Search for: numerical-algorithms
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    A plasticity model for metal powder forming processes

    , Article International Journal of Plasticity ; Volume 17, Issue 12 , 2001 , Pages 1659-1692 ; 07496419 (ISSN) Lewis, R. W ; Khoei, A. R ; Sharif University of Technology
    In this paper, a double-surface plasticity model, based on a combination of a convex yield surface consisting of a failure envelope, such as a Mohr-Coulomb yield surface and, a hardening cap model, is developed for the nonlinear behaviour of powder materials in the concept of a generalized plasticity formulation for the description of cyclic loading. This model reflects the yielding, frictional and densification characteristics of powder along with strain and geometrical hardening which occur during the compaction process. The solution yields details on the powder displacement from which it is possible to establish the stress state in the powder and the densification is derived from... 

    Nonlinear Programming Without a Penalty Function or a Filter

    , M.Sc. Thesis Sharif University of Technology Abdollahi, Fahimeh (Author) ; Mahdavi Amiri, Nezamedin (Supervisor)
    A new method, recently introduced in the literature, is discussed for solving equality constrained nonlinear optimization problems. This method does not use a penalty or a barrier function, or a filter, and yet its global convergence to first-order stationary points can be proved. The method uses different trust regions to cope with the nonlinearities of the objective function and the constraints, and admits inexact SQP steps not lying exactly in the nullspace of the local Jacobian. We implement the method in MATLAb 7.7 software environment and test the resulting program on a collection of CUTEr problems. The numerical results are promising and confirm the global convergence of the method  

    A directional renumbering strategy for improving unstructured grid data structure

    , Article 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, FL, 4 January 2010 through 7 January 2010 ; 2010 ; 9781600867392 (ISBN) Fouladi, N ; Darbandi, M ; Schneider, G. E ; Sharif University of Technology
    A very simple and computationally low cost numerical algorithm is developed to generate a quasi-structured data structure for an unstructured grid. To achieve this purpose, the data structure in the matrices of an unstructured grid is classified to address the element layers and node lines in the computational domain. In this regard, elements and nodes of the unstructured grid are renumbered in a directional ordering-based strategy. The elements and nodes arrangement in each layer and line is accomplished in a unique direction around an interior object of the grid either clockwise or counterclockwise. Furthermore, a new searching scheme is introduced which guarantees a quick search inside... 

    An efficient numerical algorithm for stability testing of fractional-delay systems

    , Article ISA Transactions ; Volume 48, Issue 1 , 2009 , Pages 32-37 ; 00190578 (ISSN) Merrikh Bayat, F ; Karimi Ghartemani, M ; Sharif University of Technology
    This paper presents a numerical algorithm for BIBO stability testing of a certain class of the so-called fractional-delay systems. The characteristic function of the systems under consideration is a multi-valued function of the Laplace variable s which is defined on a Riemann surface with finite number of Riemann sheets where the origin is a branch point. The stability analysis of such systems is not straightforward because there is no universally applicable analytical method to find the roots of the characteristic equation on the right half-plane of the first Riemann sheet. The proposed method is based on the Rouche's theorem which provides the number of the zeros of a given function in a... 

    On the contact mechanics of a rolling cylinder on a graded coating. Part 2: Numerical results

    , Article Mechanics of Materials ; Volume 66 , 2013 , Pages 134-159 ; 01676636 (ISSN) Guler, M. A ; Alinia, Y ; Adibnazari, S ; Sharif University of Technology
    The analytical formulation of the fully coupled and the uncoupled rolling contact mechanics problems for all possible stick/slip regimes are derived in Part I (Alinia et al., 2013). In this part, we focus on the numerical algorithm, the iteration procedure and the numerical results. The coupled and the uncoupled solutions corresponding to each of the assumed stick/slip regimes are provided. The uncoupled solution provides an acceptable approximation to the problem for small coefficient of friction values. However, for high values of the coefficient of friction the problem should be solved in fully coupled form. In addition, the effect of several parameters such as the stiffness ratio, the... 

