Search for: numerical-methods
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    Integrating load reduction into wholesale energy market with application to wind power integration

    , Article IEEE Systems Journal ; Vol. 6, issue. 1 , 2012 , p. 35-45 ; ISSN: 19328184 Parvania, M ; Fotuhi-Firuzabad, M ; Sharif University of Technology
    Renewable energy resources, notably wind power, are expected to provide considerable portion of the world energy requirements in the near future. Many system operators around the world are challenged by the problems associated with integrating these intermittent resources into the grid. As one of the potential solutions, demand response (DR) is expected to play a major role for mitigating integration issues of intermittent renewable energy resources. In this context, this paper proposes a DR program which helps to integrate wind power by reshaping the load of the system. The DR program provides a framework to procure load reduction from DR resources in the wholesale energy market. The... 

    Design and routine test optimization of modern protection systems with reliability and economic constraints

    , Article IEEE Transactions on Power Delivery ; Vol. 27, issue. 1 , 2012 , p. 271-278 ; ISSN: 08858977 Etemadi, A. H ; Fotuhi-Firuzabad, M ; Sharif University of Technology
    This paper approaches the topic of protection system reliability from an economic point of view by 1) designing an optimal modern protective relay based on a prescribed level of reliability subject to economic constraints and 2) determining optimal routine test intervals by balancing the cost of routine tests and losses due to relay failure. This paper proposes a method that enables the designer to optimally select hardware and software components of a digital protective relay to gain the highest possible overall reliability with a restricted budget. This design can be viewed as a reliability and redundancy allocation problem for which a new easy-to-implement algorithm is proposed. In... 

    Introducing structural approximation method for modeling nanostructures

    , Article Journal of Computational and Theoretical Nanoscience ; Vol. 7, Issue 2 , 2010 , p. 423-428 ; ISSN: 15461955 Momeni, K ; Alasty, A ; Sharif University of Technology
    In this work a new method for analyzing nanostructured materials has been proposed to accelerate the simulations for solid crystalline materials. The proposed Structural Approximation Method (SAM) is based on Molecular Dynamics (MD) and the accuracy of the results can also be improved in a systematic manner by sacrificing the simulation speed. In this method a virtual material is used instead of the real one, which has less number of atoms and therefore fewer degrees of freedom, compared to the real material. The number of differential equations that must be integrated in order to specify the state of the system will decrease significantly, and the simulation speed increases. To generalize... 

    Study on non-equilibrium effects during spontaneous imbibition

    , Article Energy and Fuels ; Vol. 25, issue. 7 , June , 2011 , p. 3053-3059 ; ISSN: 08870624 Mirzaei-Paiaman, A ; Masihi, M ; Standnes, D. C ; Sharif University of Technology
    Spontaneous imbibition of water into the matrix blocks because of capillary forces is an important recovery mechanism for oil recovery from naturally fractured reservoirs. In modeling this process, it has been assumed classically that local equilibrium is reached and, therefore, capillary pressure and relative permeability functions are only a function of water saturation, resulting in the appearance of the self-similarity condition. In some works published in the last 2 decades, it has, however, been claimed that local equilibrium is not reached in porous media, and therefore, opposite the classical local-equilibrium/self-similar approach, non-equilibrium effects should be taken into... 

    Improved S2 approximations

    , Article Annals of Nuclear Energy ; Vol. 77 , 2014 , Pages 257-264 ; ISSN: 03064549 Safari, M. J ; Bahrami, M ; Sharif University of Technology
    Discrete ordinates method relies on approximating the integral term of the transport equation with the aid of quadrature summation rules. These quadratures are usually based on certain assumptions which assure specific symmetry rules and transport/diffusion limits. Generally, these assumptions are not problem-dependent which results in inaccuracies in some instances. Here, various methods have been developed for more accurate estimation of the independent angle in S2 approximation, as it is tightly related to valid estimation of the diffusion coefficient/length. We proposed and examined a method to reduce a complicated problem that usually is consisting many energy groups and discrete... 

