Sharif Digital Repository

Porous silica microspheres were used as hard template to produce porous polyacrylamide microspheres. The microspheres were modified with ethylenediamine and used for the removal of hexavalent chromium [Cr(VI)] from aqueous solution. Scanning electron microscopy, thermogravimetry analysis, and Fourier transform infrared spectroscopy were utilized to characterize the adsorbent. Adsorption of Cr(VI) was conducted in batch and dynamic modes, and effect of various parameters including solution pH, adsorbent dose, initial concentration of Cr(VI) and agitation time on the adsorption process was studied. The optimum pH for the maximum adsorption (124 mg Cr(VI)/g dry polymer) was found to be 3.... 

An agent-based model is employed for simulating the price of oil futures. The model proceeds as follows: On each time step agents choose their rule for price expectation formation. Next, they bid and ask based on their price and trend expectations. The new price is formed using “the market mechanism”. Finally, the time steps forward and the process is repeated in the next day. The agents use 6 different rules to make price and trend expectations. Brent future prices in a 2-year-period (2010-2011) and in 2012 are used for model calibration and validation, respectively. It was shown that market participants weigh U.S. stocks data more than other factors, while OECD stock’s data were not that... 

A high sensitive upper critical solution temperature (UCST) copolymer was synthesized through free radical copolymerization with acrylamide (AAm) and vinyl acetate (VA). The UCST behavior is due to a combination of hydrogen bonds and electrostatic potentials between copolymer chains. Turbidimetry, dynamic light scattering, and rheology studies were used to determine the phase transition and aggregation behavior. It is indicated that copolymer with 80% AAm has a phase transition temperature between 38 and 38.2 °C, which is suitable for releasing fever medicine. Also, the temperature gradient for the complete dissolution of the copolymer is 2 °C. The molecular dynamics simulations at a... 

Asphaltic concrete has been used as waterproofing core in embankment dams, since 1948. In this application, the asphaltic core is surrounded by granular filter materials. The interaction of the asphaltic concrete and the granular materials has not been sufficiently investigated. In this paper the mechanical behavior of the interface between a natural smooth sand filter and asphaltic concrete at different levels of normal stresses and a constant shear strain rate has been studied. Small scale direct shear test has been conducted in this study, in which the shear surface is considered as the interface. Asphalt concrete specimens used in the shear test were cut in square shape (10×10×2.5 cm)... 

In this paper, an analysis of kinematics of the isotropic elastic Cosserat continuum is presented in infinitesimal and finite deformations. Emphasis is given on the applicability of corotational stress rates for hypoelasticity in micro-polar continua. A non-linear finite element analysis is performed with an explicit formulation of tangent stiffness matrices in the case of Truesdell stress and couple stress rates. A comprehensive path-dependent procedure is employed based on the arc-length method to calculate the stability points and handle the snap-back problem. Finally, the accuracy and efficiency of method are illustrated by numerical examples  

The state of solids mixing can be considered as a key factor in accomplishment of high efficiency and a uniform product in gas-solid fluidized-beds operating with several different particle sizes. In the present work, coupled DEM-CFD approach was used to carefully investigate the mixing process of binary particles in flat-bottom spouted beds. In this regard, simultaneous effects of particle specifications, operating conditions, and bed dimensions on the equilibrium mixing state of solid mixture were evaluated in detail. Moreover, taking into account the contribution of various parameters, the obtained simulation results were used to draw a correlation for predicting the equilibrium mixing... 

Estimation of efficiency of axial flow gas turbines under variety of conditions i.e. different speed and pressure ratio has been hampered by lack of reliable experimental data and experiments cost. Because the flow in an axial turbine is complex and many mechanisms of the flow losses in turbine have not been known well, loss models are necessary not only in the preliminary process of mean line prediction, but also in the further process of through flow calculation in the simulation and optimization of turbines. Present study has been carried out using 1-D modeling. Simulation computer code is prepared for one-stage axial turbine based on Ainley&Mathieson method with some modifications in the... 

We analyze the interaction of a propagating guided electromagnetic wave with a quantum well embedded in a dielectric slab waveguide. First, we design a quantum well based on InAlGaAs compounds with the transition energy of 0.8 eV corresponding to a wavelength of 1.55 μm. By exploiting the envelope function approximation, we derive the eigenstates of electrons and holes and the transition dipole moments. Next, we calculate the electrical susceptibility of a three-level quantum system (as a model for the 2-D electron gas trapped in the waveguide), by using phenomenological optical Bloch equations. We show that the 2-D electron gas behaves as a conducting interface, whose conductivity can be... 

Pre-compensators are used in multivariable control systems to reduce (or eliminate) open loop system interactions. Classical methods for the design of pre-compensator are traditionally based on static designs. Static pre-compensators are preferred for their simplicity, but are highly unsystematic in the amounts of achievable diagonal dominance. In many applications, only the more powerful dynamic pre-compensators are able to deliver the desired amounts of decoupling. This paper proposes a new method for the design of dynamic pre-compensators which is based on a Quadratic Programming (QP) optimization. Using the proposed approach the total pre-compensator is found through several smaller... 

Sanctions imposed by a country against another country are considered as a foreign policy tool. Many studies have investigated different aspects of sanctions including the efficacy of sanctions to alter the target country’s behaviour. However, the effects of a potential fight-back strategy adopted by a target country have been discussed rarely. In this paper, the outcomes of a possible fight-back strategy are simulated using game theoretical analysis. To find the matching state of game theory and sanction regimes, we have conducted a critical and systematic review of 12 sanctions, from 1950 to 2012. The results show that the capability of a target country to implement fight-back strategies... 

