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    Analytical extraction of leaky modes in circular slab waveguides with arbitrary refractive index profile

    , Article Applied Optics ; Volume 46, Issue 36 , 2007 , Pages 8656-8667 ; 1559128X (ISSN) Sarrafi, P ; Zareian, N ; Mehrany, K ; Sharif University of Technology
    2007
    Abstract
    Circular slab waveguides are conformally transformed into straight inhomogeneous waveguides, where-upon electromagnetic fields in the core are expanded in terms of Legendre polynomial basis functions. Thereafter, different analytical expression of electromagnetic fields in the cladding region, viz. Wentzel-Kramers-Brillouin solution, modified Airy function expansion, and the exact field solution for circular waveguides, i.e., Hankel function of complex order, are each matched to the polynomial expansion of the transverse electric field within the guide. This field matching process renders different boundary conditions to be satisfied by the set of orthogonal Legendre polynomial basis... 

    Hermite polynomial expansion for analysis of electromagnetic slow waves in coupled nano conducting layer with Gaussian profile

    , Article LAPC 2007: 3rd Loughborough Antennas and Propagation Conference, Loughborough, 2 April 2007 through 3 April 2007 ; 2007 , Pages 265-268 ; 1424407761 (ISBN); 9781424407767 (ISBN) Sarrafi, P ; Mehrany, K ; Rashidian, B ; Sharif University of Technology
    2007
    Abstract
    The possibility of electromagnetic slow wave propagation in dielectric slab waveguides, when free two-dimensional interface charge layers are generated at film-substrate and film-cover interfaces, is already explored. In this paper, the propagation of electromagnetic slow waves supported by induced charge layers of nonzero thickness, whose induced charge densities have Gaussian profiles, is investigated for the first time. In order to analyze the effects of inhomogeneity in the conductivity profile, the electric field is expanded in terms of Hermite polynomials and each eigenmode is sought in the complete space spanned by them. © 2007 IEEE  

    Passivity enforcement via residues perturbation for admittance representation of power system components

    , Article Iranian Journal of Science and Technology, Transaction B: Engineering ; Volume 31, Issue 3 , 2007 , Pages 273-288 ; 03601307 (ISSN) Porkar, B ; Vakilian, M ; Shahrtash, M ; Iravani, M. R ; Sharif University of Technology
    2007
    Abstract
    Employing an admittance representation in the form of a black box model approximated by rational functions for linear power system components or network equivalents to be included in electromagnetic transient studies is a well-known method which improves calculation efficiency. All of the methods that have been proposed to solve the rational approximation problem have made efforts to overcome the problem of preserving the passivity of the final model. Passivity is a vital property, since a non-passive model may lead to an unstable transient simulation in the time domain. In this paper a post-processing technique for passivity enforcement through an iterative process for the detection and... 

    Buckling of cracked cylindrical thin shells under combined internal pressure and axial compression

    , Article Thin-Walled Structures ; Volume 44, Issue 2 , 2006 , Pages 141-151 ; 02638231 (ISSN) Vaziri, A ; Estekanchi, H. E ; Sharif University of Technology
    2006
    Abstract
    Linear eigenvalue analysis of cracked cylindrical shells under combined internal pressure and axial compression is carried out to study the effect of crack type, size and orientation on the buckling behavior of cylindrical thin shells. Two types of crack are considered; through crack and thumbnail crack. Our calculations indicate that depending on the crack type, length, orientation and the internal pressure, local buckling may precede the global buckling of the cylindrical shell. The internal pressure, in general, increases the buckling load associated with the global buckling mode of the cylindrical shells. In contrast, the effect of internal pressure on buckling loads associated with the... 

    Simple model for exact heave motion of tension leg platform

    , Article WSEAS Transactions on Mathematics ; Volume 5, Issue 5 , 2006 , Pages 500-506 ; 11092769 (ISSN) Tabeshpour, M. R ; Golafshani, A. A ; Seif, M. S ; Sharif University of Technology
    2006
    Abstract
    In this paper the dynamic response of the leg of a tension leg platform (tether) subjected to the load simulated as ocean wave 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. Two continuous models are proposed to present the structural system and the mentioned effects. The problem is solved by means of non-harmonic Fourier expansion in terms of eigenfunctions obtained from a non-regular Sturm-Liouville system  

    Damping Critical Electromechanical Oscillations in Power System Through Remedial Actions Using Wide Area Measurement System (WAMS)

