مجله مکانیک سازه ها و شاره ها
سال ششم شماره 4 (زمستان 1395)

At the bending process, sheet deforms plastically between tool and die to take its final shape. Deviation of final bending angle from intended angle after removing load is the most important defect which occurs in sheet metal forming processes. This phenomenon which is called springback, occurs because of elastic behavior of sheet after unloading. In order to increasing geometrical accuracy of products, the bending parameters should be chosen in such a way that the intended bending angle be obtained after unloading. In recent years, due to fuel crisis; aerospace and automotive industries trends to use lightweight materials to decrease fuel consumption. Fiber Metal Laminates are attractive materials for industries due to their suitable specifications such as high strength to weight ratio, low cost, high chemical resistance and high acoustic and vibration damping. In the present study, the springback behavior of Fiber metal laminates has been investigated by using theoretical and experimental procedures. The results of this study shows that the developed theoretical method can predict springback with mean error of 17%.

In this article, wavelet-based spectral finite element (WSFE) is formulated for time domain and wave domain dynamic analysis of Timoshenko beam subjected to a uniform axial tensile or compressive force (prestressed). Daubechies wavelet basis functions transform the time and space-dependent governing partial differential equations into a set of coupled space-dependent ordinary differential equations (ODE). The resulting ODEs are decoupled through an eigenvalue analysis and then solved exactly to obtain the shape functions and dynamic stiffness matrix. In the WSFE model, a beam can be divided into only a single element, but larger number of elements may be used in a finite element (FE) model. The accuracy of present WSFE model is validated by comparing its results with those of FE method. The results display advantages of WSFE model compared to FE one in reducing number of elements as well as increasing numerical accuracy. These advantages are more visible in higher frequency content excitations. In addition, the effects of axial tensile or compressive force on time domain analysis and system natural frequencies are investigated. Divergence instability of beam subjected to critical axial compressive force is investigated.

Controlling and optimizing natural frequencies of structures is an important issue in mechanical, aerospace and civil engineering. In this paper, some new problems for cross-section area optimization of 2D and 3D truss structures considering different frequency objective functions are introduced and investigated. Three algorithms are developed in order to 1- increase the difference of the first two natural frequencies, 2- increase the first natural frequency and 3- increase of the second natural frequency with a limited change of the first natural frequency. Using a discrete sensitivity analysis, the cross-section area of the truss members are changed while the total mass of the structure is maintained fixed in order to achieve optimum natural frequencies. Modal analysis, sensitivity analysis and optimization process are performed by a developed APDL code in ANSYS software. Several two and three dimensional truss optimization examples are presented to demonstrate the efficiency of the methods. The presented examples show that natural frequencies of a truss structure can be optimized significantly by using the proposed methods.

This paper presents an optimization algorithm based on Simulated Annealing in design optimization of 2D continuum structures. The algorithm – denoted as CMLPSA (Corrected Multi-Level & Multi-Point Simulated Annealing) – implements an advanced search mechanism where each candidate design is selected from a population of trial points randomly generated. The multi-point strategy is adopted for both feasible and infeasible intermediate designs. CMLPSA includes a multi-level annealing strategy where trial points are generated by perturbing all design variables simultaneously (global level) or one by one (local level). In this paper, the effects of rejection ratio and evolutionary rate are investigated in design optimization of 2D continuum structures. The results of this study are also compared with other researches reported in the literature. It is concluded that for different values of initial discarding and rate of discarding parameters the final volume or von Mises stress of the optimum structure do not change considrably, but the final shape for different rate of discarding is changed. However, final volume and von Mises stress of the optimum structure are improved in comparison with other methods.

In this paper, at first stress distribution in a single-lap joint between composite laminates by first order shear deformation theory with and without the presence of longitudinal strain has been evaluated. Adhesive layer supposed to be isotropic and adherents assumed to be orthotropic laminates. A new improved method of displacement theory has been used to determine stresses distribution. The results of this analytical solution have been compared by the results of finite element and it has been revealed that both methods have similar results. The advantage of this new method compared to classic method is that it has a higher Compatibility by finite element results. In shear stress there is no significant improvement. In the other hand it has been declared that the method has assumed length strain in adhesive layer has a higher accuracy in its results. In shear stress distribution, considering of length strain has no significant benefit, but in peeling stress this difference will improve the results significantly.

