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    A continuum-atomistic multi-scale technique for nonlinear behavior of nano-materials

    , Article International Journal of Mechanical Sciences ; Volume 148 , 2018 , Pages 191-208 ; 00207403 (ISSN) Khoei, A. R ; Sameti, A. R ; Kazerooni, Y. N ; Sharif University of Technology
    Elsevier Ltd  2018
    In this paper, a hierarchical RVE-based continuum-atomistic multi-scale procedure is developed to model the nonlinear behavior of nano-materials. The atomistic RVE is accomplished in consonance with the underlying atomistic structure, and the inter-scale consistency principals, i.e. kinematic and energetic consistency principals, are exploited. To ensure the kinematic compatibility between the fine- and coarse-scales, the implementation of periodic boundary conditions is elucidated for the fully atomistic method. The material properties of coarse-scale are modeled with the nonlinear finite element method, in which the stress tensor and tangent modulus are computed using the Hill-Mandel... 

    Multi-scale Analysis of Dislocation Emission for Nano-crystalline Structures

    , M.Sc. Thesis Sharif University of Technology Fattahi Faradonbeh, Mehran (Author) ; Khoei, Amir Reza (Supervisor) ; Jahanshahi, Mohsen (Supervisor)
    In this study, a new multi-scale hierarchical technique has been employed to investigate the role of edge dislocation on nano-plates with hex atomic structure in large deformation. Two multiscale hierarchical atomistic/molecular dynamics (MD)–finite element (FE) coupling methods are proposed to illustrate the influence of temperature on mechanical properties of Magnesium in large deformation. The atomic nonlinear elastic parameters are obtained via computing second-order derivative of Representative atom’s energy and RVE’s strain energy density with respect to deformation criterions (deformation gradient and Green strain tensor) to bridge between atomistic and continuum level, the... 

    Fluctuations in the order of System Size in the Avalanche-Size Distribution of Sandpiles Model

    , M.Sc. Thesis Sharif University of Technology Saadat, Elaheh (Author) ; Moghimi Araghi, Saman (Supervisor)
    Since the concept of Self-Organized Criticality was introduced in terms of BTW Sandpiles model, its major features have been known as broad power law distributions without any tuning parameters. In some selforganized critical systems like brain and neural networks, some evidences and experiments show a periodic or non-power law distribution of avalanches in addition to the power-law distributions of avalanches. In this thesis we try to observe the same phenomenon in the well-known SOC models, namely the BTW and Manna sandpile models. We have considered small lattice sizes with periodic boundary conditions and a small amount of dissipation. Within such conditions we observe a periodic-like... 

    Analysis of transient heat conduction in a hollow cylinder using Duhamel theorem

    , Article International Journal of Thermophysics ; Volume 34, Issue 2 , 2013 , Pages 350-365 ; 0195928X (ISSN) Fazeli, H ; Abdous, M. A ; Karabi, H ; Moallemi, N ; Esmaeili, M ; Sharif University of Technology
    The objective of this paper is to derive the mathematical model of two-dimensional heat conduction at the inner and outer surfaces of a hollow cylinder which are subjected to a time-dependent periodic boundary condition. The substance is assumed to be homogenous and isotropic with time-independent thermal properties. Duhamel's theorem is used to solve the problem for the periodic boundary condition which is decomposed by Fourier series. In this paper, the effects of the temperature oscillation frequency on the boundaries, the variation of the hollow cylinder thickness, the length of the cylinder, the thermophysical properties at ambient conditions, and the cylinder involved in some... 

    Study of cut-off radius and temperature effects on water molecular behavior using molecular dynamics method

    , Article ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2011, 19 June 2011 through 22 June 2011, Edmonton, AB ; Volume 2 , 2011 , Pages 277-282 ; 9780791844649 (ISBN) Darbandi, M ; Khaledi-Alidusti, R ; Abbaspour, M ; Abbasi, H. R ; Schneider, G ; Sharif University of Technology
    Water molecules are one of the important molecules in nanofluidics. Its structure and its behavior can change with Temperature and cut-off distance parameters. In this study temperature and cut-off distance effects on the nano-scale water molecules behavior are investigated by molecular dynamics simulations. Many water molecular models have been developed in order to help discover the structure of water molecules. In this study, the flexible three centered (TIP3P-C) water potential is used to model the inter- and intramolecular interactions of the water molecules. In this simulation, we have been studied 512 water molecules with periodic boundary conditions and in a simulation box with 25... 

