Sharif Digital Repository

In the present study, a one step hydrothermal process was employed to synthesize magnetite nanoparticles using oleic acid as surfactant agent at 140 °C. Effects of reaction time and alkalinity were studied on particles size and morphology. By changing these parameters, some monodisperse spherical nanoparticles with mean particle size between 2.71 and 13.88 nm were synthesized and characterized via TEM, XRD, VSM, TGA and FT-IR techniques. Assuming the Avrami behavior of particles formation, a kinetics equation was proposed for the transformation rate at 140 °C. Using some simplifying assumptions, nucleation and growth rates were calculated for the hydrothermal formation of magnetite... 

Although satisfiability checking is known to be an effective approach in classical planning, it has scarcely been investigated in the field of temporal planning. Most notably, the usage of ∃-step semantics for encoding the problem into a SAT formula, while being demonstrably quite efficient for decreasing the size of the encodings in classical planning, has not yet been employed to tackle temporal planning problems. In this paper, we define temporal versions of classical ∀-step and ∃-step plans. We show that when the casual and temporal reasoning phases of a SAT-based temporal planner are separated, these semantics can be used to translate a given temporal planning problem into a SAT... 

Micropumps are among useful equipment in microsystems. The magnetically actuated mercury micropump, which has been introduced for less than a decade, is an innovative kind of micropumps which uses mercury droplets motion as a pumping agent. The equations governing this micropump are complex and their numerical solution is a time-consuming process, due to electromagnetic, hydrodynamic, and unsteady effects. In the present study, for the first time, using simplifying assumptions, the performance of a Magneto Mercury Reciprocating (MMR) micropump with electromagnetic actuation is studied through electrical analogy and then, the components and operational stages of the micropump are simulated... 

Analysis of thermal-hydraulics fluctuations in pressurized water reactors (e.g., local and global temperature or density fluctuations, as well as primary and charging pumps fluctuations) has various applications in calculation or measurement of the core dynamical parameters (temperature or density reactivity coefficients) in addition to thermal-hydraulics surveillance and diagnostics. In this paper, the thermal-hydraulics fluctuations in PWRs are investigated. At first, the single-phase thermal-hydraulics noise equations (in the frequency domain) are originally derived, without any simplifying assumptions. The fluctuations of all the coolant parameters, as well as the radial distribution of... 

High resolution range profile (HRRP) is a widely noted tool in radar target recognition. However, its high sensitivity to the target's aspect angle makes it necessary to seek solutions for this problem. One alternative is to assume consecutive samples of HRRP identically and independently distributed in small frames of aspect angles, an assumption which is not true in reality. Based on this simplifying assumption, some models, such as the hidden Markov model, have been developed to characterise the sequential information contained in multi-aspect radar echoes. As a result, these models consider only the short dependency between consecutive samples. Considering such issues, in this study, the... 

This paper is concerned with the problem of guidance filter design for a homing air defense missile using measurements of an on-board pitch-yaw gimballed seeker mechanism. This problem possess a highly nonlinear dynamic which is influenced by various error sources such as seeker's stabilization loop torque disturbances originated from pursuer's accelerations and angular velocities, significant channel couplings, time-delayed and noisy measurements of optical sensor besides the unknown target maneuvers. To handle the inherent nonlinearity of problem a wide variety of assumptions are made in literature to derive simplified engagement kinematics. In present work, performance degradations which... 

In this paper size-dependent static and dynamic behavior of nonlinear Euler-Bernoulli beams made of functionally graded materials (FGMs) is investigated on the basis of the strain gradient theory. The volume fraction of the material constituents is assumed to be varying through the thickness of the beam based on a power law. As a consequence, the material properties of the microbeam (including length scales) are varying in the direction of the beam thickness. To develop the model, the usual simplifying assumption which considers the length scale parameter to be constant through the thickness is avoided and equivalent length scale parameters are introduced for functionally graded microbeams... 

A depleted naturally fractured gas condensate reservoir with extremely tight matrix permeability in the center of Iran is used for gas storage. Due to the tightness of the matrix, simulation of this process may present a unique challenge in terms of the importance of diffusion process in flow behavior of the injected gas in matrix blocks. This article presents a multimechanistic (Darcian-type flow and Fickian-type flow) mathematical model to investigate the importance of diffusion process as a function of important rock and fluid parameters. The authors' approach consists of the following steps: (a) development of a 1-D governing equations for a single matrix block consists of a gas... 

A water balance model for a local location across a PEFC was previously developed. Below a critical current density, water removal out of the cathode catalyst layer (CCL) to cathode side proceeds by vapor diffusion. Above the critical current density, excessive water generation leads to the build-up of an excess liquid pressure in the CCL, which acts as a driving force for hydraulic fluxes to polymer electrolyte membrane (PEM) and gas diffusion media (GDM) of cathode. These fluxes are a function of local cell operating conditions such as the local relative humidity in cathode feed channel (CFC). Assuming no water condensation/evaporation in the CFC, water vapor balance along CFC under steady... 

