Sharif Digital Repository

This paper presents a model to analyze contact phenomenon in microsystems actuated by ramp voltages, which has applications in frequency sweeping. First-order shear deformation theory is used to model dynamical system using finite element method, while finite difference method is applied to model squeeze film damping. The model is validated by static pull-in results. The presented hybrid FEMFDM model is utilized to compute values of contact time and dynamic behavior. Considering this model, effects of different geometrical and mechanical parameters on contact time are studied. The influence of imposing the additional reverse voltage on dynamic characteristics of the system is also... 

In this paper, the development of a neutron noise simulator for hexagonal-structured reactor cores using both the forward and the adjoint methods is reported. The spatial discretisation of both 2-D 2-group static and dynamic equations is based on a developed box-scheme finite difference method for hexagonal mesh boxes. Using the power iteration method for the static calculations, the 2-group neutron flux and its adjoint with the corresponding eigenvalues are obtained by the developed static simulator. The results are then benchmarked against the well-known CITATION computer code. The dynamic calculations are performed in the frequency domain which leads to discarding of the time... 

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... 

The maximally flat (MF) fractional-delay (FD) allpass filter, also known as Thiran's allpass filter, is one of the most popular IIR FD systems which is typically deployed in its causal forms. It is shown that if this causality constraint is removed, MFFD allpass filters with considerably wider bandwidths can be obtained. In many applications this extra bandwidth is worth having a non-causal system  

The sloshing phenomena in a partially filled tank can affect its stability. Modifications of tank instability due to the movement of the tank carrier, are key design points for the stability of a carrier. Even though the sloshing phenomenon has already been investigated using the BEM-FDM technique, the research in this paper covers this phenomenon in a porous media, which is new in 2-D coordinates. For this purpose, a Laplace equation has been used for potential flow, and kinematic and dynamic boundary conditions have been applied to the free surface. Also, a formulation has been developed for a free surface in porous media. BEM has been used for solving the governing equation and FDM... 

This paper reports the design of an optimal controller to prevent suppressvertical vibration due to undesired out of plane excitations generated by environment or gripper during manipulation for a CMOS-MEMS Nano-Newton capacitive force sensor applied for biomedical applications. Undesired out of plane excitations generated by environment or gripper during manipulation is the most prevalent source of vertical vibration in this type of sensors. To suppress the vibrational movement a PZT 5A is used as actuation mechanism. Discrete element method DEM model and Modal analysis were used to find dominant natural frequencies and mode shape vectors. To eliminate out of plane excitation an optimal... 

In this paper, we propose a new packet prioritization scheme in order to exploit the full-duplex (FD) capability of the access point (AP) more efficiently, in an asymmetric IEEE 802.11-based WLAN, i.e., only the AP has the in-band FD communications capability. In this respect, we consider a modified version of IEEE 802.11 MAC protocol such that at any transmission opportunity in which AP has the role of the transmitter or the receiver, it does the best to select a partner packet to be simultaneously received or transmitted, respectively. The key feature of our proposed partner packet selection scheme is to reduce the idle time intervals that in an FD transmission opportunity, due to... 

In this paper, the resource allocation and scheduling problem for a full-duplex (FD) orthogonal frequency-division multiple-access network is studied where an FD base station simultaneously communicates with multiple pairs of uplink (UL) and downlink (DL) half-duplex (HD) users bidirectionally. In this paper, we aim to maximize the network sum-rate through joint UL and DL user pairing, OFDM subchannel assignment, and power allocation. We formulate the problem as a non-convex optimization problem. The optimal algorithm requires an exhaustive search, which will become prohibitively complicated as the numbers of users and subchannels increase. To tackle this complex problem more efficiently, we... 

A high-order compact finite-difference scheme is applied and assessed for the numerical simulation of structural dynamics. The two-dimensional elastic stress-strain equations are considered in the generalized curvilinear coordinates and the spatial derivatives in the resulting equations are discretized by a fourth-order compact finite-difference scheme. For the time integration, an implicit second-order dual time-stepping method is utilized in which a fourth-order Runge-Kutta scheme is used to integrate in the pseudo-time level. The accuracy and robustness of the solution procedure proposed are investigated through simulating different two-dimensional benchmark test cases in structural... 