In this paper, a new model reference adaptive controller (MRAC) for uncertain switched linear systems is developed. A class of uncertain switched linear systems with parametric mismatched and input matched uncertainties, control input effectiveness and unmodeled dynamics is studied in this paper. The difference in input matrix for different switching modes which means the input channel degradation is investigated through this paper. By using common Lyapunov function method and developing new linear matrix inequality based sufficient conditions, uniform ultimate boundedness of the reference tracking error is guaranteed by switched MRAC and by using a novel nonlinear controller term, the size... 

The best energy transfer scenario (BETS) is a situation under which the optimum transfer of power (or energy) from the electric utility to an electric power consumer is achieved. Such a scenario is important because it is used as a reference point to evaluate how efficient the load operates. The index of power factor (PF) is used to quantify such an efficiency. The closer the PF to unity corresponds to more efficient performance of the load. This paper is to show that the conventional definition of the BETS is inadequate and results in an unfair value for the PF unless the transmission line is perfect and lossless (which is an unrealistic condition). The paper also investigates an... 

Contrary to the widespread understanding, this paper shows by means of mathematics and numerics that a perfect power factor correction cannot be made at the load terminals only. To achieve a perfect power factor correction, the supply commitment is required as well. In the absence of such commitment, the power factor cannot reach unity unless we refine the very definition of power factor. But the refined power factor is then slightly higher than the conventional one. The refined expression for power factor is a function of the load parameters as well as the line characteristics. For an ideal power system (with no line losses), this coincides with the conventional formula. © 2008 IEEE  

This paper proposes a control strategy for the autonomous (islanded) operation of a four-wire, electronically-coupled distributed generation (DG) unit which can feed a highly unbalanced load, e.g., due to the presence of single-phase loads. In the grid-connected mode, the power-electronic interface of the DG unit enables the exchange of real and reactive power with the distribution network, based on the conventional dq-frame current control strategy. The current control scheme is disabled subsequent to the detection of an islanding event, and the proposed controller is activated. The proposed control strategy utilizes: i) an internal oscillator to maintain the island frequency and ii) a... 

This paper presents a control strategy for the autonomous (islanded) operation of a distributed generation (DG) unit. The DG unit supplies a balanced load through a voitage-sourced converter (VSC). To maintain the autonomous operation in the islanded mode, the DG unit should provide its dedicated load with a sinusoidal voltage with a constant magnitude and a constant frequency. The dynamic model of the islanded DG system is represented by a set of nonlinear equations. Since the objective is to regulate voltage and frequency of the islanded DG about their rated values, the nonlinear model is linearized about the operating point. The obtained linearized model represents a multivariable LTI... 

In this paper, we analyze a cylindrical waveguide consisting of two layers of bianisotropic material with anti-symmetric magnetoelectric coupling tensors. The analysis is carried out in terms of pseudo-electric and pseudo-magnetic fields which satisfy Maxwells' equations with gyrotropic permittivity and permeability tensors. We show that the rotationally symmetric modes of the waveguide are unidirectional with transverse pseudo-electric and transverse pseudo-magnetic modes propagating in opposite directions. These modes are surface waves whose electromagnetic field is concentrated near the interface between the two anisotropic materials. They follow the contour of the interface even in the... 

The authors regret to inform that the corresponding author is altered from Hamed Karimi Motaalegh Mahalegi to Amir Mardani with the email address of: Amardani@sharif.edu updated as above. The authors would like to apologise for any inconvenience caused. © 2020 Elsevier Ltd  

In the present work, we report a chemically modified polyacrylamide/silica nanoporous composite adsorbent for the removal of reactive black 5 (RB5) azo dye from aqueous solutions. The composite adsorbent was synthesized in a packed bed and modified by ethylenediamine (EDA). The adsorbent was characterized by Fourier transformation infrared (FT-IR), thermogravimetric analysis (TGA), thermoporometry, Brunauer, Emmett and Teller (BET) method and scanning electron microscopy (SEM). Mechanical stability of the adsorbent was examined in a packed bed by following the back-pressure of the column. Pore diameter of the composite adsorbent in dry and wet states was estimated to be about 18.71 nm and... 

We show theoretically that unidirectional surface waves can propagate on the surface of homogeneous bi-anisotropic layers with an anti-symmetric chirality tensor. These materials mimic the electromagnetic behavior of an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. The unidirectional waves have transverse pseudo-electric or magnetic polarizations and pass through an obstacle without backscattering if the obstacle does not cause polarization change. The bi-anisotropic medium can be realized as a metamaterial comprising omega particles tailored to achieve the constitutive parameters desired. © 2018 IEEE  

Moderate or intense low-oxygen dilution (MILD) combustion of liquid fuels has attracted attention because of its advantages in industrial burners and gas turbine applications. Here, numerical investigation has been conducted on an experimental MILD turbulent spray burner. The H∥ flame of Delft spray in a hot co-flow burner is selected, and the Reynolds averaged Navier-Stokes/eddy dissipation concept framework with 40 species/180 reversible reactions through a skeletal chemical mechanism is used in addition to unsteady Lagrangian tracking of spray droplets to investigate the flame structure and chemical kinetic of reacting flow field. At first, current numerical results were compared with...