نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 7 (مهر 1392)

Fuel control unit (FCU) is one of the most essential parts in a gas turbine engine; therefore it is necessary to be studied as an important part of the fuel control system. This paper report the use of Nonlinear Auto Regressive with eXogenous input (NARX) neural network model for modeling of the jet engine FCU. Therefore، To measure and recording data from the FCU inputs and output، the test bench including hydraulic system، data acquisition system and induction motor control system are designed and constructed. This setup is a mechatronic collection which includes mechanical design، discharge and pressure sensors، tachometer، control unit and piping systems. The process of modeling is carried out in MATLAB software. The identified model is evaluated with validation data and its response is compared with the real system response. Results demonstrate the effectiveness of the NARX neural network model and show that the real system is estimated by the NARX neural network model accurately.

Complex nonlinear unloading behavior following plastic deformation on metals has been observed by researchers. However، a constant chord modulus has been used by most of the researchers to describe the unloading behavior. In this study، a new hypoelastic model is proposed in order to describe this complex nonlinear behavior. The elastic modulus is assumed to be the initial Young’s modulus at the beginning of each loading reversal and is exponentially decreased with the elastic deformation and finally saturates to a particular value at the start of plastic deformation. The model can be easily utilized in any plasticity constitutive law. Furthermore، its numerical implementation is fairly straightforward. As an example، a simple constitutive law was developed using the von-Mises yield function، isotropic hardening law and the proposed hypoelastic model. This constitutive law was implemented into ABAQUS commercial package via a user material subroutine UMAT. A comparison between the experimental response of DP980 and its simulated response showed that the model is able to describe the material response fairly well.

Mixed convection flow of a water-copper nanofluid in a channel under magnetic field effects has been numerically investigated. The fluid flow and temperature fields as well as the heat transfer rate have been determined by solving the Navier-Stocks and energy equations. In this study، the effects of various parameters such as the Richardson number، the Hartmann number، the solid volume fraction and the channel angle on the thermal performance of the channel have been examined. The results showed that at high Richardson numbers، the heat transfer rate decreased as the Hartman number increased. It was also found that the heat transfer rate increased as the Richardson number، the solid volume fraction and the channel angle increased. The maximum flow reversal was observed to occur in a vertical channel.

Smoothed particle hydrodynamics (SPH) is a fully Lagrangian particle method which solves a problem without using any mesh or grid. Pressure fluctuation is one of the main drawbacks of the weakly compressible SPH (WCSPH) method that leads to an inaccurate pressure distribution. In the present work، a diffusive term is added to the continuity equation to suppress the density and consequently pressure fluctuations. In contrast to the mesh-based methods، flow separation and inflow/outflow boundary conditions are two challenging issues in the SPH method. To overcome these problems، a new algorithm for inflow/outflow boundary condition as well as a particle shifting method is utilized for simulation of flow past a cylinder. Comparing the results with those of literature، it is shown that the method is capable to decrease the pressure fluctuations and solve problems including open boundaries as well as flow separation.

Chatter vibration is one of the limiting factors in increasing the material removing rate in machining operations. In this paper، the integrated mechatronic modeling of a lathe machine equipped with MR damper، and design of a fuzzy semi-active controller are presented. To suppress chatter، the structural dynamic characteristics (i. e.، real and imaginary part of the frequency response function)، which are the main parameters in drawing the stability lobes، are varied semi-actively using magnetorheological damper. Modified Bouc-Wen model is employed for MR damper modeling، which enables us to account for the input voltage to the damper as the control input of the system. Since the structure becomes nonlinear in the presence of MR damper، a time-domain approach for generation of stability lobes is presented. In controller design and as a feedback of the chatter level، a novel chatter detection index (CDI) is developed، which can also be employed for experimental chatter detection. The obtained results show that the proposed system has been successful in enhancing the stability of the lathe machine.

In a vertical wind tunnel، in order to prevent persons or an experimental model from falling، a protective screen should be installed at the end of the nozzle section. Since the air has the maximum velocity at this section، the pressure drop due to the protective screen will be significantly high. On the other hand، as the protective screen is alternatively exposed to dynamic forces due to free fall of the floating persons or the model، the screen wires will experience fatigue. To prevent this، multi strand cables should be used in the manufacturing of these protective screens. In this research work، using the momentum difference method، drag coefficient for the multi strand cables and circular rods has been measured and compared. For this purpose، a hot wire anemometer (HWA) with a one-dimensional probe has been used. Results show that at Reynolds number in proximity of 2 × 103، the drag coefficient for the multi strand cable exceeds that of a circular rod by 16% and that this amount decreases with further increase in Reynolds number. The trend is such that for Reynolds number of 104، the drag coefficients of the multi strand cable and circular rod are almost equal.

In this article، effect of axial angle of injection nozzles on the flow field structure in a Low-Pressure vortex tube has been investigated by computational fluid dynamics (CFD) techniques. Numerical results of compressible and turbulent flows are derived by using the standard k-ε turbulence model. The dimensions of studied vortex tubes are kept the same for all models and the performance of machine is studied under 6 different axial angles (β) of nozzles. Achieving to a minimum cold exit temperature is the main goal of this numerical research. Our investigation shows that utilizing this kind of nozzle changes the energy separation and flow characteristic. Considering total pressure of cold flow، a new parameter، ξ is defined and results shows that changing the amount of ξ can affect the cold exit temperature directly. Finally، some results of the CFD models are validated by the available experimental data which show reasonable agreement.

