Showing 16 results for Mohammadi
A. R. Azimian and S. Mohammadivand,
Volume 18, Issue 2 (7-1999)
Abstract
A. H. Shirmohammadi,
Volume 19, Issue 2 (1-2001)
Abstract
The present study is intended to develop a Preventive Maintenance (PM) policy for industrial applications, while considering two different objectives: the first objective is to optimize the total maintenance costs, i.e., the Preventive Maintenance (PM) and Emergency Maintenance (EM) Costs Per Unit Time. (CPUT). The second objective is to design the policy in such a way that the simplicity of its application in a production industry environment is improved.
A simulation model is constructed and, using a Monte Carlo simulation, the model is run for a sufficient number of cycles in order to determine the optimum value of a decision parameter (TX). The TX value provides the PM planning section with the possibility of economically postponing the PM action to its next scheduled date. Numerical examples are given to show the validity of the model and comparisons are made with existing PM policies in industry and in the literature to evaluate the cost reduction values that can be obtained through the adoption of the policy, while the implementation simplicity is also maintained
M. Khan-Mohammadi and M. S. Marefat,
Volume 25, Issue 1 (7-2006)
Abstract
To assess the performance criteria of the reinforced-concrete, five-storey residential buildings common in Iran, an
experimental study in the structural laboratory of the University of Tehran has been conducted. The test program includes cyclic and monotonic load tests of six beams that represent three-to-five storey buildings with rigid frame structures. Using definitions given in FEMA-356 and ATC-40, stages of immediate occupancy, life safety, and collapse prevention have been identified on the drift- force curves of all specimens. Based on the test results, values of the plastic rotation, ductility, strain in concrete cover and in longitudinal bar, crack width, damage index, and length of plastic region at different levels have been determined. It was found that the recommended values of plastic rotation and ductility for reinforced concrete beams by FEMA-356 are conservative. The length of plastic hinge region in the stage of immediate occupancy is about half the plastic hinge length in the stage of life safety and it increases by 20% from life safety to collapse prevention
H.a. Abyaneh, R. Mohammadi H. Torkaman, F. Razavi, and E. Afjei,
Volume 27, Issue 2 (1-2009)
Abstract
All algorithms for impedance calculation use an analog-to-digital converter. The high accuracy of the impedance seen by a distance relay is an important factor in the correct isolation of the faulty part of power systems. To achieve this, a novel
technique based on third order interpolation is used in this paper. According to this technique, the times and the values of the obtained samples are changed to real ones. To evaluate the new technique, it is applied to six digital distance algorithms, namely, Discrete Fourier Transform (DFT), Half-cycle Discrete Fourier Transform, Least Square, Mann-Morrison, Least Square with Delete Dc, and Prodar70. The technique is found to be capable of accurately computing the impedance in the algorithms mentioned. Comparisons are made among the results to show the efficiency of the new technique for decreasing errors in all algorithms.
A. Karami Mohammadi, N. Aleali,
Volume 34, Issue 1 (7-2015)
Abstract
: In this paper, a nonlinear model of clamped-clamped microbeam actuated by electrostatic load with stretching and thermoelastic effects is presented. Free vibration frequency is calculated by discretization based on DQ method. Frequency is a complex value due to the thermoelastic effect that dissipates the energy. By separating the real and imaginary parts of frequency, quality factor of thermoelastic damping is calculated. Both stretching and thermoelastic effects are validated against the results of the reference papers. The variations of thermoelastic damping versus elasticity modulus, coefficient of thermal expansion and geometrical parameters such as thickness, gap distance, and length are investigated and these results are compared in the linear and nonlinear models for high values of voltage. Also, this paper shows that since for high values of electrostatic voltage the linear model reveals a large error for calculating the thermoelastic damping, the nonlinear model should be used for this purpose.
S. Hashemi , S. Mohammadi,
Volume 34, Issue 2 (1-2016)
Abstract
The phase transformation phenomenon due to the crystallographic change of shape memory alloys subjected to mechanical or thermal loading is very complicated. Regarding the thermo-mechanical coupling effects in shape memory alloys, in case of high loading rates, heat generation/absorption during the forward/reverse transformation, will lead in temperature-dependent variation and consequently affects its mechanical behavior. In this paper, a numerical algorithm based on the finite element method is proposed to investigate complex mechanical, thermal, and coupled behavior of shape memory alloys, including both exclusive behaviours of these alloys, that are superelasticity and shape memory effect. Several key examples are simulated and discussed to assess the efficiency and accuracy of proposed algorithm.
