مجله مهندسی مکانیک
سال چهل و یکم شماره 1 (پیاپی 61، بهار و تابستان 1390)

The effect of installation of turbulence promoter wire on the wake flow characteristics such as average velocity and turbulence intensity about an airfoil is investigated experimentally at a Reynolds number of 3780. The wire is installed at the maximum thickness of NACA0012 airfoil and its diameter is changed to obtain different turbulente flows. This study shows that the width and minimum of velocity profile in the wake is increased by increasing the wire diameter. Also by increasing x/c and the wire diameter, the half width at half depth (b1/2) is increased. The velocity outside of wake region is approximately equal for all cases. This investigation also shows that the drag coefficient is increased by increasing the wire diameter.

The main subject of this article is the simulation of the cavitation including partial and super cavitation over the axisymmetric bodies using a boundary element method (BEM) based on potential by re-entrant jet model in closure zone of cavity. Re-entrant jet model is one of the closure models of cavity which is in agreement with physics of unsteady cavitating flows. To simulate the cavitation region, cavity and body surfaces are modeled with plane panels. Then, by applying the integral expression of Green theory, cavitation can be modeled by distributing the source rings on cavity surfaces and dipole rings on cavity/body surfaces. The length of the cavity and re-entrant jet are assumed to be fixed as initial predictions. The cavitation number and shape of cavity are the main results of this simulation. Furthermore, the pressure coefficient in partial cavitation and drag coefficient in super cavitation are predicted. The comparison between these results with experimental and analytical ones show that simulation of partial cavitation using BEM with re-entrant jet model modified the overshoot -that exists in results of BEM simulation without using re-entrant jet model- at the end of the cavity pressure coefficient. The implemented BEM method with re-entrant jet in super cavitation led to very good results in prediction of cavity shape and drag coefficient. The main advantage of this method is shortening the time of the desirable convergence.

In this research, a new continuous model for vibration analysis of a simply support cracked Euler Bernouli beam is proposed. The effect of the crack on the stress, strain and displacement field is modeled as a continuous disturbance function. The equation of motion and the corresponding boundary conditions for bending vibration of a cracked beam with an open edge crack, are derived by using Hu-Washizu variational principle. By employing the Galerkin method, the natural frequencies and mode shapes are obtained, and the influence of the crack parameters (crack depth and location) on the vibration behavior of the cracked beam is studied. The results have been validated by the experimental and theoretical dada reported in the literature. There is a good agreement between the results obtained through the proposed method and those obtained from the reported experimental data.

In this paper a two-dimensional axisymmetric numerical model with variable porosity for premixed methane/air combustion in porous media has been developed. For this purpose، multi step mechanisms such as GRI 1. 2، GRI. 2. 11، GRI. 3. 0، Skeletal and a mechanism with 17 species and 58 chemical reactions has been used and the effects of the mechanisms on the temperature profile، mass fraction of species and emission of pollutants have been investigated. This model solves the continuity، Navier Stokes، the gas and solid energy and the chemical species transport equations by using the finite volume method. The pressure and velocity have been coupled with the SIMPLE algorithm. The results showed that the combustion mechanisms have the same accuracy in predicting of temperature profile and mass fraction of species in order to the GRI. 1. 2، GRI. 2. 11، GRI. 3. 0 mechanisms are the same accuracy and also the skeletal and 17 species mechanisms are the same in predicting of temperature profile and mass fraction of species. Also the effects of increasing the inlet velocity and the volumetric heat transfer coefficient at combustion zone with variation porosity are investigated. The results showed that by increasing in inlet velocity، the peak temperature and the amount of NOx emission will be decreased and also by decreasing in volumetric heat transfer، the peak temperature will be decreased.

Alumina (Al2O3) with having excellent properties such as high hardness has disadvantages such as low fracture toughness and various types of additives such as nanoscale silicon carbide (SiC) powder can added to Al2O3 to improve physical and mechanical properties In this study، silicon carbide nanoparticles containing 2 5، 5، 7 5، 10 and 15 volumetric were added to Al2O3 powder containing 0 05 weight magnesium oxide (MgO) nanoparticles. The combined powder was ball milled in isopropanol using high purity tungsten carbide (WC) balls in a planetary ball mill The milled mixtures were dried and compacted by the cold press Sintering of the powder compacts was carried out the graphite furnace in argon atmosphere and employ hot press method at 1600، 1650، 1700 and 1750°C temperatures and constant pressure. The effect of raw materials composition and temperature of synthesis to the physical and mechanical properties and the results indicate significant increases in mechanical properties with addition nanoscale SiC powder.

Various volume fractions of vanadium carbide (VC) (5-20 vol%) was added to TaC- nano Gr composites produced by the pressure-less sintering at the sintering temperatures 2200 °C and 2300 °C. Physical and mechanical properties of the samples such as density, modulus elasticity, hardness and microstructure were studied. The results indicated increasing remarkable of the density, modulus elasticity and hardness. The density increased from 79 to 95.6% for the samples consisting 10 vol% and 20 vol%, respectively at the sintering temperature 2300 °C. Also, the hardness 23.22 GPa was obtained for the composition consisting 20 vol%, sintered at temperature 2200 °C. X-ray diffraction pattern analysis confirmed the formation solid solution between of TaC and VC.

Numerical model such as finite element analysis can be used for simulation of tillage process and movement of blade in soil. In this paper a non-linear 3-D finite element model for analyzing of wide tillage tool moving in soil was developed. It is assumed that soil is completely homogenous and modeled based on drucker-pragers elastic perfectly plastic model. Contact element was used for modeling of contact surface between soil and tool. By this model، horizontal and vertical forces needed for blade moving in soil were predicted and failure surfaces made by tillage operation، distribution of elastic and plastic stress in soil and displacement vectors of soil obtained. This model was compared with other models and showed a good agreement. Results showed that this method can be used for prediction of tillage operation.