Tuned mass damper (TMD) has been widely used as an adopted strategy for vibration control of mechanical and structural systems. Tuning of TMD parameters plays an important role in its performance. In this paper, novel mathematical models based on regression analysis scheme are presented for optimum tuning of TMD parameters in a damped main system subjected to white noise base acceleration. For this purpose, a database of optimum frequency and damping ratio of TMD is created and then models based on regression analysis scheme are proposed for optimum tuning of TMD parameters. Considering the confidence index as a statistical measurement, the efficiency of the proposed mathematical models is compared with other explicit models in the literature. The results show that the proposed models are simple and, due to having the lowest estimated errors and the best agreement with optimum tuning from database, they are able to provide more accuracy than other explicit mathematical models for optimum tuning of TMD parameters. Also, the proposed models are more efficient and simple than the search-based optimization algorithms. Therefore, they can readily be used for engineering applications without the need of time-consuming calculations. Furthermore, it is found that the optimum TMD parameters are not influenced by the predominant frequency of filtered white-noise excitation. At the end, the efficiency of the proposed mathematical models is shown for a structure subjected to different earthquakes.

In this paper, robust control and improvement of vehicle dynamic and stability is considered. Direct yaw control as a means of corrective moment generating could be effective during vehicle maneuvers. Due to the differential brake forces, a corrective moment is generated at the center of gravity of vehicle to be effective in an emergency diversion and loss of the stability conditions. The control system is designed to produce the corrective torque required in top-level and the brake torque of each wheel in low-level controller. The corrective moment is designed by developed quantitative feedback theory as a robust controller method. This robust controller considers the vehicle system uncertainties existed by change of center of gravity, road conditions, tire inflation pressure and many reasons. The uncertainties firstly considered by a statistical method named Taguch to find the most effective parameters in the model. Base of the linear model derived from nonlinear vehicle dynamics, all the uncertainties studied by a pre-filter and compensator control and then applied in comprehensive nonlinear model. Moreover, the brake torque is generated by a set of simple rules. A double lane change maneuver with the low friction road is employed to show the robust performance of the controller.

The manufacturing processes mostly generate residual stresses in structures. Welding as well, can cause these types of stresses which can be useful or detrimental in different cases. Expensive and time-consuming tests should be conducted to measure residual stresses, however, modal testing is widely available providing results conveniently and quickly. In cases in which the qualitative changes in stress is required, experimental modal testing is a suitable substitute for residual stress measurement processes. In this paper, experimental modal analysis have been conducted on an aluminum specimen, also the same procedure has been done on welded specimens. Natural frequencies are compared before and after the welding along with verification of experimental modal analysis integrity using Euler-Bernoulli relations. In addition to experimental modal analysis, finite element modeling of welding process has been done comparing the numerical and experimental results. The results obtained from the investigation have shown that welding made the structure harder leading to elevation of its natural frequencies. This increase in frequencies are associated with residual stresses generated in welding process. By comparing natural frequencies of the specimens, a quantitative relation can be drawn between the residual stress caused by welding and changes of natural frequencies.

Time depended deformation and critical buckling load of viscoelastic thick plates were studied using finite strip method with the trigonometric functions in longitudinal direction and the polynomial functions in transverse direction. The plates were considered to be thick and the third order shear deformation theory was used to consider the effect of shear stresses in thickness. The mechanical properties of the material were considered to be linear viscoelastic by expressing the relaxation modulus in terms of Prony series. Time history of maximum deflection of viscoelastic plates subjected to transverse loading and unloading on plates was calculated using a fully discretized formulation. In addition, the critical in-plane load of plates was calculated by a nonlinear procedure in different times of loading. Moreover, the effect of thickness and the interaction of biaxial in-plane loading on critical load of plate were studied. The results show that the interaction curves of biaxial critical load of viscoelastic plates have the linear behavior for all time of loading.

In this study, free vibration analysis of isotropic sector plates coupled with piezoelectric layers based on first-order shear deformation theory has been analyzed. Governing differential equations of the dynamic behavior of free vibration of isotropic sector plates coupled with piezoelectric layers are derived using Hamilton's principle and the electrostatic Maxwell equations. The Coupled governing differential equations of the vibrating composite plate are solved by applying the method of separation of variables and auxiliary potential functions. The presented analytical exact solution in this paper is validated with available data in the literature. Using numerical results the effect of opening angle of sector plate , the host thickness to radius ratios , inner to outer radius ratios , the ratio of thickness of the piezoelectric layer respect to thickness of the host plate and different boundary conditions on the natural frequencies of the vibrating sector plates coupled with piezoelectric layers are obtained. Also impact, manner and conditions of each parameter on the natural frequencies areconsidered and discussed.

