فصلنامه مدل سازی در مهندسی
پیاپی 36 (بهار 1393)

This paper presents an optimal design of Fuzzy Power System Stabilizer (FPSS) using Harmony Search Algorithm (HSA) in multi-machine power system. The PID controller is one of the appropriate techniques to damp low frequency oscillation in nonlinear systems. The proposed controller is based on their rule base and membership function information. In conventional techniques the information is obtained by trial and error which needs too much time. So، finding the optimum value of rule base or membership function is really important. Accordingly، HAS is proposed in this paper to find the optimum value of rule base by considering the power system constrains and factors. This newly developed control strategy mixed the advantage of HSA and Fuzzy controller with simple structure while is easy to implement. Hence، the proposed controller is tested over the New England 10-unit 39-bus standard power system and 9 buses IEEE power system، under various system configurations and loading conditions، to illustrate the performance of the proposed method. The effectiveness of proposed controller is compared with other techniques. Eigenvalue analysis and nonlinear simulation results show the effectiveness of the proposed controller.

This paper presents the new method for separation of payload from launch vehicle. In this method، in addition to helical compression spring mechanism used in usual separation systems، two retro rockets mounted symmetrically on the second stage were used. To protect on-going stage from disturbance، the retro rocket performance was initiated a few millisecond after payload separation. Linear and rotational equations were solved using MATLAB and the numerical analysis was done by ADAMS. The comparison among analytic، numeric and test results showed that this method could be used as an appropriate tool for payload separating from spent body.

Appropriate estimation of the reliability index is important to evaluate the failure probability in structural reliability analysis. Therefore، some methods such as: HL-RF and the Gradient method have been developed and commonly used to determine the reliability index of structures. In this paper، a new method is proposed for determining of the reliability index which is formulated using the nonlinear conjugate gradient method. First، the new iterative method was presented and then، efficiency and robustness of the proposed method were investigated using several examples. Accurate convergence and iterations of the new iterative algorithm were compared with the previous methods such as: the Hasofer-Lind approach، Gradient method، stability transformation method (STM) and Mont Carlo simulation، for examples. The results indicate that the proposed method is more robust than the old first order reliability methods i. e. the HL-RF and Gradient method. So that، this old methods were not converged for some examples but، the proposed method was converged for all problems. On the other hand، the results of proposed method are accurate and as similar as the STM but more efficient and was converged with less number of iterations in comparison with the STM.

In this research، in order to study the surface melting phenomena of metals for different crystallographic planes، we present a thermodynamic model on the example of copper. This model includes interfacial energies of thin layers of solid with gas، surface of melted layers with gas and interface between solid and liquid of metals. Moreover، the effect of surface energy of different planes، number of layers and crystallographic orientation of metallic layers on melting transformation was considered. Therefore، by applying the effect of energy and orientation of surface layers on the variation of Gibbs free energy value of solid phase (thin films) and molten metal، surface melting point for solid thin films has been evaluated. Furthermore the effect of surface energies of (100)، (110) and (111) planes has been investigated. The results of our calculations show good agreement with experimental results and other theoretical predictions in literature. By using this model we are also able to calculate and analyze the melting phenomena of metallic thin films layer by layer from some nm scale up to thick (bulk metallic) cases.

In this paper، a six-phase switched reluctance motor for applications such as vertical transportation in a linear elevator is considered. The driving force ripple in a linear switched reluctance is high and the use of these machines in low speeds are accompanying with high speed ripple. In fact، the proposed construction consists of the combination of two 3-phase series linear switched reluctance motors which make a 6-phase structure in order to provide a better torque specification. In this paper we tried to reach a better speed profile in low speeds (0. 5 m/s up to 1 m/s) which is normally the case in elevators. The simulation results show the superiority of the proposed structure over conventional structure of the switched reluctance motors. The proposed structure reduces the speed ripple from 16. 8% to 1. 24% at 0. 5 m/s. This result shows the high performance improvement of the 6-phase structure of the motor.

One of the most important problems related to performing railway on soft subgrade، is the problem of bearing capacity and embankment controlling settlement relied on these subgrades. The most provided concentrated solutions to solve this problem on improvement methods is based on ground soil such as compaction، injection، preloading، performing stone and cement columns. One of the new methods in which used in building way and roadway based on soft embankment used successfully، it can be mention to usage of light material of poly styrene or geofoam instead of soil in embankment. The literature show that till now there is no wide research about the possibility of using this material to build the embankment of Railway in soft subgrades. Therefore this research، focusing on two effective index on usage of Railway ballast on serviceability such as bearing and settlement embankmentand. To this end، with developing behavior model (hardening) specially geofoam material in finite difference software in FLAC2D، to investigated behavior of this material on ballast Railway embankment effected by static loading model of LM71and also Harmonic dynamic loading and as index out put، stability of embankment with difference heights and also the counter of their settlement is investigated، in additional to، by out puts from numerical model، using multivariable regression، the relation closed form analysis between settlement of static and dynamic with ductility، cohesion of ground، and ductility of geofoam parameters، embankment height and train axle load is derived. At final part، as for as the limitation in general settlement in rail way embankment، usage allowable domain of this material is drived.

Due to wide spread applications of adhesive T-joints in various industries، it necessitates a review of properties and strength of their fracture modes under static and dynamic loadings. By defining the ability of failure in the joint material and fracture of adhesive in the numerical model، fracture modes of sandwich T-joints have been investigated. This paper presents numerical results on the performance of sandwich T-joints subjected to static tensile and static transverse loading. The results of available experiments in the literature have been used to validate the detailed numerical models capable of simulating the damage processes observed. In general، the failure load predicted by the finite element (FE) analysis is within 5% of the experimental results.