MODARES TECHNICAL AND ENGINEERING
Year:2002 | Volume:- | Issue:6|Papers Count:10

Following the studies of Moen et at. [1986], Shepherd [1986], and Lee [1991, 1993], regarding the dependence of some dynamic parameters of detonation on the "detonation instability," the present work has been performed to investigate the role of "detonations instability" on the direct initiation problem. This study has been carried out by numerical integration of the one-dimensional Euler equations in aplanar geometry. For the chemical kinetics model, asingle-step Arrhenius law was assumed. The present results show that initiation process has the same mechanism for both stable and unstable detonations. It is shown that the ZND induction length is not an appropriate length scale to specify the highly transient processes in the so-called quasi-steady period. However, for unstable detonations when the activation energy is very high, no unique value can be defined for the critical initiation energy. It was found that analytical models based on the Zeldovich criterion cannot predict the critical initiation energy over the full range of activation energies considered in this study. This is because the Zeldovich criterion does not consider any dynamic effects during the quasi steady period.

The existence of thermo-mechanical stresses, due to the frequent start-ups and shutdowns of gas turbines. combined with high working temperatures may cause creep and fatigue failure of the blades. This paper describes a fracture-mechanics life assessment of a gas turbine blade. Initially, the distributions of thermal and mechanical stresses were obtained by using the finite element method. Accordingly; the crack modeling was performed in a high stress region at the suction side surface of the blade. Several crack growth increments were observed and the related crack tip parameters were calculated. Finally; the creep-fatigue crack growth in each cycle was calculated and the total number of start-stop cycles was determined.

Temperature measurement and control for large steam turbines were considered. A Turbomax system was observed. Due to being depended to the simulated measurement equipment and complexity of the design, theoretical methods were suggested. Theoretical methods discussed in this research work are based on modeling the temperature drop in a turbine shaft by using calculation methods. These methods were conducted and the results were compared with the values obtained from a power plant steam turbines, equipped with turbomax. The agreement between the results (theoretical and turbomax) was very good. It was concluded that theoretical methods which are suggested here could be used as a valuable procedure to substitute turbomax system.

Joint compliance in Robots structure causes less accuracy and lower efficiency. For higher accuracy and high speed operation the compliance should be limited in a way to be compatible with dynamic forces. In this paper the vibrations of robots structure have been studied based on two assumptions.(a): The whole structure compliance is considered to be located at the joints.(b): A continuous path of robot has been already studied and the displacements as well as velocities increments are given in small values.The model has eight degree of freedom. The kinetic and potential energy equations have been used along with variational method. The equations of motion are obtained by using Lagrange's equations. These equations are non-linear and coupled. An example of the results has been discussed.

Penetration analysis of long rod projectiles into target plates has been studied by investigators of impact mechanics in the past decades. A basic theory in this field, developed by Alekseevskii and Tate. has been used as a standard reference model for the past 30 years. Their model has been very useful in ballistic penetration analysis of long rods into semi-infinite targets.In this paper. a brief explanation of the model is presented and then the model is extended to multi-layer targets. Comparison between results of the extended model and those of other investigators shows good agreement. Besides, a parametric study has been performed and also, application of the extended model in designing multi-layer armors against long rod projectiles is presented.

A mathematical model for the analysis of pulsatile blood flow in deformable elastic arteries is presented and solved for the duration of one pulse by an analytical numerical method. The purpose of solving the obtained initial-boundary value problem is to find out the fluid velocity profile. flow rate. tensile stress at the artery wall, pressure and artery radius. The blood is assumed as a homogenous. incompressible fluid. Due to the radius of artery, the blood is assumed as a Newtonian fluid. The flow is axisymmetric, two dimensional. laminar and time dependent, the artery wall is taken as elastic. incompressible and homogeneous material and the internal pressure is a function of artery radius.Governing equations are: Navier-Stoke's equations for the radial and axial motions, continuity equation. and an auxiliary equation which relates the internal pressure and artery radius. The functions chosen for boundary and initial conditions are so that a periodic (pulsatile) solution could he obtained.Another feature of the solution is to employ a finite polynomial as a candidate for axial velocity profile and then solving the obtained differential equation system by discreting them in the axial direction and using the Range-Kutta of order 4 method to integrate the equation with respect to the time. The results of the present study are proved to be in good agreement with the results reported in the literature of physiological and theoretical studies. The present study applies a more accurate mathematical model with less simplifying assumptions; therefore, it offers solutions in more convenient way.

In order to study the dynamic behavior of engines a software was developed; the actual pressure and also the friction forces were considered in the governing equations. Considering the torque as the input, the crank speed and acceleration were calculated. The cylinder gas pressure was detected with a photo sensor installed in the spark plug. The predicted and actual crank velocities were compared and the differences were explained.

Computer aided simulation of three-dimensional deformation behavior of sheet metal during cold roll forming process is described In this simulation, the shape of deformed sheet metal is expressed by an approximated mathematical equation called "Shape Function". By means of this shape function, stress and strain distribution and dissipated energy in the deformed sheet metal can be calculated by a computerized incremental procedure using Hooks law in the elastic range and Levy - Mises equations in the plastic range. The optimum shape of deformed sheet can then be predicted by minimization of the calculated energy.By the simulation, it was possible to explain various characteristics of the cold roll forming process. The authors believe that using Levy - Mises equations simplifies the previously proposed methods and at the same time the results seem to be quite encouraging.