Modares Mechanical Engineering
Year:2012 | Volume:12 | Issue:2|Papers Count:17

In the present paper, the parameters of Johnson-Cook (JC) constitutive model for two steels have been identified, based on the Hopkinson pressure bar data found in the literature. Using the measured strain pulses, the experimental stress-strain and deformation-time curves can be extracted. The experimental data have been processed using two different methods. In the first method strain rate assume to be constant during deformation and in the other one the deformation has been applied to a modeled specimen. In each method, an optimal set of material constants for JC constitutive model have been computed by minimizing the standard deviation of the numerically obtained stress-strain curve from the experimental data. Also a sensitivity analysis has been performed on JC constitutive model parameters and temperature changes during test have been investigated. The obtained results show that using constant strain rate method, leads to considerable error in results; for example in this study the minimum error is about 14%.

In this paper an analytical model for investigating of the ballistic impact behavior of two dimensional woven E-glass/epoxy composites is presented on the basis dividing the impact duration to several time intervals and calculating the energy absorbed during each time interval. The major components of energy lost by projectile during ballistic impact are identified, namely the cone kinetic energy formed on the back face of the target, the secondary yarns deformation energy, the tensile failure energy of primary yarns, the delamination and matrix cracking energy. It is assumed that the shear plug formation is not observed for glass reinforced composites and the energy lost in overcoming the frictional force between projectile and composite is negligible. Analytical formulations have been presented for calculating energy absorbed by each mechanism in each time interval. Finally, a good correlation has been observed, comparing the analytical model presented in this paper to the experimental results presented by others investigators.

In this study, suspension system in vertical direction of depressed center wagon with six axles is optimized for transporting of high weight transformers on straight tracks. For this purpose, dynamics of depressed center wagon simulated in two ways, first by ADAMS / Rail software and then by Newton-Euler analysis method. In Newton-Euler analysis method non-linear equations of motion and kinematical constraints have been solved in time domain. A function for optimization has been defined based on Non Operating Shock Specification (NOSS) factor that is factor of vulnerability in transformer transportation. By performing many simulations on variation interval of selected parameters, suspension of this wagon has been optimized for transporting of transformers. Results show that for improvement of dynamics behavior of depressed center wagon, stiffness of suspension element must be changed a little.

This article presents a transient model of a solar adsorption cooling system. A computer program is developed to simulate the operation of a two-bed silica gel- water adsorption cooling system as well as flat plate collectors and the hot water storage tank. This program is then utilized to simulate the performance of a sample solar adsorption cooling system used for cooling a set of rooms that comprises an area of 52 m2 located in Ahwaz city in Iran. The results include the temperature profiles of hot, cooling and chilled water in addition to adsorption/desorption beds, evaporator and condenser, collector hot water temperature, auxiliary heater fuel consumption and solar fraction of the system. Furthermore, the effect of the cycle time on COP (coefficient of performance), refrigeration capacity, fuel consumption and solar fraction is studied. The results show that the cycle time increases COP and SF of the cycle but decreases cooling capacity and supplementary heater fuel consumption.

In this paper, numerical solution of beta-type Stirling engine was presented considering its non-ideal regenerator. To this end, the second-order model including thermal and hydraulic losses of regenerator was used and their effect on the output power and efficiency of the engine was obtained. Then, a numerical code was used for calculating dimensional and functional optimum values of regenerator. To confirm the obtained result, the functional and geometrical parameters of the engine made by General Motors Corporation called GPU-3 were used. According to the obtained results, the values of hydraulic and thermal losses in the regenerator were considerable and led to the decrease in the engine power and efficiency by 23% and 8%, respectively. Using the obtained results and numerical code, the amounts of porosity, frequency and length of the regenerator were suggested as less than 0.7, 35 Hz and 24 mm, respectively, in the optimum physical and geometrical conditions of the engine.

In the present study the phenomenon of drying a ceramic mud is analyzed. Strain-Stress equations coupled to heat and mass transfers during drying of deformable two-phase media has been model and both 2D and 3D arrangements have been studied. In order to compare the results, ceramics with similar composition is used for both configurations. The product is considered as a two phase, homogeneous, isotropic, and highly shrinkable medium. The principal equations of the model, because of the shrinkage behavior are written in a Lagrangian formulation. The model is solved numerically by a finite difference method And Validation of results is achieved by comparing the numerical and experimental data. The simulation allows the derivation of the time and space evaluating moisture, strain, and stress. A significant difference was observed between the results obtained for the two different configurations particularly in intensity of the stress causing cracking.

