Scientia Iranica
Volume:11 Issue: 4, 2005

This paper describes a theoretical study of the self-condensation of non-equilibrium wet steam during supersonic expansion in a de Laval nozzle. Nucleation and droplet growth theories have been combined with the equations of one-dimensional gas dynamics and the system integrated numerically on a computer. The method of solution has been applied to predict the self-condensation zone in the supersonic region of a converging-diverging nozzle and it has been shown that the condensation of vapour in divergent part of the nozzle causes a pressure rise in flow. The results have been confirmed experimentally and, on the strength of this agreement, the treatment has been extended to predict the effects of friction factor, stagnation pressure and temperature on the location of the condensation shock. The work can be extended to predict the formation and growth of liquid droplets in steam turbines.

$\phi$A two-stage nonlinear programming penalty method for discrete optimization of pipe networks is presented in this paper. The problem of pipe network optimization is formulated as an unconstrained optimization problem via use of an iterative penalty method, which is then solved to get the continuous solution for the pipe diameters. In the second stage, a second optimization problem is defined to get the discrete solution starting from the already available continuous solution as a good initial guess. The search space for the discrete diameters is restricted to the upper and lower diameter limits of the optimal continuous solution. An all-purpose optimization toll, DOT, is used in both stages to obtain the solutions. The method is shown to be capable of producing results comparable to the existing algorithms with much less computational time. The method is used to find the optimal solution to some of the benchmark pipe networks and the results are presented. The results obtained for the networks consisting of pipes are encouraging. Further research is underway to extend the method for the optimization of pumped networks.

The stability of CJ detonations has been investigated numerically with a two-step kinetics model. The reaction model consists of a non-heat release induction step, followed by an exothermic step. Both steps are governed by the Arrhenius kinetics model. The effect of activation energies associated with these steps on the detonation front behavior has been studied. This study was arranged in two stages. At each stage, one of the activation energies was kept constant and the other one was changed. In the steady detonation structure, the activation energies of the first and second steps (Ea_1, Ea_2) control the induction and reaction lengths, respectively. Increasing Ea_1 (for a fixed Ea_2) increases induction length and destabilizes a detonation, the same behavior as a one-step model. Increasing Ea_2 first increases reaction length and has a stabilizing effect (i.e., the amplitude of oscillation decreases). Further increasing Ea_2 has a destabilizing effect. The present study shows that the ratio of the reaction length to the induction length characterizes general features of detonation stability."

In this paper, a new analytical method is presented that can be used to determine the behavior of a particular steel beam-to-column extended end plate connection in linear and nonlinear regions. A common means of forming a rigid joint between a universal beam and column is to weld an end plate to the beam end and then to bolt it to the column. A physically based analytical method that can predict the behavior of bolted and extended end plate eave connections, using the connection dimensions as input, is resented. This article demonstrates an analytical procedure for the stablishment of elastic and plastic parts of the M-\theta curve of this form of connection. An analytical method is proposed for the extended end plate joints having four bolts in the tension region and without any stiffened plate. However, the presented technique can also properly be extended to other types of this form of connection. An extensive approach to estimate the plastic stiffness of the connection has also been performed. Comparison is made on a series of test results for a range of bolted end plate moment connections and good agreement is achieved. Furthermore, the authors believe that the method presented shall efficiently serve design engineers in real design conditions.

In this paper, the development of a resistivity probe for measurement of air concentration and bubble count in high-speed air water flow is described. One advantage of this type of probe is in its ability for real time measurement. The sampling frequency of the new probe is increased up to 250 kHz with 72 seconds sampling time. Polarization of the probe tip was noticed in this research work and the probe circuit was designed to avoid it. Sensitive microampere meters were installed on the probe to detect possible weak leakage currents. Electronic tests were performed to check the probe circuit, as well as more than 30 laboratory tests in air-water flow to set the threshold voltage of the probe circuit and to test its accuracy.

The choice of technology to transport passengers in large metropolitan areas is an important issue everywhere. There are many factors involved in this choice. This paper deals with the possibility of the objective use of available information in the analysis of the suitability of a rail public transport system for a city. A database has been made from publications on public city transportation and country level information. Logit models of choice have been calibrated by the maximum likelihood and nonlinear least square methods based on the acquired information. Each city is treated as an "individual", choosing rail or non-rail modes for its trips. Only cities with a population of more than one million have been included in the analysis to ensure the instigation of mode diversification in these cities. Selected models have been validated and then used to suggest the desirability of a rail public transport mode in some sample cities, according to world practice

In this paper, a theoretical analysis of the combined heat and mass transfer over cooled horizontal tubes is presented. 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 absorber tube bundle. The effects of important parameters, such as solution flow rate absorber pressure and tube radius, are discussed on the overall heat and mass transfer for a tube and tube bundle.

In this paper, a three-dimensional code is developed to solve turbulent supersonic flows over a blunt-nose-cylinder at 32\degree\ and 44\degree\ angles of attack. The method used is an explicit finite-volume Runge-Kutta time stepping model for unsteady, three-dimensional, full, Navier-Stokes equations. This model can handle arbitrary geometries by using general coordinate transformations. The flowfields under consideration contain extensive regions of crossflow separation. The Reynolds shear stress terms are modeled algebraically with modifications to correct the turbulent length and velocity scales in separated regions. Calculations performed using the developed code require a computational memory accessible on most personal computers. Numerical results are in good agreement with experimental measurements. Comparisons of turbulent flow with graphical visualization by means of helicity revealed that the Runge-Kutta time stepping algorithm conserves symmetry at high angles of attack.

