مجله مکانیک سازه ها و شاره ها
سال چهارم شماره 2 (تابستان 1393)

This paper presents a novel robust control approach for robotic manipulators by using a neural network estimator and the voltage control strategy. The proposed estimator is an adaptive multi-layer neural network that its parameters are regulated by a propagation algorithm. The novelty of the proposed design is the use of voltage control strategy that is different with the common torque control strategy. As an advantage، it is free from the dynamical model of the robotic manipulator. Compared with the conventional robust control، it does not require the determination of the bounds of uncertainty and bounding functions. In addition، the problem of lacking data for estimation of uncertainty is solved. In fact، the back-propagation algorithm uses the tracking error and its derivative instead of the estimation error. Stability of the control system is proven by analysis. The efficiency of the proposed control approach and the estimator of uncertainty are shown by simulation of the SCARA robot driven by permanent magnet dc motors. The control performance of the proposed control is compared with a control approach which uses a fuzzy system to estimate the uncertainty. The simulation results show the superiority of the proposed control in tracking control، set point control and estimation of uncertainty.

Design of structures in aerospace needs minimum weight. Cylindrical thin-walled pressure vessels are one of the most functional among these structures and one of the main ways to reduce weight of these structures is using stiffener rings. In this research in order to find optimum structure weight، internal stiffener ring dimensions were optimized with genetic algorithm coupled with a neural network. In other hand، for joining the stiffener rings to vessel، welding is a conventional way. However، this process can produce welding residual stresses and distortions، which add to external loads and cause structure failure. So in order to predict and control these effects، a comprehensive simulation tool was developed to predict distortions and residual stresses generated in double sided fillet-welded internal stiffener ring on a cylindrical pressure vessel. Results showed that the T-stiffener ring have the best results and the optimum ring dimensions were determined and reported. In addition، magnitude of residual stresses was near to half of base metal yield strength and in weld region، hoop residual stress was tensile and axial residual stress was tensile on inner surface and compressive on the outer surface of vessel. Moreover Effect of tack welds were appeared on residual stress distribution like tiny peaks and have more effect on hoop residual stress distribution on stiffener ring position.

Collapse fragility curves represent exceedance probability of collapse in structures while being excited by strong ground motions of earthquakes. Epistemic uncertainty sources have considerable effects on collapse fragility curves. In this paper، effects of epistemic uncertainties due to variation of modified Ibarra-Krawinkler moment-rotation parameters of steel structures are involved in collapse fragility curves applying Monte Carlo simulation based on Artificial Neural Networks (ANNs). To train the networks، input data is obtained by limited numbers of simulations of modeling parameters based on their probability distributions and output data is considered as resultant means and standard deviations of collapse fragility curves. Two two-layered artificial neural networks are trained and validated by obtained data. Monte Carlo simulation is implemented through application of trained neural networks and resultant collapse fragility curves are derived. Efficiency of the proposed method is demonstrated by comparing with response surface based Monte Carlo method. Prediction errors are reduced 22% and 2% applying ANN-based Monte Carlo simulations for mean and standard deviation of collapse fragility curves، respectively.

Dampers can be implemented into the structure in order to dissipate energy and decrease the damages. Damages are much better distributed in the damped frames. In this paper، fluid viscous dampers (FVDs) will be used. There are many design procedures for this kind of dampers. In this study، Performance-Based Plastic Design (PBPD) method will be utilized to achieve the enhanced response of structures to earthquake. The required formulas are derived for the design process. In order to validate the proposed technique، four steel moment frames with and without viscous dampers will be designed. After a series of nonlinear dynamic analysis، the performance objective of the structure can be well verified. Dampers can be implemented into the structure in order to dissipate energy and decrease the damages. Damages are much better distributed in the damped frames. In this paper، fluid viscous dampers (FVDs) will be used. There are many design procedures for this kind of dampers. In this study، Performance-Based Plastic Design (PBPD) method will be utilized to achieve the enhanced response of structures to earthquake. The required formulas are derived for the design process. In order to validate the proposed technique، four steel moment frames with and without viscous dampers will be designed. After a series of nonlinear dynamic analysis، the performance objective of the structure can be well verified.

