Sharif Digital Repository

Acidizing of carbonate oil-wet rocks saturated with oil and saline formation water is subjected failure in some cases due to acid-induced damage such as sludge and emulsion formations. This condition may also lead to mineral precipitation, oil film barrier between acid and rock and diversion chemical malfunctions. Therefore, pre-flush process has been proposed as one of the most efficient stage for oil-wells matrix acidizing to reduce these challenges significantly. Besides, the pre-flush stage would result in more clean rock as the reservoir fluids are pushed back from the near wellbore regions, restoring rock wettability to more water wet state, preventing direct acid-oil contact and... 

Fading and path loss are the major challenges in practical deployment of free space optical communication systems. In this paper, a cooperative free space communication via an optical amplify-and-forward relay is considered to deal with these challenges. We use photon counting approach to investigate the system bit error probability (BEP) performance and study the effects of atmospheric turbulence, background light, amplified spontaneous emission, and receiver thermal noise on the system performance. We compare the results with those of the multiple-transmitter (MT) system. The results indicate that the performance of the relay-assisted system is much better than that of the MT system in... 

Despite their attractive features, free-space optical (FSO) communications suffer from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, multiple input multiple output (MIMO) and cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in radio frequency (RF) communications have greatly been investigated, there have not been remarkable researches on cooperative transmissions in FSO communications. In this study, the authors consider a relay-assisted FSO communication and investigate the outage performance of two well-known cooperative protocols, namely amplify-and-forward (AF)... 

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... 

This paper studies simultaneous lot sizing and scheduling problem in a job shop environment over a finite number of periods. The machines are flexible such that the production manager is able to change their working speeds. Also, the production schedule of the items over machines should be in consistence with technological precedence relationships. The problem is formulated as an integer linear program. Then, a number of valid inequalities are developed based on the problem structures. The proposed valid inequalities are added to the problem formulation in a "cut and branch" manner to accelerate the search process by common solvers. Finally, performance of the valid inequalities is... 

The heterobinuclear cyclometalated PtII-AuI complexes of the type [MePt(bhq)(μ-dppy)AuL]PF6, in which, bhq = benzo{h}quinoline, dppy = 2-diphenylphosphinopyridine, L = PPh3, 5; PPh2Me, 6; PPhMe2, 7; have been prepared by reaction of the mononuclear platinum(II) complex [MePt(bhq)(dppy)], 1, with 1 equiv. [AuL]+. Also, the reaction of complex 1 with 1 equiv AuCl(SMe2) at -60 °C gave the neutral heterobinuclear complex [MePt(bhq)(μ-dppy)AuCl], 8. All of the complexes were characterized using NMR spectroscopy and elemental microanalysis and complexes 5 and 8 were further identified by single crystal X-ray structure determination. Time-dependent density functional theory (TD-DFT) is used to... 

Free-space optical (FSO) communication suffers from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, spatial diversity based on MIMO techniques has been proposed for FSO systems. User Cooperation diversity is a new form of spatial diversity which is introduced to overcome some limitations of MIMO structures. Although the promising effects of cooperative transmission in RF communications have greatly been considered so far, there have been a few notable research on cooperative diversity in FSO. In this paper, we first consider a 3-way FSO communication setup, in which the cooperative protocol can be applied to achieve the... 

In Part I, a new internally coded time hoping optical code division multiple access (TH/OCDMA) scheme for fiber optic communication systems has been proposed and its multiple access performance has been evaluated using optical orthogonal code (OOC). Due to low cardinality of OOCs with a correlation value of 1, the capability of the proposed scheme could not be utilized effectively for an increasing number of simultaneous users. In this part, we consider applying the internally coded technique introduced in part one to the frame time hopping/OCDMA scheme. We evaluate the multiple access performance of the system for the three detectors introduced in Part I. Our results demonstrate the... 

In this paper, a new internally coded time-hopping optical code-division multiple-access (TH/OCDMA) scheme for fiber-optic communication systems is proposed, and its multiple-access performance is evaluated using optical orthogonal codes (OOCs). In the proposed method, the duration of each bit is divided into Ns frames, each one containing L chips. Two signature codes, namely, one pseudonoise (PN) sequence and one OOC, are assigned to each user. During each bit interval, based on the output of an channel encoder and the user's dedicated PN sequence, one of the Ns frames is selected, in which W pulses are transmitted in W chips, marked by the user's OOC. We consider three detectors at the... 

Nonlinear behavior of functionally graded (FG) sandwich circular sector plates with simply supported radial edges under transverse loading is investigated using the first-order shear deformation theory with von Karman geometric nonlinearity. The nonlinear governing equations are reformulated and solved via single-parameter perturbation and Fourier series methods. Verification is performed by comparing numerical results with the available ones in the literature and the ones obtained via ABAQUS code. The effects of non-linearity, material constant, lay-up, and boundary conditions on bending of FG sandwich sector plates with FG core/homogenous face sheets and metallic core/FG face sheets are... 

