Ghalee A.,Tafresh University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014
We consider a scalar field with a kinetic term nonminimally coupled to gravity in an anisotropic background. Various potentials for the scalar field are considered. By explicit examples, we show how the anisotropy can change the dynamics of the scalar field compared with the isotropic background. © 2014 American Physical Society.
Kazemy A.,Tafresh University
Ocean Engineering | Year: 2017
This paper focuses on the problems associated with designing a robust mixed H∞/passive controller for offshore steel jacket platforms with structured uncertainty. The objective of this paper is to design a controller that is meant to stabilize the offshore platform dynamic when exposed to structured uncertainties. This designation must also satisfy a mixed H∞/passive performance index. In order to design such a controller, at first, the instantaneous state feedback controller ought to be considered. Then, mixed time-delayed states and instantaneous states are to be considered in such a way that the control law covers various kinds of practical situations. Based on the Lyapunov-Krasovskii Functional (LKF), some stability criteria are obtained and expressed in the form of Linear Matrix Inequalities (LMIs). According to these stability criteria, some sufficient conditions for the existence of the state feedback controller gain are to be met. At the end, the designed controllers are applied to the model of the offshore platform with uncertainty before numerically simulating the effectiveness of the proposed methods. © 2017 Elsevier Ltd
Gandomi A.H.,Tafresh University |
Alavi A.H.,Iran University of Science and Technology
Information Sciences | Year: 2011
This paper presents a new multi-stage genetic programming (MSGP) strategy for modeling nonlinear systems. The proposed strategy is based on incorporating the individual effect of predictor variables and the interactions among them to provide more accurate simulations. According to the MSGP strategy, an efficient formulation for a problem comprises different terms. In the first stage of the MSGP-based analysis, the output variable is formulated in terms of an influencing variable. Thereafter, the error between the actual and the predicted value is formulated in terms of a new variable. Finally, the interaction term is derived by formulating the difference between the actual values and the values predicted by the individually developed terms. The capabilities of MSGP are illustrated by applying it to the formulation of different complex engineering problems. The problems analyzed herein include the following: (i) simulation of pH neutralization process, (ii) prediction of surface roughness in end milling, and (iii) classification of soil liquefaction conditions. The validity of the proposed strategy is confirmed by applying the derived models to the parts of the experimental results that were not included in the analyses. Further, the external validation of the models is verified using several statistical criteria recommended by other researchers. The MSGP-based solutions are capable of effectively simulating the nonlinear behavior of the investigated systems. The results of MSGP are found to be more accurate than those of standard GP and artificial neural network-based models. © 2011 Elsevier Inc. All rights reserved.
Yang X.-S.,National Physical Laboratory United Kingdom |
Gandomi A.H.,Tafresh University
Engineering Computations (Swansea, Wales) | Year: 2012
Purpose - Nature-inspired algorithms are among the most powerful algorithms for optimization. The purpose of this paper is to introduce a new nature-inspired metaheuristic optimization algorithm, called bat algorithm (BA), for solving engineering optimization tasks. Design/methodology/approach - The proposed BA is based on the echolocation behavior of bats. After a detailed formulation and explanation of its implementation, BA is verified using eight nonlinear engineering optimization problems reported in the specialized literature. Findings - BA has been carefully implemented and carried out optimization for eight well-known optimization tasks; then a comparison has been made between the proposed algorithm and other existing algorithms. Originality/value - The optimal solutions obtained by the proposed algorithm are better than the best solutions obtained by the existing methods. The unique search features used in BA are analyzed, and their implications for future research are also discussed in detail. © Emerald Group Publishing Limited.
Madady A.,Tafresh University
International Journal of Control, Automation and Systems | Year: 2013
This paper presents a new iterative learning control (ILC) for discrete-time single-input single-output (SISO) linear time-invariant (LTI) systems. To establish this ILC, the input of the controlled system is modified by using a novel four-parametric algorithm. This algorithm is called the extended proportional plus integral and derivative (EPID) type, since by eliminating the fourth parameter of it one would get to the PID type ILC, therefore PID type ILC is a special case of it. The convergence of the proposed ILC is analyzed and an optimal method is presented to determine its parameters. It is shown that the given ILC has a better performance than the PID-type one. Three illustrative examples are included to demonstrate the effectiveness and the preference of the presented ILC. © 2013 Institute of Control, Robotics and Systems and The Korean Institute of Electrical Engineers and Springer-Verlag Berlin Heidelberg.