    Comparison of boundary slip for two variants of immersed boundary method in lattice Boltzmann framework

    , Article Physica A: Statistical Mechanics and its Applications ; Vol. 404 , 2014 , Pages 200-216 ; ISSN: 03784371 Farnoush, S ; Manzari, M. T ; Sharif University of Technology
    In this paper, the Immersed Moving Boundary-Lattice Boltzmann (IMB-LB) method is compared with the single relaxation time and multiple-relaxation-time versions of the Immersed Boundary-Lattice Boltzmann (IB-LB) method in terms of the amount of numerical velocity slip produced on solid boundaries. The comparisons are performed for both straight and curved boundaries based on the effects of thickness of virtual domain used in the IB method for the first time, and relaxation time parameter(s) of the LB method. For the straight boundaries, a shear flow problem is studied while for the curved boundaries, a falling circular cylinder in an infinite channel is investigated. First, sensitivities of... 

    Multi-scale modeling of surface effects in nano-materials with temperature-related Cauchy-Born hypothesis via the modified boundary cauchy-born model

    , Article International Journal for Numerical Methods in Engineering ; Vol. 97, issue. 2 , 2014 , pp. 79-110 ; ISSN: 00295981 Khoei, A. R ; Ghahremani, P ; Dormohammadi, H ; Sharif University of Technology
    In nano-structures, the influence of surface effects on the properties of material is highly important because the ratio of surface to volume at the nano-scale level is much higher than that of the macro-scale level. In this paper, a novel temperature-dependent multi-scale model is presented based on the modified boundary Cauchy-Born (MBCB) technique to model the surface, edge, and corner effects in nano-scale materials. The Lagrangian finite element formulation is incorporated into the heat transfer analysis to develop the thermo-mechanical finite element model. The temperature-related Cauchy-Born hypothesis is implemented by using the Helmholtz free energy to evaluate the temperature... 

    Boundary control of a marine riser pipe conveying fluid

    , Article ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) ; Vol. 4A , 2014 ; ISBN: 9780791846476 Nojoumian, M. A ; Shirazi, M. J ; Salarieh, H ; Sharif University of Technology
    In this paper, the model of pipe conveying fluid, like marine risers, is modeled with the effect of the interaction between the fluid and structure. A torque is assumed at one end of the pipe to control the vibration of the pipe. The stability of the pipe under time varying disturbance in forced vibration using boundary control law is proved. Exponential stability can be achieved under the free vibration condition. The proposed control law is simple and could be attained by some simple sensors at the end of the pipe. The control law is independent from system parameters so the controller is robust to disturbances and environmental conditions. The controller is obtained via a Lyapunov... 

    Finite volume coupling strategies for the solution of a Biot consolidation model

    , Article Computers and Geotechnics ; Vol. 55, issue , January , 2014 , p. 494-505 ; ISSN: 0266352X Asadi, R ; Ataie-Ashtiani, B ; Simmons, C. T ; Sharif University of Technology
    In this paper a finite volume (FV) numerical method is implemented to solve a Biot consolidation model with discontinuous coefficients. Our studies show that the FV scheme leads to a locally mass conservative approach which removes pressure oscillations especially along the interface between materials with different properties and yields higher accuracy for the flow and mechanics parameters. Then this numerical discretization is utilized to investigate different sequential strategies with various degrees of coupling including: iteratively, explicitly and loosely coupled methods. A comprehensive study is performed on the stability, accuracy and rate of convergence of all of these sequential... 

    Minimum power Miller-compensated CMOS operational amplifiers

    , Article Scientia Iranica ; Vol. 21, Issue. 6 , 2014 , pp. 2243-2249 ; e-ISSN :23453605 Meghdadi, M ; Bakhtiar, M. S ; Sharif University of Technology
    A new approach for the design of two-stage Miller-compensated CMOS op amps is presented. The paper studies the basic relations between power consumption, unitygain bandwidth, the biasing region, technology parameters, and the external capacitive load. As a result, simple and efficient design guides are provided to achieve the minimum possible power consumption for the given specifications and for short-channel devices. It is shown that the conventional design procedures do not always result in minimum power op amps. The presented results are also verified by Spectre simulations  

    Investigation on the effect of mold constraints and cooling rate on residual stress during the sand-casting process of 1086 steel by employing a thermomechanical model

