نشریه مهندسی مکانیک مدرس
سال چهاردهم شماره 6 (شهریور 1393)

In this paper، a LQG/LTR controller is proposed for attitude control a geostationary satellite at nominal phase. Basically، proposed methodology includes three parts: LQR regulator، EKF، and loop transfer recovery. Controller design is based on the linearized equations of the spacecraft dynamics using reduced quaternion model. Reduced quaternion model solve uncontrollable problem in some subspaces in the linearized state space quaternion model using all four components of quaternion. Spacecraft actuators are reaction wheels and attitude determination sensors are sun and earth sensors. LQR controller is ideal and it doesn’t account for the model uncertainty and sensor noise and it uses the feedback of the full states. To consider the model uncertainty and sensor noise، we have designed EKF which is used by LQG and LQG/LTR controllers. Controller gain coefficients are obtained using a reduced quaternion model، and based on linearization around the equilibrium point and the natural frequency of the closed loop system. To increase the robustness of the design with respect to solar radiation disturbance، singular values of LQG are approximated to Kalman filter، in LTR section. The results demonstrate that LQG/LTR performance is better than LQG’s and LQG/LTR has a good robust stability margin with respect to disturbances.

In this paper، the sound behavior of a double walled composite with an intermediate porous layer has been conducted using the classical laminated plate theory (CLPT). The main objective of the paper is devoted to considering the analytical study of various boundaries on porous layers as well as parameter study on power transmission through the structure. Thus، viscous and inertia coupling in a dynamic equation، as well as stress transfer، thermal and elastic coupling of porous material are considered based on Biot theory. In addition، the equation of wave propagation are extracted according to vibration equation of composite layers. Then، with applying the various boundaries on the structures along with solving these equations simultaneously، the Transmission Loss (TL) is calculated. The analytical results are compared with both numerical ones obtained from Statistical energy Analysis (SEA) as well as empirical results and an excellent agreement is observed. The parametric studies are presented to investigate the effects of boundary conditions on TL. The results indicate that the interface of porous-composite layers as well as stacking sequences of the composite layers would play an important role in reduction of power transmission through the structure.

In this paper، contact boundary condition between two pieces of the processed meat at different temperatures (two pieces with cold and room temperatures)، that were suddenly in contact with each other in an inverse non-Fourier heat conduction problem was estimated. The direct problem was non-Fourier heat conduction that expressed with fractional single phase model and this problem involved only the piece of the processed meat in the room temperature. In this problem، it was assumed that governing equation، thermophysical properties and initial and boundary conditions were known and then it was solved numerically using the modified Mac-Cormack method. In the inverse problem، the estimation of the unknown contact boundary condition in the fractional non-Fourier model as a new work is done using the parameter estimation version of conjugate gradient method without/with adjoin problem that is one of the efficient deterministic methods in inverse analysis. In addition، in order to obtain the measured temperature of the inverse problem، a linear dual phase lag model validated with experimental data، was used. Finally، these two methods were compared to each other. Their results of these two methods showed the efficient estimation of the unknown contact boundary condition in fractional non-Fourier heat conduction.

This study examines the reinforcing effect of organically modified layered clay on the structure and mechanical properties of the amine-cured epoxy/clay nanocomposites. The epoxy resin system is made of a diglycidyl ether of bisphenol A، Epon 828، as the epoxy prepolymer and Epikure 3234، namely TETA، as the curing agent. The organically modified clay، Closite 30B، is dispersed into the epoxy system in a 0%، 1. 5%، 3% and 5% ratio in weight with respect to the matrix. The state of dispersion was characterized by X-ray diffraction method. The results of XRD show that the clay has been further intercalated by the epoxy matrix. The tensile and flexural properties of the epoxy/clay nanocomposites were investigated according to the standard tests. It is found that the tensile and flexural modulus and flexural Strength of the epoxy/clay nanocomposites increase with adding clay loading into the resin. However، the other mechanical properties such as tensile strength، strain at break and energy to break decrease as the clay nanoparticles is added into the resin. The results also approved that the TETA-cured epoxy resin can be more brittle when the clay nanoparticle is added into the epoxy resin.

In this paper، numerical solution of non-Newtonian fluid flow through a channel with a cavity is studied. Carreau-Yasuda non-Newtonian model which represent dependence of stress on shear rate well is used and the effect of n index of model on attribute of flow is considered. Governing equations are discretized using finite difference method on staggered mesh and the form of allocating flow parameters on staggered mesh is based on marker and cell method. For dependence between continuity and momentum equations، artificial compressibility method is used. Numerical results express that with decrease of n index، the developing length is increased and the velocity in center of channel and pressure drop of flow are decreased.

