixed electroosmotically and pressure-driven flow in a parallel plate microchannel with a step change in wall temperature is considered in the framework of an extended Graetz problem. Both Joule heating and viscous dissipation effects are taken into consideration. Expressions for the dimensionless temperature and Nusselt number in the form of infinite series are presented. In general, the associated eigenvalue problem is solved numerically. Nevertheless, an analytical solution is also presented for axial locations close to the entrance. Comparisons are made between the present results and those obtained by approximating the electroosmotic velocity with the... 

    Transmit beampattern synthesis using eigenvalue decomposition in MIMO radar

    , Article ICICS 2011 - 8th International Conference on Information, Communications and Signal Processing, 13 December 2011 through 16 December 2011 ; December , 2011 , Page(s): 1 - 5 ; 9781457700309 (ISBN) Shadi, K ; Behnia, F ; Sharif University of Technology
    2011
    Abstract
    MIMO radar is the next generation radar which transmits arbitrary waveforms at each one of its apertures. It has been shown that design of waveforms for MIMO radars in order to synthesize a desired spatial beampattern is mapped into a waveform correlation matrix (R) design in the narrowband case. Searching for desired R has been modeled as a convex optimization problem which demands considerable processing power. There are also some close form solutions for special cases like rectangular beampatterns. Here we deal with the problem from a matrix eigenvalue theory perspective and show how close form solutions can be found for more general cases relaxing high computational power demand. Our... 

    On the existence of an analytic solution to the 1-D Ising model with nearest and next-nearest neighbor interactions in the presence of a magnetic field

    , Article Phase Transitions ; Volume 84, Issue 1 , Dec , 2011 , Pages 77-84 ; 01411594 (ISSN) Taherkhani, F ; Daryaei, E ; Abroshan, H ; Akbarzadeh, H ; Parsafar, G ; Fortunelli, A ; Sharif University of Technology
    2011
    Abstract
    To solve the controversy, regarding the existence of an analytic solution to the 1-D Ising model with nearest-neighbor (NN) and next-nearest-neighbor (NNN) interactions in the presence of a magnetic field, we apply the transfer matrix method to solve the 1-D Ising model in the presence of a magnetic field, taking both NN and NNN interactions into account. We show that it is possible to write a transfer matrix only if the number of sites is even. Even in such a case, it is impossible to diagonalize the transfer matrix in an analytic form. Therefore, we employ a numerical method to obtain the eigenvalues of the transfer matrix. Moreover, the heat capacity, magnetization, and magnetic... 

    Active control of robotic manipulators vibration via feedback control

    , Article 17th International Congress on Sound and Vibration 2010, ICSV 2010, 18 July 2010 through 22 July 2010 ; Volume 1 , 2010 , Pages 464-471 ; 9781617822551 (ISBN) Moradi, H ; Bakhtiari Nejad, F ; Sadighi, M ; Alasty, A ; Sharif University of Technology
    Abstract
    In this paper, a robotic manipulator modelled as a cantilever rotating Euler-Bernoulli beam is considered. Control objective is achieving a desired angular rotation of the manipulator tip while its lateral vibration is suppressed. An external driving torque is the control input of the system. Two dynamic transfer functions are derived to describe beam tip motion and angular rotation in terms of the desired angular rotation. After state-space representation of the problem, an observer is designed to estimate state variables of the system. Then, a feedback control is designed for both regulation and tracking objectives. Eigenvalues are chosen such that an appropriate response is achieved while... 

    Optical anisotropy of schwarzschild metric within equivalent medium framework

    , Article Optics Communications ; Volume 283, Issue 7 , April , 2010 , Pages 1222-1228 ; 00304018 (ISSN) Khorasani, S ; Rashidian, B ; Sharif University of Technology
    2010
    Abstract
    It is has been long known that the curved space in the presence of gravitation can be described as a non-homogeneous anisotropic medium in flat geometry with different constitutive equations. In this article, we show that the eigenpolarizations of such medium can be exactly solved, leading to a pseudo-isotropic description of curved vacuum with two refractive index eigenvalues having opposite signs, which correspond to forward and backward travel in time. We conclude that for a rotating universe, time-reversal symmetry is broken. We also demonstrate the applicability of this method to Schwarzschild metric and derive exact forms of refractive index. We derive the subtle optical anisotropy of... 

    Multiple antenna spectrum sensing in cognitive radios

    , Article IEEE Transactions on Wireless Communications ; Volume 9, Issue 2 , 2010 , Pages 814-823 ; 15361276 (ISSN) Taherpour, A ; Nasiri-Kenari, M ; Gazor, S ; Sharif University of Technology
    2010
    Abstract
    In this paper, we consider the problem of spectrum sensing by using multiple antenna in cognitive radios when the noise and the primary user signal are assumed as independent complex zero-mean Gaussian random signals. The optimal multiple antenna spectrum sensing detector needs to know the channel gains, noise variance, and primary user signal variance. In practice some or all of these parameters may be unknown, so we derive the Generalized Likelihood Ratio (GLR) detectors under these circumstances. The proposed GLR detector, in which all the parameters are unknown, is a blind and invariant detector with a low computational complexity. We also analytically compute the missed detection and... 

