نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 1 (فروردین 1392)

In this paper, an algorithm is presented based on using bspline function for optimizing tank cross section. This process minimizes fluid c.g. height and overturning moment and improve rollover threshold of tank vehicles. This algorithm receives tank capacity specifications as inputs and offers fourth order bspline function with 10 control points that has more roll stability, and then optimizes it for different filling conditions. This algorithm is based on the third order bspline function with 8 control points, initially. Therefore, with averaging and optimizing, range of control points is modified and the numbers of control points and degree of bspline function are increased. The results show that, the mutation rate is better to be between 4 and 6%, and the number of individuals in each generation should be at least 40. The algorithm presented in this paper, is a fast and accurate method for optimization of tank cross section in different filling conditions. The Algorithm based on GA maintains simplicity applicable for industries and specially has a rollover threshold of 10% higher than conventional tanks.

The most prominent feature of sheet material forming process is an elastic recovery phenomenon during unloading which leads to springback and side wall curl. Therefore evaluation of springback and side wall curl is mandatory for production of precise products. In this paper، the effects of some parameters such as friction coefficient، sheet thickness، yield strength of sheet and blank-holder force on the springback and side wall curl radius in U-bending of dual phase steel sheets were investigated by performing experimental tests and finite element method. ABAQUS software was used for finite element simulation. Comparison of experimental and finite element results shows good agreement. The results of this research shows that increasing of sheet thickness، reduces springback and side wall curl and increasing of yield strength increases springback and side wall curl. Springback and side wall curl initially increase with raising the friction coefficient and blank-holder force but they decrease again when they go beyond certain values.

In this paper، simulation and analysis of thin steel cylindrical shells of various lengths and diameters and thickness with triangular cutouts have been studied. In this research buckling and post-buckling analyses were carried out using the finite element method by ABAQUS software. Moreover، the effect of cutout position and the length-to-diameter (L/D) and diameter-to-thickness (D/t) ratios on the buckling and post-buckling behavior of cylindrical shells have been investigated. In this work the cylindrical shells used for this study were made of mild steel and their mechanical properties were determined using servo hydraulic machine. Then buckling tests were performed using a servo hydraulic machine. In order to numerical analyze the buckling subject to axial load similar to what was done in the experiments; a displacement was applied to the center of the upper of the specimens. The results of experimental tests were compared to the results of the finite element method. A very good correlation was observed between numerical simulation and experimental result.

In this research، initiation and propagation of delamination are investigated using finite element analysis and existing theories for isotropic and composite double cantilever beam (DCB) specimens. These theories work based on the well-known traction-separation laws such as linear، bilinear and exponential laws. In addition، the effects of cohesive zone parameters، i. e.، critical strain energy release rate and maximum interfacial stress، transverse shear deformations and fiber bridging law are studied. The results show that the introduced theories and finite element analysis based on bilinear cohesive law are not capable to predict initiation and propagation of delamination in unidirectional composite specimen with fiber bridging effect and neglecting this region in CZM cause significant error in prediction of delamination growth. For this purpose، bilinear CZM considering bridging law is modified and implemented in 3D finite element analysis. Comparing numerical results with available experimental data in the literature shows that finite element models based on modified CZM can predict initiation of delamination as well as propagation accurately.

In the present study، the ventilation performance of a one-sided wind-catcher positioned in the wake of an upstream structure is experimentally investigated by measuring the induced airflow rates and employing smoke flow visualization techniques. Wind-catchers are usually used in places of high urban densities in Middle East regions; however، their potential to provide natural ventilation depends on the presence of upstream structures. The present study focuses on the ventilation performance evaluation of a one-sided wind-catcher located in the wake of an upstream structure model. The influence of the upstream structure height and its distance relative to the wind-catcher on the flow structure within and around the wind-catcher is investigated. Moreover، the ventilation performance is evaluated by measuring the ventilation flow rate using a hot wire anemometer for upstream air velocities of 10 and 15 m/s. The results show that the presence of an upstream object influences the rate and the direction of air flowing from the wind-catcher to the house. Placing a short upstream object increases the induced air flow rate. However، by increasing the height of upstream object، the airflow direction is reversed inside the wind-catcher and the air may flow backward from the ventilated space to the wind-catcher.

In order to produce shell parts with diffrents applications using the new method in sheet metal forming is inevitable. In this article is intruduced a new process that movement of forming tool is compeltely gradual and controlled. In this method can creat complicated shapes in sheet metal. Also in order to create movement of tool is used Computer numerical Controlled (CNC) machine. In this process with inserting a punch under the sheet and gradual movement of tool in the special path creates a deformation in accoddance to punch shape. In this research by using experimental tests and theoretical analysis (slab analysis) is presented a comprehensive study of the governing equations in process. With calculating of stress field can present applied load at tool and sheet. ge for Calculation of this force is a suitable gauge for choosing kind of CNC machine equipment، sheet type and etc. Also according to analysis results can make decision about the effect of immeasurable important parameters in this process.

