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    Design and optimization of a heat driven thermoacoustic refrigerator

    , Article Applied Thermal Engineering ; Volume 62, Issue 2 , 25 January 2014 , Pages 653–661 Ghorbanian, K ; Karimi, M ; Sharif University of Technology
    Abstract
    The present paper deals with the design and optimization of a heat driven thermoacoustic refrigerator. A simplified model is developed which enables to pinpoint and examine the most important physical characteristics of a compact traveling wave thermoacoustic refrigerator driven by a traveling wave thermoacoustic engine. The model can explain the so-called traveling standing wave effect in thermoacoustics very well. The position, length and hydraulic radius of the refrigerator are optimized for the maximum total COP. The prime mover efficiency, refrigerator COP and dimensionless dissipation and their impacts on total COP are investigated and discussed. The results indicate that a COP of... 

    Rigorous derivation of temporal coupled mode theory expressions for travelling and standing wave resonators coupled to optical waveguides

    , Article 7th International Conference on Photonics, Optics and Laser Technology, PHOTOPTICS 2019, 25 February 2019 through 27 February 2019 ; 2019 , Pages 201-208 ; 9789897583643 (ISBN) Zarif, A ; Memarian, M ; Mehrany, K ; Ribeiro P ; Raposo M ; Andrews D ; Institute for Systems and Technologies of Information, Control and Communication (INSTICC) ; Sharif University of Technology
    SciTePress  2019
    Abstract
    Temporal coupled mode theory (CMT) has so far been applied phenomenologically in the analysis of optical cavity-waveguide structures, and relies on a priori knowledge of the to-be-excited resonator mode. Thus a rigorous derivation from Maxwell's equations, and without any prior knowledge of the resonator type is needed. In this paper we derive temporal CMT of optical cavities coupled to waveguides. Starting from Maxwell's equations and considering a proper expansion of the modes of the waveguide and resonator, and using mode orthogonality, the temporal CMT for this structure is obtained. We show that this formulation is general and can be applied to both traveling wave and standing wave type... 

    Rigorous derivation of temporal coupled mode theory expressions for travelling and standing wave resonators coupled to optical waveguides

    , Article 7th International Conference on Photonics, Optics and Laser Technology, PHOTOPTICS 2019, 25 February 2019 through 27 February 2019 ; 2019 , Pages 201-208 ; 9789897583643 (ISBN) Zarif, A ; Memarian, M ; Mehrany, K ; Sharif University of Technology
    SciTePress  2019
    Abstract
    Temporal coupled mode theory (CMT) has so far been applied phenomenologically in the analysis of optical cavity-waveguide structures, and relies on a priori knowledge of the to-be-excited resonator mode. Thus a rigorous derivation from Maxwell's equations, and without any prior knowledge of the resonator type is needed. In this paper we derive temporal CMT of optical cavities coupled to waveguides. Starting from Maxwell's equations and considering a proper expansion of the modes of the waveguide and resonator, and using mode orthogonality, the temporal CMT for this structure is obtained. We show that this formulation is general and can be applied to both traveling wave and standing wave type... 

    Analysis of in-plane vibration and critical speeds of the functionally graded rotating disks

    , Article International Journal of Applied Mechanics ; Volume 11, Issue 2 , 2019 ; 17588251 (ISSN) Bagheri, E ; Jahangiri, M ; Sharif University of Technology
    World Scientific Publishing Co. Pte Ltd  2019
    Abstract
    In this paper, the in-plane free vibration analysis of the functionally graded rotating disks with variable thickness is presented utilizing DQM. It is assumed that the rotational velocity of the disk is constant and the thickness and material properties including modulus of elasticity and density vary along the radial coordinate. The distribution of the forward and backward traveling waves versus the angular velocity is demonstrated for several modal circles and nodal diameters with respect to the fixed and rotating coordinate systems. After presenting the accuracy and convergence of the numerical method, the derived formulation and the solution method are validated by comparing the results... 

    Numerical investigation of the forward and backward travelling waves through an undulating propulsor: performance and wake pattern

    , Article Ships and Offshore Structures ; Apr , 2015 ; 17445302 (ISSN) Ebrahimi, M ; Abbaspour, M ; Sharif University of Technology
    Taylor and Francis Ltd  2015
    Abstract
    Recently, the mechanisms of natural undulatory locomotion of aquatic animal swimming have become one of the most significant issues for the researchers, swimmers and engineers. This study aims to elucidate and compare the propulsive vortical signature and performance of backward (negative undulation) and forward (positive undulation) travelling waves through a typical fishlike propulsor by a systematic numerical study. The numerical approach uses a pressure-based finite volume method solver to solve Navier–Stokes equations in an arbitrary Lagrangian–Eulerian framework domain containing a two-dimensional NACA 0012 foil moving with prescribed kinematics. Some of the important findings are: (1)... 