    , Ph.D. Dissertation Sharif University of Technology Setareh, Mohammad (Author) ; Parniani, Mostafa (Supervisor) ; Aminifar, Farrokh (Co-Supervisor)
    Abstract
    Nowadays, by proliferation the basis of the phasor-based synchronous wide area measurement system (WAMS), its application for improving electromechanical oscillations damping has been considered by researchers. In interconnected power systems, inter-area modes are usually critical modes. There are two approaches for damping these modes by utilizing WAMS: 1) the traditional approach of using power system stabilizers with wide area signals; and 2) discrete corrective actions by changing the operating conditions. The first approach requires a thorough study at the design stage, installation and testing. Besides, the controller is designed for a specific operating point. Nevertheless, in the... 

    Robust regulation and tracking system design for multivariable control of the tape transport mechanism

    , Article Microsystem Technologies ; Vol. 18, Issue. 5 , 2012 , pp. 557-573 ; ISSN: 09467076 Moradi, H ; Bakhtiari-Nejad, F ; Alasty, A ; Sharif University of Technology
    Abstract
    Tape mechanisms must be able to transport the tape with a constant velocity for achieving high rate of data transfer. In this paper, a multivariable model of tape transport mechanism including the take-up and supply reel servos for tape tension control and capstan servo for speed control is considered. To achieve desired performance,pole-placement approach based on general canonical control form (GCCF) is used. Instead of using expensive tension sensors, an observer is designed. A regulator system is developed for disturbance rejection; and a modified integral control is designed for perfect tracking of desired setpoints in tape speed and tensions. Since the real dynamic model associates... 

    A successive boundary element model for investigation of sloshing frequencies in axisymmetric multi baffled containers

    , Article Engineering Analysis with Boundary Elements ; Volume 37, Issue 2 , 2013 , Pages 383-392 ; 09557997 (ISSN) Ebrahimian, M ; Noorian, M. A ; Haddadpour, H ; Sharif University of Technology
    2013
    Abstract
    This study presents a developed successive Boundary Element Method to determine the symmetric and antisymmetric sloshing natural frequencies and mode shapes for multi baffled axisymmetric containers with arbitrary geometries. The developed fluid model is based on the Laplace equation and Green's theorem. The governing equations of fluid dynamic and free surface boundary condition are also applied to proposed model. A zoning method is presented to model arbitrary arrangement of baffles in multi baffled axisymmetric tanks. The influence of each zone on neighboring zones is applied by introducing interface influence matrix which correlates the velocity potential of interfaces to their flux. By... 

    Development of two-dimensional, multigroup neutron diffusion computer code based on GFEM with unstructured triangle elements

    , Article Annals of Nuclear Energy ; Volume 51 , 2013 , Pages 213-226 ; 03064549 (ISSN) Hosseini, S. A ; Vosoughi, N ; Sharif University of Technology
    2013
    Abstract
    Various methods for solving the forward/adjoint equation in hexagonal and rectangular geometries are known in the literatures. In this paper, the solution of multigroup forward/adjoint equation using Finite Element Method (FEM) for hexagonal and rectangular reactor cores is reported. The spatial discretization of equations is based on Galerkin FEM (GFEM) using unstructured triangle elements. Calculations are performed for both linear and quadratic approximations of the shape function; based on which results are compared. Using power iteration method for the forward and adjoint calculations, the forward and adjoint fluxes with the corresponding eigenvalues are obtained. The results are then... 

    A VLSI architecture for multiple antenna eigenvalue-based spectrum sensing

    , Article 2012 19th IEEE International Conference on Electronics, Circuits, and Systems, ICECS 2012, 9 December 2012 through 12 December 2012 ; December , 2012 , Pages 153-156 ; 9781467312615 (ISBN) Safavi, S. M ; Shabany, M ; Sharif University of Technology
    2012
    Abstract
    An Eigenvalue-based detection (EBD) scheme, is proposed as an efficient method to overcome the noise uncertainty and the SNR wall problem in conventional energy detection (ED) schemes. Despite remarkable efforts made to analyze the EBD performance, a VLSI implementation is missing in literature. In this paper, a new FFT-based EBD algorithm is introduced, which eliminates the need for filter banks and discrete wavelet packet transform to channelize the input signal. The proposed method enables the utilization of the EBD algorithm in high-resolution spectrum sensing approaches. Moreover, it enables the detection of signals with SNRs as low as -10 dB. A low-power, area-efficient yet real-time... 