Two-fluid models are the most accurate and complex models for analysis of two-phase flows. There are two different two-fluid models for analyzing compressible isothermal two-phase flows which are Single Pressure Model (SPM) and Two-Pressure Model (TPM). In spite of capabilities of these models in capturing two-phase flow behavior، it is not possible to express them in conservative form due to existence of non-conservative term in momentum equation of phases. Therefore، the classical Rankine-Hugoniot condition across discontinuities in the flow filed is not applicable for these equations and there would be difficulty in using classical numerical methods for solving these equations. In this paper a new path-conservative method is used to overcome this difficulty. In this method، one can apply general Rankine-Hugoniot condition along a path connecting left and right states of the discontinuity. After expressing path-conservative form of the employed central numerical methods which are Lax-Fridriches، Lax-Wendroff and Rusanove، water faucet and large relative velocity shock tube problems are solved by using these schemes. Grid independence was achieved using different grid sizes. For water faucet problem، comparison of numerical results with analytical solution show good agreement and for shock tube problem، the results indicate that this method is highly capable in capturing discontinuities in two-phase flow.

In this paper the problem of coupled radiation and natural convection is investigated in a square porous cavity using local thermal non-equilibrium model. The radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM) and the energy and momentum equations are solved using finite element method. The results of the present study are compared with that of the other investigations which have used another method to solve radiative transfer equation. Effective parameters on heat transfer and fluid flow characteristics such as Planck number، inter-phase heat transfer coefficient and scattering albedo are studied and the results are presented in the group of dimensionless parameters. The results indicate that the solving method of the radiative transfer equation would have significant effect on the fluid flow and heat transfer characteristic. In the case of low optically thick media، discrete ordinates method (DOM) is more precise than the other methods which used in other literature.

This research develops a Lamb wave technique to determine the dispersion curves of a two layered bonded component: an aluminum sheet attached to a composite layer by means of a cohesive. A commercial finite element code (ABAQUS Explicit) is used to determine the dispersion curves of the Aluminum-cohesive-composite multilayer component. The finite element model includes three plain strain layers that the middle one is cohesive. Then a lamb wave is propagated in the model and some output signals are received. The dispersion curves are obtained by using 2D Fourier transformation of finite element model output signals. In addition، to produce various modes، experiments are carried out on a composite-aluminum assembly using two 2 MHz variable angle transducers. Comparison of modes obtained from finite element method and experiments shows that group velocities are almost identical. Hence، good agreement between finite element method results and experimental results indicates that finite element is reasonably accurate for determination of dispersion curves.

The main goal of this paper is to present a mathematical model for inverse dynamic equation of elastic robotic manipulator with revolute-prismatic joints. Due to the fact that there is no limitation on the number of mechanical arms، the proposed model must be extracted based on a systematic and automotive algorithm. Also according to the high computational complexity، the equations should be formed by a recursive formulation. Hence، a recursive and systematic methodology for deriving the equation of motion of elastic robotic arm with revolute-prismatic joints is presented. The inverse dynamic equations for this robotic system are obtained based on Gibbs-Applle formulation. All dynamic expressions of a link are expressed in the same link local coordinate system. Finally، in order to show the ability of this formulation in deriving and solving the equation of motion of such systems، a computational simulation for a flexible single robotic arm with revolute-prismatic joint is presented.

To prevent unpleasant incidents، preservation high-speed railway vehicle stability has vital importance. For this purpose، the Railway vehicle dynamic is modeled using a 38-DOF includes the longitudinal، lateral and vertical displacements، roll، pitch and yaw angles. A heuristic nonlinear creep model and the elastic rail are used for simulation of the wheel and rail contact. To solve coupled and nonlinear differential equations، Matlab software and Runge Kutta methods are used. In order to study stability، bifurcation analyses are performed. In bifurcation analysis، speed is considered as the bifurcation parameter. These analyses are carried out for different wheel conicity and radius of the curved track. It is revealed that critical hunting speed decreases by increasing the wheel conicity or decreasing the radius of the curved track.

This paper discusses about the effects of square wave pulsation on the turbulent flow and heat transfer from slot jet impinging to a concave surface. The RNG k-ε turbulence model is applied for modeling the turbulent flow and heat transfer filed in the present 2-D slot jet flow. The effects of jet Reynolds number، nozzle to surface distance and pulsation frequency on time-averaged Nusselt number distribution are studied carefully. Results show that applying the pulsating jet in the range of 10 Hz to 50 Hz can increase heat transfer from the concave surface in comparison with the steady jet. Increasing jet Reynolds number ranged from 4740 to 9590 significantly increases the time-averaged local Nusselt number. Also، in steady jet، decreasing the nozzle to surface distance، consequences increasing the Nusselt number near the impingement zone. While in pulsating jet، it causes both increasing/ decreasing the Nusselt number all over the concave surface.

In this paper، the Meshless Local Petrov-Galerkin (MLPG) method is used to analyze the fracture of an isotropic FGM plate. The stress intensity factor of Mode I and Mode II are determined under the influence of various non-homogeneity ratios، crack length and material gradation angle. Both the moving least square (MLS) and the direct method have been applied to estimate the shape function and to impose the essential boundary conditions. The enriched weight function method is used to simulate the displacement and stress field around the crack tip. Normalized stress intensity factors (NDSIF) are calculated using the path independent integral، J*، which is formulated for the non-homogeneous material. The Edge-Cracked FGM plate is considered here and analyzed under the uniform load and uniform fixed grip conditions. To validate results، at first، homogeneous and FGM plate with material gradation along crack length was analyzed and compared with exact solution. Results showed good agreement between MLPG and exact solution.