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Total 132 records

    Open-and short-circuit switch fault diagnosis for nonisolated DC-DC converters using field programmable gate array

    , Article IEEE Transactions on Industrial Electronics ; Volume 60, Issue 9 , October , 2013 , Pages 4136-4146 ; 02780046 (ISSN) Shahbazi, M ; Jamshidpour, E ; Poure, P ; Saadate, S ; Zolghadri, M. R ; Sharif University of Technology
    2013
    Abstract
    Fault detection (FD) in power electronic converters is necessary in embedded and safety critical applications to prevent further damage. Fast FD is a mandatory step in order to make a suitable response to a fault in one of the semiconductor devices. The aim of this study is to present a fast yet robust method for fault diagnosis in nonisolated dc-dc converters. FD is based on time and current criteria which observe the slope of the inductor current over the time. It is realized by using a hybrid structure via coordinated operation of two FD subsystems that work in parallel. No additional sensors, which increase system cost and reduce reliability, are required for this detection method. For... 

    The dynamic pull-in instability and snap-through behavior of initially curved microbeams

    , Article Mechanics of Advanced Materials and Structures ; Volume 19, Issue 6 , Jul , 2012 , Pages 485-491 ; 15376494 (ISSN) Zand, M. M ; Sharif University of Technology
    T&F  2012
    Abstract
    In this study, dynamic pull-in instability and snap-through buckling of initially curved microbeams are investigated. The microbeams are actuated by suddenly applied electrostatic force. A finite element model is developed to discretize the governing equations, and Newmark time discretization is employed to solve the discretized equations. The static pull-in behavior is investigated to validate the model. The results of the finite element model are compared with finite difference solutions and their convergence is examined. In addition, the influence of different parameters on dynamic pull-in instability and snap-through buckling is explored  

    Upwind compact implicit and explicit high-order finite difference schemes for level set technique

    , Article International Journal of Computational Methods in Engineering Science and Mechanics ; Volume 13, Issue 4 , 2012 , Pages 308-318 ; 15502287 (ISSN) Nouri Borujerdi, A ; Kebriaee, A ; Sharif University of Technology
    2012
    Abstract
    This paper investigates implementation of upwind compact implicit and explicit high-order finite difference schemes for solution of the level set equation. The upwind compact implicit and explicit high-order finite difference schemes are well-known techniques to descritize spatial derivatives for convection term in hyperbolic equations. Applying of upwind high-order schemes on the level set equation leads to less error and CPU time reduction compared to essential non-oscillatory (ENO), weighted essential non-oscillatory schemes (WENO), and even different particle level set methods. The results indicate the error based on area loss decreases drastically with applying high-order upwind,... 

    Effect of pressure on heat transfer coefficient at the metal/mold interface of A356 aluminum alloy

    , Article International Communications in Heat and Mass Transfer ; Volume 39, Issue 5 , 2012 , Pages 705-712 ; 07351933 (ISSN) Fardi Ilkhchy, A ; Jabbari, M ; Davami, P ; Sharif University of Technology
    Abstract
    The aim of this paper is to correlate interfacial heat transfer coefficient (IHTC) to applied external pressure, in which IHTC at the interface between A356 aluminum alloy and metallic mold during the solidification of casting under different pressures were obtained using the inverse heat conduction problem (IHCP) method. The method covers the expedient of comparing theoretical and experimental thermal histories. Temperature profiles obtained from thermocouples were used in a finite difference heat flow program to estimate the transient heat transfer coefficients. The new simple formula was presented for correlation between external pressure and heat transfer coefficient. Acceptable... 

    Coupled lattice boltzmann - Discrete element method for numerical modelling of sand production

    , Article Particle-Based Methods II - Fundamentals and Applications, 26 October 2011 through 28 October 2011 ; 2011 , Pages 371-382 ; 9788489925670 (ISBN) Ghassemi, A ; Pak, A ; Sharif University of Technology
    Abstract
    In this study, a coupled numerical approach based on Lattice Boltzmann Method (LBM) and Discrete Element Method (DEM) is employed for 2D simulation of fluid flow in porous media comprising of movable circular particles. The developed model is used for simulation of sand production which is one of the important problems in petroleum industry. The numerical tool has proved to have the capability of investigating the mechanisms involved in sand production problem. The results show that the rate of sand production is strongly affected by flow rate and confining pressure  

