Sharif Digital Repository

Due to several advantages, three-phase Dual Active Bridge (DAB) converter is widely used in numerous applications nowadays. On the other hand, this converter is very vulnerable to Transistor Open-Circuit Fault (TOCF). Therefore, a fault-tolerant (FT) scheme has been proposed in this paper to solve the problem. First, normal and faulty conditions are investigated, and according to the results, a fault-diagnosis (FD) approach is introduced. Using the outcomes of FD unit, a new post-fault strategy is proposed for the converter. The FD method is based on the DC component of transformer phase currents, and the basis of FT technique is shedding the faulty phase. Some benefits of the proposed... 

When cells exposed to an electric field, localized changes in the distribution of the electric field will be induced and these changes in turn lead to electrical stresses on cell surface. The electrical stresses play a key role in the cell membrane structural changes which leads to important phenomena like hydrophilic pores formation on the cell membrane resulting in the cell permeability. In this work, protoplast cell interaction with direct current (DC) electric field is investigated. The electrical stresses acted on the cell membrane in the presence of electric field are investigated numerically by a modified finite difference method, fast Immersed Interface Method (IIM). Exact solution... 

The main objective of this work is to develop a novel moving-mesh finite-volume method capable of solving the seepage problem in domains with arbitrary geometries. One major difficulty in analysing the seepage problem is the position of phreatic boundary which is unknown at the beginning of solution. In the current algorithm, we first choose an arbitrary solution domain with a hypothetical phreatic boundary and distribute the finite volumes therein. Then, we derive the conservative statement on a curvilinear co-ordinate system for each cell and implement the known boundary conditions all over the solution domain. Defining a consistency factor, the inconsistency between the hypothesis... 

In this paper, two very simple and accurate equations are introduced that enable geotechnical engineers to determine the undrained ultimate bearing capacity of shallow strip foundations resting on two-layered clays. Results of the analyses show that in the case of weak-overstrong clay, usually general shear failure occurs, and in the case of strong-over-weak clay, the soil will most likely experience punching failure with a large amount of plastic settlement before it reaches its ultimate bearing capacity. This study applied the finite-difference method using a computer program to evaluate the undrained bearing capacity for all cases analyzed. The Mohr-Coulomb failure criterion was used for... 

SnO2-CuO is one the most promising systems for detection of detrimental H2S gas. Although previous experimental research has suggested a sulfidation reaction to explain selectivity toward H2S, little is known about the origin of change of electrical response of this system by changing the H2S gas concentration. In this study the relation between sensing response of continuous SnO2-CuO bilayer thin film and H2S gas concentration is computed based on changeability of chemical composition of covellite CuxS. For this purpose, chemical activity of sulfur as a function of atomic fraction in covellite copper sulfide is estimated using Gibbs energies of formation and chemical thermodynamics. By... 

To describe immobilized cells in porous microcapsule membranes with straight pores, a novel model called corrugated parallel bundle model (CPBM) was utilized. In this model, a network was developed with 10 main pores each composing 10 pore elements. Cell growth kinetic in the network was examined using non-structural models. Effectiveness factor and pore plugging time were calculated by solving reaction-diffusion equation set via finite difference method. The findings revealed that diffusion coefficient for lower order reactions will create a lesser impact on the reduction of effectiveness factor. These findings also indicated that the use of such supporting carrier for cell immobilization... 

Karun Bridge is part of a national highway project underway in southern Iran. The bridge is a 336 m steel arch structure that weighs more than 2500 tons and crosses the Karun River 270 meters above the river valley. The bridge will be the largest suspended bridge in the Middle East. Cantilever construction is to be used to construct the bridge from both sides simultaneously. The weight of the bridge is to be carried by two abutments and four piers that are anchored to exposed weathered rock mass. Loads on the abutments and piers include significant cantilever loads and moments during bridge erection and wind loads. Daily rapid temperature changes also will impart significant thermal loads on... 

In the present study a finite difference method has been developed to model the transient fluid flow and heat transfer. A single fluid has been selected for modeling of mold filling and The SOLA-VOF 3D technique was modified to increase the accuracy of simulation of filling phenomena for shape castings. The model was then evaluated with the experimental methods. Refereeing to the experimental and simulation results a good consistency and the accuracy of the suggested model are confirmed. © 2005 Published by Elsevier B.V  

The absorber is an important component in absorption machines and its characteristics have significant effects on the overall efficiency of absorption machines. This article reports on the results of numerical studies on the characteristics of falling film LiBr-H2O solution on a completely wetted horizontal tube and the associated vapor absorption in the Reynolds number range of 5 < Re < 100. The boundary layer assumptions are used for the transport of mass, momentum and energy equations and the finite difference method is employed to solve the governing equations in the film flow. The heat and mass transfer coefficients are expressed in the forms of Reynolds number, Prandtl number and... 

Short-circuit fault (SCF) detection is mandatory in a high voltage DC power supply (HVPS) to prevent fatal damage. The majority of converters employ a single sensor to detect the SCF. This attribute increases the interference vulnerability of the fault detection (FD) system in the presence of noise. Therefore, miss detections and false alarms are possible to occur. Miss detections and false alarms are harmful catastrophes in most applications. A commonly used method to suppress the noise impacts is using a low-bandwidth low-pass filter. However, the use of the low-bandwidth low-pass filter reduces the speed of FD due to the filter delay. This paper proposes a fast FD algorithm based on... 