A moving vehicle, owing to its vibration and mass inertia, has significant effects on the dynamic response of structures. Most bridge codes define a factor called the dynamic load allowance, which is applied to the maximum static moment under static loading due to traffic load. This paper presents how to model an actual truck on bridges and how the maximum dynamic stresses of bridges change during the passage of moving vehicles. Furthermore, an algorithm to solve the governing equation of the bridge simultaneous with the equations of motion of an actual European truck is presented. Subsequently, 32 dynamic analyses of different bridges with different spans, road profiles and boundary... 

Mesh point positions in Finite Difference Method (FDM) of discretization for the neutron diffusion equation can remarkably affect the averaged neutron fluxes as well as the effective multiplication factor. In this study, by aid of improving the mesh point positions, an enhanced finite difference scheme for the neutron diffusion equation is proposed based on the neutron importance function. In order to determine the neutron importance function, the adjoint (backward) neutron diffusion calculations are performed in the same procedure as for the forward calculations. Considering the neutron importance function, the mesh points can be improved through the entire reactor core. Accordingly, in... 

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... 

The secondary flow of PTT fluids in rectangular cross-sectional plane of microchannels under combined effects of electroosmotic and pressure driving forces is the subject of the present study. Employing second-order central finite difference method in a very refined grid network, we investigate the effect of electrokinetic and geometric parameters on the pattern, strength and the average of the secondary flow. In this regard, we try to illustrate the deformations of recirculating vortices due to change in the dimensionless Debye–Hückel and zeta potential parameters as well as channel aspect ratio. We demonstrate that, in the presence of thick electric double layers, significant alteration... 

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.... 

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... 

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... 

Absorber is an important component in absorption machines and its characteristics have significant effects on the overall efficiency of absorption machines. This article reports a model of simultaneous heat and mass transfer process in absorption of refrigerant vapor into a lithium bromide solution of water - cooled vertical plate absorber in the Reynolds number range of 5 < Re < 150. The boundary layer assumptions were used for the transport of mass, momentum and energy equations and the fully implicit finite difference method was employed to solve the governing equations in the film flow. Dependence of lithium bromide aqueous properties to the temperature and concentration and film... 

Nonlinear ultrasound modeling is finding an increasing number of applications in both medical and industrial areas where due to high pressure amplitudes the effects of nonlinear propagation are no longer negligible. Taking nonlinear effects into account makes the ultrasound beam analysis more accurate in these applications. One of the most widely used nonlinear models for propagation of 3D diffractive sound beams in dissipative media is the KZK (Khokhlov, Kuznetsov, Zabolotskaya) parabolic nonlinear wave equation. Various numerical algorithms have been developed to solve the KZK equation. Generally, these algorithms fall into one of three main categories: frequency domain, time domain and... 

The characteristic boundary conditions are applied and assessed for the solution of incompressible inviscid flows. The two-dimensional incompressible Euler equations based on the artificial compressibility method are considered and then the characteristic boundary conditions are formulated in the generalized curvilinear coordinates and implemented on both the far-field and wall boundaries. A fourth-order compact finite-difference scheme is used to discretize the resulting system of equations. The solution methodology adopted is more suitable for this assessment because the Euler equations and the high-accurate numerical scheme applied are quite sensitive to the treatment of boundary... 

This study reports on the simulation of temperature distribution of human tooth under a laser beam based on non-Fourier models. The temperature in the tooth depth that directly results from the conduction heat transfer process is caused by the lengthy thermal relaxation time in the tooth layers. A detailed tooth composed of enamel, dentin, and pulp with unstructured shape, uneven boundaries, and realistic thicknesses was considered. A finite difference scheme was separately adopted to solve time-dependent equations in solid layers and soft tissue of the tooth. In this study, a dual-phase-lag non-Fourier heat conduction model was applied to evaluate temperature distribution induced by laser...