    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 
    Abstract
    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 
    Abstract
    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... 

    Highly accurate and east convergent diffractive interface theory for fast analysis of metasurfaces

    , Article IEEE Journal of Quantum Electronics ; Volume 52, Issue 7 , 2016 ; 00189197 (ISSN) Nekuee, S. A. H ; Khavasi, A ; Akbari, M ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc 
    Abstract
    Recently, an approximate formalism [Opt. Express 23, 2764, (2015)] called diffractive interface theory has been reported for the fast analysis of the optical response of metasurfaces, subwavelength two-dimensional periodic arrays. In this method, the electromagnetic boundary conditions are derived using the susceptibility distribution of the metasurface, such that the analysis of metasurface is possible without solving any eigenvalue equation inside the grating layer. In this paper, we modify the boundary conditions to achieve more accurate results. In addition, in this paper, correct Fourier factorization rules are also applied leading to faster convergence rate. The obtained results are... 

    Deriving surface impedance for 2-d arrays of graphene patches using a variational method

    , Article IEEE Journal of Quantum Electronics ; Volume 53, Issue 1 , 2017 ; 00189197 (ISSN) Barzegar Parizi, S ; Tavakol, M. R ; Khavasi, A ; Sharif University of Technology
    Abstract
    In this paper, we extract the fundamental resonant mode of a graphene patch using a variational method. We use 2-D eigenvalue problem obtained from the integral equation governing the surface current on graphene patterns under quasi-static approximation. To compute the eigenvalues, we propose three trial eigenfunctions, which meet the boundary conditions. We investigate the accuracy of these eigenfunctions with comparing to the results obtained by full wave simulations. Finally, we analyze square-lattice arrangements of graphene patches using the most accurate proposed eigenfunction and derive a very accurate surface impedance for it. The proposed surface impedance is much more precise than... 

    Optimal and robust waveform design for MIMO radars

    , Article 2009 IEEE International Conference on Acoustics, Speech, and Signal Processing, ICASSP 2009, Taipei, 19 April 2009 through 24 April 2009 ; 2009 , Pages 2085-2088 ; 15206149 (ISSN); 9781424423545 (ISBN) Naghibi, T ; Behnia, F ; Institute of Electrical and Electronics Engineers; Signal Processing Society ; Sharif University of Technology
    2009
    Abstract
    Waveform design for Target identification and classification in MIMO radar systems has been studied in several recent works. While the previous works considered signal independent noise, here we extend the results to the case where signal-dependent noise, clutter, is also present and then we find the optimum waveform for several estimators differing in the assumptions on the given statistics. Computing the optimal waveforms for MMSE estimator leads to the Semi-definite programming (SDP) problem. Finding the optimal transmit signals for CSLS estimator results in a minimax eigenvalue problem. Finally it is shown that equal power waveforms are the best transmit signals for the SLS estimator.... 

    Vibrations and stability analysis of double current-carrying strips interacting with magnetic field

    , Article Acta Mechanica ; 2020 Hosseinian, A. R ; Firouz Abadi, R. D ; Sharif University of Technology
    Springer  2020
    Abstract
    Interactive vibrations and buckling of double current-carrying strips (DCCS) are investigated in this study. Considering the rotational and transverse deformation of the strip, four coupled equations of motion are obtained using Hamilton’s principle. Using the Galerkin method, mass and stiffness matrices are extracted and the stability of the system is determined by solving the eigenvalue problem. Effects of pretension and elevated temperature on the stability of DCCS are studied for three types of materials and various arrangements. Finally, the effect of horizontal or vertical distance between strips on the critical current value is investigated. According to the results, the effects of... 

    Stabilization of nonlinear dynamic systems over limited capacity communication channels

    , Article IEEE Transactions on Automatic Control ; Volume 65, Issue 8 , 2020 , Pages 3655-3662 Sanjaroon, V ; Farhadi, A ; Seyed Motahari, A ; Hosain Khalaj, B ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc  2020
    Abstract
    This article addresses the stabilization of noiseless nonlinear dynamic systems over limited capacity communication channels. It is shown that the stability of nonlinear dynamic systems over memory-less communication channels implies an inequality condition between the Shannon channel capacity and the summation of the positive equilibrium Lyapunov exponents of the dynamic system or, equivalently, the logarithms of the magnitude of the unstable eigenvalues of system Jacobian. Furthermore, we propose an encoder, decoder, and a controller to prove that scalar nonlinear dynamic systems are stabilizable under the aforementioned inequality condition over the digital noiseless and the packet... 