    Effects of microstructural morphology on formability, strain localization, and damage of ferrite-pearlite steels: experimental and micromechanical approaches

    , Article Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science ; Volume 52, Issue 2 , January , 2021 , Pages 711-725 ; 10735623 (ISSN) Isavand, S ; Assempour, A ; Sharif University of Technology
    Springer  2021
    This paper attempts to predict how the microstructural features and mechanical properties of the individual constituents affect the deformation behavior and formability of ferrite-pearlite steels under quasi-static loading at room temperature. For this purpose, finite element simulations using representative volume elements (RVEs) based on the real microstructures were implemented to model the flow behavior of the ferrite-pearlite steels with various microstructural morphologies (non-banded and banded). The homogenized flow curves obtained from the RVEs subjected to periodic boundary conditions together with displacement boundary conditions were validated with the experimental results of the... 

    Scattering Analysis from Periodic Rough Surfaces Using Spectral-FDTD

    , M.Sc. Thesis Sharif University of Technology Kianinejad, Amin (Author) ; Shishegar, Amir Ahmad (Supervisor)
    In this thesis the scattering analysis of periodic rough surfaces is studied using the Spectral Finite Difference Time Domain (SFDTD) method. This method takes advantages of both wideband frequency response and stability criterion for all incident angles compared to other methods in solving the periodic problems. In the SFDTD method a wave with Constant Transverse Wavenumber (CTW) is used as the incident wave. As a result, the problem of time delay in periodic boundary is solved and Periodic Boundary Condition (PBC) can be implemented in time domain directly. Periodic rough surfaces are important part of electromagnetic propagation surfaces. Therefore analysis of scattering from these... 

    Developing Advanced Models to Simply Simulate the HRSG of Class F Turbine in Full Scale

    , M.Sc. Thesis Sharif University of Technology Mohammad, Ali (Author) ; Darbandi, Masoud (Supervisor)
    Todays, heat recovery steam generator (HRSG) systems are used extensively in combined power plant cycles (consisting of Rankine cycles and Brighton gas cycle) to maximize their efficiencies. The role of HRSG is essential in providing additional energy needed in the steam cycle part. The HRSG has many pipes or harps. They recover the heat from the incoming gas from the gas turbine outlet, which may be boosted up by the duct burner unit. The main challenge in simulating heat recovery units such as tube bundles is to provide ultra-large and cost-efficient grids. In such situations, alternative models are used to simplify the simulation of the steam generator in full scale, including the... 

    Molecular dynamics investigation of β-SiC behavior under three-axial tensile loading

    , Article Journal of Computational and Theoretical Nanoscience ; Volume 8, Issue 11 , 2011 , Pages 2187-2192 ; 15461955 (ISSN) Mortazavi, B ; Simchi, A ; Besharati Givi, M. K ; Rajabpour, A ; Sharif University of Technology
    Molecular dynamics (MD) simulations were used to study the mechanical behaviour of β-SiC at nano-scale under tensile loading. Effects of loading rate and tensile temperature on the mechanical properties and failure were studied. Modified embedded-atom method (MEAM) potential and Berendsen thermostat were utilized for modelling. Periodic boundary conditions were employed and the behaviour of material was analyzed under three-axial loading condition at which the stress- strain relation was acceptably size independent. It is shown that with increasing the loading rate from 5 m/s to 70 m/s, the failure strain increases without a remarkable change in the stress-strain relationship. The MD... 