In this work، mixed convection of Cu-water nanofluid in a trapezoidal enclosure with heat source on lateral walls has been numerically investigated. Vertical walls of the enclosure are kept at constant temperatures of Th and Tc، while horizontal walls are insulated. The mixed convection flow has been generated by passing the fluid through the enclosure and natural convection has been، also، investigated by holding the left wall at a temperature higher than the right wall. In order to examine the effect of the ports position، two cases were considered. Comparison between the results indicates that the rate of heat transfer is higher when the inlet port is near the cold wall than the hot wall. The results have been presented for various volume fractions، Richardson and Reynolds numbers. It was observed that for the considered Reynolds numbers and Richardson number، at a given Reynolds number and solid volume fraction، the Nusselt number increases with increasing the Richardson number. Moreover، at a given Richardson number and solid volume fraction، increasing the Reynolds number results in an increase in the Nusselt number. For the higher Richardson and Reynolds numbers، the nanofluid has more effect on the increase of the heat transfer performance.

In this study، repeated low velocity impacts on aluminum plate are investigated experimentally and numerically. In order to investigate the failure mechanism، Lemitre''s model of the continuum damage mechanics is used. Numerical simulation is carried out employing a Vumat subroutine in Abaqus FE package. Repeated impacts are performed on the plate with the same level of energy. Plastic deformation is observed on the plate in the first impact. During the subsequent impacts and prior to crack initiation، the effect of strain hardening on the aluminum plate is observed. After crack initiation، the stiffness of the structure decreases. As the impacts continue، stiffness further decreases and the damage area increases، finally perforation and penetration appear on the plate. Also، the present model is validated by the experimental results. Comparison of numerical with experimental results shows a good agreement for the force-time and force-displacement histories.

Flexible roll forming is a modern process for producing profiles with changing cross section. One of the important defects in this process is the web warping of product that causes failure to obtain dimensional and geometrical tolerances. In this paper، mechanism of web warping occurrence was investigated by finite element simulation in ABAQUS/CAE software. Results of simulation indicated that inadequate longitudinal strain in the edge of profile’s flange in transition zone is the reason of profile’s web warping. Furthermore، the effect of geometric parameters of product such as flange length، bend angle، radius of transition zone and thickness on the web warping were determined. Analysis of variance showed flange length and bend angle are recognized as the most effective factors on warping of profiles with specific thickness. An equation for prediction of warping was proposed in terms of geometrical parameters of product. In order to verify the finite element model، the longitudinal strain of deformed strip edge was obtained from simulations and compared with the experimental results of other researchers. A good agreement between them confirmed the accuracy of the finite element model.

Tornado is a destructive phenomenon which causes severe damage every year. To improve resistance of structures which face tornado، the flow field and factors which affect damage patterns of tornado need to be investigated. In this paper، numerical simulations of stationary and translating tornadoes are carried out using Ward-type simulator results and large eddy turbulence model. Validation for stationary case has been done with experimental work of Baker. The effects of peak winds، duration of intense winds and acceleration of translating tornado on damage patterns have been investigated. Results show that destruction is more intense at the side of the tornado that translational velocity and tangential wind velocity are added up. Moreover، peak wind velocity and duration of intense winds are important factors that have important effects on the destruction pattern of tall structures. However، the value of the translational acceleration of tornado is important for the design of all structures regardless of their heights.

In this paper the effect of electromagnetic field lengths to change simultaneously is simulated on the temperature distribution and flow velocity of a MHD micropump considering the lateral electromagnetic diffusive regions. The geometry of flow is a two-dimensional channel between two parallel plates and the flow is assumed to be incompressible، steady and laminar. In addition، thermophysical properties such as the dynamic viscosity and electric conductivity of fluid are considered to be the function of temperature. The governing equations of both flow and electromagnetic fields have been solved using the finite volume numerical method a comprehensive analytical solution including velocity، pressure and temperature filed distributions has been derived for an special case. The numerical results show that by changing the length of electromagnetic fields and considering the fluid (water) properties as a function of temperature، for flow in a 1000 mm2 cross-section channel، magnetic field intensity 0. 025 Tesla and electric field strength 20 volt/mm، the flow rate reaches 250 mLit/s and the mean cup temperature from 25 0C at entrance reaches to 40 0C at the exit of channel. However for constant properties، the flow rate and the mean cup temperature reach 70 mLit/s and more than 60 0C respectively.

In the papers published on compressible fluid using Pseudo Bond Graph approach، isentropic flow is assumed. However in a converging- diverging nozzle، for a specific pressure ratio، the assumption of isentropic flow is invalid. For the purpose of considering normal shock effects، this paper introduces a new field (NIKE-field) to the pseudo bond graph. The output of the new field can be also used to determine normal shock position and to extract momentum equation as well. In the following، the methodology developed in this paper has been applied a simple pedagogic example. Simulation result is validated by comparison with the analytical result. As the new field can be modeled non-isentropic flow، it can be used to for modeling rockets motors and thrusters in transient state. One of another advantage of new field (NIKE-field) is that it can be easily used in many software applications like MS1، SYMBOLS2000 and 20SIM®; therefore، With regard to the systematic derivation of a mathematical model from a bond graph in these softwares، there is no need to derive any state equations and their solutions.