Here a special state variable feedback or state feedback for short is introduced with the main objective of providing a convenient method for decoupling a MIMO transfer matrix. Decoupling in general and decoupling via state feedback in particular offer obvious advantages in the design and synthesis of controllers for MIMO systems. The early parts of this paper deal with the eigenvalues, poles, and zeros as well as the controllability and observability properties of the proposed closed-loop system as compared with those of the open-loop system. The proposed method is tested on a model of a realistic physical system and results are illustrated. © 2008 IEEE  

Analytic finite sample equivalents for autoregressive-model order selection criteria are introduced. These equivalents are based on approximately analytic formulas and can be used in the case where the estimation method is least-squares-forward. In fact, these criteria are approximately analytic equivalents of existing empirical finite sample criteria. The performance of the finite sample criteria is better than that of their asymptotic equivalents in the finite sample case. In the large sample case, the criteria introduced in this paper converge to the existing criteria  

In this paper, the thermal conductivity of water-based hematite Fe2O3 and magnetite Fe3O4 nanofluids have been investigated in the absence and presence of a uniform magnetic field. The experiments have been performed in the volume concentration range of 0 % to 4.8 % and the temperature range of 20∘C to 60∘C. The effects of the particle volume fraction, temperature, and magnetic field strength on the thermal conductivity have been analyzed. Results show that the thermal conductivity of iron oxide nanofluids has a direct relation with the particle volume fraction and temperature, without the presence of a magnetic field. But surprisingly, when the magnetic field is applied, it is observed that... 

This paper presents mathematical modeling of joint lot sizing and scheduling problem in a job shop environment under a set of realistic working conditions. One main realistic assumption of the current study is dealing with flexible machines able to change their working speeds, known as process compressibility. The production schedules should be subject to limited available time in every planning period. Also, the model assumes that periodical sequences should be determined in a way that they obey from a fixed global sequence. Another complicating aspect of the problem is about consideration of precedence relationships for the needed processes of an item type over the corresponding machines.... 

Anaerobic digestion processes and the conductive electron transfer approach were used to describe the bioenergy production processes in a microbial fuel cell (MFC), respectively. The present model is a far more completed form of conduction-based modeling which is able to predict performance of an MFC fed with complex substrates and inoculated with multi-population culture. One-dimensional spatial distributions of the different microorganisms, as biocatalysts of processes and intermediates produced in the different steps of the anaerobic digestion processes in the biofilm, as well as the dynamic behavior of the anolyte including syntropic interactions among different groups of microorganisms... 

In this study, a new model of microbial fuel cell (MFC) was obtained for the first time. The modeled MFC was made using a combination of two approaches; the conduction-based method and two-step anaerobic digestion. Performance of the MFC was based on calculations for current evolution and polarization curves with different subsequent variables of the biofilm and anolyte. The model was able to make predictions for performance of the MFC for a simple substrate to more complex ones. The model was successfully validated with a variety of substrates (acetate, glucose and dairy wastewater) and the results were compared with previously published measurements. The model polarization results showed... 

In this study, a blind steganalysis approach which accurately discloses low rate data hiding schemes is proposed. The absolute values of neighbouring joint density (absNJ) are used for feature extraction. In this way, the intra- and inter-block situations are employed providing a variety of different features. Aside from the absolute values of discrete cosine transform (DCT) coefficients, the differential DCT coefficients are also exploited to extract the features. Moreover, the absNJ features will be extended and used over differential DCT coefficients. It is shown that applying the Pth power of the DCT coefficients instead of their first power gains more discriminative features. Then,... 

The objective of this study was to construct a novel model to be applied in a general manner to simulate microbial electrochemical cells (MXCs); for both microbial fuel cell (MFC) and microbial electrolysis cell (MEC). The liquid bulk was modeled based on the organic matters degradation to acetate via the anaerobic digestion process. Biofilm simulation was established based upon one-dimensional distribution and the dynamical electron transfer was completed by means of the conduction-based mechanism. We, for the first time, introduced biofilm local potential modeling for MEC simulation with general and simplified linear boundary conditions. The MFC-related part of the model was evaluated... 

ABSTRACT: Experimental study of the performance of a gas centrifuge can be appreciably simplified if instead of isotopic mixtures, a binary mixture of gases with large molecular weight difference is used. The current study undertook this approach by injecting a 53%–47% (w/w) mixture of “Freon12-Freon22” into a gas centrifuge. The two parameters, whose investigation was the objective of the current study were: the feed flow rate (F), and the clearance between tail scoop and the rotor wall (d). The results demonstrated that changing the scoop-wall clearance has the most significant effect on the cut (θ), so that by fixing “d”, “θ” becomes nearly invariant. The head separation factor (α)... 

Asphalt concrete is a complex mixture, the behavior of which is dependent on its components while showing different nonlinear behaviors under various conditions. Application of rest time between compressive loading cycles causes hardening-relaxation (H-R) behavior in the sample and increases the deformation potential of the sample. This process might induce changes in the microstructure and the pattern of the placement and rotation of the aggregates. These changes cause a relaxation in the hardening of the material, pose a higher potential of reception of viscoplastic strain in the following cycles, and increase the rate of accumulation of the permanent deformation in contrast with creep...