Two thixoforming process parameters like billet temperature and die profile were experimentally investigated on a commercial 6061 Aluminum alloy which is typically used in aerospace industries. The results show globular microstructure can be achieved using semisolid forming. The optimum semisolid material temperature was obtained in 620 °C by observing the microstructural and mechanical properties test results. It is important to notice that obtained results are at a condition that the post heat treatment process was not performed for thixoextruded parts but high elongation value was obtained at a suitable temperature of the semisolid material. Microstructural evaluation revealed that there is not important difference between specimens microstructure which were thixoformed at conic and curve dies، but according to tensile test result، it was exposed that thixoformed sample which was extruded by curve die has higher yield strenght and ultimate tensile strenght with higher elongation value and better ductility in comparison to another one because of higher residual stress resulted higher stress concentration in the conical die.

In this paper، We introduce a new method for the stability of discrete-time two-dimensional systems defined by the Roesser model، at first we introduce discrete-time two-dimensional linear systems described by Rosser model. Then، with using of this property that stability of discrete-time two-dimensional linear systems has a similar manner with stability of discrete time linear systems، we study stability of Rosser discrete-time two-dimensional systems. As for the stability of discrete time systems it is necessary to place all of its eigenvalues in unit circle. We use the partial eigenvalues assignment method for replace the desired eigenvalues instead eigenvalues of the open loop system which are outside the unit circle. For solving this problem، with using of partial Schur decomposition method، we decompose big matrix A to smaller matrices. Then with applying similarity transitions method for linear control systems، we allocate the desired spectrum to the system، so system will be stable. Finally، an illustrative example is presented.

Wing in ground effect are suitable vehicle in military industries and marine transportations that can transport with sophisticated aerodynamic characteristics and appropriate speed. Using numerical simulation can reduce the costs and save the time for investigating wing''s aerodynamic characteristics، that possesses a more acceptable precision in comparison with experimental data. In this study، the results obtained from the experimental examination conducted on Naca6409 are used to validate the simulation and the best turbulence model with an appropriate number of mesh elements has been chosen. Also، the effect of ground effect and the angle of attack on important aerodynamic parameters have been calculated and static stability of these wings have been determined by investigating the effect of moving and fix wall on aerodynamic characteristics of WIG. Finally، with changes in wing''s geometry factors such as twist angle، dihedral angle، sweep angle and taper ratio، the effect of these factors have been examined. As result sweep has not used for stability and also positive twist angle has used to decrease drag and delay in stall and taper ratio with increasing in aerodynamic characteristics، is better in trailing state.

The MHD equations solution for small plasma Beta using characteristics-splitting schemes which have low numerical dissipation is frequently diverged. Simultaneous increasing of magnetic energy (due to high discharge current) and kinetic energy (due to strong gas-dynamic expansion) leads to decreasing of internal energy and finally the pressure value is negative near the electrodes tip. In this research، to obtain a stable solution، the HLLE approximate Riemann solver has been used. This method can produce necessary numerical dissipation to prevent entropy violation. To achieve a high order accurate solution، new modification of MUSCL technique has been employed. This method is called OMUSCL2 technique which has lower dispersion and dissipation errors. For simulation of non-equilibrium ionization mechanism، a 7-species chemistry model has been implemented. Numerical results of a lab-scale thruster are presented، whereby comparison with other experimental and numerical data shows good agreement between the predicted and measured enclosed current and thrust.

In this study at first، the transient performance of an solar absorption cooling system is simulated for an educational place in Tehran with a 1600 m^2 area by the commercial TRANSYS software including flat plate collector، stratification storage tank، natural gas fired auxiliary boiler، cooling tower، fan coil and etc، then The effects of different parameters such as collector’s area and slope، tank’s volume and auxiliary heater’s set temperature on the system performance from energetic and economic points of view are also studied and in the final step Two-objective optimization of solar absorption cooling system based on the solar fraction and life cycle saving as in objective functions is carried out by means of genetic algorithms and GMDH neural network. Simulation results of cooling system show that the best temperature of heater has to be set as minimum in the operating range. Moreover، with changing in collector area and tank volume simultaneously، it is observed that maximum solar fraction is captured with maximum collector area and maximum tank volume، and also maximum life cycle saving is captured with minimum collector area and minimum tank volume in present range. The final results indicate that the energy and economical performance stand in opposite behavior