    MR artifact reduction in the simultaneous acquisition of EEG and fMRI of epileptic patients

    , Article 16th European Signal Processing Conference, EUSIPCO 2008, Lausanne, 25 August 2008 through 29 August 2008 ; 2008 ; 22195491 (ISSN) Amini, L ; Sameni, R ; Jutten, C ; Hossein Zadeh, G. A ; Soltanian Zadeh, H ; Sharif University of Technology
    2008
    Abstract
    Integrating high spatial resolution of functional magnetic resonance imaging (fMRI) and high temporal resolution of electroencephalogram (EEG) is promising in simultaneous EEG and fMRI analysis, especially for epileptic patients. The EEG recorded inside an MR scanner is interfered with MR artifacts. In this article, we propose new artifact reduction approaches and compare them with the conventional artifact reduction methods. Our proposed approaches are based on generalized eigenvalue decomposition (GEVD) and median filtering. The proposed methods are applied on experimental simultaneous EEG and fMRI recordings of an epileptic patient. The results show significant improvement over... 

    First order perturbation solution for axial vibration of tension leg platforms

    , Article Scientia Iranica ; Volume 14, Issue 5 , 2007 , Pages 414-423 ; 10263098 (ISSN) Golafshani, A. A ; Tabeshpour, M. R ; Seif, M. S ; Sharif University of Technology
    Sharif University of Technology  2007
    Abstract
    The dynamic response of the leg (tether) of a Tension Leg Platform (TLP), subjected to axial load at the top of the leg, is presented. The structural model is very simple, but several complicated factors, such as foundation effect, buoyancy and simulated ocean wave load, are considered. As an application, the effect of added mass fluctuation on the dynamic response of the leg subjected to such a load is presented. This effect is important in the fatigue life study of tethers. A first order perturbation method is used, in order to formulate and solve the problem. The differential equation is solved by means of non-harmonic Fourier expansion, in terms of eigenfunctions obtained from a... 

    Color PCA eigenimages and their application to compression and watermarking

    , Article Image and Vision Computing ; Volume 26, Issue 7 , 2008 , Pages 878-890 ; 02628856 (ISSN) Abadpour, A ; Kasaei, S ; Sharif University of Technology
    Elsevier Ltd  2008
    Abstract
    From the birth of multi-spectral imaging techniques, there has been a tendency to consider and process this new type of data as a set of parallel gray-scale images, instead of an ensemble of an n-D realization. However, it has been proved that using vector-based tools leads to a more appropriate understanding of color images and thus more efficient algorithms for processing them. Such tools are able to take into consideration the high correlation of the color components and thus to successfully carry out energy compaction. In this paper, a novel method is proposed to utilize the principal component analysis in the neighborhoods of an image in order to extract the corresponding eigenimages.... 

    Free vibration analysis of cross-ply layered composite beams with finite length on elastic foundation

    , Article International Journal of Computational Methods ; Volume 5, Issue 1 , 2008 , Pages 21-36 ; 02198762 (ISSN) Jafari Talookolaei, R. A ; Kargarnovin, M. H ; Ahmadian, M. T ; Sharif University of Technology
    2008
    Abstract
    In this paper, free vibration analysis of cross-ply layered composite beams (LCB) with finite length and rectangular cross-section rested on an elastic foundation is investigated by finite element method. Based on the Timoshenko beam theory which includes the shear deformation and rotary inertia, the stiffness and mass matrices of a LCB are obtained using the energy method. Then, the natural frequencies are calculated by employing eigenvalue technique. The obtained results are verified against existing data in the literatures for a LCB with no foundation and uniform cross-section. Good agreements are observed between these cases. In the same way, the natural frequencies of a specific case,... 

    An efficient reduced-order modelling approach based on fluid eigenmodes and boundary element method

    , Article Journal of Fluids and Structures ; Volume 23, Issue 1 , 2007 , Pages 143-153 ; 08899746 (ISSN) Shahverdi, H ; Nobari, A. S ; Behbahani Nejad, M ; Haddadpour, H ; Sharif University of Technology
    2007
    Abstract
    This paper presents an efficient reduced-order modelling approach based on the boundary element method. In this approach, the eigenvalue problem of the unsteady flows is defined based on the unknown wake singularities. By constructing this reduced-order model, the body quasi-static eigenmodes are removed from the eigensystem and it is possible to obtain satisfactory results without using the static correction technique when enough eigenmodes are used. In addition to the conventional method, eigenanalysis and reduced-order modelling of unsteady flows over a NACA 0012 airfoil, a wing with NACA 0012 section and a wing-body combination are performed using the proposed reduced order modelling... 

    Graph homomorphisms and nodal domains

    , Article Linear Algebra and Its Applications ; Volume 418, Issue 1 , 2006 , Pages 44-52 ; 00243795 (ISSN) Daneshgar, A ; Hajiabolhassan, H ; Sharif University of Technology
    2006
    Abstract
    In this paper, we derive some necessary spectral conditions for the existence of graph homomorphisms in which we also consider some parameters related to the corresponding eigenspaces such as nodal domains. In this approach, we consider the combinatorial Laplacian and co-Laplacian as well as the adjacency matrix. Also, we present some applications in graph decompositions where we prove a general version of Fisher's inequality for G-designs. © 2006 Elsevier Inc. All rights reserved