Modares Mechanical Engineering
Year:2014 | Volume:13 | Issue:13|Papers Count:22

In this study design, construction and test of a solar bread cooker has been undertaken for solar radiation status in Kashan. In design step and in order to produce the required energy, use of a parabolic reflective surface for concentrating the solar flux on the back of a cooking plate was considered. For cooking thin bread with 45 cm diameter the diameter of the reflective surface was calculated 130 cm and the cooking plate diameter and thickness were considered 48 cm and 6 mm, respectively. Since the sun orientation changes with time the reflective surface became capable of being adjusted around a north-south as well as east-west axes; also it may be moved vertically up or down. For evaluating the performance, tests and measurements were performed on different days of summer, 1391. The results indicated the validity of calculations and showed the overall efficiency of the solar bread cooker is around 50%. With the solar cooker it is possible to cook 12 beards per hour each with 200 gr weight of dough in eight months of the year at least for six hours in every sunny day; also it may be used for cooking other foods.

In this study, classical and generalized dual phase lag bioheat transfer models are applied for investigate thermal damage to skin tissue exposed to the transient heat flux. The analytical bioheat transfer analysis with transient heat flux on skin tissue has only been studied by Pennes and thermal wave models. This paper, for the first time, provides the analytical solution of the dual phase lag model in skin tissue under transient surface heating using Laplace transform method and inversion theorem. Since the dual phase lag model under certain circumstances reduces to the Pennes and thermal wave models, comparisons of the temperature responses and thermal damages between the these three models are carried out. The influence of porosity factor and coupling factor between blood and tissue on the thermal damage of tissue is investigated and the results demonstrate that increases in these factors respectively leads to the higher and lower tissue thermal damage and the effects of these factors on the thermal damage in the depth of tissue is lower than near the surface.

In this study a feed forward back propagation artificial neural network (ANN) model was established to predict Vickers microhardness in aluminum-alumina nanocomposites which have been synthesized by mechanical alloying and hot pressing. Volume percent of reinforcement, size of nanoparticles, force in microhardness test; and mechanical alloying parameters, such as time, ball to powder ratio (BPR) and speed of ball mill were used as the inputs and Vickers microhardness as the output of the model. Effective parameters in training such as learning rate, hidden layers and number of neurons, were determined by trail and error due to amount and percentage of errors. Regression analysis in train, validation and test stages; and mean squared error were used to verify the performance of neural network. Average error of predicted results was 2.67% or 2.25 Vickers. Also mean squared error for validation data was 7.76. As can be expected, ANN methods reduce the expenses of experimental investigations, by predicting the optimum parameters.

In this paper, exact closed-form solutions in explicit forms are presented to investigate small scale effects on the buckling of Lévy-type rectangular nanoplates based on the Reddy’s nonlocal third-order shear deformation plate theory. Two other edges may be restrained by different combinations of free, simply supported, or clamped boundary conditions. Hamilton’s principle is used to derive the nonlocal equations of motion and natural boundary conditions of the nanoplate. Two comparison studies with analytical and numerical techniques reported in literature are carried out to demonstrate the high accuracy of the present new formulation. Comprehensive benchmark results with considering the small scale effects on buckling load ratios and non-dimensional buckling loads of rectangular nanoplates with different combinations of boundary conditions are tabulated for various values of nonlocal parameters, aspect ratios and thickness to length ratios. Due to the inherent features of the present exact closed-form solution, the present findings will be a useful benchmark for evaluating the accuracy of other analytical and numerical methods, which will be developed by researchers in the future. Also, the present study may be useful for static and dynamic analysis of thicker nano scale plate-like structures, multi-layer graphene and graphite as composite or sandwich structures.

Two-phase flow modeling has been the subject of many investigations. However, fewer studies are corresponded for two-phase flow within a porous medium, because of additional complications. In this paper, two-phase flow with the density and viscosity ratio of 1, within a porous medium is simulated by Shan and Chen model. Due to inherent limitations and weaknesses of this approach in an independent control of surface tension, investigation of parameters such as Reynolds number, Froude and Weber is not applicable. However, porous medium parameters such as Darcy number and contact angle could be studied by changing the porous medium and contact angle. Competition between opposing forces against the drop and the capillary effect because of increasing the number of particles in the porous media is described using the Darcy number. Also the effect of the contact angle between liquid-gas phases and the solid surface is evaluated on the droplet penetration inside the porous medium.

