Scientia Iranica
Volume:23 Issue: 1, 2016

Radial forging without a mandrel offers a cost effective method for production of tubular components. However, nonuniform deformation, thickness variation, poor surface quality and undesirable residual stress distribution are the problems which should be overcome in order to achieve an optimal process. In this paper, radial forging dies with curved-shape profiles are proposed to alleviate some of the mentioned difficulties. Finite element simulations are used to prove the advantage of the proposed dies over the conventional linear dies. A novel analytical approach based on the slab method of analysis is developed to verify the finite element modeling. The obtained results provide useful guidelines for design and optimization of the radial forging process without mandrel.

In this work, the effect of ductile damage on distributions of strain and displacement components in rotating annular disks with variable thicknesses under plane stress condition is studied using semi-analytical and finite element methods. The plastic behavior of disks under mechanical loading is studied on the basis of continuum damage mechanics. The semi-analytical method is developed using the Prandtl-Reuss relations, the method of successive elastic solution and damage plasticity model proposed by Xue and Wierzbicki that is used for the finite element analysis as well. The proposed damage plasticity model incorporates effects of both hydrostatic stress and the Lode angle to define the fracture envelope. The results obtained by semi-analytical method are then compared with the results obtained by the finite element method. Numerical calculations for different ranges of thickness parameter with and without damage effect are carried out and results are compared. It is shown that the damage has a significant effect on values of maximum von Mises stress and the limit angular velocity of annular disks. Results of the present study confirm the credibility of the proposed model in predicting the damage limit angular velocity and can be extend to other state of loadings.

The main aim of this paper is to develop an efficient aeroelastic tool for predicting the flutter speed of a typical section in transonic regime. An implicit mesh-less method based on the Euler and Navier-Stokes equations are conducted to simulate the transonic fluid flow around an airfoil. This technique is applied directly to the differential form of the aerodynamic governing equations and the time integration is carried out using a dual-time implicit time discretization scheme. The capabilities of the flow solution method are demonstrated by flow computations around NACA0012 airfoil at different flow conditions. For structural dynamics simulation, a typical section model with pitching and plunging motion capability is considered. Finally, the aeroelastic analysis of the 2D model is performed by the consecutive simulation of both structural and aerodynamic domains. Also, the effect of viscosity and time interval choice between two structural and aerodynamic solvers on the flutter instability is studied. The comparison between the obtained results and those available in the literature shows the good accuracy of the present method.

An inverse solution technique is applied to the design of radiant enclosures when design variables are discrete and radiation is the dominant mode of heat transfer.The enclosure contains an absorbing, emitting and linear anisotropic scattering medium in radiative equilibrium. The discrete ordinate method is employed to solve the radiative transfer equation. The goal of the design problem is to find the best number and location of discrete equally power heaters which produce the desired (specified) temperature and heat flux profile over the design surface of enclosures. The inverse problem is formulated as an optimization problem and is solved using a micro-genetic algorithm. Results show that the micro genetic algorithm is able to find the optimal solution by just searching a few percent of feasible solutions.The ability of this methodology is demonstrated by finding the optimal number and location of heaters in an irregular enclosure.Then, the effect of some thermophysical properties, such as extinction coefficient, scattering albedo, scattering phase function, and design surface emissivity, on the optimal solution is considered.

Design of an optimal fuzzy scheme, for a fuel cell/battery vehicle to control the power flow between the main components, fuel cell, electric motor, and battery, at various driving conditions is considered in this paper. For this purpose, firstly, the optimum sizes of the main components are calculated by means of particle swarm optimization (PSO) algorithm. Subsequently, a fuzzy logic controller (FLC) is devised for the control of the power flow. Finally, the FLC is optimized for various driving patterns and an optimal control scheme, based on PSO application, is proposed for energy management of the fuel cell vehicle (FCV) at various traffic conditions. In each one of the mentioned stages, the same optimization process is conducted by applying genetic algorithm (GA) for comparison with the result of the PSO. The results of the computer simulation are compared over diverse driving conditions. The results give an acceptable indication of progress in fuel economy at various driving patterns, using the proposed optimal fuzzy controller.

The function and morphology of endothelial cells (ECs) play a key role in atherosclerosis. The mechanical stimuli of ECs such as wall shear stress (WSS) and arterial wall strains greatly influence the function and morphology of these cells. The present article deals with computations of these stimuli for a 3D model of a healthy coronary artery bifurcation. The focus of the study is to propose an accurate method for computations of WSS and strains. Two approaches are considered. The coupled simultaneous simulation of arterial wall and blood flow called Fluid-Structure Interaction (FSI) simulation and decoupled one,which simulates each domain (fluid and solid domain) separately. The study demonstrates that the computed circumferential strain resulting from both methods are identical. However, longitudinal strain and WSS are very different from these two approaches. The resulting time averaged wall shear stress (TAWSS) from decoupled fluid model is always higher than corresponding value from FSI simulation. While, oscillatory shear index (OSI) from rigid wall model is lower than the values resulting from FSI. Therefore, the decoupled simulation may underestimate the atheroprone sites of the artery and suggests that using FSI simulation for mechanical stimuli of ECs is inevitable.

