Computational Methods in Engineering

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In this paper, the equation of motion of an elastic 2 DoF wing model has been derived using Lagranges method. The aerodynamic loads on the wing were calculated via the Strip-Theory and the effect of compressibility was included. Wing deflections due to bending and twist motions were determined using the Assume-Mode method. The aeroelastic equations were solved numerically using the V-g method. The results obtained for different types of wings were in good agreement with experimental data.

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The optimal path planning of cooperative manipulators is studied in the present research. Optimal Control Theory is employed to calculate the optimal path of each joint choosing an appropriate index of the system to be minimized and taking the kinematics equations as the constraints. The formulation has been derived using Pontryagin Minimum Principle and results in a Two Point Boundary Value Problem, (TPBVP). The problem is solved for a cooperative manipulator system consisting of two 3-DOF serial robots.

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An intensive experimental investigation was conducted to study the effect of vertical tail, single and twin (with different cant angles) on the flow field and the corresponding aerodynamic forces and moments of a model of a fighter A/C. Aerodynamic forces under different flight conditions and different vertical tail settings were measured in a supersonic wind tunnel. Furthermore, effects of vertical tail on the model wake at subsonic speed were investigated. In addition to the force and pressure measurements, schlieren system was used to visualize the shock formation and movement oat various locations on the model. The results show existence of a pair of symmetric vortices for the model equipped with a 22 degree vertical tail cant angle. The vortices burst symmetrically at moderate angle of attack. The drag coefficient increases with increasing cant angle at low to moderate alpha and decreases when alpha is further increased.

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The blades of wind turbines are the most important parts in producing power output. In this study, a section of a 660 KW wind turbine blade will be installed in Iran in near future was tested in a wind tunnel. In addition to steady tests, various unsteady tests including the effects of reduced frequency, mean angle of attack, and amplitudes were carried out. The preliminary results show strong effects of reduced frequency on the aerodynamic coefficients of the airfoil. Moreover, increasing the reduced frequency delays dynamic stall angle of attack but increases lift and drag coefficients compared to the static results. Further, the values of the aerodynamic coefficients in the upstroke motion (increasing angle of attack) are different from their corresponding values in the downstroke (decreasing angle of attack). These differences create a hysteresis loop where its width and shape are strong functions of reduced frequency, mean angle of attack, and amplitudes.

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Production of nickel-silver by mechanical alloying was investigated. Effects of parameters such as milling duration, ball to powder weight ratio, and chemical composition on mechanical alloying process, and alloys color and microstructure were studied. The milled powders were characterized, using XRD and SEM. Results showed that nickel-silvers could be produced by mechanical alloying in a wide range of compositions. Alloyed powder with a bright silvery contrast and less than 15 nm grain size could be obtained by optimization of milling parameters. Zinc content of the powder mixture had a significant effect on the minimum alloying time. Ball to powder ratio up to 25 also reduced minimum alloying time but it had no significant effect above this value.

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Polypropylene (PP) has poor adhesion to metals and other surfaces for its chemical structure. Hence, chemical modification of PP is necessary for metal surface coating application. In this research, grafting of maleic anhydride (MA) onto co(propylene-b-ethylene) in the presence of a dicumyl peroxide (DCP) was accomplished in a single screw extruder. Characteristics of the modified polymer were determined by Infra-red Spectroscopy (IR), Scanning Electron Microscopy (SEM), and adhesion test. Maximum grafting of MA was found to be 1.2832% for 1.5 pph of MA. Adhesion test showed that the samples containing 1 pph of MA (degree of grafting is 0.5816%) had better adhesion to steel surface (17.25 kgf).

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In welded tubular joints, when the fatigue crack depth is less than 20% of chord wall thickness, the crack growing process is highly affected by weld geometry. Hence, T-butt solution and weld magnification factor (Mk) are applicable tools for evaluating the crack growth rate in this domain. In this research, the capability of Artificial Neural Network (ANN) for estimating the Mk of weld toe cracks in T-butt joints is investigated. Four Multi-Layer Perceptron (MLP) networks are designed and trained to predict the Mk in deepest point and ends of weld toe cracks under membrane and bending stresses. Training and testing data of networks are extracted from a reputable resource on finite element modeling. Comparison of the results obtained and those from the most recently published equations shows that using ANN seems to be very beneficial in this field

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In this paper, two suboptimum detectors are proposed for coherent radar signal detection in K-distributed clutter. Assuming certain values for several initial moments of clutter amplitude, the characteristic function of the clutter amplitude is approximated by a limited series. Using the Pade approximation, it is then converted to a rational fraction. Thus, the pdf of the clutter amplitude is obtained as a sum of simple exponential functions. Using such a pdf, we develop the suboptimum detectors PGLR and PAALR, which are simplified forms of the GLR and AALR. Computer simulations show that the suggested detectors have appropriate performance compared to OLD, GLR and AALR detectors.