Recently، great attention has been focused on epoxy polymers in different industrial and scientific activities، owing to superior mechanical properties and their stability in different environmental conditions. In this study، the molecular dynamics method was used to study the structure of cross-linked epoxy polymers and predict glass their transition temperature (Tg). The epoxy polymer with a certain degree of cross linking was constructed through the previously proposed cross linking procedure. A temperature cycle (300-600 K) with a constant rate was then applied to the cross-linked epoxy، and a rough estimate of the glass transition region was obtained through mean squared displacement curves. Thereafter، variation of density in terms of temperature was utilized to precisely calculate Tg. The estimated Tg was found to be in good agreement with experimental observations. Radial distribution function was finally used to investigate the effects of temperature and cross linking on the local structure of simulated polymer.

The impact of the different integral scales of two isotropic turbulent fields on the dynamics of a shear-free turbulent mixing layer is investigated. To this end، two-dimensional incompressible Navier-Stokes equation is numerically solved using pseudo-spectral method. Governing equations are considered in the vorticity-stream function formulation to guarantee the divergence freeness of the velocity field. Dynamics of the turbulent interaction is examined through relevant statistical parameters such as skewness and kurtosis of the velocity components and their spatial derivatives. Moreover، the efficiency of mixing is investigated by considering the length and curvature of the mixing layer. It has been observed that increasing the difference between the initial integral length scales of two isotropic turbulent fields increases the mixing and anisotropic level of interaction.

In this article، after the design of a CCHP system for office buildings in Tehran، a mathematical analysis of the CCHP system following thermal demand management in comparison to separate system is presented. In order to have a comprehensive evaluation of the performance of the CCHP system، four criteria including primary energy saving، CO2 emission reduction، operational cost reduction and rate of return are employed for a typical office building in Iran. Also a sensitivity analysis of rate of return based on increasing natural gas and electric price is performed. Results show that the CCHP system with selling electricity to grid has much better performance than separate system when all of the criteria are involved. Also without selling electricity to grid the CCHP system achieves more benefits than separate system but these benefits are less than the benefits of the situation with selling electricity to grid. The sensitivity analysis shows that in the situation with selling electricity to grid، with increasing natural gas and electric price the ROR will be increased but in the situation without selling electricity to gird، with 40% increase in natural gas price the ROR will become less than Interest Rate.

Gears are one of the important sources of vibrations and noise in industrial rotating machinery and power transmission systems. In order to design and develop an optimal and quiet geared power transmission system, this paper presents the design of an active vibration control for gear-bearing system. A dynamic model of the geared system is presented, where some undesired parameters in the design such as manufacturing errors, teeth deformations, mounting errors as well as external excitations resulting from distributions of applied torque are included. An active control system is presents in order to control and attenuate the disturbance impress on the system vibrations. The idea behind the design of this control system is to reduce vibration transmissibility by the introduction of the excitation forces in the bearing. The controller is investigated and designed by using feedback control and based on the H-infinity control approach. It can be presented as an optimization problem. To solve this optimization problem, Particle swarm optimization (PSO) algorithm is used, which is one of the optimization methods available among artificial intelligence. The simulation results are performed to investigate performance of the control system.

Sandwich composites are widely used in structural applications because of their appropriate mechanical properties and low strength/weight ratio. Delamination is common failure mode in these structures that lead to a reduction in strength and stiffness of composite. In this paper, using acoustic emission, initiation and propagation of delamination in sandwich composite specimens was investigated. The specimens were loaded under mode I loading. Then the characteristics of the signals related to different damage mechanisms were specified. The acoustic emission signals were classified based on their frequency ranges. Then the acoustic emission signals were recorded during the test specimens were processed using wavelet transform. Thus the percentage of energy in each components of the acoustic emission signal was specified. Each of these components has a certain frequency range corresponding to a damage mechanism. Thus the percentages of different damage mechanisms in each specimen were specified. The Scanning Electron Microscopy (SEM) was also employed to verify the results which were obtained from acoustic emission and wavelet transform method. The results showed acoustic emission is efficient tool for identification and separation of different damage mechanisms in sandwich structures.