    , Article Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science ; Vol. 45, issue. 3 , 2014 , pp. 1157-1169 ; ISSN: 1073-5615 Baghani, A ; Davami, P ; Varahram, N ; Shabani, M. O ; Sharif University of Technology
    In this study, the effects of mold constraints and cooling rate on residual stress were analyzed during the shaped casting process. For this purpose, an H-shaped sample was designed in which the contraction of its middle portion is highly restricted by the mold during the cooling process. The effects of an increasing cooling rate combined with mold constraints were analyzed by reducing the thickness of the middle portion in the second sample. A three-dimensional coupled temperature-displacement analysis was performed in finite-element code ABAQUS to simulate residual stress distribution, and then numerical results were verified by the hole-drilling strain-gauge method. It was concluded that... 

    Robust and rapid converging adaptive beamforming via a subspace method for the signal-plusinterferences covariance matrix estimation

    , Article IET Signal Processing ; Vol. 8, Issue. 5 , July , 2014 , pp. 507-520 ; ISSN: 17519675 Rahmani, M ; Bastani, M. H ; Sharif University of Technology
    The presence of the desired signal (DS) in the training snapshots makes the adaptive beamformer sensitive to any steering vector mismatch and dramatically reduces the convergence rate. Even the performance of the most of the existing robust adaptive beamformers is degraded when the signal-to-noise ratio (SNR) is increased. In this study, a high converging rate robust adaptive beamformer is proposed. This method is a promoted eigenspace-based beamformer. In this paper, a new signal-plus-interferences (SPI) covariance matrix estimator is proposed. The subspace of the ideal SPI covariance matrices is exploited and the estimated covariance matrix is projected into this subspace. This projection... 

    Size-dependent dynamic behavior of microcantilevers under suddenly applied DC voltage

    , Article Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science ; Vol. 228, Issue. 5 , May , 2014 , pp. 896-906 ; ISSN: 09544062 Rahaeifard, M ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
    This paper investigates the dynamic behavior of microcantilevers under suddenly applied DC voltage based on the modified couple stress theory. The cantilever is modeled based on the Euler-Bernoulli beam theory and equation of motion is derived using Hamilton's principle. Both analytical and numerical methods are utilized to predict the dynamic behavior of the microbeam. Multiple scales method is used for analytical analysis and the numerical approach is based on a hybrid finite element/finite difference method. The results of the modified couple stress theory are compared with those from the literature as well as the results predicted by the classical theory. It is shown that the modified... 

    A new multi-winding traction transformer equivalent circuit for short-circuit performance analysis

    , Article International Transactions on Electrical Energy Systems ; Vol. 24, issue. 2 , 2014 , pp. 186-202 ; ISSN: 20507038 Azizian, D ; Vakilian, M ; Faiz, J ; Sharif University of Technology
    Current and force calculations in different short-circuit conditions are required for short-circuit performance analysis of a multi-winding traction transformer which is one of the most important requirements in its design process. This paper extends the available low-frequency three-winding star equivalent circuits to develop a novel equivalent circuit for the four-winding traction transformers. The leakage inductances of the traction transformer are determined and employed to calculate the parameters of this developed star model. It is shown that the star equivalent circuit is a valid and appropriate model to simulate the steady-state and dynamic performance of the traction transformer... 

    Multi-conductor transmission line networks in analysis of side-coupled metal-insulator-metal plasmonic structures

    , Article Optics Communications ; Vol. 313, issue , 2014 , pp. 375-381 ; ISSN: 00304018 Eshaghian, A ; Bahadori, M ; Rezaei, M ; Khavasi, A ; Hodaei, H ; Mehrany, K ; Sharif University of Technology
    An approximate and accurate enough multi-conductor transmission line model is developed for analysis of side-coupled metal-insulator-metal (MIM) waveguides. MIM waveguides have been already modeled by single conductor transmission lines. Here, the side coupling effects that exist between neighboring plasmonic structures are taken into account by finding appropriate values for distributed mutual inductance and mutual capacitance between every two neighboring conductors in the conventional single-conductor transmission line models. In this manner, multi-conductor transmission line models are introduced. Closed-form expressions are given for the transmission and reflection of miscellaneous MIM... 