In this study, the numerical and experimental study of energy absorption and deformation of thin-walled conical tubes under axial and oblique loading is studied. The purpose of this study was to investigate the effect of geometry on the energy absorption of clamped conical tubes and effect of foam-filled tubes to absorb more energy under axial and oblique crushing. In the experimental part, empty aluminum tubes filled with solid polyurethane foam were prepared and then the quasi-static tests with static loading rates were performed on samples and the load-compression diagrams in each test were obtained. In the last part of this study simulation of the phenomenon of axial and oblique on thin sections was carried out with the ABAQUS software. The comparison of numerical and experimental results showed that the present model provides an appropriate procedure to determine the collapse mechanism and load-compression curve. The model is used to evaluate the effects of important parameters defined in empty and foam-filled clamped samples such as wall thickness and semi-apical angle of conical samples. A dynamic amplification factor is considered to relate the quasi-static results to dynamic response of conical shock absorber.

In this work vibrational-damping properties of a viscoelastic material of a Russian vibration absorber coating named склг-27м is investigated. For this purpose ASTM E756-05 standard is employed and corresponding experimental tests are performed step by step. This standard test method is useful to obtain vibrational-damping properties of materials and can makes it possible to apply viscoelastic material in passive vibration control applications in a beneficial manner. Since the investigated vibration absorber coating is of type of constrained layer damping or CLD, a sandwich specimen is fabricated according to the standard instructions. Then by performing experimental modal testing on the sandwich specimen and base steel beams as cantilever beams, the desired outputs including variations in loss factor and shear modulus of the viscoelastic material versus frequency are obtained. Also the accuracy of conducted tests are checked out using the standard criteria which the outcomes confirm the validity of tests and results.

The Darrieus lift-based VAWT has been considered by many scientists due to the simplicity of design and independence to the wind direction. Due to increasing angle of attack at low tip speed ratios, the straight-bladed Darrieus wind turbines have the inherent problem of self-starting inability and have less power coefficient as compared to the horizontal axis wind turbines. In this study, has been showed that use of the variable pitch angle turbines is a suitable solution to overcome the self-starting problem and increasing the output power of Darrieus wind turbine. So, the effect of variable blade pitch angle mechanism with different amplitudes on output torque, flow field around the rotor and self-starting of Darrieus wind turbine has been investigated. Also, unsteady two-dimensional simulation is conducted using computational fluid dynamics with SST kω turbulence model and moving mesh technique is used for simulation of the rotating rotor. The numerical investigation shows that variable pitch mechanism decreases the angle of attack and makes the turbine have a chance to produce more torque at different azimuthal angles. Investigation of the flow around the blade also shows that the use of variable pitch blades increases the pressure difference between the low and high pressure of around the blade and delays stall. So, the Darrieus turbine with variable pitch angle in comparison with zero fixed pitch angle has the ability to produce more power at the middle and lower tip speeds ratios.

The heat transfer performance of passive two-phase cooling devices such as heat pipes and vapor chambers is mainly depend on the topology and geometry of wick and evaporator microstructures. The desired characteristics of evaporator microstructures are high permeability, high wicking capability and large extended meniscus area that sustains thin-film evaporation which choices of scale and porosity lead to trade-offs between the desired characteristics. In the present study, the free-surface shapes of the static liquid meniscus in spherical microstructures and conical micropillar were modeled using gradient decent algorithm of Surface Evolver software and the non-dimensional capillary pressure was determined based on curvature of surface. Permeability and heat transfer coefficient were computed using Fluent software as functions of the nondimensional geometrical parameter and the contact angle between the liquid and solid. Based on these performance parameters, due to high heat contact surface, decrease of cross section, increase of permeability and increase of thin film liquid due to extention of liquid on inclined surface, conical microstructures provide more efficient and desirable geometry for wicking and thin-film evaporation. The solid-liquid contact angle and geometrical parameter that yield the best performance were also identified.

In this study, the flow and temperature fields affected by electric field applied to the fine wire are numerically investigated for the incompressible, turbulent, and steady flow over a single slot. The numerical modeling is based on solving electric, flow, and energy equations with the finite volume approach. The computed results are firstly compared with the experimental data in case of flat plate and the results agree very well. Then, the effect of different parameters such as the applied voltage, Reynolds number, and the emitting electrode position on the heat transfer coefficient and pressure drop is evaluated. The numerical results show that the heat transfer coefficient with the presence of electric field increases by incrementing of the Reynolds number but then decreases. And it also increases by the incrementing of the applied voltage. Moreover, the reduction of distance between the emitting electrode and the slot edges can obviously effect on the heat transfer enhancement, power consumption and pressure drop.