Ghalee A.,Tafresh University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013
We propose a model for the dust matter in the cosmological context. The model contains a scalar field with a kinetic term nonminimally coupled to gravity. By investigating the background and perturbative equations, it is demonstrated that the scalar field has the same dynamics as the dust matter. We have also considered the cosmological constant in the model. It turns out that the model has not exotic behavior. Thus, a universe including the scalar field and the cosmological constant evolves just as our Universe. Moreover, we have added the quadratic term in the action. It is shown that the quadratic term can be ruled out by its consequences. © 2013 American Physical Society.
Rezaei Z.,Tafresh University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012
The open string tachyon and U(1) gauge field as longitudinal fluctuations and the velocity as a transverse fluctuation of an arbitrary dimensional D-brane are considered as boundary deformations of a closed superstring free action. The path integral approach will be applied to calculate the corresponding generalized boundary states using supersymmetrized boundary actions. Obtaining the disk partition functions from the boundary states and studying the effect of tachyon condensation on both of them in the Neveu-Schwarz-Neveu-Schwarz and Ramond-Ramond sectors, leads to results that differ from the established ones. © 2012 American Physical Society.
Ghalee A.,Tafresh University
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013
We consider the dynamics of a scalar field non-minimally coupled to gravity in the context of cosmology. It is demonstrated that there exists a new phase for the scalar field, in addition to the inflationary and dust-like (reheating period) phases. Analytic expressions for the scalar field and the Hubble parameter, which describe the new phase are given. The Hubble parameter indicates an accelerating expanding Universe. We explicitly show that the scalar field oscillates with time-dependent frequency. Moreover, an interaction between the scalar field in the new phase and other fields is discussed. It turns out that the parametric resonance is absent, which is another crucial difference between the dynamics of the scalar field in the new phase and dust-like phase. © 2013 Elsevier B.V.
Golmakani H.R.,Tafresh University
International Journal of Production Research | Year: 2012
In condition-based maintenance (CBM) with periodic inspection, the item is preventively replaced if failure risk, which is calculated based on the information obtained from inspection, exceeds a pre-determined threshold. The determination of optimal replacement threshold is often based on minimisation of long-run average maintenance costs per unit time due to preventive and failure replacements. It is assumed that inspections are performed at equal time intervals and that the corresponding cost is negligible. However, in many practical situations where CBM is implemented, e.g. manufacturing processes, inspections require labours, specific test devices, and sometimes suspension of operations. Thus, when inspection cost is considerable, it is reasonable to inspect less frequently during the time the item is in healthier states, and, more frequently as time passes and/or the item degrades, namely, a condition-based inspection scheme. This paper proposes a novel two-phase approach for determination of replacement threshold and a condition-based inspection scheme for CBM. First, it takes into account failure and preventive replacement costs to determine the optimal replacement threshold assuming that inspections are performed at equal time intervals with no cost. This assumption is, then, relaxed and its consequences on total average cost are evaluated using a proposed iterative procedure to obtain a cost-effective condition-based inspection scheme. The proposed approach can be utilised in many CBM applications. For the sake of simplicity of presentation, the approach is illustrated through a simplified case study already reported by some researchers referenced in the paper. © 2012 Copyright Taylor and Francis Group, LLC.
Ghalee A.,Tafresh University
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012
Recently Maleknejad and Sheikh-Jabbari have proposed a new model for inflation with non-Abelian gauge fields (Gauge-flation), and they have studied the model by numerical methods (Maleknejad and Sheikh-Jabbari, 2011 ). In this model, the isotropy of space-time is recovered by suitable combination of gauge configurations, and a scalar field is constructed by gauge field and the scale factor, which produces inflation period. In this work, exact solutions for the scalar field and the Hubble parameter are presented and we provide analytic solutions for the numerical results. We explicitly present Hubble parameter and fields as functions of time and it is also demonstrated that in some conditions they are damped oscillator. Moreover, reheating period in the model is discussed. © 2012 Elsevier B.V.