    Neutron noise simulation by GFEM and unstructured triangle elements

    , Article Nuclear Engineering and Design ; Volume 253 , 2012 , Pages 238-258 ; 00295493 (ISSN) Hosseini, S. A ; Vosoughi, N ; Sharif University of Technology
    2012
    Abstract
    In the present study, the neutron noise, i.e. The stationary fluctuation of the neutron flux around its mean value, is calculated in 2-group forward and adjoint diffusion theory for both hexagonal and rectangular reactor cores. To this end, the static neutron calculation is performed at the first stage. The spatial discretization of equations is based on linear approximation of Galerkin Finite Element Method (GFEM) using unstructured triangle elements. Using power iteration method, forward and adjoint fluxes with the corresponding eigenvalues are obtained. The results are then benchmarked against the valid results for BIBLIS-2D and IAEA-2D benchmark problems and DONJON computer code. The... 

    Application of subsynchronous damping controller (SSDC) to STATCOM

    , Article International Journal of Electrical Power and Energy Systems ; Volume 43, Issue 1 , December , 2012 , Pages 418-426 ; 01420615 (ISSN) Ghorbani, A ; Mozaffari, B ; Ranjbar, A. M ; Sharif University of Technology
    2012
    Abstract
    In this paper a novel supplementary subsynchronous damping controller (SSDC) is proposed for the STATic synchronous COMpensator (STATCOM) which is capable of damping out subsynchronous oscillations in power system with series compensated transmission lines. An auxiliary subsynchronous damping controller (SSDC) for a STATCOM using the generator rotor speed deviation signal as the stabilizing signal has been designed to damp subsynchronous oscillations. Eigenvalue analysis and transient simulations of detailed nonlinear system are considered to investigate the performance of the controller. Robustness of the controller has been analyzed by facing the system with disturbances leading to... 

    Source enumeration in large arrays using moments of eigenvalues and relatively few samples

    , Article IET Signal Processing ; Volume 6, Issue 7 , 2012 , Pages 689-696 ; 17519675 (ISSN) Yazdian, E ; Gazor, S ; Bastani, H ; Sharif University of Technology
    IET  2012
    Abstract
    This study presents a method based on minimum description length criterion to enumerate the incident waves impinging on a large array using a relatively small number of samples. The proposed scheme exploits the statistical properties of eigenvalues of the sample covariance matrix (SCM) of Gaussian processes. The authors use a number of moments of noise eigenvalues of the SCM in order to separate noise and signal subspaces more accurately. In particular, the authors assume a Marcenko-Pastur probability density function (pdf) for the eigenvalues of SCM associated with the noise subspace. We also use an enhanced noise variance estimator to reduce the bias leakage between the subspaces.... 

    Hippocampal shape analysis in the Laplace Beltrami feature space for temporal lobe epilepsy diagnosis and lateralization

    , Article Proceedings - International Symposium on Biomedical Imaging ; 2012 , Pages 150-153 ; 19457928 (ISSN) ; 9781457718588 (ISBN) Shishegar, R ; Gandomkar, Z ; Soltaman Zadeh, H ; Moghadasi, S. R ; Sharif University of Technology
    IEEE  2012
    Abstract
    Shape analysis plays an important role in many medical imaging studies. One of the recent shape analysis methods uses the Laplace Beltrami operator which is also used in this paper for hippocampal shape comparison. We proposed a feature vector which consists of size measures and shape descriptors based on Laplace Beltrami eigenvalues and eigenfunctions. The aforementioned feature space is utilised for automatic differentiating normal subjects from epileptic patients as well as distinguishing epileptic patients with left temporal lobe epilepsy (LTLE) from patients with right temporal lobe epilepsy (RTLE). Achieved results are diagnostic accuracy of 93% with 95% sensitivity and lateralization... 

    Robust regulation and tracking system design for multivariable control of the tape transport mechanism

    , Article Microsystem Technologies ; Volume 18, Issue 5 , 2012 , Pages 557-573 ; 09467076 (ISSN) Moradi, H ; Bakhtiari Nejad, F ; Alasty, A ; Sharif University of Technology
    2012
    Abstract
    Tape mechanisms must be able to transport the tape with a constant velocity for achieving high rate of data transfer. In this paper, a multivariable model of tape transport mechanism including the take-up and supply reel servos for tape tension control and capstan servo for speed control is considered. To achieve desired performance,pole-placement approach based on general canonical control form (GCCF) is used. Instead of using expensive tension sensors, an observer is designed. A regulator system is developed for disturbance rejection; and a modified integral control is designed for perfect tracking of desired setpoints in tape speed and tensions. Since the real dynamic model associates... 