    An FEM approach for three - Dimensional thermoviscoelastic stress analysis of orthotropic cylinders made of polymers

    , Article Advanced Materials Research, Dubai ; Volume 685 , 2013 , Pages 295-299 ; 10226680 (ISSN) ; 9783037856765 (ISBN) Ashrafi, H ; Keshmiri, H ; Bahadori, M. R ; Shariyat, M ; Sharif University of Technology
    The objective of this study is to develop a general finite element formulation associated with an incremental adaptive procedure which established for thermoviscoelastic stress analysis of orthotropic cylinders made of polymers. This paper concerned with development of a numerical algorithm for the solution of the quasistatic initial/boundary value problems involving the linear viscoelastic media with thermal and mechanical deformations. The viscoelastic constitutive equations, represented in an integral form and involving relaxation functions, are transformed into an incremental algebraic relation. An incremental relaxation is then developed for the finite element formulation to deal with... 

    A method for reducing the complexity, and increasing the accuracy of field emission electron gun simulations

    , Article Vacuum ; Volume 95 , 2013 , Pages 50-65 ; 0042207X (ISSN) Yasrebi, N ; Rashidian, B ; Sharif University of Technology
    Problems regarding simulation of field emitter array (FEA) electron guns are discussed. A simple method is proposed to significantly reduce computational requirements such as computation power, system memory, and time of FEA electron gun simulation and modeling. The method can be applied to any numerical solver regardless of its meshing technique. In order to extract field emission parameter from any experimental cathode I-V curve, a partly numerical algorithm, which uses the presented truncation method at the heart of its solver, is proposed. The proposed method and algorithm are applied to a number of examples, including a double-gated FEA problem, and its effectiveness in terms of error... 

    On the capacity of level and type modulations in Molecular communication with ligand receptors

    , Article IEEE International Symposium on Information Theory - Proceedings, 14 June 2015 through 19 June 2015 ; Volume 2015-June , 2015 , Pages 1951-1955 ; 21578095 (ISSN) ; 9781467377041 (ISBN) Aminian, G ; Mirmohseni, M ; Nasiri Kenari, M ; Fekri, F ; Sharif University of Technology
    Institute of Electrical and Electronics Engineers Inc  2015
    In this paper, we consider the bacterial point-to-point communication problem with one transmitter and one receiver by considering the ligand receptor binding process. The most commonly investigated signalling model, referred to as the Level Scenario (LS), uses one type of a molecule with different concentration levels for signaling. An alternative approach is to employ multiple types of molecules with a single concentration level, referred to as the Type Scenario (TS). We investigate the trade-offs between the two scenarios for the ligand receptor from the capacity point of view. For this purpose, we evaluate the capacity using numerical algorithms. Moreover, we derive an upper bound on the... 

    Analytical performance modeling of elastic optical links with aligned spectrum allocation

    , Article Computer Networks ; Volume 88 , September , 2015 , Pages 40-50 ; 13891286 (ISSN) Vaezi, K ; Akar, N ; Sharif University of Technology
    Elsevier  2015
    Abstract Elastic optical networking has recently been proposed for use in optical transport networks to cope with increasingly heterogeneous and dynamic demand patterns. In this paper, we study the blocking performance of a multi-class elastic optical link for which a demand needs to be allocated a contiguous subset of the entire spectrum. This problem is different than the well-known blocking problem in multi-class multi-server loss systems due to the contiguous allocation constraint. We first propose a non-work-conserving aligned spectrum allocation policy which is shown to outperform the conventional first fit-based work-conserving allocation policy without alignment. Subsequently, for... 

    Transient thermal behavior of radial fins of rectangular, triangular and hyperbolic profiles with temperature-dependent properties using DTM-FDM

    , Article Journal of Central South University ; Volume 24, Issue 3 , 2017 , Pages 675-682 ; 20952899 (ISSN) Mosayebidorcheh, S ; Rahimi Gorji, M ; Ganji, D. D ; Moayebidorcheh, T ; Pourmehran, O ; Biglarian, M ; Sharif University of Technology
    Central South University of Technology  2017
    This work focuses on transient thermal behavior of radial fins of rectangular, triangular and hyperbolic profiles with temperature-dependent properties. A hybrid numerical algorithm which combines differential transformation (DTM) and finite difference (FDM) methods is utilized to theoretically study the present problem. DTM and FDM are applied to the time and space domains of the problem, respectively. The accuracy of this method solution is checked against the numerical solution. Then, the effects of some applicable parameters were studied comparatively. Since a broad range of governing parameters are investigated, the results could be useful in a number of industrial and engineering... 