This paper has been presented a new method for optimum design of multi-stage axial flow compressor blades considering overall performance and stage matching. This tool is made up of an optimization algorithm, a flow solver and a parametric geometry generation system. A two-dimensional streamline curvature code has been improved to evalute the compressor performance parameters and flow field. Design parameters consist of geometrical parameters and aerodynamic performance parameters include the minimum loss of blade and allowable range of incidence. The efficiency increment of a 10-stage compressor has been investigated to evaluate the proposed method. The geometry of three front stages of compressor is fixed, the geometry of three middle stages is optimized and four rear stages have been re-staggered, At the first, compressor is optimized by re-designing middle stages and second, it is optimized by redesigning middle stages and re-staggering rear stages. In best case, compressor efficiency has been improved by 1.18 in nominal speed and 1.83 percent in 95% of nominal speed.

nonholonornic mobile robots are widely used in industrial environments due to their extended workspace. Also, to increase the productivity and efficiency of the mobile robot, their path planning is an important task, and is attracted attention of many of robotic scientists. In this paper, the optimal path planning of the wheeled mobile robots are performed considering their nonlinear dynamic equations and the nonholonomic constraints. Problem of the trajectory optimization is formulated, and conditions of the optimality are derived as a set of nonlinear differential equations by means of indirect method of optimal control method. Then, the optimality equations are solved numerically, and variant simulations are executed. To verify the simulation study, some experimental analysis are done for the Scout mobile robot and compared to the simulation results. The experimental analysis verifies the simulation results, and demonstrates the applicability of the proposed method for the optimal path planning of the mobile robots.

source in a horizontal channel is performed. The walls of the channel are adiabatic and the heat source is placed at the bottom wall of the channel. Free flow at cold temperature enters channel and takes heat from heat source. Discretization of the continuity, momentums and energy equations are achieved through a finite volume method and solved with SIMPLE method. The Brownian motion of nanoparticles is simulated to determine the thermal conductivity of the nanofluid. The results show that using the nanofluid caused to heat diffusion and average temperature of source to increase. Also, increase in solid volume fraction causes increase in heat transfer especially at high Reynolds number. It is understand that with increase in ratio of length to height of source in its constant area, heat transfer decreases first and then increases.

In this research, the aerodynamic design of a centrifugal compressor is carried out using an inverse design method. At the first step of the aerodynamic design, the shape modification capability of compressor meridional plane is generated by linking up the Ball-Spine inverse design algorithm as a shape modification algorithm and quasi 3D analysis code as a flow solver. Then, the meridional plane is modified by improving the hub and shroud pressure distribution and applying it to the inverse design code. At the second part of this research, by developing a novel design method on the blade to blade plane, and incorporating it into the quasi 3D flow solver, the 3D profiles of impellers will be obtained in order to reach the higher blade loading. Finally, to check the outcome of design process, the current and the modified impellers are analyzed using the full 3D flow solver, CFX. The results are the representatives of about 5 percent enhancement in compressor total pressure ratio.

in this article, jet flow from the nozzle exit was considered numerically in transient two-dimensional axsymmetric turbulent condition using volume of fluid method (VOF). The hydrodynamical instability for jet flow was analyzed and the breakup length was calculated, the nozzle diameter of 0.11875mm, 0.2375mm and 0.475 mm, the center line velocity of the fluid in the 15.416 m/s and 7.708 m/s and the fluid inside the nozzle are water and gasoil used for the calculation. Fluent6.3.26 was used for two-phase flow analysis. The results obtained from the present numerical simulation compared with the experimental results of the previous researches, good agreement was obtained. It was concluded that the breakup length decreases as the relative velocity between the phases increase or the liquid density decreases. The nozzle diameter is an important parameter which effect of the nozzle outlet regime and the breakup length.

This paper introduces a new approach for the reduction of sound radiation from the mechanical structures. A combination of genetic algorithm method and geometry modification concept minimizes the root mean square level of structure borne sound for a square plate over a specific frequency range. The structure's local geometry modification values at the selected surface key-points considered as design variables. The model is under three non-symmetric harmonic excitations. An iterative approach is used to develop new modified model until when a termination criterion is reached. The results show that this approach could produce significant reduction in the value of radiated sound power level of the structure.

In the present study, natural convection in a porous cavity in the presence of a non-isothermal biochemical heat source is investigated numerically. The porous cavity is assumed to be homogenous and the Darcy model is used for the momentum equation. The porous cavity includes two species namely biomass and substrate. The heat source generated in the cavity is equal to the rate of consumption of the substrate by the biomass. The bio-chemical source in the energy equation is proportional to the generation rate of solute concentration governed by a Monod model. Heat generation in the porous media, the rate of consumption of a substrate by a biomass, temperature distribution in the cavity, and the local Nusselt number are analyzed completely. Increasing the porosity leads to an increase in the biochemical heat generation in the cavity.

This research is dedicated to the estimation of the effective elastic modulus of polymer concrete by a new proposed micromechanical model. The capability of the equivalent inclusion methods have been proved by numerous researches. The Mori- Tanaka (M-T) model is the most used homogenization scheme for two-phase composite materials. The M-T model provides good estimates of the stiffness tensor for two-phase composites with low to moderate volume fraction of aligned inclusions. However, when the volume fraction of reinforcing phase is high, the M-T model is unable to predict the stiffness tensor accurately. The major disadvantage of the M- T model is that when volume fraction is high, in the associated isolated inclusion medium (AIIM) the properties of the reinforcing phase does not affect the matrix properties. The idea of the current proposed model is that in high volume fraction in the associated isolated inclusion medium the matrix phase must be affected by the reinforcing phase properties. In order to evaluate this model with experimental results, twelve mixes with two different compositions were manufactured and tested. In addition, the results from other researches were used to evaluate this model. The validation of the proposed model with the experimental data and the results by other researchers shows remarkable predictive capability of this model.