In this article, non-evaporating and non-reacting diesel spray is modeled under ultra-high injection pressure using an Eulerian-Lagrangian scheme. This is accomplished in order to probe into the effects of injection pressure, nozzle diameter, and ambient density on spray characteristics. An advanced hybrid breakup model that takes into consideration the transient processes during spray injection has been added to the open source code OpenFOAM. Reynolds-Average Navier-Stokes (RANS) equations are solved using standard turbulence model and fuel droplet is tracked by a Lagrangian scheme. Published experimental data have been used for validation of spray characteristics at 15 kg/m3 ambient density and injection pressures of 100, 200 and 300MPa. Also, three nozzle diameters of 0.08, 0.12 and 0.16mm have been implemented for investigating the effect of this parameter on spray formation. Computed spray shape, jet penetration, spray volume, equivalent ratio along the injector axis and Sauter Mean Diameter (SMD) illustrate good agreement with experimental data of single hole nozzle and symmetric spray. The effects of fuel injection pressure, nozzle hole diameter and ambient density on main spray parameters are presented. It is concluded that numerical model presented here is quite suitable for accurately predicting diesel spray shapes under ultra-high injection pressures.

A general variational formulation for free vibration analysis of hybrid (metalcomposite) plates with a trapezoidal platform is presented in this paper. The plate is composed of two distinguished parts in the span direction, where the inboard section is assumed to be made of an isotropic metal, and the outboard section is from a laminated composite material. Kinematics, corresponding to the First-order Shear Deformation plate Theory (FSDT), is used to take into account the non-classic e ects of transverse shear deformation and rotational inertia for both sections in the analysis. The developed approach is based on the global Ritz method, where the transverse de flection and two rotations of the plate are independently approximated by simple and Legendre polynomials. The algorithm allows one to obtain an approximate analytical solution for the hybrid plate with di erent geometric aspect ratios, numbers of layers, staking sequences and metal to composite ratios. The results of the present work for special cases, including pure metal and pure composite plates, are in good agreement with previous works.

A cell migration numerical simulation is presented to mimic the motility of endothelial cells subjected to the concentration gradients of Forebrain embryonic cortical neuron conditioned medium (CM). This factor was previously shown to induce the directional chemotaxis of endothelial cells with over-expressed G protein coupled receptor 124 (GPR 124). A cell simulator program incorporates basic elements of the cell cytoskeleton including membrane, nucleus and cytoskeletons. The developed 2D cell model is capable of responding to concentration gradients of biochemical factors by changing the cytoskeleton arrangement. Random walk force, cell drag force and the cell inertial effects are also implemented into the cell migration to complete the simulation of the phenomenon. The obtained results of cell migration were calibrated with experimental cell chemotaxis data. This model can be implemented for prediction of cell behavior during cell chemotaxis and also it provides a powerful tool to explain the cell migration phenomenon mechanistically.

In this paper, a two-dimensional (2D) hierarchical computational model was developed for analysis of heat transfer in unidirectional composites filled with doubly periodic natural fiber bundle. The reinforcement in the composite encloses large number of small lumens, which hints the composite consisting of matrix and natural fiber bundles involves structures at several level scales. In the model, the unit representative volume element (RVE) of composite with fibers arranged periodically was taken into consideration and equivalent models were converted from differently scaled RVEs by a two-step homogenized procedure. Subsequently, numerical simulation of heat–transfer process in each model was performed by finite element analysis and the overall transverse thermal conductivity of each model was obtained numerically. To verify the developed composite models, an optional interrelationship between the overall thermal conductivity of the equivalent natural fiber bundle and the solid region phase in it was obtained for the first-step homogenization and then was compared with analytical results or numerical results from other methods. Finally, a sensitivity analysis was conducted with the models to investigate how changes in the values of important variables such as thermal conductivity and volume fraction of constituent can affect the effective thermal properties of the composite.

This paper is aimed to study the operation mechanism of Stirling pulse tube refrigerators (PTRs) by tracing working gas elements characteristics at the cold end of the system using both Eulerian and Eulerian-Lagrangian (E-L) methods. The main objective of the investigations is to demonstrate non-symmetry effects in the pulse tube section of the system. Elemental cyclic-enthalpy transfer of simple (S), double-inlet (DI) and multi-mesh regenerator (MM) PTRs are also investigated to demonstrate the effects of DI and MM systems on the refrigeration mechanism of PTRs. It is shown that the elemental cyclic-enthalpy transfer of SPTR is less than those of DIPTR, MMPTR and MMDIPTR, though it''s elemental cyclic-temperature reduction is more than the others; because, the mass flow rate upon cold end of the SPTR is less than the others. Regarding the reduction of losses in DIPTR and MMPTR, their cold end mean pressure and density increase which consequently lead the cold end mass flow rate and cyclic-enthalpy transfer to increases. Increase in enthalpy transfer of MM-PTR, MM-DIPTR and DIPTR consequently improves their cooling performance.

This paper investigates the nonlinear resonant behavior of a capacitive micro-beam based on the nonlocal theory of elasticity. The micro-beam is deflected by a DC voltage, where it acts as a micro-resonator by superimposing an AC voltage. Taking into account stretching effects, the Galerkin projection method is used to discretize the partial differential equations into a set of nonlinear, ordinary differential equations. Multiple-scales method is used to obtain an approximate analytical solution to construct the nonlinear resonant curves of the transverse vibration amplitude. Taking into account the classical and nonlocal elasticity theories, the frequency response curves are plotted for different values of DC voltage. Effects of mid-plane stretching on the resonant curves are also examined. The hardening behavior of the system is shown to decrease due to the presence of the nonlocality as well as the DC voltage. However, mid-plane stretching increases the hardening effects. The results show that, in spite of the existence of nonlinearity in the system, this conflict effect can result in a linear frequency response curve for some values of the nonlocal parameter.

Realistic simulation of tool-tissue interactions can help to develop more effective surgical training systems and simulators. This study used a finite element and meshless modeling approaches to simulate the grasping procedure of a large intra-abdominal organ, i.e., kidney, during laparoscopic surgery. Results indicated that the accuracy of meshless method was comparable with that of the finite element, with the root mean square errors in the range of 0.8 to 2.3 mm in different directions. For the model presented in this study, the computational cost of the meshless method, was much less than that of the finite element model.