    Numerical modeling of incline plate LiBr absorber

    , Article Heat and Mass Transfer/Waerme- und Stoffuebertragung ; Volume 47, Issue 3 , November , 2011 , Pages 259-267 ; 09477411 (ISSN) Karami, S ; Farhanieh, B ; Sharif University of Technology
    2011
    Abstract
    Among major components of LiBr-H 2O absorption chillers is the absorber, which has a direct effect on the chillier size and whose characteristics have significant effects on the overall efficiency of absorption machines. In this article, heat and mass transfer process in absorption of refrigerant vapor into a lithium bromide solution of water-cooled incline plate absorber in the Reynolds number range of 5

    Simulation of the interaction between nonspherical particles within the CFD–DEM framework via multisphere approximation and rolling resistance method

    , Article Particulate Science and Technology ; 2015 , Pages 1-11 ; 02726351 (ISSN) Akhshik, S ; Behzad, M ; Rajabi, M ; Sharif University of Technology
    Taylor and Francis Inc  2015
    Abstract
    The particle shape is an important factor playing critical role in evaluation of the interactions between particles in high-concentration particle-fluid flows. In this paper, the well-known multisphere (MS) approximation approach and the novel rolling resistance approach are utilized to examine their performance in order to simplify the generalized shaped particle’s interactions within the framework of discrete element method (DEM) and computational fluid dynamics (CFD). The performance of two approaches are compared with the perfect particle’s shape geometry, for the limited cases of cubic-shaped and disk-shaped particle flows in a horizontal well drilling process as a reference scenario.... 

    Simulation of two-phase liquid-vapor flows using a high-order compact finite-difference lattice Boltzmann method

    , Article Physical Review E - Statistical, Nonlinear, and Soft Matter Physics ; Volume 92, Issue 5 , November , 2015 ; 15393755 (ISSN) Hejranfar, K ; Ezzatneshan, E ; Sharif University of Technology
    American Physical Society  2015
    Abstract
    A high-order compact finite-difference lattice Boltzmann method (CFDLBM) is extended and applied to accurately simulate two-phase liquid-vapor flows with high density ratios. Herein, the He-Shan-Doolen-type lattice Boltzmann multiphase model is used and the spatial derivatives in the resulting equations are discretized by using the fourth-order compact finite-difference scheme and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient two-phase flow solver. A high-order spectral-type low-pass compact nonlinear filter is used to regularize the numerical solution and remove spurious waves generated by flow nonlinearities in smooth regions... 

    Numerical simulation of sand production experiment using a coupled Lattice Boltzmann-Discrete Element Method

    , Article Journal of Petroleum Science and Engineering ; Volume 135 , November , 2015 , Pages 218-231 ; 09204105 (ISSN) Ghassemi, A ; Pak, A ; Sharif University of Technology
    Elsevier  2015
    Abstract
    In this study, a coupled numerical approach based on Lattice Boltzmann Method (LBM) and Discrete Element Method (DEM) is employed for two-dimensional simulation of fluid flow in deformable particulate media comprising of movable circular particles. The developed LB-DE code is validated against the results of a bi-axial shear test as well as two well-known benchmark problems including settling of a circular particle under gravity force inside a viscous fluid, and motion of a neutrally buoyant particle released in a Poiseuille flow. The verified code is then utilized for simulation of "Sand Production" phenomenon which is of importance for oil producing wells in weakly cemented sandstone... 

    Congestion effect on maximum dynamic stresses of bridges

    , Article Structural Engineering and Mechanics ; Volume 55, Issue 1 , 2015 , Pages 111-135 ; 12254568 (ISSN) Samanipour, K ; Vafaia, H ; Sharif University of Technology
    Techno Press  2015
    Abstract
    Bridge behavior under passing traffic loads has been studied for the past 50 years. This paper presents how to model congestion on bridges and how the maximum dynamic stress of bridges change during the passing of moving vehicles. Most current research is based on mid-span dynamic effects due to traffic load and most bridge codes define a factor called the dynamic load allowance (DLA), which is applied to the maximum static moment under static loading. This paper presents an algorithm to solve the governing equation of the bridge as well as the equations of motions of two real European trucks with different speeds, simultaneously. It will be shown, considering congestion in eight case... 