In this paper, ballistic impact on sandwich panel with composite face sheet made of Glass/Epoxy and aluminum honeycomb core has been studied. The solution is derived from a wave propagation model. At first both analytical and numerical solutions were clarified and their results were compared with experimental results. Some deformation patterns, failure modes and energy absorption mechanisms were identified by observation, such as: dynamic movement of the target, stretching, bending deformation, delamination, debonding, shear fracture honeycomb, tensile fracture of Glass/Epoxy and plug and petal formation in composite facings. The solution involves a four-stage and effective masses of the face sheets and core as the shock waves travel through sandwich panel are derived using Lagrangian mechanics. The resulting non-linear differential equation of motion was solved considering the local damage effects and corresponding energy absorptions. Also numerical model, analysis of the penetration process was performed by a nonlinear explicit finite element code, LSDYNA. The results of analytical solution and numerical simulation are compared with experimental tests. Ballistic impact tests is carried out on the samples by flat-ended projectile with 8/5 gr mass and 10 mm diameter in difference velocities.

The present paper investigated the capability of various non-linear k–ε models for predicting flow field and pollutant dispersion around a cubical model building with a stack vent located on its roof center within the turbulent boundary layer. One quadratic model proposed by Nisizima and Yoshizawa, and two cubic models, proposed by Lien et al. and Ehrhard and Moussiopoulos were examined by comparing their simulation results with the wind tunnel data and standard k–ε model. All the computations were performed by using the self-developed object-oriented C++ programming in OpenFOAM CFD package, which contains applications and utilities for finite volume solvers. The standard k–ε model provided inadequate results for the flow field, because it could not reproduce the basic flow structures, such as reverse flow on the roof. By contrast, the non-linear models were able to predict anisotropic stresses and correctly showed the dominant stress over the roof to be the streamwise Reynolds stress. The non-linear models were able to predict the concentration field better than the SKE model due to inclusion of the quadratic and cubic terms. Among the RANS models, the Ehrhard model showed the best agreement with the experimental data. It was shown that concentrations predicted by all turbulence models were less diffusive than those of the experiment, although the non-linear k–ε models have reduced this difference.

A two dimensional generalized plane strain micromechanical model is developed to study electro-elastic behavior of piezoelectric fiber reinforced composites (PFRC) with transverse polarization. A small repeating area of the composite, representing a quarter of fiber surrounded by matrix is considered as representative volume element (RVE). The composite system consists of long parallel piezoelectric fibers with transversely isotropic properties and perfectly bounded to the isotropic matrix in a square array arrangement. In addition, the constituents are assumed to have both linear elastic and electrical behavior, whereas, the matrix is piezoelectrically passive. The element free Galerkin method is employed to obtain solution for the governing system of partial differential of equations. In this method, the Moving Least Square shape functions are used to approximate the field variable at arbitrary point. Comparison of the presented results with other techniques available in the literature reveals good agreement. It is demonstrated that the piezoelectric coefficient “e31” in the transverse polarization is considerably improved in comparison with corresponding coefficient of pure piezoelectric material. Furthermore, as a result, it is found that fibers with elliptical cross section may enhance the amount of electrical sensitivity of PFRC several times than circular fibers in a specific direction.

Ductile iron pipes are widely used in modern drinking water and wastewater networks. They often produce by horizontal centrifugal casting process. In this reasearch, the finite element base package ANSYS software has been used for thermal simulation of horizontal centrifugal casting process of ductile iron pipes.The simulation includes obtaining temperature distribution of mold and cast during different temperature cycles. In the simulation, latent heat due to solidification, temperature-dependent thermo-physical properties of material, heat transfer coefficient in metal-mold interface due to mold coating and air gap and thermal boundary conditions proportional to practical conditions, are considered. In this paper, pouring process to get transient thermal distribution in main body of mold and cast are also simulated. The results of the thermal simulation show good agreement with the experimental results conducted in this study and literature. The results can be used as input data for the numerical model to estimate thermal fatigue life of a permanent mold. The results of simulation have shown that, the thermal resistance of the air gap and mold coating has a significant effect on the temperature distribution in the pipe and the mold. Pouring process causes temperature gradients in the axial direction in the mold and the cast.

The problem of bone fracture in medicine due to an accident, aging or diseases, has existed from times when humans started to work and activity. The process of bone drilling is an essential part of internal immobilization in orthopaedic and trauma surgery. The force required to chip formation in drilling process, resulting in heat generation in drill site that leads to the occurrence of thermal necrosis. This research experimentally investigates the effect of ultrasonic vibration on thrust force in drilling of bovine femur bone. This method induces high-frequency and law-amplitude vibration in the feed direction during cutting, and has the potential to spread tiny cracks in bone and decrease friction leading to reduce of cutting forces and also increase the speed of chip disposal leading to reduction of machining forces, totally. Experimental results demonstrate that ultrasonic assisted drilling of bone produces fewer thrust force than conventional drilling and rotational speed of 1000 rpm is the optimal speed to achieve at minimum thrust force for all feed rates. Moreover, this method is due to the force independence of the feed rate in the rotational speed of 1000 rpm, is applicable in orthopedic surgery.