    Numerical investigation of the forward and backward travelling waves through an undulating propulsor: performance and wake pattern

    , Article Ships and Offshore Structures ; Volume 11, Issue 5 , 2016 , Pages 517-539 ; 17445302 (ISSN) Ebrahimi, M ; Abbaspour, M ; Sharif University of Technology
    Taylor and Francis Ltd  2016
    Abstract
    Recently, the mechanisms of natural undulatory locomotion of aquatic animal swimming have become one of the most significant issues for the researchers, swimmers and engineers. This study aims to elucidate and compare the propulsive vortical signature and performance of backward (negative undulation) and forward (positive undulation) travelling waves through a typical fishlike propulsor by a systematic numerical study. The numerical approach uses a pressure-based finite volume method solver to solve Navier–Stokes equations in an arbitrary Lagrangian–Eulerian framework domain containing a two-dimensional NACA 0012 foil moving with prescribed kinematics. Some of the important findings are: (1)... 

    Non-Reciprocity in Transmission Line Resonator with Time Varying Boundary Condition

    , M.Sc. Thesis Sharif University of Technology Shams, Raheleh (Author) ; Mehrany, Khashayar (Supervisor) ; Memarian, Mohammad (Supervisor)
    Abstract
    Structures with time-varying electromagnetic properties can potentially yield non-reciprocity, amplification, harmonic generation, and other interesting wave propagation phenomena. To date, some studies have dealt with time-varying media and circuits, including time-varying optical ring resonators having degenerate travelling wave resonances. In time-varying ring resonators, the clockwise and counter clockwise resonances lose their degeneracy, and non-reciprocity is observed. One of the main challenges of such research is the need to induce time-variations in the entirety of the resonator or structure, which makes their implementation very limited or impossible in practice. The purpose of... 

    The Existence and Stability of Classical Solutions in the Neural Fields Equations

    , M.Sc. Thesis Sharif University of Technology Shafiee, Ahmad (Author) ; Fotouhi Firouzabadi, Morteza (Supervisor)
    Abstract
    In this thesis, first, the modeling method of neural fields is precisely presented. Then, Existence and Stability of different solutions of one dimensional neural fields like Standing Pulses, Traveling Waves and ... are investigated in three different models of neural fields. In order for proving Existence and Stability of the solutions the mathematical tools like Fourier transform and Evans function are applied. All the models which analysed in this thesis have the following Integro-Differential Equation form:
    τ
    ∂u(x, t)
    ∂t
    = −u(x, t) +
    ∫ +∞
    −∞
    w(x, y)f[u(y, t)]dy + I(x, t) + s(x, t)
    and also in some models the parameters might be changed  

    Design and optimization of a heat driven thermoacoustic refrigerator

    , Article Applied Thermal Engineering ; Volume 61, Issue 2 , 2013 , Pages 653-661 ; 13594311 (ISSN) Ghorbanian, K ; Karimi, M ; Sharif University of Technology
    2013
    Abstract
    The present paper deals with the design and optimization of a heat driven thermoacoustic refrigerator. A simplified model is developed which enables to pinpoint and examine the most important physical characteristics of a compact traveling wave thermoacoustic refrigerator driven by a traveling wave thermoacoustic engine. The model can explain the so-called traveling standing wave effect in thermoacoustics very well. The position, length and hydraulic radius of the refrigerator are optimized for the maximum total COP. The prime mover efficiency, refrigerator COP and dimensionless dissipation and their impacts on total COP are investigated and discussed. The results indicate that a COP of... 

    Neural fields with fast learning dynamic kernel

    , Article Biological Cybernetics ; Volume 106, Issue 1 , January , 2012 , Pages 15-26 ; 03401200 (ISSN) Abbassian, A. H ; Fotouhi, M ; Heidari, M ; Sharif University of Technology
    Abstract
    We introduce a modified-firing-rate model based on Hebbian-type changing synaptic connections. The existence and stability of solutions such as rest state, bumps, and traveling waves are shown for this type of model. Three types of kernels, namely exponential, Mexican hat, and periodic synaptic connections, are considered. In the former two cases, the existence of a rest state solution is proved and the conditions for their stability are found. Bump solutions are shown for two kinds of synaptic kernels, and their stability is investigated by constructing a corresponding Evans function that holds for a specific range of values of the kernel coefficient strength (KCS). Applying a similar... 