    Unbalance-induced rub between rotor and compliant-segmented stator

    , Article Journal of Sound and Vibration ; Volume 429 , 2018 , Pages 96-129 ; 0022460X (ISSN) Behzad, M ; Alvandi, M ; Sharif University of Technology
    Academic Press  2018
    Today, turbomachinery designers are trying to attain zero clearances for optimizing performance and efficiency of machine by designing advanced compliant-segmented seals. This trend involves secondary issues such as rotor-to-stator contact interaction which is to be treated with more technical knowledge in rotordynamic design and condition monitoring programs. The aim of this paper is to gain insight into dynamic interaction between a flexible rotor and a set of compliant arc-shaped segments as stator system. Specifically, the unbalanced-induced forward rubbing response is investigated in tight clearance condition during resonance-passing situations. A Jeffcott rotor modeling is used for... 

    A pressure-based algorithm for internal compressible turbulent flows through a geometrical singularity

    , Article Numerical Heat Transfer, Part B: Fundamentals ; Volume 75, Issue 2 , 2019 , Pages 127-143 ; 10407790 (ISSN) Nouri Borujerdi, A ; Shafiei Ghazani, A ; Sharif University of Technology
    Taylor and Francis Ltd  2019
    Compressible turbulent flow through the abrupt enlargement in pipes is studied numerically by means of Advection Upstream Splitting Method (AUSM+-up). In low Mach numbers, a pressure correction equation of elliptic type is derived. This equation is compatible with the nature of governing equations and retrieves hyperbolic characteristic at higher Mach numbers. It is shown that the proposed numerical algorithm is computationally more efficient than the preconditioned density-based methods. The flow parameters such as reattachment length, pressure loss coefficient and wall shear stress are predicted. It is found that the loss coefficient of the compressible flow rises drastically with... 

    Numerical investigation of flow pattern and hydrodynamic forces of submerged marine propellers using unsteady boundary element method

    , Article Proceedings of the Institution of Mechanical Engineers Part M: Journal of Engineering for the Maritime Environment ; Volume 233, Issue 1 , 2019 , Pages 67-79 ; 14750902 (ISSN) Najafi, S ; Abbaspoor, M ; Sharif University of Technology
    SAGE Publications Ltd  2019
    In this study, a numerical model of unsteady potential flow around submerged marine propellers has been developed. The boundary element approach in combination with time stepping method to model free wake dynamics has been implemented. An important feature of this method in the simulation of pressure-dominant problems is a proper balance between time and accuracy in the numerical process. Another advantage of time stepping method is that there is no need to define wake geometry before modeling. Due to inherent instability of boundary integral equations, a smoothing function to damp the effect of singularities is imposed to the solution. The main innovative idea of this work is that the... 

    Connectivity analysis of one-dimensional ad hoc networks with arbitrary spatial distribution for variable and fixed number of nodes

    , Article IEEE Transactions on Mobile Computing ; Volume 11, Issue 10 , 2012 , Pages 1425-1435 ; 15361233 (ISSN) Sharif Nassab, A ; Ashtiani, F ; Sharif University of Technology
    In this paper, we propose an analytical approach to compute the probability of connectivity for one-dimensional ad hoc networks. The proposed analysis gives the exact probability of connectivity for an arbitrary distribution of nodes, provided that nodes are independently and identically distributed. We conduct separate analyses for two cases; in the first case, the number of nodes varies by time under a stationary distribution and in the second case, there is a fixed (known) number of nodes in the network. Using the approaches presented in this work, we are able to derive closed-form formulas for the probability of connectivity for some spatial distributions, while for more complicated... 

    Unsteady multiphase modeling of cavitation around NACA 0015

    , Article Journal of Marine Science and Technology ; Volume 18, Issue 5 , 2010 , Pages 689-696 ; 10232796 (ISSN) Asnaghi, A ; Jahanbakhsh, E ; Seif, M. S ; Sharif University of Technology
    The present study focuses on the numerical simulation of cavitation around the NACA 0015. The unsteady behaviors of cavitation which have worthwhile applications are investigated. The cavitation patterns, velocity fields and frequency of the cavitating flow around hydrofoil is obtained. For multi phase simulation, single-fluid Navier-Stokes equations, along with a volume fraction transport equation, are employed. The bubble dynamics model is utilized to simulate phase change. SIMPLE algorithm is used for velocity and pressure computations. For discretization of equations the finite-volume approach written in body fitted curvilinear coordinates, on collocated grid, is used. In this study,... 