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The mechanical behavior of cold rolled sheets is significantly related to residual stresses that arise from bending and unbending processes. Measurement of residual stresses is mostly limited to surface measurement techniques. Experimental determination of stress variation through thickness is difficult and time-consuming. This paper presents a closed form solution for residual stresses, in which the bending-unbending process is modeled as an elastic-plastic plane strain problem. An anisotropic material is assumed. To validate the analytical solution, finite element simulation is also demonstrated. This study is applicable to analysis of coiling-uncoiling, leveling and straightening processes.

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This paper presents a nonlinear mixed-integer programming model to minimize the stoppage cost of mixed-model assembly lines. Nowadays, most manufacturing firms employ this type of line due to the increasing varieties of products in their attempts to quickly respond to diversified customer demands. Advancement of new technologies, competitiveness, diversification of products, and large customer demand have encouraged practitioners to use different methods of improving production lines. Minimizing line stoppage is regarded as a main factor in determining the sequence of processing products. Line stoppage results in idleness of operators and machines, reduced throughput, increased overhead costs, and decreased overall productivity. Due to the complexity of the model proposed, which belongs to a class of NP-hard problems, a meta-heuristic method based on a genetic algorithm (GA) is proposed to obtain near-optimal solutions in reasonable time, especially for large-scale problems. To show the efficiency of the proposed GA, the computational results are compared with those obtained by the Lingo software.

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This paper presents a discounted cash-flow approach to an inventory model for deteriorating items with the two-parameter Weibull distribution. According to our proposed model, two shortages are considered: back-orders and lost-sales, in which the back-order rate is a varying function of the time when the shortage happens. In general, the demand rate is a linear function of the selling price. The objective of this model is to determine the optimal pricing policy and the optimal throughput time in such a way that the total net present value of profits is maximized in the given planning horizon. Finally, a numerical example is provided to solve the model presented using our proposed three-stage approach.

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Among the adaptive-grid methods, redistribution and embedding techniques have been the focus of more attention by researchers. Simultaneous or combined adaptive techniques have also been used. This paper describes a combination of adaptive-grid embedding and redistribution methods on semi-structured grids for two-dimensional invisid flows. Since the grid is semi-structured, it is possible to use different algorithms for combining adaptive-grid embedding and redistribution methods. To evaluate the accuracy and efficiency of the method, this combination is used to solve two model problems, transonic and supersonic inviscid flows in channels with circular arc bump. The results show that combination of adaptive-grid embedding and redistribution methods on semi-structured grids remarkably increases the accuracy at the cost of a slight increase in computational time in comparison with the embedding method alone.

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Stability of reinforced slopes is almost always carried out using limit equilibrium methods and controlled by the shear strengths of the slope materials and the extension force of reinforcements. According to limit equilibrium methods, the stability of slopes is assessed by dividing the whole failure wedge into several vertical elements. In order to determine the safety factor of the reinforced slopes, a new approach is proposed based on the inclined slices method. According to this approach, a 4n formulation is introduced which uses fewer unknowns and a simpler formulation to calculate the extension forces of reinforcements and safety factors of the slopes. Additionally, moment and forces equilibrium in all slices are taken into account while the tensile force of each reinforcing element is independently calculated. Comparisons revealed differences at 5 to 10 percent level between analytical results obtained from this method and those of ReSSA software.

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In this research, a new method called elastic surface algorithm is presented for inverse design of 2-D airfoil in a viscous flow regime. In this method as an iterative one, airfoil walls are considered as flexible curved beams. The difference between the target and the current pressure distribution causes the flexible beams to deflect at each shape modification step. In modification shape algorithm, the finite element equations of two-node Timoshenko beam are solved to calculate the deflection of the beams. In order to validate the proposed method, various airfoils in subsonic and transonic regimes are studied, which show the robustness of the method in the viscous flow regime with separation and normal shock. Also, three design examples are presented here, which show the capability of the proposed method.

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In this paper, considering all the linear and nonlinear inertia terms of moving masses on a flexible beam, the dynamic response and dynamic stability of the beam are studied. Homotopy perturbation method is used to perform the analysis and results are provided in a stability map for the different values of mass and velocity of the moving masses. It is concluded that there is a borderline in the diagram that separates the stable and unstable regions. For the first time, this borderline is determined semi-analytically. Results of the stability analysis are validated using the Floquet theory. In addition to this borderline, it is also concluded that the Homotopy perturbation method is capable of evaluating the new critical values for mass and velocity which cause vibration resonance in the beam. The locus of these resonant points, which is totally a new finding in dynamic analysis of beam-moving mass problem, is determined semi-analytically. Finally, the effect of the friction between the beam and the moving mass is studied on the stability of the system and resonant conditions. Accuracy of the results for this case is also evaluated with a numerical simulation.