Scientia Iranica
Volume:21 Issue: 4, 2014

In recent years, the research work in material science has encouraged the development of advanced ceramic materials. Boron carbide has many applications in industries for manufacturing of products such as water jet nozzle and abrasive particles. In this paper several experimental tests have been conducted to study the effect of process parameters in machining of boron carbide «B4C» by using the electrical discharge machining process “EDM”. The effect of voltage variation, current intensity, on-pulse and off-pulse durations on surface roughness (Ra) and material removal rate (MRR) were studied simultaneously by utilizing the design of experiment (DOE) technique. This research work indicated that, the “EDM” process can be used to machine semi-conductor materials such as “B4C” successfully. In addition, it was found that, the quantity of material removal rate and the quality of workpart surface roughness could be affected by the variations of these process parameters. Finally, the experimental results obtained, were compared with the published results reported by other scientist and found a good agreement between them.

Renormalization group method (RGM) is a simple and powerful method to obtain analytical solution for differential equation. In this paper, with some examples, we show that the application of RGM to second order form of differential equation to determine the higher order approximation may give solution that is at variance with those solutions obtained with multiple scales method (MSM) and generalized method of averaging (GMA). However, transforming the differential equation to complex-variable form and then applying RGM, one may obtain solutions in agreement with the MSM and GMA solutions. Furthermore, we consider a Hamiltonian 2DOF system and observe that the application of RGM to second order form, results in non-Hamiltonian RG equations and the result is at variance with the MSM and GMA solutions. Again, this problem can be overcame by applying the RGM to the complex-variable form of equations and obtaining solutions, which are derivable from a Lagrangian and are in agreement with the MSM and GMA solutions. Therefore, in use of RGM correct results may be obtained by treating the equations in complex-variable form.

An analysis has been carried out to study the heat transfer analysis and MHD stagnation-point. ow of a viscous. uid over an unsteady stretching sheet in a porous medium with slip condition. For the present problem, the govern- ing equations are transformed into a system of non-linear ordinary di¤erential equations by means of similarity transformations. This system is solved both analytically by an analytictechnique, namely the homotopy analysis method (HAM) and numerically using a shootingmethod with Runge-Kutta algorithm. The in.uences of the involved parameters on the. ow and temperature. elds are graphically illustrated and analyzed. The results obtained bymeans of both methods are compared and found in excellent agreement.

A finite volume method is developed for simulation of oscillatory compressible flow in the pulse tube part of an orifice pulse tube refrigerator. Governing equations for control volumes are written in 1D discretized form. Second order upwind is used for the convective terms as well as Euler implicit method for temporal derivatives. The results include the temperature and mass flow rate as functions of time and position, and the buffer pressure as a function of time. A typical pulse tube is modeled by using the numerical model. The results show that the present numerical method has good agreement with previously published results. The cold end (inlet) mass flow rate makes an angle of 39.60 with the pressure vector. Hot end mass flow rate is in phase with the pressure vector. Previously, it was mentioned that in one-dimensional models, overshoots never disappear completely, whereas in the present one-dimensional model they are eliminated. The overshoots in the both ends for present results are less than those for 3D results. By using the results of Eulerian coordinate, Lagrangian approach is used to track the movement of the gas parcels to get their pressure, temperature, and velocity in a thermodynamic cycle.

This study considers blood flow in total cavopulmonary connection (TCPC) morphology, created in Fontan surgical procedure in patients with a single ventricle heart disease. Ordinary process of TCPC operation reduces pulmonary blood flow pulsatility; since the right ventricle being bypassed. This reduction may limit the long term outcome of Fontan circulation. There is an idea of increasing pulmonary flow pulsations by keeping main pulmonary artery (MPA) partially open while it was closed in ordinary TCPC operation. The purpose of the present study is to verify the effects of antegrade flow (AF) coming through stenosed MPA on pulmonary flow pulsations. The 3-D geometry is reconstructed from CT angiography scan of a patient who has undergone an ordinary TCPC procedure. The stenosed MPA or pulmonary stenosis (PS) is virtually added to the original geometry. We applied a 3D-1D coupled method to simulate blood flow in this situation more precisely. The results show that adding AF increases pulsatility index (PI) in both left and right pulmonary artery (LPA and RPA respectively). Moreover, adding AF leads to an increase in energy loss. It also increases the pulmonary-to-systemic flow ratio leading to increase in total cardiac flow rate and hence heart power.