In this article, nonlinear bending analysis of single-layered circular graphene sheet is studied. The equilibrium equations are derived based on the nonlocal continuum mechanics and principle of virtual work and first order shear deformation plate theory (FSDT). Differential quadrature method is used to discretize the equilibrium equations. In this method a non-uniform mesh point distribution (Chebyshev- Gauss- Lobatto) is used for provide accuracy of solutions and convergence rate. The effect of nonlocal parameter, thickness, number of grid points and lateral loading are investigated on deflection of graphene sheet. The results are compared with valid results reported in the literature.

This paper presents a new method to design optimal thrusters’ configuration for geostationary satellite in order to reduce the fuel consumption and increase the control accuracy. The thrusters configuration generally contains information about thrusters fixed on the satellite body structure, including their location, orientation. One important factor playing a key role in thrusters’ configuration design is satellite force-torque analysis. The proposed configuration, however, should lead to fulfill specified attitude maneuver when the set of force and torque produced by satellite thruster system is adequate. For this purpose, two optimization methods using genetic algorithm (GA) and differential evolution (DE) has been applied to determine the optimal thrusters configuration on the communication satellite body. The cost function employed to minimize both the fuel consumption and error generated by thrusters installation and uncertainties. Moreover, this work allows applying some different constraints in the proposed formulation including minimization of the thruster plume impingement effect on the satellite outer structure and on the solar arrays and the second one is the satellite dimension and geometry. Simulation results show that DE outperforms GA in terms of accuracy and CPU time. Effectiveness of differential evolution algorithm is illustrated in the paper when compared with GA results.

This paper studies the vibration characteristics of a cracked Timoshenko beam with a varying transverse cross-section using Differential Transform Method (DTM). The effects of the crack location and the crack size in calculating the natural frequencies and mode shapes are investigated. The result have been validated for a beam with and without the crack against those obtained from experimental modal test, Abaqus software and some other methods reported in the literature and a good agreement between the results is observed. The results show that the Timoshenko theory predicts fewer values for the natural frequencies because there is less rigidity, especially for large values of cross-section moment of inertia. Then, the inverse problem is investigated. For this reason, the position and depth of the crack of the beam with a varying transverse cross-section are estimated using the genetic algorithm and then, the natural frequencies are obtained from the modal test. It is seen that the numerical results have a suitable agreement with the actual position and depth of the crack that indicates the effectiveness of this method in determining the parameters of the crack in the Timoshenko beam.

In this paper, the effect of size of an atomic force microscope (AFM) with an assembled cantilever probe (ACP) on resonant frequencies and their sensitivities are investigated using the strain gradient elasticity theory. The proposed ACP comprises a horizontal microcantilever, an extension and a tip located at the free end of the extension, which make the AFM capable of scanning the sample sidewall. First, the governing differential equation of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of the strain gradient elasticity theory and Hamilton principle. Afterwards, the flexural resonant frequency and sensitivity of the proposed AFM microcantilever are obtained numerically. The results of the proposed method are compared with those of modified couple stress and classical beam theories. The comparison shows that the difference between the results predicted by the strain gradient elasticity theory with those obtained by couple stress and classical beam theories become significant when the horizontal cantilever thickness comes approximately close to the material length scale parameter.

Airflow over a passenger train has been investigated experimentally and numerically in this research. The experimental model was a 1: 26 scale model of a real train including a locomotive with one wagon behind it. A total of 16 pressure tabs for train were employed to measure the air pressure at various points on the model for different air flow velocity. Turbulent, incompressible and 3D model of air flow has been applied in numerical simulation. The numerical results of pressure coefficients were compared with the results obtained by the experimental investigation for the numerical simulation verification. The wagon number affect on the train drag coefficient and air pressure distribution on the symmetry plane of the train have been investigated numerically. The results show that the drag coefficient increases to 1.2336 for a locomotive and 7 wagons behind it but the air flow velocity has not a sensible affect on the drag coefficient. The averaged drag coefficient of each intermediate wagon has been obtained 0.1321.