In this investigation, a numerical method based on potential flow has been developed for aerodynamic analysis of morphing MAVs. At first, results of this method are validated with experimental data and then, effect of taper ratio greater than one has been studied on aerodynamic characteristic. Results show that increasing taper ratio, whether is the range of zero to one or bigger one, cause increasing lift coefficient and improving performance of the MAVs. The only disadvantage of taper ratio of bigger one is appearing large pitching moments. Of course, in MAVs is negligible due to the small size. In this study, also a mechanism for creation of different taper ratios has been discussed. Finally, the modeling of insect wings hav been presented for use in the MAVs. The aerodynamic coefficients were compared with conventional wings. The results show a decrease in slope Cm - CL Curve and this represents an increase in longitudinal stability MAV.

In this paper, the effect of fuel injection and intake systems on upgrading the power of RK215 HD diesel engine investigated. The process simulated in four state with full load performance by AVL-Fire V8.3.1 software.
The first state is about the original engine with semi-triangular discharge curve, 139mg fuel injection per cycle, nozzle diameter of 3mm and air intake pressure of 4.2bar. In the second state the effect of changing the discharge curve to rectangular type and reducing the nozzle diameter and increasing fuel injection quantity is investigated which is resulted in 0.2, 21, 16.6 and 27 percent increase in peak pressure, output power, NO and soot emissions, respectively. In the third state, only the effect of air intake pressure increase (5bar) with respect to the second state is considered. In this state the amount of peak pressure, output power and NO emission is increased by 15.3, 26, 9 percent, respectively and soot emission reduced by 4.6%. In the fourth state the nozzle diameter is reduced with respect to the third state. The result is 10.4 and 24 percent increase in peak pressure and output power, respectively. In the other hand, the Soot and NO emissions are decreased about 22 and 4.6 percent, respectively.

In this paper three dimensional numerical study of melting of phase change material (PCM) in a triplex tube heat exchanger is studied. Water is used as heat transfer fluid (HTF) which flows through the inner and outer tubes while the shell side is filled with RT35 as the PCM. The main purpose of this study is to investigate the effect of increasing number of inner hot fluid tube and its arrangement on melting behavior of PCM. Also a comparison between triplex and double tube heat exchanger is done. Enthalpy porosity method is used for modeling the phase change process. The result shows that at the initial steps of the melting process, the major mechanism of heat transfer is conduction while afterward natural convection will be main heat transfer mechanism. Increasing number of inner tubes increases heat transfer surface thus the natural convection intensifies in shell side which considerably diminishes the melting time. Increasing number of inner tube in a triplex tube heat exchanger from 1 to 4 leads to 29 percent decrease in melting time. Arrangement of inner tubes in distributed case intensifies the melting. In a triplex tube heat exchanger in comparsion with double pipe melting time is 80 percent less.

In this study, the process of heat transfer in a fluidized bed of particles belonging to group A of Geldart classification was investigated. For this, an experimental setup was installed, in which a bubble fluidity regime was formed with the combination of hot air and alumina powder belonging to group A of Geldart classification. In each experiment, the temperature of solid phase and outlet gas were measured and recorded over the time, by keeping temperature constant at the inlet opening. Due to the low humidity level of solid particles, mass transfer between the phases was ignored. In the following, using three phase modeling equations and experimental data, a correlation has been provided for predicting heat transfer coefficient between the solid and interstitial gas phases. In this study, heat capacity of alumina powder has been considered as a function of temperature. In addition, finite volume method has been used for solving three-phase model equations. The results of this study show very good conformity between experimental data and numerical simulation so that the maximum error of numerical modeling with experimental data is 9% and the highest temperature variation in the bed happens early in 2 minutes of the process that demonstrates the high rate of heat transfer is in this type of beds.