Smoothed Particle Hydrodynamics method is used to explore the effects of design parameters on the mixing efficiency of two types of active micromixers. First, the complex flow field and the mixing process of two separated fluids in a square mixing chamber with nine symmetric microstirrers are simulated. The influence of design parameters such as the microstirrer rotation arrangement and the angular velocity of the microstirrer on the mixing performance are investigated. Mixing index parameter on ten control points is calculated and the average mixing index is compared for different cases. Simulations illustrate that the rotation arrangement of microstirrers is a key parameter in the mixing process. Then, a parametric study is performed for a horizontal and a Y-shape active micromixer where the influence of the length, the angular velocity, the rotation or the oscillation movement of the microstirrer, the inlet Reynolds number, and the inlet stream angle is investigated. It is also illustrated that the inlet Reynolds number and the Struhal number are the prominent parameters that affect the mixing efficiency of this type of active micromixers.

Mixed convection of air with Prandtl number of 0.7 in eccentric horizontal annuli with isothermal wall conditions was numerically investigated. The inner cylinder is at a higher temperature and the inner and outer cylinders rotate either clockwise or counter-clockwise at a constant angular velocity. The effects of parameters such as the eccentricity of the axes, the Rayleigh number and rotation direction were studied. The two-dimensional Navier-Stokes and energy equations were numerically solved in an orthogonal grid by employing a finite-volume method based on the SIMPLE algorithm. This orthogonality was achieved by employing a cylindrical bipolar coordinate system. Use of this coordinate system has the privileges of orthogonal coordinate and simplicity of computing different fluxes involved. Mean and local heat transfer results were obtained. The physics of the flow and the heat transfer is revealed by the streamlines and the isotherms obtained from the numerical solutions. Flow and heat transfer are influenced by the orientation and eccentricity of the inner cylinder. The present numerical results are in good agreement with the results of others for pure natural convection.

In this paper, strain gradient thermo-elasticity formulation for axisym- metric Functionally Graded (FG) thick-walled cylinders is presented. For this purpose, elastic strain energy density function is considered to be a function of gradient of strain tensor in addition to the strain tensor. The material properties are assumed to vary according to a power law in radial direction. Using the constitutive equations and equation of equilibrium in the cylindrical coordinates, a fourth order non-homogenous governing equa- tion for thermo-elastic analysis of thick-walled FG cylinders subjected to thermal and mechanical loadings is obtained and solved numerically. Re- sults show that the intrinsic length parameter a ects the stress distribution in FG thick-walled cylinders greatly and increasing the intrinsic length pa- rameter reduces the maximum radial and hoop stresses. Also, the effect of FG power indices on the radial and hoop stresses are studied.

Natural gas is considered to be the most promising alternative fuel considering economy and clean burning. At the present, emission regulations for NG are restricted with Euro IV, V and even for Euro VI and these regulations are expected to become more restricted in future. Emission generation characteristics of pure natural gas is fairly known, however more experimental investigation of natural gas combustion is necessary. In this research, a turbocharged natural gas SI engine is investigated experimentally. Emission and performance characteristics of the engine at WOT and lean burn conditions are studied and validated experimentally and theoretically. A computer code is also developed in MATLAB environment for predicting engine performance and NO emissionsand the results are validated with experimental data. Thermal boundary layer, as a novel approach, is considered in the simulation and a better agreement of predicted mass fraction burned was achieved at end part of combustion process when compared with experimental results. Experimental results have revealed that turbocharger match at mid engine speed, waste-gate opening and increase of friction losses at high engine speed would decrease the torque and resulted in better torque back-up. Boost pressure, controlled by opening range of waste gate based on compressor outlet pressure, remained nearly constant at speeds higher than 1450 rpm. NOx emission reduced with engine speed increase due to shorter timethat burned gases remain at high temperatures, although they have higher temperature at high engine speeds.NO emission increased with the increase of excess air ratio until it reached to its peak value at about 1.1 from stoichiometric mixture and then decreased. NO emission decreased substantially with increase of spark timing retard. Brake specific UHC and CO2 emissions were min at mid speed range and WOT.