    Equations-of-motion method for triplet excitation operators in graphene

    , Article Journal of Physics Condensed Matter ; Volume 24, Issue 9 , February , 2012 ; 09538984 (ISSN) Jafari, S. A ; Baskaran, G ; Sharif University of Technology
    2012
    Abstract
    The particlehole continuum in the Dirac sea of graphene has a unique window underneath, which in principle leaves room for bound state formation in the triplet particlehole channel (Baskaran and Jafari 2002 Phys. Rev. Lett. 89 016402). In this work, we construct appropriate triplet particlehole operators and, using a repulsive Hubbard-type effective interaction, we employ equations of motion to derive approximate eigenvalue equations for such triplet operators. While the secular equation for the spin density fluctuations gives rise to an equation which is second order in the strength of the short range interaction, the explicit construction of the triplet operators obtained here shows that,... 

    Assessment of three preconditioning schemes for solution of the two-dimensional Euler equations at low Mach number flows

    , Article International Journal for Numerical Methods in Engineering ; Volume 89, Issue 1 , 2012 , Pages 20-52 ; 00295981 (ISSN) Hejranfar, K ; Kamali Moghadam, R ; Sharif University of Technology
    Abstract
    Three preconditioners proposed by Eriksson, Choi and Merkel, and Turkel are implemented in a 2D upwind Euler flow solver on unstructured meshes. The mathematical formulations of these preconditioning schemes for different sets of primitive variables are drawn, and their eigenvalues and eigenvectors are compared with each other. For this purpose, these preconditioning schemes are expressed in a unified formulation. A cell-centered finite volume Roe's method is used for the discretization of the preconditioned Euler equations. The accuracy and performance of these preconditioning schemes are examined by computing steady low Mach number flows over a NACA0012 airfoil and a two-element... 

    A comparative study of two preconditioners for solving 3D inviscid low speed flows

    , Article Applied Mechanics and Materials ; Volume 110-116 , 2012 , Pages 423-430 ; 16609336 (ISSN) ; 9783037852620 (ISBN) Hejranfar, K ; Moghadam, R. K ; Sharif University of Technology
    2012
    Abstract
    In the present study, two preconditioners proposed by Eriksson, and Choi and Merkel are implemented on a 3D upwind Euler flow solver on unstructured meshes. The mathematical formulations of these preconditioning schemes for the set of primitive variables Q→p 1=[ρ u v , w p] T are drawn and their eigenvalues and eigenvectors are compared with each others. A cell-centered finite volume Roe's method is used for discretization of the 3D preconditioned Euler equations. The accuracy and performance of these preconditioning schemes are examined by computing low Mach number flows over the ONERA M6 wing for different conditions  

    Free vibration analysis of FGM beams with different boundary conditions using RKPM meshless method

    , 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 1187-1191 ; 9780791854785 (ISBN) Saljooghi, R ; Ahmadian, M. T ; Sharif University of Technology
    Abstract
    This paper presents free vibration analysis of functionally graded material (FGM) beams with different boundary conditions, using RKPM (Reproducing Kernel Particle Method), which is a meshless method. System of equations of motion is derived by using Lagrange's method under the assumption of Euler-Bernoulli beam theory. Boundary conditions of beam are taken into account by using Lagrange multipliers. It is assumed that material properties of the beam vary continuously in the thickness direction according to the power-law form. RKPM is applied to obtain eigenvalue equation of vibration and natural frequencies are obtained. It should be noted that for special cases where the beam is uniform,... 

    Recursive spectral analysis of natural time series based on eigenvector matrix perturbation for online applications

    , Article IET Signal Processing ; Volume 5, Issue 6 , 2011 , Pages 515-526 ; 17519675 (ISSN) Mirmomeni, M ; Lucas, C ; Araabi, B. N ; Moshiri, B ; Bidar, M. R ; Sharif University of Technology
    2011
    Abstract
    Singular spectrum analysis (SSA) is a well-studied approach in signal processing. SSA has originally been designed to extract information from short noisy chaotic time series and to enhance the signal-to-noise ratio. SSA is good for offline applications; however, many applications, such as modelling, analysis, and prediction of time-varying and non-stationary time series, demand for online analysis. This study introduces a recursive algorithm called recursive SSA as a modification to regular SSA for dynamic and online applications. The proposed method is based on eigenvector matrix perturbation approach. After recursively calculating the covariance matrix of the trajectory matrix, R-SSA...