Sharif Digital Repository

In the present work, a new structure of the strained quantum-well (QW) laser diode is designed and simulated. In this structure, the active region consists of tow 6H-SiC barrier and 3C-SiC quantum well (QW)which is sandwiched between two layers of 6H-SiC that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroel ectricity of 6H using an industrial-based numerical simulator. The basic design goal was to decrease the threshold current by using only silicon carbide polymers. We could obtain a working model at stable optical wavelength of 0.83μm. This paper provides key results of the device characteristics, including the light power... 

This paper investigates the relation between information technology (IT) and the change of business operations. We evaluate the geographical demand of companies for customer service interactions. The structure is a separate selection model in which districts play the role of distinguished products. The approximated demand structure is used to evaluate the results of IT on customer size, place selection, and cost preserving. The results verify the higher cost vulnerability of IT-thorough companies but propose that capability to utilize cost different is highly company-specified and that the influence of geographically restricted externalities is even existent. Because the productivity of many... 

In this study, numerical simulation of collective cell migration is presented in order to mimic the group migration of endothelial cells subjected to the concentration gradients of a biochemical factor. The developed 2D model incorporates basic elements of the cell, including both the cell membrane and the cell cytoskeleton, based on a viscoelastic cell mechanic model. Various cell processes - including cell random walk, cell-cell interactions, cell chemotaxis, and cellular cytoskeleton rearrangements - are considered and analyzed in our developed model. After validating the model by using available experimental data, the model is used to investigate various important parameters during... 

For the first time, the mechanism of Fischer esterification between acetic acid and ethanol over silica-functionalized propylsulfonic acid (silica-propyl-SO3H) catalyst was explored by means of computational modeling techniques. For this purpose, 6-edge-atom cage-like cluster comprising Si-O-Si sequences has been selected to represent the surface of the catalyst. The results indicate that the reaction goes through concerted transition states. In all optimized structures no proton (H+) transfer occurs from catalyst to the substrates and the role of the catalyst is via the activation of the substrates through the formation of strong hydrogen bonds (H-bonds). Furthermore, the energetic diagram... 

A circuit model has been proposed for two-dimensional metallic arrays of metallic square holes located on a homogeneous substrate. Although the proposed circuit model holds true, the surface admittance between the metallic hole array and the cover was erroneously calculated since the presence of the cross-polarization terms had been neglected. Furthermore, there have been mistakes in numerical simulation and in analytical calculations based on the presented formulas. The correct formulation is now given, and erroneous figures are replotted  

In this paper, an attractive and novel algorithm for improving irreducible model identification of continuous time(CT) MIMO systems has been presented. The algorithm is based on least - squares (LS) estimates of Markov parameters (MP) using input output data and residual whitening. By choosing a linear-in-parameters model structure, the estimation becomes linear and asymptotically robust to zero-mean additive disturbances. CT Markov parameters may result in diverging approximations even for stable systems. To remove the existing limitations in the case of systems with low or zero damping, Markov Poisson parameters have been used to lend much flexibility to the estimation model. The MIMO... 

In this paper we investigate the effect of the number of dissipative sites in an Abelian sandpile model. The dissipative sites are considered to be in the bulk rather than in the boundary. In such systems, statistics of avalanches smaller than a certain size obey a power law. We have seen that the exponents associated with these power law behaviors change slightly as the number of dissipative sites is decreased. We have found that the zero dissipation limits of these exponents are independent of size. Therefore we suggest that the previously-seen dependence of the exponent on the system size is because of the number of dissipative sites rather than the system size. Also it is observed that... 

We study a system of ordinary differential equations which describes the metapopulation SIR model. The main target is to control the epidemic spreading using the structure of connections between cities. For this purpose, we formulate optimal control strategy that the control represents a drug treatment and Prevention strategies . Some methods are given in numerical example section such as control through spectral radius of movement matrix or optimal control through travelling between cities. Existence results for the optimal control are studied  

Cooling water is used to remove the heat from stator bar of large turbogenerators and prevent additional temperature rise. Blockage of these channels leads to heat traps along the stator bar and its temperature failure. In addition, transposition of strands in a bar causes accumulation of heat in some parts of stator bar because of non-uniform arrangement of cooling channels especially when some of channels are blocked. In this paper, effect of cooling water channels blockage is studied using a precise thermal model. It is shown that such heat traps cause additional temperature rise in stator bars and this phenomenon can not be detected by thermal sensors of turbogenerator. Laboratory and... 

In this study, natural super-cavitating flow around three different conical cavitators with wedge angles of 30°, 45° and 60° is investigated. We apply the k−ϵ turbulence model and the volume of fluid (VOF) technique to numerically study the three-dimensional cavitating flow around the cavitators. The turbulence approach is coupled with a mass transfer model which is implemented into the finite-volume package. Simulations are performed for different cavitation numbers. Finally, the effects of some important parameters such as the cavitation index, inlet velocity, Froude number and wedge angle of cavitators on the geometrical characteristics of the super-cavities are discussed. Our numerical... 