A. R. Ghasemi, M. Mohammadi,
Volume 35, Issue 2 (2-2017)
Abstract
In this study, Circular Disk Model (CDM) has been developed to determine the residual stresses in twophase and three- phase unit cell. The two-phase unit cell is consisting of carbon fiber and matrix. The three-phase unit cell is consisting of carbon fiber, carbon nanotubes and matrix in which the carbon fiber is reinforced with the carbon nanotube using electrophoresis method. For different volume fractions of carbon nanotubes, thermal properties of the carbon fiber and carbon nanotube in different linear and lateral directions and also different placement conditions of carbon nanotubes have been considered. Also, residual stresses distribution in two and three phases has been studied, separately. Results of micromechanical analysis of residual stresses obtained from Finite Element Method and CDM, confirms the evaluation and development of three dimensional CDM.
H. Lakzian, A Karami Mohammadi, A. Jalali,
Volume 36, Issue 1 (9-2017)
Abstract
The present work studies the performance of linear and nonlinear dynamic vibration absorbers mounted on Euler–Bernoulli beams subjected to moving loads. Absorbers used in this work consist of one mass, two springs and one linear damper.The springs may be considered either linear or non-linear. The objective is to compare the performance of these absorbers with classical dynamic and nonlinear absorbers. The partial differential equations governing the problem are reduced to a set of ordinary differential equations by means of Galerkin–Bubnov method. The performance of the dynamic absorbers in reduction of the beams’ vibration is estimated through the maximum amplitude of vibration and the portion of energy dissipated by the dynamic damper. Finally, after optimizations, the effectiveness of the dynamic absorbers is determined for different conditions and applications.
H. Zohali, B. Naderi, M. Mohammadi,
Volume 36, Issue 2 (3-2018)
Abstract
This paper addresses the lot sizing and scheduling problem for a number of products in flexible flow shop with identical parallel machines. The production stages are in series, while separated by finite intermediate buffers. The objective is to minimize the sum of setup and inventory holding costs per unit of time. The available mathematical model of this problem in the literature suffers from huge complexity in terms of size and computation. In this paper, a new mixed integer linear program is developed for delay with the huge dimentions of the problem. Also, a new meta heuristic algorithm is developed for the problem. The results of the numerical experiments represent a significant advantage of the proposed model and algorithm compared with the available models and algorithms in the literature.
M. Mohammadimehr, S. Alimirzaei,
Volume 36, Issue 2 (3-2018)
Abstract
In this research, the nonlinear buckling analysis of Functionally Graded (FG) nano-composite beam reinforced by various distributions of Boron Nitrid Nanotube (BNNT) is investigated under electro-thermodynamical loading with considering initial geometrical imperfection. The analysis is performed based on nonlocal elasticity theory and using the Finite Element Method (FEM). Various distributions of BNNT along the beam’s thickness are considered as uniform and decreasing-increasing functionally graded; and the extended mixture model is used to estimate the properties of nano-composite beam. The elastic medium around the smart nano-composite beam is modeled as elastic foundation. The governing equations of equilibrium are derived using energy method and nonlocal elasticity theory; and the critical buckling load is obtained for various boundary conditions such as simply-simply supported (S-S) and clamped-clamped (C-C) using the FEM. The results indicate that with an increase in the geometrical imperfection parameter, the stiffness of nano-composite beam increases and consequently the stability of the system increases. The effect of FG-X distribution type is more than uniform distributions. Also, the critical buckling load of nano-composite beam increases with an increase in the electric field and elastic foundation.
M. Bashi Varshosaz, B. Naderi, M. Mohammadi,
Volume 37, Issue 1 (9-2018)
Abstract
The purpose of this research is to deal with the problem of two-stage assembly flow shop scheduling. A number of single-item products (identical) each formed of several different parts are ordered. Each part has m operations done at the first stage with m different machines. After manufacturing the parts, they are assembled into a final product with some non-identical machines. The purpose of the problem is to find the optimal sequence of the parts in the manufacturing stage, allocation and the optimal sequence of the products in the assembly stage. A mixed integer linear programming model and two metaheuristic algorithms, which are particle swarm with local search (MPSO) and simulated annealing (SA), are presented to solve this problem. Computational experiments are conducted to evaluate the performance of the proposed model and algorithms. The results show that the MPSO algorithm performs better than the SA one.