    Calculation of thermodynamic properties of Ni nanoclusters via selected equations of state based on molecular dynamics simulations

    , Article Solid State Communications ; Volume 151, Issue 14-15 , 2011 , Pages 965-970 ; 00381098 (ISSN) Akbarzadeh, H ; Abroshan, H ; Taherkhani, F ; Parsafar, G. A ; Sharif University of Technology
    We present an approach for constant-pressure molecular dynamics simulations. This approach is especially designed for finite systems, for which no periodic boundary condition applies. A molecular dynamics (MD) simulation for Ni nanoclusters is used to calculate their pressurevolumetemperature (pvT) data for the temperature range 200 K≤T≤400 K, and pressures up to 600 kbar. Isothermal sets of pvT data were generated by the simulation; each set was fitted by three equations of state (EoSs): Linear Isotherm Regularity-II (LIRII), BirchMurnaghan (BM), and EOS III. It is found that the MD data are satisfactorily reproduced by the EoSs with reasonable precision. Some features of the EoSs criteria,... 

    A quasi-three-dimensional thermal model for multi-stream plate fin heat exchangers

    , Article Applied Thermal Engineering ; Volume 157 , 2019 ; 13594311 (ISSN) Niroomand, R ; Saidi, M. H ; Hannani, S. K ; Sharif University of Technology
    Elsevier Ltd  2019
    In this study, a novel pseudo-three-dimensional model is developed to find out both fluid and solid temperature distributions in multi-stream plate fin heat exchangers. In this simulation algorithm, heat exchangers can be in either parallel flow or cross flow configuration. The model considerations include: heat leakage of cap plates and side plates, conduction throughout the solid matrix of the heat exchanger, variable physical properties, and inlet mass flow rate maldistribution. Using the computational code, the effects of different factors such as: the number of layers, mass flow variation, inlet mass flow rate maldistribution, and stacking pattern on the thermal performance of the heat... 

    A general multi-scale modeling framework for two-phase simulation of multi-stream plate-fin heat exchangers

    , Article International Journal of Heat and Mass Transfer ; Volume 156 , 2020 Niroomand, R ; Saidi, M. H ; Hannani, S. K ; Sharif University of Technology
    Elsevier Ltd  2020
    Compact heat exchangers are among the vital components used in various industries. In this study, a general framework has been developed with a multi-scale point of view for three-dimensional simulation of multi-stream plate-fin heat exchangers. The most important features in the MSPFHEs simulation, such as phase change phenomena, multi-component mixtures, multiple streams, transversal, lateral and longitudinal conduction, non-uniformity of inlet flow, variable fluid properties, and heat leakage are simultaneously considered in this model. The modular form of the model structure has facilitated layer-by-layer simulation of cross flow heat exchangers as well as parallel flow ones. Our model... 

    Micromechanics and constitutive modeling of connective soft tissues

    , Article Journal of the Mechanical Behavior of Biomedical Materials ; Volume 60 , 2016 , Pages 157-176 ; 17516161 (ISSN) Fallah, A ; Ahmadian, M. T ; Firozbakhsh, K ; Aghdam, M. M ; Sharif University of Technology
    Elsevier Ltd 
    In this paper, a micromechanical model for connective soft tissues based on the available histological evidences is developed. The proposed model constituents i.e. collagen fibers and ground matrix are considered as hyperelastic materials. The matrix material is assumed to be isotropic Neo-Hookean while the collagen fibers are considered to be transversely isotropic hyperelastic. In order to take into account the effects of tissue structure in lower scales on the macroscopic behavior of tissue, a strain energy density function (SEDF) is developed for collagen fibers based on tissue hierarchical structure. Macroscopic response and properties of tissue are obtained using the numerical... 

    Effect of different geometries in simulation of 3D viscous flow in francis turbine runners

    , Article Scientia Iranica ; Volume 16, Issue 4 B , 2009 , Pages 363-369 ; 10263098 (ISSN) Firoozabadi, B ; Dadfar, R ; Pirali, A. P ; Ahmadi, G ; Sharif University of Technology
    Overall turbine analysis requires large CPU time and computer memory, even in the present days. As a result, choosing an appropriate computational domain accompanied by a suitable boundary condition can dramatically reduce the time cost of computations. This work compares different geometries for numerical investigation of the 3D flow in the runner of a Francis turbine, and presents an optimum geometry with least computational effort and desirable numerical accuracy. The numerical results are validated with a GAMM Francis Turbine runner, which was used as a test case (GAMM workshop on 3D computation of incompressible internal flows, 1989) in which the geometry and detailed best efficiency...