فصلنامه مدل سازی در مهندسی
پیاپی 31 (زمستان 1391)

In this paper، the seismic behavior of steel dual frames with large-scaled concentrically bracings have been evaluated at special ductility level، and then the ductility reduction factor and the over-strength reduction factor for this type of structural system have been calculated. To do so، multi-story buildings with large scaled X- bracing form were considered. Then، the push-over static analysis، non-linear incremental dynamic analysis and linear dynamic analysis were performed to determine the effect of building height on the behavior factor. This study shows that the seismic behavior، the ductility reduction factor and the over-strength reduction factor decrease when the number stories of building increase. But the decrease rate in ductility reduction factor is more than the decrease rate in over-strength reduction factor. Finally، the behavior factor for each building has been calculated separately and، mean values، 5. 0 and 7. 5 have been recommended for limit-states design and permissible (or Allowable) stress approaches، respectively.

Concrete Filled Double Steel Tubular columns are from the concrete filled tubular columns’ (CFT) family formed of two steel walls with square or circular sections. They are assembled concentrically and the space between the two walls is filled with concrete. Since CFDST columns do not need concrete casting and are easy to assemble and carried and are well protected against damages، they have some advantages over other columns. They are generally used in offshore structures، piers of the bridges with large openings، and nuclear canals in power plants. They have recently been used in the frames of high rising buildings in Japan as well. The present study attempts to investigate the behavior of these new columns under axial pressure forces and bending moment through finite element method and ABAQUS 6. 10 program. In order to make sure the finite element modeling is correct، the results of numerical analyses of CFT columns were compared to the laboratory results and the results were satisfying. The present study also investigated the effects different parameters of column thinness، the cross section’s geometric features، and characteristics of strength of materials and found that these columns’ capacity for load is more than CFT columns and are lighter and more economical compared to them.

In this paper، the combustion chamber of SGT600 gas turbine with 18 similar burners is numerically simulated from compressor outlet to turbine inlet with ANSYS CFX software to investigate the flow field and combustion properties. The three-dimensional simulation is carried out with a combustion model based on a turbulent flame speed closure (TFC) model for the 2-equation turbulence model، k-ε، in SGT600. Excellent agreement between the results of numerical analyses and experimental data was observed so that the error rate in calculated outlet temperature was 0. 1 percent. The numerical model predicted the temperature at the inner surface of the heat shield about 1390˚K. Also the maximum temperature of liner was 1285˚K. The temperature reduction shows the effects of cooling flow. Due to results، the CFD model enables the prediction of the temperature of heat shield and combustor’s wall and could determine the critical points of operation.

Among the effects of strain hardening، strain-rate hardening، and temperature softening، it has been long argued about which effect is predominant in governing the material flow stress in machining. In addition، there is a belief that the temperature and strain rate counterbalance the effect of each other; therefore there is no need to consider temperature softening and strain-rate hardening while modeling material flow behavior. It means that similar to metal forming processes، the flow stress will be the function of strain only; whereas، the magnitudes of strain-rates، and temperatures involved in machining are several orders higher than those generated in forming processes. On the other hand، measuring material flow stress under high strain-rates and temperatures in controllable conditions needs especial costly equipments; therefore it is worth to discuss whether the stress-strain data obtained in the simple standard tensile test can be used in machining. This paper discusses some requirements for mathematical form of material models applicable to describe the yield surface in simulation of machining. As a result the first predominant factor governing the material flow stress is strain hardening. It is demonstrated that strain-rate and temperature do not counterbalance the effects of each other in all machining zone. Therefore implemented material model in machining should include both considerable effects in a suitable way. It is demonstrated that friction is an important heat source independent to plastic dissipated energy; therefore its contribution in material flow should be considered in convenient method via implementing temperature parameter in mathematical equation of material model. In addition، the velocity of material deformation is another parameter governing the material flow behavior in machining. It is indicated that the strain rate parameter plays the role of suitable corresponding field variable; therefore it should be appeared in mathematical form of material model to include related effects.

In this paper an analytical model considering die arc and variable thickness for dome portion of the deformed sheet is proposed for axisymmetric sheet hydroforming process. The aim of the analysis is to calculate the thickness variation along the final shape and the required forming pressure in two cases of high and low blank holder forces. For analysis، first a mathematical relation for the thickness distribution in dome portion is proposed. Then، by applying the incompressibility law in plastic deformation the thickness distribution is calculated. After that، with the calculated thickness variation، the strains and the stresses all over the deformed sheet and the strain energy are determined. By equating the total strain energy with the external work، the forming pressure is calculated. The predictions of the proposed analytical model are validated by comparing them with the results available in the literature and they are found to be in good agreement.

Friction is one of the most effective parameters in the forging process، which its analysis is needed. Efficient lubrication is best way in controlling friction and wear in this process. Because of the low cost of lubrication in comparison with the reductions that it creates in cost، testing the lubricants is important in forging. One of the best ways of testing the lubricant is the ring compression test، which with finite element method، the friction coefficient is calculated. In this research the effect of mixed graphite in water and also the effect of the amount of graphite in the lubricant in the reduction of friction was calculated using the ring test. Effect of the lubricant was easily recognized in the final dimensions of the specimens in result of the reduced friction; this was seen in the changing of sticking friction to sliding friction. Nevertheless، the changes in the amount of graphite in water did not have a great effect on the reduction of the friction coefficient. Furthermore، after viewing differences in the friction coefficients calculated by the means of finite element analysis ABAQUS software and the frictional calibration curves، it was concluded that the sole calibration curves could not be used for hot working conditions.

The Purpose of this paper is to study the effective parameters on the performance of the eddy interaction model (EIM) for droplet dispersion and deposition in a wave-plate mist eliminator. The gas droplet flow inside the eliminator has been studied using Eulerian-Lagrangian method. Reynolds stress model (RSM) with enhanced wall treatment has been applied for gas flow simulations and the gas flow field inside the bends of the eliminator has been studied. The results show that eddy lifetime constant (CL) should be changed for different gas flow velocities and the value of this parameter changes with gas flow Reynolds number. For validation of numerical simulations، the results are compared with available experimental and numerical data in the literature which shows the good agreement. Finally، effects of changes in number of bends on pressure drop and removal efficiency of a wave-plate mist eliminator has been investigated.