Carbon nanotubes (CNTs) are viewed as rolled graphene. Thus, an appropriate formulation describing the behavior of CNTs must contain the key information about both their initial configuration as graphene and final configuration as CNT.On this note, to date, somemodels, in particular based on the Cauchy- Born rule, for the description of CNTs behavior exist. A simplifying assumption in some of these models is that the length and perimeter of the CNT equal the corresponding dimensions of the unrolled initial configuration, thus neglecting the induced hoop and longitudinal strains. On the other hand, the present work offers a purely nonlinear continuum model suitable for the description of the... 

In this paper, a 3-dimensional mathematical model for one cell of an anode-supported SOFC (solid oxide fuel cells) is presented. The model is derived from the partial differential equations representing the conservation laws of ionic and electronic charges, mass, energy, and momentum. The model is implemented to fully characterize the steady state operation of the cell with countercurrent flow pattern of fuel and air. The model is also used for the comparison of countercurrent with concurrent flow patterns in terms of thermal stress (temperature distribution) and quality of operation (current density). Results reveal that the steady-state cell performance curve and output of simulations... 

Effective diffusion coefficient is an important parameter which needs to be determined in different fields of study, such as cathode catalyst layers of PEM fuel. For this purpose, a Loschmidt diffusion cell can be used. When a porous medium is placed in Loschmidt apparatus, the effective gas diffusion coefficient (EGDC) of this section must be correlated by diffusion coefficient in absence of a porous medium. In the previous researches studying the Loschmidt diffusion cell, a simplifying infinite-length assumption was used in the analytical solution. Therefore, the solution is only applicable for a short time range, and this can result in high error. In order to overcome this challenge, the... 

One of the main concerns in using commercial software for finite element analyses of dam-foundation-reservoir systems is that the simplifying assumptions of the massless foundation are unreliable. In this study, an appropriate direct finite element method is introduced for simulating the mass, radiation damping and wave propagation effect in foundations of dam-foundation-reservoir systems using commercial software ABAQUS. The free-field boundary condition is used for modeling the semi-infinite foundation and radiation damping, which is not a built-in boundary condition in most of the available commercial software for finite element analysis of structures such as ANSYS or ABAQUS and thus... 

Attitude control of a satellite having only two reaction wheels is a challenging issue. To address this problem, previously published researches considered some simplifying assumptions on the satellites such as diagonality of the moment of the inertia matrix. On the other hand, in some works, the total angular momentum of the satellite is assumed to be zero. In this paper, a linear time-variant model predictive control (LTV MPC) is designed to control a satellite with two reaction wheels. This control method can be applied to a satellite with a non-diagonal inertial matrix in the presence of external torques, to rotate the satellite toward the desired directions in the space and orbit. The... 

This paper presents a data-driven approach that utilizes the gathered experimental data to model and control a test rig constructed for the high-powered gearboxes. For simulating a wide variety of operational conditions, the test rig should be capable of providing different speeds and torques; this is possible using a torque-applying system. For this purpose, Electro-Hydraulic Actuators (EHAs) are used. Since applying accurate torque is a crucial demand as it affects the performance evaluation of the gearboxes, precise modelling of the actuation system along with a high-performance controller are required. In order to eliminate the need for to solve complex nonlinear equations of EHA that... 

This paper studies the statistical properties of the signal-to-noise ratio (SNR) of the dual-hop relaying link in a cooperative wireless communication system in the presence of channel estimation errors for fixed-gain (FG) and variable-gain (VG) relays. The SNR expression is derived and three different analytical approaches with different simplifying assumptions are proposed to obtain the probability distribution function of the SNR and the outage probability in each mode. All but one approach result in closed-form expressions for the outage probability. The simplest approach in each mode has been used to find an optimum power allocation scheme for pilot and data symbols transmission at the... 

With the rising penetration of wind power into electric power systems, more accurate and comprehensive studies are required to identify the interaction between wind farm(s) and power system. This requires accurate models of different types of wind turbine-generators. This paper describes modeling and small signal analysis of a grid connected fixed speed wind turbine generator (FSWTG). A complete model of FSWTG is derived and some reduced-order models are deduced. Eigenvalues and participation factors of the system for these models are calculated. Then, using the eigen analysis results, the models are compared and the simplifying assumptions which have been considered in the literatures are... 

Spine musculoskeletal (MS) models make simplifying assumptions on the intervertebral joint degrees-of-freedom (rotational and/or translational), representation (spherical or beam-like joints), and properties (linear or nonlinear). They also generally neglect the realistic structure of the joints with disc nuclei/annuli, facets, and ligaments. We aim to develop a novel MS model where trunk muscles are incorporated into a detailed finite element (FE) model of the ligamentous T12-S1 spine thus constructing a gold standard coupled MS-FE model. Model predictions are compared under some tasks with those of our earlier spherical joints, beam joints, and hybrid (uncoupled) MS-FE models. The coupled...