    Travelling-wave Ka-band frequency scanning Antennas for millimeter-wave imaging applications

    , Article 2016 8th International Symposium on Telecommunications, IST 2016, 27 September 2016 through 29 September 2016 ; 2017 , Pages 591-595 ; 9781509034345 (ISBN) Ranjbar Naeini, M ; Fakharzadeh, M ; Farzaneh, F ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc  2017
    Abstract
    In this paper, a novel travelling-wave printed Frequency Scanning Antenna (FSA), which can be used for imaging purposes, is proposed. All sample designs scan the space symmetrically about the broadside direction. For different scanning ranges, two low profile antennas are designed on a 6.6 mils thick Rogers 4350 board substrate layer. Impedance bandwidth of the first one covers 6.5GHz, from 29.5GHz to 36GHz, and this antenna scans 56° in angular space. The second antenna has 5.6GHz bandwidth from 30.6GHz to 36.2GHz, and scans 98° in angular space by means of switching the input port. The average gain and the Side-lobe level (SLL) of the first and the second antennas are 11.46dBi, 12.8dB, and... 

    The Impact of Non-reciprocal One-way Propagation on the Coupling between Electromagnetic Resonators and Waveguides

    , M.Sc. Thesis Sharif University of Technology Zarif, Arezoo (Author) ; Mehrany, Khashayar (Supervisor) ; Rajaei, Behzad (Co-Advisor)
    Abstract
    Subwavelength localization of electromagnetic energy with intense local fields, also known as electromagnetic hotspots, has received significant attention over the past few decades. In most cases the hotspot is achieved through the resonant concentration of electromagnetic fields. One way recently considered to get hotspot is through reflection of electromagnetic waves in nonreciprocal one-way structures, in plasmonics and via magneto optic effect.However these hotspots are less confined compared to hotspots caused by resonances. So combining nonreciprocal structures with resonance can provide better hotspots.Our aim in this thesis is to study nonreciprocal resonant structures using coupled... 

    Analysis and Improvement of Output Voltage Ripple in High Voltage DC Power Supply

    , M.Sc. Thesis Sharif University of Technology Ziaoddini, Mojtaba (Author) ; Kaboli, Shahryar (Supervisor)
    Abstract
    The TWT is highly sensitive to the ripple amplitude of its power supply output voltage. If the amplitude of the output voltage's ripple exceeds a certain limit, the amplitude of the undesired harmonics in the power spectrum of the TWT is greatly increased. Increasing the amplitude of undesired harmonics in the TWT output power spectrum reduces the accuracy and performance of the TWT. Therefore, for optimum TWT performance, a low-ripple power supply is essential. Typically, a low-frequency rectifier or high-frequency switching structure is used to build a high-voltage DC power supply. In this thesis, the different structures of high voltage DC power supplies are studied. Next, by focusing on... 

    Numerical Investigation of Jet Pump Performance in Traveling Wave Thermoacoustic Engine

    , M.Sc. Thesis Sharif University of Technology Barzegari, Ali (Author) ; Ghorbanian, Kaveh (Supervisor)
    Abstract
    Thermoacoustic engines are a promising alternative to the conventional heat engines due to the absence of moving parts in the hot region and the small temperature difference required for operation. However, the traveling wave thermoacoustic engine configuration has a relatively lower efficiency due to a phenomenon called Gedeon streaming. In this regard, a jet pump might be used to lower the negative effects of Gedeon streaming. The jet pump, which is simply an element consisting of one or more tapered holes, induces an oscillating flow through the asymmetric device and hence resulting in a time-averaged pressure drop across it. As a result, by balancing this time averaged pressure drop with... 