    Modeling of large deformation - Large sliding contact via the penalty X-FEM technique

    , Article Computational Materials Science ; Volume 48, Issue 3 , May , 2010 , Pages 471-480 ; 09270256 (ISSN) Khoei, A. R ; Taheri Mousavi, S.M ; Sharif University of Technology
    In this paper, an extended finite element method is employed to simulate the presence of discontinuities caused by the contact surface. In X-FEM, the need for mesh adaption to interface is neglected and the process is accomplished by partitioning the domain with some triangular sub-elements whose Gauss points are used for integration of the elements. The modified level set technique and the Heaviside enrichment function are employed to approximate the discontinuous displacement field of elements located on the contact surface. The penalty method is used to impose the contact constraints and establish the non-penetration condition. An efficient numerical algorithm is employed to model the... 

    Aerosol nano-particle charger simulations and optimizations

    , Article Scientia Iranica ; Volume 23, Issue 5 , 2016 , Pages 2220-2229 ; 10263098 (ISSN) Shaygani, A ; Saidi, M. S ; Sani, M ; Sharif University of Technology
    Sharif University of Technology 
    Size distribution of aerosol particles is prevalently obtained through electrical characterization techniques and study of charged particles' dynamics in the presence of electric field. In this work, a wire-cylinder corona charger is presented, redesigned, and aero dynamically optimized. An initial 2D axisymmetric geometry of the charger was employed for the simulations using the Computational Fluid Dynamics (CFD) commercial code FLUENT 6.3.26. Through successive attempts, a new geometry was obtained by streamlining the walls to eliminate the undesired vortices produced in the flow field of the previous ones. The process optimized the charger by minimizing losses and dilutions of the... 

    Simulation of high-velocity impact to concrete structures using damage-plasticity model considering large deformation

    , Article International Journal of Solids and Structures ; Volume 155 , 2018 , Pages 275-290 ; 00207683 (ISSN) Eslahi, R ; Kazemi, M. T ; Sharif University of Technology
    Elsevier Ltd  2018
    In this manuscript, an advanced damage-plasticity model is utilized to simulate response of concrete structures under high-velocity impact. Due to presence of large deformations, it is necessary to incorporate the damage-plasticity model into the finite deformation framework. In high-velocity impact, severe numerical problems could be encountered while updating stress values due to the sudden increase in their levels, especially for the complicated material model used in this study. To overcome these obstacles, an enhancement is made in the nonlinear system of equations of stress- updating procedure. In addition, an adaptive multi-step numerical algorithm is introduced which improves the... 

    Dynamic analysis of a shape memory alloy beam with pseudoelastic behavior

    , Article Journal of Intelligent Material Systems and Structures ; Volume 29, Issue 9 , 1 May , 2018 , Pages 1835-1849 ; 1045389X (ISSN) Razavilar, R ; Fathi, A ; Dardel, M ; Arghavani Hadi, J ; Sharif University of Technology
    SAGE Publications Ltd  2018
    This article aims at developing a semi-analytic approach for studying the free and forced vibrations of a pseudoelastically behaving shape memory alloy beam. Based on the Euler–Bernoulli beam theory, equations of motion were derived through Hamilton principle, and the obtained partial differential equations were decomposed by applying the Galerkin approach and were solved using Newmark integration method. A three-dimensional phenomenological model of shape memory alloy, which is capable of identifying the main properties of the shape memory alloy, was employed to model the behavior of the shape memory alloy beam. A closed-form numerical algorithm was introduced to simulate the governing... 

    A thermodynamic-based large deformation viscoplastic constitutive relationship for asphalt concrete compaction

    , Article International Journal of Solids and Structures ; Volume 165 , 2019 , Pages 192-216 ; 00207683 (ISSN) Karimi, M. M ; Darabi, M. K ; Tabatabaee, N ; Sharif University of Technology
    Elsevier Ltd  2019
    This research proposes a large deformation, time-dependent viscoplastic constitutive relationship to enhance the prediction of the compaction degree of asphalt concrete materials under laboratory and field conditions. A large-deformation thermodynamic-based framework is presented. The Helmholtz free energy and rate of energy dissipation functions were assumed to derive rate-dependent constitutive relationships to relate multi-axial state of stresses to the recoverable and non-recoverable deformation response of asphalt concrete during compaction. A straightforward method that allows the calibration of the proposed model against laboratory compaction data (e.g., data from Superpave Gyratory...