Wind generated sea states are more accurately modeled by short-crested wave fields. Whether or not these short-crested waves can induce larger response amplitudes in floating offshore structures is of great concern to offshore engineers. In this paper, the hydrodynamics of a moored semisubmersible in short-crested wave fields is investigated. Morison-based motion equations with nonlinear damping terms are used to analyze the dynamic behavior of the structure. A generalized three-parameter short-crested wave field is introduced as the incident wave. The effect of transverse phase lag, wave propagation angle and different ratios of in-line and transverse wave numbers are studied on the force and response amplitudes of the floating structure. Finally, the results are compared to that of the long-crested wave fields and critical cases are specified.

In this paper, a new method for performance based earthquake analysis and design has been introduced. In this method, the structure is subjected to accelerograms that impose increasing dynamic demand on the structure with time. Specified damage indexes are monitored up to the collapse level or other performance limit that defines the endurance limit point for the structure. Also, a method for generating standard intensifying accelerograms has been described. Three accelerograms have been generated using this method. Furthermore, the concept of Endurance Time has been described by applying these accelerograms to single and multi degree of freedom linear systems. The application of this method for analysis of complex nonlinear systems has been explained. Endurance Time method provides a uniform approach to seismic analysis and design of complex structures that can be applied in numerical and experimental investigations

Under strong laser radiation action on solids and liquids, all the thermophysical parameters which characterize these media become dependent on the medium temperature. Assuming for the coefficients \lambda(T) and c\rho(T) to be linearly changed by the temperature, the non-linear inverse problem of heat conductivity is resolved. The problem is resolved for two cases: 1) For solids and fixed liquids and 2) For heat conductivity with liquid laminar convection due to laser radiation action. The parameter \gamma, describing the gradient of coefficient of the heat conductivity, is calculated and the influence of liquid convection on \gamma is estimated. It is considered how the Bi number affects the final result. A developed algorithm for solving inverse problem may be used for finding the exact analytical solution of some problems of diffusion and fluid mechanics.

In this paper, the mixed mode experimental results of 24 notched beams of concrete with various notch depths and locations are reported. The test results for conventional critical stress intensity factors and crack trajectories are demonstrated. It is noted that with the larger thickness, which results in conditions closer to plane strain, the crack path can be better predicted by linear elastic fracture mechanics criteria. The effects of applied load and specimen weight on the fracture are considered with the use of separate stress intensity factors. It is observed that the final failure angles, based on the crack path''s intersection point with the beam''s top side, are better predicted than the crack initiation angles, from the maximum principal stress criterion. Conventional mixed mode fracture toughness increases with an increase in the mode II to mode~I stress intensity factors ratio.

In this paper, the effect of morphology, size and volume fraction of the most important microstructural constituents of cast A356 aluminum alloy on its main mechanical properties has been studied. The investigated variables consist of Dendrite Arm Spacing (DAS) of \alpha aluminum phases, spheroicity of silicon particles in eutectic areas and 2-D micro porosity areas. The variations of Quality Index (Qi) with DAS and spheroicity of eutectic silicon particles follow a linear relationship. For the micro-porosity area of a polished section of the studied samples, two linear relationships were found, one for values less than 1.25% and another for higher values. As the basis for a quantitative analysis of the microstructure, some relationships have been proposed to estimate the quality index, which is, for the most part, a compromising result of three mentioned components of the microstructure.

In this work, the production and properties of Al 6061/SiC composites, made using a squeeze casting method, were investigated. SiC preforms were manufactured by mixing SiC powder, having a 16 and 22 \mu m particle size, with colloidal silica as a binder. 6061 Al melt was squeeze cast into the pores of the SiC preform to manufacture a DRA composite containing 30v/o reinforcement. The aging behavior, tensile properties and fracture mechanism of the cast material were studied. The results show that higher hardness, yield strength, tensile strength and Young''s modulus can be obtained by the addition of SiC particles to 6061 Al alloy, whereas tensile elongation decreases. This is mainly caused by a thermal mismatch between the metal matrix and the reinforcement, which leads to a lower grain size of the matrix with more dislocation density. It was also found that the precipitation kinetics of GP zones in the composite material was accelerated, owing to the heterogeneous nucleation capability of metastable phases on the SiC particles. Decreasing the SiC particle size resulted in better mechanical properties and a faster aging response. Nevertheless, the decohesion of the interface between the metal matrix and SiC particles led to the formation of voids which, subsequently, coalesced to generate the ductile rupture of the metal matrix.

The present study is undertaken to calculate the rate of change of pressure and residual air mass in die casting for vacuum venting under choked flow conditions. In these calculations, the influence of friction factor, due to roughness and vent air velocity change through the Mach number, has been taken into account. The results show that there is a critical area ratio below which the pressure and vent inlet Mach number increase with time and above which decrease with time. In addition, for an area ratio less than the critical area, the rate of change of residual air mass seems to be more changed at the late stages than at the early stages of the filling time. The picture is reversed for a larger area ratio. This critical area ratio depends on vent area, filling time, evacuated volume, the initial pressure and temperature of the air in the die cavity