    Investigation of thermal effects on machining chatter based on FEM simulation of chip formation

    , Article CIRP Journal of Manufacturing Science and Technology ; Vol. 7, issue. 1 , 2014 , p. 1-10 Hajmohammadi, M. S ; Movahhedy, M. R ; Moradi, H ; Sharif University of Technology
    In this paper, thermo mechanical finite element analysis of orthogonal machining process with a flexible tool is carried out to study the dynamic behaviour of the machining system and its stability against chatter vibration. By combining the simulation of chip formation in metal cutting with the dynamic phenomena leading to chatter development, the influence of various phenomena, including thermal effects resulting from friction and plastic deformations on the stability of the process are investigated. The novelty of this model is in its ability to evaluate the effect of temperature rise in the cutting zone on the stability of cutting process. Process stability is analyzed and compared for... 

    An accurate derivation of spatial green's function for finite dielectric structures using characteristic green's function-perfectly matched layer method

    , Article IEEE Transactions on Antennas and Propagation ; Vol. 62, Issue. 6 , 2014 , pp. 3201-3211 ; ISSN: 0018926X Torabi, A ; Shishegar, A. A ; Sharif University of Technology
    A closed-form spatial Green's function for a truncated dielectric slab is derived by using a combination of the characteristic Green's function (CGF) and perfectly matched layer (PML) method. The original structure is terminated by PML that is backed by perfect electric conductor (PEC) in semi-infinite layer at the top and/or bottom. The eigenmodes of the closed structure by PML construct the continuous spectrum contribution of the original structure efficiently. Derived Green's function is exact for separable finite structure while it is approximate for nonseparable one especially in the corners. Generalized scattering matrix (GSM) of truncating surface which contains possible conversions... 

    A computational and analytical study into the use of counter-flow fluidic thrust vectoring nozzle for small gas turbine engines

    , Article Applied Mechanics and Materials ; Vol. 629, issue , 2014 , pp. 97-103 ; ISSN: 16609336 Banazadeh A ; Banazadeh, F ; Sharif University of Technology
    This paper provides an understanding of counter-flow fluidic thrust vectoring, in the presence of the secondary air vacuum, applied to the exhaust nozzle of a micro-jet engine. An analytical and numerical study is performed here on a divergent collar surface adjacent to the cylindrical exhaust duct system. The vectoring angle is controlled by manipulating the momentum flux through a vacuum gap that is located on a circle concentric to the main nozzle. Three dimensional numerical simulations are conducted by utilizing a computational fluid dynamics model with two-equation standard k-ε turbulence model to study the pressure and velocity distribution of internal flow and nozzle geometry.... 

    A fuzzy vendor managed inventory of multi-item economic order quantity model under shortage: An ant colony optimization algorithm

    , Article International Journal of Production Economics ; Vol. 155 , September , 2014 , pp. 259-271 Roozbeh Nia, A ; Hemmati Far, M ; Akhavan Niaki, S. T ; Sharif University of Technology
    In this study, a multi-item economic order quantity model with shortage under vendor managed inventory policy in a single vendor single buyer supply chain is developed. This model explicitly includes warehouse capacity and delivery constraints, bounds order quantity, and limits the number of pallets. Not only the demands are considered imprecise, but also resources such as available storage and total order quantity of all items can be vaguely defined in different ways. An ant colony optimization is employed to find a near-optimum solution of the fuzzy nonlinear integer-programming problem with the objective of minimizing the total cost of the supply chain. Since no benchmark is available in... 

    Surveying the jagorood river's self-purification

    , Article 6th International Conference on Environmental Informatics, ISEIS 2007 ; 21-23 November , 2014 Nazari-Alavi, A ; Mirzai, M ; Sajadi, S. A. A ; Alamolhoda, A. A ; Sharif University of Technology
    the development and enlargement of city structure has caused the environmental problems on watershed of Latian dam which is one of the drinking water resourses of Tehran. The enlargement of Lavasan town and existence of an army garrison that it's effluents of treatment plant discharge to this river necessitate a water quality surveillance in entrance area of the dam. The present research includes the survey of this river's self-purification in the mentioned-above area. Numerical analysis is used in different methods to explain self- purification. Frist, we have the numerical analysis and analytical methods for a known sample in order to determine the accuracy and deviation between two...