Accurate estimation of the pressure losses for non-Newtonian drilling fluids inside annulus is quite important to determine pump rates and select mud pump systems during wellbore drilling operation. The aim of this study is to simulate non-Newtonian (power law and Herschel-Bulkly) foam flow in underbalanced drilling condition through wellbore annulus using finite volume method. The effect of various operational parameters on pressure loss such as fluid rheology، foam fluid velocity، foam quality، drillpipe rotation and wellbore eccentricity، have been considered. Simulation results were compared with the previously published experimental data. The agreement was close with a relative error less than 5%. The results of numerical method are closer to experimental data for Herschel Bulkly model for foam fluid. Also، the results of numerical method، showed that pressure drop increases with increasing the foam fluid velocity and quality and it decreases with increasing eccentricity، but drillpipe rotation don’t have noticeable effect on pressure drop.

Wire electrical discharge machining (wire-EDM) has a significant position among production technologies mainly due to its capacity of machining hard materials and intricate shapes. One of the major problems with this process is the error in cutting corners. Processing forces acting on the wire and low rigidity of the wire are responsible for wire deformation، which has a direct influence on the accuracy of the corner cutting. In this research، investigation is focused on the convex corner radii errors and alternative solutions are proposed for the case of successive cuts (one roughing and two finishings). Experiments are carried out for roughing operation by considering frequency of discharges and feed speed. The residual materials on straight and curved paths are the output parameters. Results indicate that optimization of these parameters have a better influence for control of residual material thickness on straight paths than on curved corners. One important conclusion is that roughing is the most influential stage of cutting by WEDM. Then، concave corner radii produced during successive cuts، the effect of corner angle and corner radii are investigated. Errors at radii of different corner angles are identified and related to arc length and residual material thickness in the curved corner. Finally، an effective approach is presented for improving the accuracy of the small-radius concave corner radii of finishing stage. The main conclusion is that to achieve accurate corner radii، one must increase the traversed corner arc length by wire in the small-radius concave corner radii.

This paper presents the sensitivity analysis of a sample hydraulic servo valve. The sensitivity of the flow-current curve with respect to variations of geometrical and functional parameters is evaluated. This analysis is done by use of a model which simulates the valve behavior in terms of geometrical parameters and properties of components. With attention to multiplicity of the valve components and the complexity of the system، the more effective parameters have been studied more precisely. Deviation of “flow gain” And “saturation flow” -which are the main characteristics of the valve behavior- from their nominal values in the flow-current curve، is the criterion for the valve sensitivity assessment. The curve of characteristic variations versus parameter variations has been plotted. The evaluations indicated that all of the sensitivity curves are linear in the limited intervals. Also between the evaluated parameters، “nozzle-flapper equilibrium distance”، “orifice diameter” and “stiffness of spool spring” have more sensitivity effect on the valve performance، respectively.

Exoskeleton robot has attracted attention of many researchers because it realizes an old aspiration to attain a machine which is worn by man and maximizes his might via its powerful actuators. Thorough coordination between robot movements and that of user constitutes the greatest complexity of exoskeleton technology. Investigations showed that impedance control (IC) is suitable for this application but the present forms of IC are mostly dedicated to industrial robots which have significant differences with exoskeleton. In this article a versatile form of IC for the mentioned application is developed. Besides، according to perpetual uncertainties in load and measured force signals، for the first time an adaptive method for IC of exoskeleton robot is implemented. Simulating operation of robot in tracking user''s walking motion while carrying a load of 50 (Kg) and with uncertainties in load and measured forces، proves efficiency of the proposed control method. Tracking error during simulation is almost zero and torques needed at interfaces are immaterial.

Motion control of a planar nonholonomic system with four DoF is addressed in this paper. Three actuators are responsible for shape control of this system. Furthermore، assuming no external forces and zero angular momentum، imposes a nonholonomic constraint to the problem. First it is shown that although the simplified equations of motion for this system، could be converted to Heisenberg and chained-form systems، the conventional control methods for these systems، may not be applied to the considered problem. Then، using sliding modes and online path planning، two different closed-loop control laws are designed for bringing the system to and stabilizing around any desired equilibrium state started from any initial condition. Simulation results، show the efficiency of the proposed methods.

In this paper، a new method based on Pareto Simplex Search (PSS) has been employed to find the communications satellite optimal transfer trajectory from geosynchronous transfer orbit (GTO) to geosynchronous orbit (GEO) as the destination operational zone. We solve this problem through using a mathematically efficient algorithm and in all transferring phases، considering transfer orbits to intermediate orbits; it is supposed that the orbital maneuver control unit has continuous performance characteristics. The system governing equations representing the two body system of a spacecraft and primary gravitational source and thrust force applying to spacecraft، are defined in a nonlinear form. In this systematic approach we incorporate system dynamics as the problem constraints، and both minimum-time and fuel consumption using constant acceleration simultaneously as the problem strategy to find the optimal transfer trajectory between two orbits. Set of optimal trajectories are plotted in Pareto Front and transfer trajectory can be selected from these points.