The problem of unsteady three-dimensional axisymmetric stagnation-point flow and heat transfer of a viscous compressible fluid on a flat plate is intended to solve when the plate can move with any arbitrary time-dependently variable or constant velocity. An external low Mach number potential flow impinges, along z-direction, on the flat plate with strain rate a to produce three-dimensional axisymmetric stagnation-point flow where the plate moves toward or away from impinging flow, concurrently. An exact solution of the governing Navier-Stokes and energy equations is obtained by use of suitably-introduced similarity transformations. The temperature of the plate wall is kept constant which is different with of the main stream. A Boussinesq approximation is used to take into account the density variations of the fluid. The results are presented for a wide range of parameters characterizing the problem including volumetric expansion coefficient (β), wall temperature, Prandtl number and plate velocity at both steady and unsteady cases. According to the results obtained, it is revealed, that when the plate moves away from the impinging flow, thermal and velocity boundary layer thicknesses get higher values compared to the plate moving upward. Besides, it is captured that the value of β and Pr numbers does not have any significant effect on shear stress and, also, heat transfer for a plate moving away from the incoming potential flow.

This paper deals with attitude determination, parameter identification and reference sensor calibration simultaneously. A LEO satellite’s attitude, inertia tensor as well as calibration parameters of Three-Axis-Magnetometer (TAM) including scale factors, misalignments and biases along three body axes are estimated during a maneuver designed to satisfy the condition of persistency of excitation. The advanced nonlinear estimation algorithm of Unscented Kalman Filter (UKF) is a good choice for nonlinear estimation problem of attitude determination, but its computational cost is considerably larger than the widespread low accurate Extended Kalman Filter. Reduced Sigma Point Filters provide good solutions and also decrease the run time of the UKF. However, in contrast to nonlinear problem of attitude determination, parameter identification and sensor calibration have linear dynamics. Therefore, a new Marginal UKF is proposed that combines utility of Kalman Filter with Modified UKF (MUKF) which is based on Schmidt orthogonal algorithm. The proposed Marginal MUKF (MMUKF) utilizes only 14 sigma points to achieve the complete 25-dimensional state vector estimation. Additionally, a Monte Carlo simulation has demonstrated a good accuracy and lower computational burden for concurrent estimation of attitude, inertia tensor as well as TAM calibration parameters utilizing MMUKF with respect to the sole utilization of the UKF.

Simulation of manipulation is a basic tool for accurate and controllable displacement of bodies and particles at micro and nano scale. The atomic force microscope (AFM) system has become a useful tool for direct measurements of micro and nano structural parameters and unraveling the intermolecular forces at nanoscale level with atomic-resolution characterization. Friction forces are one of the parts of surface properties which play an important role in manipulation of nanoparticles. In order to gain more precise manipulation, different friction models have been developed one of which is LuGre model. In this paper sensitivity of manipulation of nanoparticle has been analyzed to dimensional and environmental parameters based on LuGre friction model using Sobol method. In previous work sensitivity analysis has been performed using graphical sensitivity analysis; hence the importance percentages of parameters are not clear but Sobol method, which is a statistical model, solves this problem. Results show that cantilever thickness is the most effective dimensional parameter on critical force value while cantilever length and width are less important. Environmental parameters such as cantilever elasticity modulus, substrate velocity and adhesion, respectively, take next orders.