    Contact time study of electrostatically actuated microsystems

    , Article Scientia Iranica ; Volume 17, Issue 5 B , SEPTEMBER-OCTOBER , 2010 , Pages 348-357 ; 10263098 (ISSN) Moghimi Zand, M ; Rashidian, B ; Ahmadian, M. T ; Sharif University of Technology
    2010
    Abstract
    This paper presents a model to analyze contact phenomenon in microsystems actuated by ramp voltages, which has applications in frequency sweeping. First-order shear deformation theory is used to model dynamical system using finite element method, while finite difference method is applied to model squeeze film damping. The model is validated by static pull-in results. The presented hybrid FEMFDM model is utilized to compute values of contact time and dynamic behavior. Considering this model, effects of different geometrical and mechanical parameters on contact time are studied. The influence of imposing the additional reverse voltage on dynamic characteristics of the system is also... 

    Comparison of numerical formulations for Two-phase flow in porous media

    , Article Geotechnical and Geological Engineering ; Volume 28, Issue 4 , 2010 , Pages 373-389 ; 09603182 (ISSN) Ataie Ashtiani, B ; Raeesi Ardekani, D ; Sharif University of Technology
    2010
    Abstract
    Numerical approximation based on different forms of the governing partial differential equation can lead to significantly different results for two-phase flow in porous media. Selecting the proper primary variables is a critical step in efficiently modeling the highly nonlinear problem of multiphase subsurface flow. A comparison of various forms of numerical approximations for two-phase flow equations is performed in this work. Three forms of equations including the pressure-based, mixed pressure-saturation and modified pressure-saturation are examined. Each of these three highly nonlinear formulations is approximated using finite difference method and is linearized using both Picard and... 

    Joint user pairing, subchannel, and power allocation in full-duplex multi-user OFDMA networks

    , Article IEEE Transactions on Wireless Communications ; Volume 15, Issue 12 , 2016 , Pages 8260-8272 ; 15361276 (ISSN) Di, B ; Bayat, S ; Song, L ; Li, Y ; Han, Z ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc 
    Abstract
    In this paper, the resource allocation and scheduling problem for a full-duplex (FD) orthogonal frequency-division multiple-access network is studied where an FD base station simultaneously communicates with multiple pairs of uplink (UL) and downlink (DL) half-duplex (HD) users bidirectionally. In this paper, we aim to maximize the network sum-rate through joint UL and DL user pairing, OFDM subchannel assignment, and power allocation. We formulate the problem as a non-convex optimization problem. The optimal algorithm requires an exhaustive search, which will become prohibitively complicated as the numbers of users and subchannels increase. To tackle this complex problem more efficiently, we... 

    Numerical simulation of structural dynamics using a high-order compact finite-difference scheme

    , Article Applied Mathematical Modelling ; Volume 40, Issue 3 , 2016 , Pages 2431-2453 ; 0307904X (ISSN) Hejranfar, K ; Parseh, K ; Sharif University of Technology
    Elsevier Inc 
    Abstract
    A high-order compact finite-difference scheme is applied and assessed for the numerical simulation of structural dynamics. The two-dimensional elastic stress-strain equations are considered in the generalized curvilinear coordinates and the spatial derivatives in the resulting equations are discretized by a fourth-order compact finite-difference scheme. For the time integration, an implicit second-order dual time-stepping method is utilized in which a fourth-order Runge-Kutta scheme is used to integrate in the pseudo-time level. The accuracy and robustness of the solution procedure proposed are investigated through simulating different two-dimensional benchmark test cases in structural... 

    Effect of boundary conditions on dynamic behaviour of bridges

    , Article Proceedings of the Institution of Civil Engineers: Structures and Buildings ; Volume 169, Issue 2 , 2016 , Pages 121-140 ; 09650911 (ISSN) Samanipour, K ; Vafai, H ; Sharif University of Technology
    Thomas Telford Services Ltd 
    Abstract
    A moving vehicle, owing to its vibration and mass inertia, has significant effects on the dynamic response of structures. Most bridge codes define a factor called the dynamic load allowance, which is applied to the maximum static moment under static loading due to traffic load. This paper presents how to model an actual truck on bridges and how the maximum dynamic stresses of bridges change during the passage of moving vehicles. Furthermore, an algorithm to solve the governing equation of the bridge simultaneous with the equations of motion of an actual European truck is presented. Subsequently, 32 dynamic analyses of different bridges with different spans, road profiles and boundary... 