In curved arteries، since geometry and hemodynamics are diffrent from straight ones، balanced oxygen distribution on the wall of artery will be disrupted. This disorder has a significant contribution in genesis and development of Atherosclerosis plaques. In this study، the blood oxygen mass transport in a 3D curved artery and its wall were investigated. In order to find the distribution of oxygen concenteration along the vessel، oxygen mass transport equations and their boundary conditions were solved، numerically. The oxygen carried by Hemoglobin and oxygen consumed in the avascular region were considered as two important physiological parameters. Results indicated that، the presence of centrifugal forces and formation of secondary flows in curved geometries، cause a considerable decrease in oxygen mass transport to the inner bend wall in comparison to outer one. Furthermore، with increase in Reynolds number and decrease in curvature radius، secondary flows become more powerful which cause an enhancement in the flow mixing and as a result reduce the oxygen concentration difference between the outer and the inner bend walls. These results help to predict the potential region for formation of Atherosclerosis plaques.

In this paper، numerical simulation of the gas flow is carried out to investigate the effects of changes in split line location on the amplification factor، side force and thrust vector magnitude of the moveable nozzle with supersonic split line. The comparisons between the numerical results and experimental data for two different deviation angles show that RNG k-ε turbulence model with enhanced wall treatment gives better results than other turbulence models. By using the mentioned turbulence model، it is observed that for greater distances between the split line location and the rotation center of the movable part، the amplification factor، side force and thrust vector magnitude of the nozzle will decrease. For greater distances، the oblique shock will occure at the section in which the flow has higher Mach number. Therefore the change in normal velocity of the flow across the oblique shock will be more which results in more reduction in exhaust flow momentum and the amplication factor. The results show that by increasing the distance between split line location and the rotation center of moveable part، the deviation angle of the thrust vector magnitude will be lower than nozzle rotation angle. Effects of surface roughness is also investigated.

The purpose of this study is performance analysis of a micro gas turbine power supply unit including an atmospheric solid oxide fuel cell from thermo-economic viewpoint. Since the fuel cell is the main source of power generation in hybrid systems، in this study، complete electrochemical، thermal and thermodynamic calculations are performed to obtain more accurate results; and unlike most studies، the cell temperature is not assumed constant. The performance analysis of the hybrid system shows that increasing the pressure and air to fuel ratio، causes to loss of electrical efficiency and increase in the electricity price because of reduction in cell and turbine inlet gas temperatures. Also، the economical investigation results confirms that the system electricity price in an optimum case is about 12. 2 cents based on lazaretto method and 19. 5 cents based on TRR method. the installation and preparation cost for hybrid system is about 500- 600 dollar per kilowatt.

Mixed convection of free and force convection heat transfer in an inclined lid driven two dimensional enclosure is studied by using Lattice Boltzmann method (LBM) in different values of inclination angle، Richardson (Ri) and Prandtl numbers (Pr). At present case، the velocity components will be affected by inclination angle، force and free convection movements. To do this، using LBM equations are modified. Comparing present results with those of other available ones implies appropriate accuracy. Results are shown as velocity، temperature and Nusselt number profiles and contours of isotherms and streamlines. It is seen that more Pr corresponds to more heat transfer rate especially at higher values of inclination angle and Ri. As a result، to estimate the averaged Nusselt number، a correlation based on Pr، Ri and inclination angle is presented. Moreover، it is seen that the averaged Nusselt number at the upper limit of the considered range of inclination angle، Richardson and Prandtl numbers variability increases by a factor of 7.

Delamination is a typical fracture mechanism in fiber reinforced composite laminates occurs due to low interlaminar strength. Some well-known sources of delamination under different loadings are free edges، cut out، low-velocity impact and fabrication defects. These damages may considerably reduce the global stiffnessand strength and then leads to a catastrophic structure failure. In this study، the influence of stacking sequence on mode I interlaminar fracture toughness of E-glass/epoxy laminated composites is experimentally investigated. On the other hand، delamination resistance and fiber bridging are investigated. Double cantilever beam (DCB) specimens with woven roving of glass fiber of stacking sequences; [0º/90º] 4s، [90º/0º] 4s and [±45º] 4s with an initial crack length are used. Load- displacement curve is obtained in DCB test and then interlaminar fracture toughness (GI) is estimated with R-curve. Results showed that interlaminar fracture toughness increased as the stacking sequence was changed from 0º to ±45º and ±45º to 90º and fiber bridging is more stable. Keywords: Mode I interlaminar fracture toughness، stacking sequence، delamination، composite panels.