The performance of turbine section of a gas turbine deteriorates over operation because of working in high temperature conditions and characteristics of the entry gas. On the other hand, due to complexity of the flow field within the turbine, three-dimensional analysis is required. This paper presents a numerical study of roughness effects on turbine flow field and performance. In this paper, effects of blade surface roughness caused by operation conditions on turbine performance were numerically calculated. Numerical calculations were carried out for the fourth stage of an axial turbine which was experimentally tested in the technical university of Hannover, using ANSYS software. Calculated results were verified with the measured data and showed a good agreement. To find out the effects of blade surface roughness on turbine stage performance and flow field, Two equivalent sand-grain roughness heights of 106㎛ (transitionally rough regime) and 400 mm (fully rough regime) in four different mass flow rates were considered. Results showed that summation of efficiency reductions of the rough stator and rough rotor approximately equals to that of the totally rough stage for each roughness height and effect of stator roughness on efficiency reduction is same as the effect of rotor roughness on stage efficiency.

The aim of this research is to study effective parameters of incompressible, viscous and unsteady flow in Turbomachinary cascades using the Spalart-Allmaras (SA) RANS-Based Delayed Detached Eddy Simulation method. Detached Eddy Simulation is a hybrid RANS-LES method that was purposed in order to reduce LES computational cost. In this method, near wall, in boundary layer, RANS turbulence model is used and away from the wall, method automatically switches to LES. To develop original DES method (DES97), DDES was purposed to solve modeled stress depletion problem. A new function is introduced to the DDES model to make the transition from RANS to LES grid cell size independent. The numerical method that is used for discretization is staggered finite-volume and the grid is Cartesian. Also hybrid differencing scheme (the scheme compound of central differencing scheme and upwind scheme) to discretization of convection terms in Navier-Stokes is used. The results of this study compared with the results of simulation with SA turbulence model.

In the present work, creep buckling of linear viscoelastic plate was studied. Pseudo-transient or Dynamic Relaxation method with finite element discritization was used for solving the nonlinear governing equations of the plate. The displacements were based on first order shear deformation theory. Von Karman assumptions were considered for strains, including initial imperfection of the plate. Central deflections of the rectangular PMMA plate as well as end-shortenings were obtained during the loading of the plates with simply supported and clamped edges. The results compared well with commercial finite element code ANSYS.

High temperatures and different properties of entering gas into the turbine of a gas turbine cycle can decrease its performance. Considering the complexity of the flow distribution inside the turbine, three-dimensional analysis to find out the flow and temperature field in the turbine stages is very important. As time passing the increasing of the roughness of blades is unavoidable.The aim of this paper is investigation of the blades roughness effects on flow field and efficiency of gas turbine with numerical calculations. In this research, a two-stage turbine is modeled in the form of three-dimensional and the results are validated with experimental data. Then the effects of blades roughness on flow field and performance of turbine in five pressure ratios is investigated. Also, in order to determine the role of stators and rotors in decreasing the turbine efficiency, in a special roughness, the first and second stators and then corresponding rotors have separately been examined and then this phenomenon affected on blades simultaneously. Results showed that the efficiency drop by applying all together on the turbine stage is approximately equal to summation of efficiency drops by applying separately.

In this study, various amounts of clay nanoparticles and titan nanoparticles (1, 3 and 5% wt.) were introduced into a vinyl ester resin matrix by high shear mixer. The influence of these nanoparticles on the mechanical properties (tensile strength, tensile modulus, flexural strength, flexural strength and fracture toughness) is investigated. To investigate the structure of nanocomposites, X-ray diffraction (XRD) and transmission electron microscopy (TEM) tests are done. The XRD test shows that the structure of clay-vinyl ester nanocomposites is exfoliated. The results of tensile, flexural and fracture toughness experiments show that clay is better than titan in the improvement of the mechanical properties. Clay- vinyl ester nanocomposite with 1% wt. of clay has the better mechanical properties than others samples.

The aim of this study is to investigate the influence of structural defects on the mechanical properties of single-walled carbon nanotubes. During the growth process and functionalization of carbon nanotubes (CNTs), the Carbon-Carbon bonds in their nano-structure are broken. To evaluate the influence of this defect on the Young’s modulus of CNT, the number of broken C-C bonds, distribution and their arrangements in the nano-structure of CNT are all treated as random parameters. In this study, the finite element model of the CNT is built using nanoscale continuum mechanics approach and then structural defects are applied randomly. The Young’s modulus of two defect types, known as Stone - Wales and Vacancy are investigated. The results reveal that the influence of Stone – Wales defect on the Young’s modulus of CNT is much less than that of Vacancy one. Moreover, a linear decrease in Young’s modulus with respect to the increase in defect density is observed. In contrast to the available researchers in open literature, three parameters consisting of the number, distribution and arrangement of defects are modeled on the basis of full stochastic simulation.