Development of thermal management methods for advanced electronic devices, especially for micro and nano-scales are important issues which directly affects system performance. MicroChannel Heat Sink (MCHS) is one of the efficient technologies improving the thermal performance and some research works have been conducted in this field recently. The aim of the present paper is to investigate the nanoparticles size effects on thermal performance of a trapezoidal MCHS numerically. In the present study, employment of Water - Alumina (〖"Al" 〗_"2""O" _"3" ) and Water – CuO nanofluids is modeled using Eulerian – Eulerian two-phase approach. Solving equations of continuity, momentum and energy in the computational domain is performed via Finite Volume Method (FVM) in FLUENT with the accuracy of 〖10〗^(-6). Also, since in most studies nanofluids are simulated as a homogeneous (single phase) fluid a comparison between the present two-phase models with homogeneous modeling is conducted and it is found that two phase modeling shows 13.8% better performance in comparison with other single phase modeling. The results showed that adding of both nanoparticles increases the heat transfer in microchannel and increasing the diameter of the nanoparticles results in a decrease of heat transfer rate as Alumina and Cu-O nano particles, show 6.63% and 5.022% reduction in heat transfer rate, respetively. Also in the meantime, Alumina nanoparticles have a lower heat transfer coefficient than CuO nanoparticles. Hence, Water - CuO nanofluid is more appropriate for trapezoid geometry and by optimizing the particle size the thermal efficiency of the system can be maximized.

In this paper, a Proton Exchange Membrane (PEM) fuel cell with new symmetrical flow channel pattern was designed and fabricated in-house. The fabricated PEM fuel cell has nominal power of 10 W, with an active area of 25 cm2 and Nafion 117 membrane. Using a fuel cell test station, the effects of temperature and relative humidity on the performance of fabricated PEM fuel cell (i.e. voltage - current density curve) was examined. Laboratory results showed that fabricated PEM fuel cell is able to produce maximum power density of about 0.45 [Wcm-2] or 11W power. A comprehensive three-dimensional, single phase and non-isothermal model is developed for the fabricated PEMFC. The presented model is a nine-layer model that consists of current collectors, flow channels, gas diffusion layers, catalysts layers at the anode and the cathode as well as the membrane. A commercial CFD software (Fluent) was used to solve the governing equation. Comparison of numerical and experimental results shows that there is a good agreement between them. The results indicate that the new flow pattern can produce uniform temperature, current density and concentration distributions over the PEM fuel cell active area.

In this research, canard and its vertical and longitudinal positions effects on pressure distribution and aerodynamic coefficients of a maneuverable aircraft wing model have been investigated numerically applying the fluent software. The investigations have been performed at the Reynolds number of 5×105 and different angles of attack, using unstructured grid and the Reynolds stress modeling. The applied wing and canard are delta shape and the supposed canard longitudinal positions are front, middle and rear, and its vertical positions are up, middle and low, respect to the main delta wing. For the case of no canard configuration model, the obtained results show that until angle of attack of 25 degrees, there is a very good agreement between the experimental data and the numerical results. Adding the canard causes the vortex on the wing upper surface become stronger and bigger. Also, for all longitudinal and vertical positions of the canard, the canard’s wake passes over the main wing surface. When the canard is at forward-up position respect to the main wing, the highest amount of lift force achieves. Furthermore, the least amount of drag force relates to the case which the canard is closer to the main wing and along with its axis. However, maximum amount of aerodynamic performance achieves when canard is at down-forward position respect to the main delta wing.

The problem first must be dimensional analyized for considering analytic and dimensional problems with free surface. The reason for doing this study is that, nondimesional numbers to be determined. After determining these numbers in analitic considering, the collection of model and prototype must be choosen in a way that nondimensional numbers of prototype with nondimensional numbers of model be the same. In free surface problems, liquid is in touch with air and there are two fluids with different parameters, so there are many nondimensioal numbers in two phases. Unfortunatly in analitic considering the attention is paid only to one of these numbers. It must take into consideration if in analitic study all nondimensional numbers are not considering, it may the analitic results itself in small or big scale in ratio to the original sample have much diffrents with real amounts and the difference is depend on the scale of model.While a lot of scientistst have focused on the vertical surface-piercing circular cylinder in free surface flow,Therefore this research has paid attention to consider the effect of models scale on the results. This research shows that experimental results depend on scale of model.

In this paper, the forced convective heat transfer of cu-water nanofluid over a wedge is investigated numerically. It is assumed that the flow of nanofluid is two-dimensional, laminar and incompressible. The boundary layer approximations are used to simplify the equations of momentum and energy. For solving the differential equations of momentum and energy, the similarity solution and the numerical Keller-Box method have been used. The effects of volume fraction of nanoparticles and wedge angle on the flow field and heat transfer rate are investigated. Numerical results for the dimensionless velocity and temperature profiles, the local friction coefficient and local Nusselt number are obtained. With the addition of copper nanoparticles , the hydrodynamic boundary layer thickness decreases and the thermal boundary layer thickness increased. The results indicate that the addition of nanoparticles increases the friction factor and Nusselt number. Also, increasing the wedge angle has the same effect on the friction factor and Nusselt number