F. Shirmohammadi, M. M. Saadatpour,
Volume 37, Issue 1 (9-2018)
Abstract
In this article spectral modal method is developed for studying wave propagation in thin plates with constant or variable thickness. Theses plates are subjected to the impact forces and different boundary conditions. Spectral modal method can be considered as the combination of Dynamic Stiffness Method (DSM), Fourier Analysis Method (FAM) and Finite Stripe Method (FSM). Using modeling of continuous distribution of mass and an exact stiffness causes solutions in frequency domain. Unlike the most numerical methods, in this method refining meshes is no longer necessary in which the cost and computational time is decreased. In this paper the important parameters of the method and their effects on results are studied through different examples.
H. Mohammadiun, M. Mohammadiun, M. H. Dibaee Bonab, M. Darabi, S. R. Hejazi, V. Janipour Bidsardareh,
Volume 39, Issue 1 (8-2020)
Abstract
: In this research, dimensionless temperature and entropy generation for the steady state flow in the stagnation point of incompressible nanofluid impinging on an infinite cylinder have been investigated. The impinging free stream is steady with a constant strain rate k. Similarity solution of the Navier-Stokes equations and energy equation is derived in this problem. A reduction of these equations is obtained using appropriate transformations introduced in this research. The general self similar solution is obtained when the heat flux on the cylinder wall is constant. All solutions brought above are presented for Reynolds numbers Re=ka^2/2vf that range from 0.1 to 1000 and the selected values of particle fractions, where a is the radius of the cylinder and υf is the kinematic viscosity of the base fluid. Results show that for Reynolds numbers examined, as the particle fraction increases, the depth of diffusion of the fluid velocity field in axial direction decreases, whereas Nusselt number is raised. Also, the maximum value of entropy generation has been calculated.
O. Bateniparvar, N. Noormohammadi, A. M. Salehi,
Volume 39, Issue 2 (2-2021)
Abstract
In this paper, Equilibrated Singular Basis Functions (EqSBFs) are implemented in the framework of the Finite Element Method (FEM), which can approximately satisfy the harmonic PDE in homogeneous and heterogeneous media. EqSBFs are able to automatically reproduce the terms consistent with the singularity order in the vicinity of the singular point. The newly made bases are used as the complimentary enriching part along with the polynomial bases of the FEM to construct a new set of shape functions in the elements adjacent to the singular point. It will be shown that the use of the combined bases leads to the quality improvement of the solution function as well as its derivatives, especially in the vicinity of the singularity.
M. Azizpooryan, N. Noormohammadi,
Volume 40, Issue 1 (9-2021)
Abstract
In this paper, static analysis of in-plane heterogeneous laminated composite plates is numerically studied. The Mindlin’s theory which considers linear transverse shear deformation has been implemented. The governing partial differential equation is satisfied by a weighted residual integration. Chebyshev polynomials of the first kind are used as basis functions and exponential functions make up the weight functions of the integration. The emerging integrals may be composed of some pre-evaluated 1D normalized ones, which effectively paces up the solution progress. To verify the method, several examples of homogeneous as well as heterogeneous plates with various lamination schemes and boundary conditions have been solved. Results are compared with those from the literature or by commercial codes, which reveal excellent accuracy of the proposed method.
H. Asadigorji, A. Karami Mohammadi,
Volume 40, Issue 1 (9-2021)
Abstract
Complex nonlinear behaviors such as chaotic motion have devastating effects on dynamic systems. In this study, nonlinear behavior of simply supported rectangular viscoelastic plates was examined during supersonic aerodynamics and compared with the nonlinear elastic plate. Classical plate theory was used to obtain the plate equations, and Von- Kármán strain-displacement relations were used to consider the nonlinear geometric effects. The Kelvin Voigt model was also used to describe the viscoelastic properties and the “first-order piston theory" was used for supersonic aerodynamic flow. The equations of motion of the rectangular plate were extracted using the Lagrangian method and then, discretized by the Rayleigh-Ritz method. Solution of the equations was performed using fourth order Runge Kutta method. To investigate the dynamic behavior of the plates, the eigenvalues of the system, time history curves, phase portraits, Poincaré maps, and bifurcation diagrams were studied and analyzed. The results show that in some aspect ratios, the threshold for the occurrence of the flutter in the viscoelastic plate will be lower than that in the elastic plate. On the other hand, when the control parameter increases, complex nonlinear behavior such as chaos in the elastic plate goes simpler in the viscoelastic plate, such as periodic motion.