    New and simple equations for ultimate bearing capacity of strip footings on two-layered clays: Numerical study

    , Article International Journal of Geomechanics ; Volume 16, Issue 4 , 2016 ; 15323641 (ISSN) Danaei Ahmadi, M. M ; Mofarraj Kouchaki, B ; Sharif University of Technology
    American Society of Civil Engineers (ASCE) 
    Abstract
    In this paper, two very simple and accurate equations are introduced that enable geotechnical engineers to determine the undrained ultimate bearing capacity of shallow strip foundations resting on two-layered clays. Results of the analyses show that in the case of weak-overstrong clay, usually general shear failure occurs, and in the case of strong-over-weak clay, the soil will most likely experience punching failure with a large amount of plastic settlement before it reaches its ultimate bearing capacity. This study applied the finite-difference method using a computer program to evaluate the undrained bearing capacity for all cases analyzed. The Mohr-Coulomb failure criterion was used for... 

    Electrokinetic and aspect ratio effects on secondary flow of viscoelastic fluids in rectangular microchannels

    , Article Microfluidics and Nanofluidics ; Volume 20, Issue 8 , 2016 ; 16134982 (ISSN) Reshadi, M ; Saidi, M. H ; Firoozabadi, B ; Saidi, M. S ; Sharif University of Technology
    Springer Verlag  2016
    Abstract
    The secondary flow of PTT fluids in rectangular cross-sectional plane of microchannels under combined effects of electroosmotic and pressure driving forces is the subject of the present study. Employing second-order central finite difference method in a very refined grid network, we investigate the effect of electrokinetic and geometric parameters on the pattern, strength and the average of the secondary flow. In this regard, we try to illustrate the deformations of recirculating vortices due to change in the dimensionless Debye–Hückel and zeta potential parameters as well as channel aspect ratio. We demonstrate that, in the presence of thick electric double layers, significant alteration... 

    Simulation of the interaction between nonspherical particles within the CFD–DEM framework via multisphere approximation and rolling resistance method

    , Article Particulate Science and Technology ; Volume 34, Issue 4 , 2016 , Pages 381-391 ; 02726351 (ISSN) Akhshik, S ; Behzad, M ; Rajabi, M ; Sharif University of Technology
    Taylor and Francis Inc 
    Abstract
    The particle shape is an important factor playing critical role in evaluation of the interactions between particles in high-concentration particle-fluid flows. In this paper, the well-known multisphere (MS) approximation approach and the novel rolling resistance approach are utilized to examine their performance in order to simplify the generalized shaped particle’s interactions within the framework of discrete element method (DEM) and computational fluid dynamics (CFD). The performance of two approaches are compared with the perfect particle’s shape geometry, for the limited cases of cubic-shaped and disk-shaped particle flows in a horizontal well drilling process as a reference scenario.... 

    A numerical study on the absorption of water vapor into a film of aqueous LiBr falling along a vertical plate

    , Article Heat and Mass Transfer/Waerme- und Stoffuebertragung ; Volume 46, Issue 2 , 2009 , Pages 197-207 ; 09477411 (ISSN) Karami, S ; Farhanieh, B ; Sharif University of Technology
    Abstract
    Absorber is an important component in absorption machines and its characteristics have significant effects on the overall efficiency of absorption machines. This article reports a model of simultaneous heat and mass transfer process in absorption of refrigerant vapor into a lithium bromide solution of water - cooled vertical plate absorber in the Reynolds number range of 5 < Re < 150. The boundary layer assumptions were used for the transport of mass, momentum and energy equations and the fully implicit finite difference method was employed to solve the governing equations in the film flow. Dependence of lithium bromide aqueous properties to the temperature and concentration and film... 

    Numerical solution of non-fourier heat transfer during laser irradiation on tooth layers

    , Article Journal of Mechanical Science and Technology ; Volume 31, Issue 12 , 2017 , Pages 6085-6092 ; 1738494X (ISSN) Falahatkar, S ; Nouri Borujerdi, A ; Najafi, M ; Mohammadzadeh, A ; Sharif University of Technology
    Abstract
    This study reports on the simulation of temperature distribution of human tooth under a laser beam based on non-Fourier models. The temperature in the tooth depth that directly results from the conduction heat transfer process is caused by the lengthy thermal relaxation time in the tooth layers. A detailed tooth composed of enamel, dentin, and pulp with unstructured shape, uneven boundaries, and realistic thicknesses was considered. A finite difference scheme was separately adopted to solve time-dependent equations in solid layers and soft tissue of the tooth. In this study, a dual-phase-lag non-Fourier heat conduction model was applied to evaluate temperature distribution induced by laser...