Numerical approximation based on different forms of the governing partial differential equation can lead to significantly different results for two-phase flow in porous media. Selecting the proper primary variables is a critical step in efficiently modeling the highly nonlinear problem of multiphase subsurface flow. A comparison of various forms of numerical approximations for two-phase flow equations is performed in this work. Three forms of equations including the pressure-based, mixed pressure-saturation and modified pressure-saturation are examined. Each of these three highly nonlinear formulations is approximated using finite difference method and is linearized using both Picard and... 

A two-dimensional Boussinesq-type model is presented accurate to O(μ)6 , μ = h0/l0, in dispersion and all consequential order for non-linearity with arbitrary bottom boundary, where h0 is the water depth and l0 is the characteristic wave length. The mathematical formulation is an extension of (4,4) the Padé approximant to include varying bottom boundary in two horizontal dimensions. A higher order perturbation method is used for mathematical derivation of the presented model. A two horizontal dimension numerical model is developed based on derived equations using the Finite Difference Method in higher-order scheme for time and space. The numerical wave model is verified successfully in... 

A two-dimensional depth-integrated numerical model is developed using a fourth-order Boussinesq approximation for an arbitrary time-variable bottom boundary and is applied for submarine-landslide-generated waves. The mathematical formulation of model is an extension of (4,4) Padé approximant for moving bottom boundary. The mathematical formulations are derived based on a higher-order perturbation analysis using the expanded form of velocity components. A sixth-order multi-step finite difference method is applied for spatial discretization and a sixth-order Runge-Kutta method is applied for temporal discretization of the higher-order depth-integrated governing equations and boundary... 

In this paper, the numerical errors associated with the finite difference solutions of two-dimensional advection-dispersion equation with linear sorption are obtained from a Taylor analysis and are removed from numerical solution. The error expressions are based on a general form of the corresponding difference equation. The variation of these numerical truncation errors is presented as a function of Peclet and Courant numbers in X and Y direction, a Sink/Source dimensionless number and new form of Peclet and Courant numbers in X-Y plane. It is shown that the Crank-Nicolson method is the most accurate scheme based on the truncation error analysis. The effects of these truncation errors on... 

The numerical errors associated with explicit upstream finite difference solutions of two-dimensional advection - Dispersion equation with linear sorption are formulated from a Taylor analysis. The error expressions are based on a general form of the corresponding difference equation. The numerical truncation errors are defined using Peclet and Courant numbers in the X and Y direction, a sink/source dimensionless number and new Peclet and Courant numbers in the XY plane. The effects of these truncation errors on the explicit solution of a two-dimensional advection-dispersion equation with a first-order reaction or degradation are demonstrated by comparison with an analytical solution in... 

Normal stress has some role in the deformation analysis of hydroforming processes. In this study, analytical modeling is pursued to evaluate the effect of normal stress on the hydro-mechanical deep drawing (HDD) process. Analyses are carried out for axisymmetric elements of the formed cup-shaped part for increments of the punch travel. The formulations are obtained using mechanical and geometrical relations and the finite difference method, thereby being solved by proper numerical algorithms. Furthermore, in the present work, part thickness is variable, the loading and straining are non-proportional, and bending/unbending effects over the part curvature are considered. The results show that... 

Dynamic modeling of a double-pipe heat exchanger is the subject of the current study. The basis of this study is the same velocity of vapor and liquid phases or, in other words, homogeneous phase, in the annulus part of the exchanger. The model can predict the temperature and vapor quality along the axial pipe from the pipe inlet up to a distance where steady state conditions are achieved. The simulation is conducted for two modes of co- and counter-flow in a one dimensional transient system. The physical properties of water are estimated from empirical correlation and a saturated vapor table with cubic spline interpolation. The exchanger model, which is a set of Ordinary Differential... 

The particle shape is an important factor playing critical role in evaluation of the interactions between particles in high-concentration particle-fluid flows. In this paper, the well-known multisphere (MS) approximation approach and the novel rolling resistance approach are utilized to examine their performance in order to simplify the generalized shaped particle’s interactions within the framework of discrete element method (DEM) and computational fluid dynamics (CFD). The performance of two approaches are compared with the perfect particle’s shape geometry, for the limited cases of cubic-shaped and disk-shaped particle flows in a horizontal well drilling process as a reference scenario.... 

The particle shape is an important factor playing critical role in evaluation of the interactions between particles in high-concentration particle-fluid flows. In this paper, the well-known multisphere (MS) approximation approach and the novel rolling resistance approach are utilized to examine their performance in order to simplify the generalized shaped particle’s interactions within the framework of discrete element method (DEM) and computational fluid dynamics (CFD). The performance of two approaches are compared with the perfect particle’s shape geometry, for the limited cases of cubic-shaped and disk-shaped particle flows in a horizontal well drilling process as a reference scenario.... 

We investigate the effect of particle shape on the transportation mechanism in well-drilling using a three-dimensional model that couples computational fluid dynamics (CFD) with the discrete element method (DEM). This numerical method allows us to incorporate the fluid-particle interactions (drag force, contact force, Saffman lift force, Magnus lift force, buoyancy force) using momentum exchange and the non-Newtonian behavior of the fluid. The interactions of particle-particle, particle-wall, and particle-drill pipe are taken into account with the Hertz-Mindlin model. We compare the transport of spheres with non-spherical particles (non-smooth sphere, disc, and cubic) constructed via the...