نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 8 (آبان 1392)

The current article presents an analytical approach، for determining the natural frequencies of a rotating cracked Euler–Bernoulli beam with a varying transverse cross-section، using the so-called differential transform method (DTM). First، the natural frequencies of the beam are obtained for different values of the crack position and depth. The results have been validated against those obtained from experimental modal test، Abaqus software and some other methods reported in the literature and a good agreement between the results is observed. Then، the inverse problem is investigated. For this reason، the position and depth of the crack of the rotating beam with a varying transverse cross-section are estimated using the genetic algorithm and then، the natural frequencies are obtained from the modal test. It is seen that the numerical results have a suitable agreement with the actual position and depth of the crack that indicates the effectiveness of this method in determining the parameters of the crack in the rotating beams.

Numerical simulation of non-isothermal transient gas flow is performed using implicit Steger-Warming finite difference method. Because of nonlinearity of the governing equations، they are linearized at each time step. The linearization either reduces computational effort or analyzes the flowfield more conveniently. In order to validate and evaluate the accuracy of current numerical method، Fanno and shock tube flows are investigated first. Then، transient flow in a gas pipeline that its inlet pressure changes with time is simulated. The results of present study show that Steger-Warming finite difference scheme can well captured the sudden changes in the flowfield. Moreover، the present method is able to analyze transient gas flows as nearly accurate as the nonlinear one with less computational effort.

This paper discusses an adaptation of modal analysis concepts to time-varying periodic systems. It will be shown that the pseudo-modal parameters preserve certain properties of the conventional modal parameters defined for LTI systems. For this reason، after definition of pseudo modal parameters for time varying systems، a new modal analysis method will be introduced in time domain and it will be shown that these parameters could explain the nature of system. For periodic time varying systems، state transition matrices are formed by an ensemble set of responses which are obtained through multiple experiments on the system with the same time varying behavior. In next step the pseudo natural frequencies of a beam with moving mass using introduced method will be extracted. In final، it will be proved that for a linear time periodic system، the pseudo natural frequency treats periodic too.

The transformation behavior and microstructural characteristics of API X65 pipeline steel were investigated by dilatometry and microstructural observation. Microhardness measurements were used to verify the observed microstructures. The test steel is imported from abroad and is used extensively in Iran natural gas transmission projects. The continuous cooling transformation curves of the test steel were constructed. The results showed that with increasing the cooling rate from 0. 5 to 40°C/s، the microstructure changes from polygonal ferrite، quasi-polygonal ferrite-pearlite to acicular ferrite. The microstructure was dominated by acicular ferrite in cooling rates higher than 5°C/s. The results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing process of the test steel.

In this research، validity of temperature-independent thermophysical properties assumption of water-Al203 nanofluid in natural convection problems within the enclosures is investigated. The numerical results are obtained utilizing an in-house finite volume code based on the SIMPLE algorithm. In order to do the validation the numerical results and those of existing correlations are compared. In order to evaluate the thermal performance of the enclosure، the average Nusselt number on the hot side wall in both temperature-independent and dependent cases is compared Results show that، in the all considered solid volume fractions، the difference in the Nusselt number in the case of temperature-independent properties is less than 10 percent in comparison with the case in which the properties are temperature-dependent when temperature difference is less than 5 ○C. As the temperature increases، the difference between Nusselt number in both cases increases and the effect of increase in solid volume fraction is to increase this difference. Results also show that the difference between these two cases is dependent solely on temperature differences between the hot and cold walls regardless of the temperature they have.

In this article، a new stabilizing mechanism for a two wheel robot is proposed. Such systems، due to inherent ‎instability، require dynamic stabilization. The conventional method for stabilizing these robots is moving the base back ‎and forth، to use its inertia effects. Therefore، such strategies drastically depend on the ground surface، besides the ‎robot is not able to reconfigure its manipulator to do any desired task. These limitations reduce the capability of the ‎robot to manipulate objects، and to perform accurate tasks. In order to omit these restrictions، in the developed ‎mechanism، a reaction wheel is used. The proposed mechanism exploits the inertia moment of reaction wheel to ‎stabilize motion of the robot. Therefore، since there is no interaction between the reaction wheel and the ground surface، ‎by using this mechanism there would be no concern about the surface that the robot moves on that. Also، manipulator ‎of the robot can track the given trajectories، without considering stability limitations. In order to show the performance ‎of proposed mechanism، a verified dynamics model of the robot is used and the control algorithm with various initial ‎conditions is simulated. ‎