    Stability and Traveling Waves of a Stage–structured (Predator-rey)Model with Holing Type-II Functional Response and Harvesting

    , M.Sc. Thesis Sharif University of Technology Keyhani Shayeste, Mozhgan (Author) ; Hesaaraki, Mahmoud (Supervisor)
    Abstract
    IN this upper ,we consider a reaction –diffusion predator-prey model with stage –structure, holling type –ll functional response. Nonlocal spatial impacat and harvesting The stability of the equilibria is investigated. Furthermore, by the cross-iteration schauder fixed point theorem, we deduce the existence of traveling wave solution which connects the zero solution and the positive constant eguilibrium  

    Closed form formula for longitudinal slot of LWAs with minimum side lobe level

    , Article 2017 25th Iranian Conference on Electrical Engineering, ICEE 2017, 2 May 2017 through 4 May 2017 ; 2017 , Pages 1820-1824 ; 9781509059638 (ISBN) Boozari, M ; Bayderkhani, R ; Alijani, M. G ; Neshati, M. H ; Sharif University of Technology
    Abstract
    In this paper a closed-form expression is introduced to calculate slot shape of long slot leaky wave waveguide antennas at its fundamental mode of operation with minimum side lobe level (SLL). The derived equation is a function of geometrical parameters of the structure. This is an advantage because they show a clear relationship between geometrical parameters of the antenna and radiated field that allows intelligent modification of the pattern. The Gaussian slot is used to obtain the best radiation pattern with lowest SLL. The results obtained by introduced formula are compared with the full wave commercial simulator such as HFSS software which confirms the accuracy of the introduced... 

    Tunable leaky-wave radiation by graphenecovered corrugated surfaces

    , Article Applied Optics ; Volume 57, Issue 30 , 2018 , Pages 8971-8977 ; 1559128X (ISSN) Tavakol, M. R ; Saba, A ; Arik, K ; Khavasi, A ; Sharif University of Technology
    OSA - The Optical Society  2018
    Abstract
    we propose a graphene-covered subwavelength metallic grating where the Fermi level of graphene is sinusoidally modulated as a leaky-wave antenna at terahertz frequencies. This structure can convert spoof surface plasmon guided waves to free-space radiation due to the tunability of graphene. Analysis and design of the proposed leaky-wave antenna are discussed based on sinusoidally modulated surface impedance. The surface impedance is obtained by an analytical circuit model. The sinusoidal surface impedance is realized using modulation of the conductivity of graphene by applying a bias voltage. The proposed leaky-wave antenna is capable of electronic beam scanning with an almost constant gain... 

    Using the hybrid DAS-SR method for damage localization in composite plates

    , Article Composite Structures ; Volume 247 , 2020 Nokhbatolfoghahai, A ; Navazi, H. M ; Groves, R. M ; Sharif University of Technology
    Elsevier Ltd  2020
    Abstract
    In this paper, the hybrid Delay-And-Sum (DAS) with Sparse Reconstruction (SR) method was further developed for damage location in composite plates. In composite materials, anisotropy leads to some challenges in using conventional damage location methods, which are developed for isotropic materials. In the hybrid DAS-SR method, the DAS and SR methods were combined as a complement of each other. To investigate the DAS-SR method for composite structures, the group velocity of the travelling wave for different directions was first measured experimentally via PZTs. The DAS and SR formulations were then modified to be compatible with the direction-dependent group velocities. The results show that... 

    Modeling and Characterizing Transient Behavior of Distillation Columns with Travelling Wave Theory

    , M.Sc. Thesis Sharif University of Technology Hemmati Alam, Narjes (Author) ; Pishvaie, Mahmoud Reza (Supervisor)
    Abstract
    Simulation and controlling nonlinear process is one of the challenging problems in academic and industrial research. Saving data for keeping history of process or arising number of calculation for optimization in control design, is forcing to decrease number of equation or order of system. One of the methods to decrease the order of distillation process is using the travelling wave theory. This solution shows god accuracy for simulation the behavior of distillation column. On the other hand finite difference method as a solver for distillation equation takes some time to converge to steady state condition and the numbers of calculation in this method are high. Finite difference method is... 

    Proposal of a new design for valveless micropumps

    , Article Scientia Iranica ; Volume 18, Issue 6 , December , 2011 , Pages 1261-1266 ; 10263098 (ISSN) Afrasiab, H ; Movahhedy, M. R ; Assempour, A ; Sharif University of Technology
    2011
    Abstract
    A new design for a valveless micropumping device has been proposed that integrates two existing pumping technologies, namely, the wall induced traveling wave and the obstacle-type valveless micropump. The liquid in the microchannel is transported by generating a traveling wave on the channel, while the placing of two asymmetric trapezoid obstacles, along the centerline of the channel inlet and outlet, leads to a significant (up to seven times) increase of the net flow rate of the device. The effectiveness of this innovative design has been proved through a verified three-dimensional finite element model. FluidStructure Interaction (FSI) analysis is performed in the framework of an Arbitrary...