A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data... 

Considering the role of black hole singularity in quantum evolution, a resolution to the firewall paradox is presented. It is emphasized that if an observer has the singularity as a part of his spacetime, then the semi-classical evolution would be non-unitary as viewed by him. Specifically, a free-falling observer inside the black hole would have a Hilbert space with non-unitary evolution; a quantum jump for particles encountering the singularity to outside of the horizon as late Hawking radiations. The non-unitarity in the jump resembles the one in collapse of wave function, but preserves entanglements. Accordingly, we elaborate the first postulate of black hole complementarity: freely... 

Asphalt concrete is a heterogeneous and multi-phase material that consists of aggregates, asphalt binder, and pores. These components create a complex microstructure. There is no consensus on how to quantify the microstructure and the required parameters from two-dimensional images. The load transfer can be managed by aggregate-aggregate or aggregate-mastic interactions that depend on the gradation, aggregate internal structure, and asphalt binder viscoelastic properties. The most important internal structure indices affecting the behavior of HMA are aggregate orientation, contact orientation, contact length, and number of contact points. In this paper, the influence of aggregate structure... 

The effect of condensate blockage on the pressure drop near the well bore has been studied. Two types of single well simulations have been conducted and pressure drop due to condensate blockage has been quantified based on variation of gas relative permeabilities in near well bore area. The relationship of condensate banking pressure drop, gas rate and reservoir quality has been presented. It is shown that for a poor reservoir quality, condensate blockage can increase the pressure drop up to 200% of the pressure drop in the tubing  

Interfacial tension is one of the very important parameters in pharmacology science, chemical engineering and petroleum engineering. The interfacial tension between pure hydrocarbons and water is widely used in chemical and petroleum engineering and so that it is highly desirable to find its accurate values. In this study a new unique correlation has been developed based on a comprehensive database of pure hydrocarbons-water interfacial tension values which contained both aliphatic and aromatic compounds (32 different hydrocarbons) with pressure ranges of 0.1-300 MPa and temperature ranges of 252.44-550 K. The proposed correlation uses the density difference between pure hydrocarbon and... 

We test a copula-based joint discrete–continuous model to unravel mode choice and travel distance decisions in a joint framework for school trips. This framework explicitly accounts for common unobserved factors that may affect both the mode choice and travel distance. Joint estimation of the models makes a significant difference in the effect of travel distance on willingness to walk to school. The absolute value of the travel distance coefficient in the mode choice model increases by 22% when a joint formulation is adopted instead of the conventional single estimations. We find a significant decrease of 19% in the coefficient of travel safety perception in the joint mode choice model... 

We demonstrate design and characterization of a polarization-independent ultra-broadband absorber of light consisting of periodic array of graphene disks on top of a lossless quarter-wavelength dielectric spacer placed on a metallic reflector. The absorber is duly designed based on impedance matching concept by proposing a fully analytical circuit model resulting in a normalized bandwidth of 100 % in the terahertz regime. © 2016, Springer Science+Business Media New York  

Social networks are getting closer to our real physical world. People share the exact location and time of their check-ins and are influenced by their friends. Modeling the spatio-temporal behavior of users in social networks is of great importance for predicting the future behavior of users, controlling the users’ movements, and finding the latent influence network. It is observed that users have periodic patterns in their movements. Also, they are influenced by the locations that their close friends recently visited. Leveraging these two observations, we propose a probabilistic model based on a doubly stochastic point process with a periodic-decaying kernel for the time of check-ins and a... 

Purpose: In this paper, the joint replenishment problem is modeled for a two-level supply chain consisting of a single supplier and multiple retailers that use the vendor-managed inventory (VMI) policy for several products. This paper aims to find the optimal number of products to order in both policies, the optimal times at which each retailer orders the products in the traditional policy and the optimal times at which the supplier orders the product in the VMI policy. Design/methodology/approach: The problem is first formulated into the framework of a constrained integer nonlinear programming model; then, the problem is solved using a teacher-learner based optimization algorithm. As there... 

We study the volume averaging of inhomogeneous metrics within GR and discuss its shortcomings, such as gauge dependence, singular behavior as a result of caustics, and causality violations. To remedy these shortcomings, we suggest some modifications to this method. As a case study we focus on the inhomogeneous structured FRW model based on a flat LTB metric. The effect of averaging is then studied in terms of an effective backreaction fluid. It is shown that, contrary to the claims in the literature, the backreaction fluid behaves like a dark matter component, instead of dark energy, having a density of the order of 10-5 times the matter density, and, most importantly, it is gauge-dependent....