In this paper، natural convection heat transfer inside an enclosure which is partially filled with porous layer is reported using lattice Boltzmann method. Generalized equations in modeling flow in porous media have been employed which are coupled with the lattice Boltzmann formulation of the momentum and energy equations. The present study investigates the effect of position of porous layer on heat transfer rate for different dimensionless parameters، such as Rayleigh number، Darcy number and porosity of the porous layer. In addition، a modified Rayleigh number is presented as an effective parameter which affects the degree of penetration of the fluid into the porous layers. The obtained results showed that the heat transfer rate in the case of vertical layer is more than that of horizontal porous layers.

Blade forging is an intricate process due to complicated geometry، high dimensional accuracy، complicated material flow، varying and sometimes very thin thicknesses. As a powerful tool، numerical simulation is used in different steps of designing process. Mechanical properties (stress-strain curves) of the material and friction factor of contact surface are of the most important inputs of the simulation. Thus، cylinder and ring compression experiments were conducted to obtain these inputs for Al-2024 used in forging of 202 MVA Siemens generator fan blade. Because of high costs of blade forging and suitability of cylinder side-pressing this experiments were used to evaluate the compression tests and simulations. Good accordance was observed between simulation and experiment. Final forging die cavity and then preforms needed to produce a sound part are modeled. Designed preforming steps include extrusion، bending and upsetting. Blade final forging step was simulated in different temperatures of die and workpiece and strain rates and the optimum condition was determined.

In this paper، it is shown how to use the recently developed Firefly Algorithm to optimize abrasive water-jet cutting as a nonlinear multi-parameter process. Back propagation neural network were developed to predict surface roughness in abrasive water-jet cutting (AWJ) process. In the development of predictive models، machining parameters of traverse speed، water-jet pressure، standoff distance and abrasive flow rate were considered as model variables. Firefly Algorithm by using back propagation neural network optimizes glass surface roughness in abrasive water-jet cutting and proposes appropriate parameters for minimum surface roughness. Testing results demonstrate that the model is suitable for predicting the response parameters. However this algorithm has not be tested for practical problems، the results showed this algorithm applicable for processes with complex nature.

In this study، convergent nozzle ejector in the PEM fuel cell system is analyzed. This method can reduce the parasitic power of the fuel cell، recycle the unconsumed hydrogen to the fuel cell to increase the fuel usage efficiency، utilize the pressure potential energy of hydrogen and regulate the anode humidity with the recycle gas. For this purpose، continuity، momentum، energy and state equations are solved by numerical methods and effects of pressure drop (through the channel towards the anode)، operating pressure and temperature of the fuel cell and also nozzle diameter on the ejector performance was analyzed. With decreasing of pressure drop، even in primary lower pressure، increasing of performance pressure the performance of ejector will improved. The temperature increase has no effect on the performance of the ejector itself، but has enormous effect on the fuel cell. Increasing the diameter ratio of the constant diameter zone to the nozzle diameter leads to increasing of recirculation anode line of the fuel in higher pressure.

In this article، analytical solutions of low velocity transverse impact on a nanobeam are presented using the nonlocal theory to bring out the effect of the nonlocal behavior on dynamic deflections. Impact of a projectile (mass) on simply supported and clamped nanobeams are investigated using nonlocal Timoshenko beam theory. In order to obtain an analytical result for this problem، an approximate method has been developed wherein the applied impulse is replaced by a suitable boundary condition and initial momentum of projectile and nanobeam. A number of numerical examples with analytical solutions for nonlocal nanobeam and classical beam (steel and aluminum) have been presented and discussed. When the value of the striker mass is increased، the frequencies are decreased and the maximum dynamic deflection at the center of the beam is increased for both of the simply supported and the clamped-clamped nanobeams. The inclusion of the nonlocal effect increases the magnitudes of dynamic deflections and decreases frequencies. Furthermore، the mass and the velocity of the nanoparticle (striker) have significant effects on the dynamic behavior of nanobeam.