In tube hydroforming, the loading path, that is the relationship between axial feeding and internal fluid pressure, is of important significance. Researchers have employed various optimization approaches to find an optimum loading path. In this research a statistical method based on finite element analysis has been developed. An accurate FEA has been used to simulate the process and to find the response of the process to the loading. The Response Surface Method (RSM) has been used to model the responses from the finite element analysis. The behavior of the process can be predicted using this model. The obtained model then used to optimize the process. Since The RSM model was initially obtained for a predefined domain of variables multilevel optimization was employed to improve the accuracy of the model. The multilevel optimized curve yielded the best thickness uniformity, the result of which are reported.

A hybrid cooling system encompassing; cooling tower, cooling coil and evaporative cooler have been discussed in present study. Thereinafter, the hybrid system model is used to predict cooling potential of the system under various operational conditions. In order to have an accurate performance prediction of the hybrid cooling system, a numerical simulation was performed and the results validated using experimental measurements. The presented hybrid cooling system provides the necessary pre-cooling effects, enabling a direct evaporative cooler that cools the air even below outdoor air wet-bulb temperature. Besides, the potential of presented hybrid cooling system to provide thermal comfort in various outdoor design conditions evaluated and compared with conventional direct evaporative cooler. Numerical simulation revealed that the hybrid system complements direct evaporative cooling. Based on the simulation results, the overall cooling effectiveness of hybrid system is tangibly magnified 10%-20% and also it is able to fulfill the comfort condition in extended climate conditions rather than stand-alone direct evaporative coolers. Also in present study water loss amount of hybrid system and direct evaporative cooler were verified in various climate conditions of Iran.

Specifying of the dynamic behavior and impedance characteristics determination of the patients’ lower limb is a necessity for interactions with rehabilitation set-up. In simple proposed models of previous researches, the effects of joint angles on the impedance properties of the leg were not considered and the parameters were identified by nonparametric methods. Hence, the results of identification were not so much appropriate to analyze the rehabilitation process. So, in this paper, a 3 DOF nonlinear dynamic model is developed regarding to the angular stiffness and damping of the lower limb joints and moment of inertia of its moving organs. Then, a linear model is presented to identify the parameters by PEM method. The linearization and identification precisions are evaluated individually. The leg’s parameters are identified for 3 subjects with different mass numbers at four distinctive configurations which are considered during the walking. The identification accuracy is evaluated by comparison between the identified and geometrically estimated mass moment of inertia. Ultimately, the interaction performance of the patients’ leg with rehabilitation pneumatic actuator is investigated considering to leg rigidity and the amount of overweight. The results demonstrate good accuracy and high performance of the linear model for parameters identification.

In this paper, the equivalent layup method to estimate the delamination initiation of asymmetric double cantilever beam (ADCB) is presented. A relation for critical strain energy release rates (SERR) of unidirectional (UD) and multidirectional (MD) laminates, as a criterion for crack growth initiation, was presented. For finite element analyses, the ADCB specimens were modeled in ABAQUS/Standard software and SERR is determined using the virtual crack closure technique (VCCT). According to this method, the ability of estimating delamination initiation without using experimental tests and finite element modeling of MD specimens is provided. This method reduces the volume of calculations FEM and experimental tests significantly. Accuracy of the proposed method has been verified by experimental data. Also, the total strain energy release rate analyzed by this method is compared with the FEM and theoretical results. Results show that SERR of MD lay-ups can be predicted by measuring SERR of the unidirectional specimens using FEM method with an error less than 10%.

It has been proved that developing a supercaviting flow over under-water projectiles has an important role on their drag reduction, so many of researchers have focused on this subject during recent decade. In this research, the geometrical characteristics of supercavitaties developed behind three different conical cavitators with conic angles of 30, 45 and 60 degrees are studied numerically and experimentally. The experiments were done in an open-loop water tunnel. The fluid flow velocity in the test section was between 27 to 38 m/s. Also the 3D multiphase fluid flow over the cavitators within the test section are modeled and analyzed numerically by solving the corresponding governing equations using finite volume method and mixture model. Good agreement was observed in comparison between the numerical and experimental results. Finally, effects of some important parameters .i.e. the cavitation index, inlet velocity and conic angle of the cavitators on the geometrical characteristics of the supercavities are discussed.