نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 14 (اسفند 1392)

In this paper, the governing equations for a vibratory beam with moderately large deflection are derived using the first order shear deformation theory. These equations which are a system of nonlinear partial differential equations with constant coefficients are solved analytically with the perturbation technique and the natural frequencies and the buckling load of the system are determined. A parametric study is performed and the effects of the geometrical and material properties on the natural frequency and buckling load are investigated and the effect of normal transverse strain and axial load on natural frequency are examined. Some results based on the first order shear deformation theory are consistent with classic theories of beams and some yield different results. Formulation presented to calculate the transverse frequency, determines the axial frequency too. Also, the natural frequencies and buckling load are calculated with the finite elements method by applying one and three-dimensional elements and the results are compared with the analytical solution.

Extended finite element method (XFEM) is one of the strongest numerical methods that its basis is finite element but regardless of mesh location respect to discountinuty solves the problems. In this method, using of enreaching the nodes and increasing of their degrees of freedom (from 2 to 4 or even upto 10) virtually and without verifying the mesh and geometry of discountinuty, one can model and develop the required governing equations of the system. In this paper, fatigue crack growth of repaired aluminum panels containing a crack is studied. The cracked panels were repaired on one side with glass/epoxy composite patches in the mixed mode condition. The extended finite element method is used to study the effects of patch lay-up configuration on crack front displacement and stress intensity factor and the effect of crack angle on stress intensity factor of the repaired panels. The results show that the plate-fiber-fiber-aluminum configuration has best effect and it could reduce the stress intensity factor (k1) by upto seventy percent.

In this study، a two-dimensional، axisymmetric، computational Algorithm has been developed to simulate the plasma flowfield in a MPD thruster for the purpose of determining the flow behavior and electromagnetic characteristics distribution. The solution employs Roe’s flux vector difference method in combination with Powell’s characteristics-splitting scheme. To ensure the stable high-accuracy solution، new modification of MUSCL technique so called OMUSCL2 method is used. According to being supersonic strong gasdynamic expansion near the electrodes tip، HHT entropy correction is employed. Further improvements to the physical model، such as the inclusion of relevant classical transport properties، a real equation of state، multi-level equilibrium ionization models، anomalous transport، and multi-temperature effects، that are essential for the realistic simulation MPD flows، are implemented. Numerical results of a lab-scale thruster are presented، whereby comparison with experimental data shows good agreement between the predicted and measured enclosed current and electric potential.

In this paper، heat transfer in a sinusoidal channel filled with nanofluid under magnetic field effect is investigated numerically. The magnetic field transversely applied to the channel. Water as a base fluid and copper as nano particles were considered. The Maxwell-Garnetts model and Brinkman model for heat conduction coefficient and dynamic viscosity were used respectively. The effects of changing some parameters such as shape، volume fraction، Hartmann number and Reynods number were considered. The results show that increasing in all mentioned parameters lead to increasing in Nusselt number. Volume fraction is mainly affect on maximum local Nusselt number in each channel’s wave while Hartmann number is affected minimum and maximum Nusselt number.

Carbon nanotubes (CNTs) are rolled form of graphene sheet with unique properties due to the covalent bonds between carbon atoms. In this research، different structures of CNTs are studied for a wide range of diameter and length to determine the influence of chiral angle on their shear and bending modulus. Covalent bands between carbon atoms are simulated using linear beam elements based on molecular mechanics and finite element method. By using finite element analysis، the effects of diameter، length and chiral angel of nanotubes on mechanical properties under torsional and bending loading conditions are studied. The results show that zigzag CNT has the least shear and bending modulus comparing the armchair and chiral structures. Chiral nanotubes with angles smaller than 17 degrees has less shear modulus comparing armchair ones. But، for larger angles، chiral nanotubes has the largest shear modulus. Also، bending modulus of chiral CNTs is larger than armchair and zigzag structures.

In this paper، optimal trajectory planning of flexible joint manipulator in point-to-point motion is presented in which besides the determining the maximum load carrying capacity، the vibration amplitude is also reduced. The solution method is on the base of the indirect solution of optimal control problem. For this purpose، an appropriate objective function is defined، dynamic equations are derived in state space form، Hamiltonian function is developed and necessary optimality conditions are obtained by using the Pontryagin maximum principle. In order to reduce the vibration of the end effector during the path، an appropriate state variables are defined and the control law is improved to omit the suddenly variation in applied torque. Then، in order to illustrate the power and efficiency of the proposed method، a number of simulation tests are performed for a two-link manipulator. To this end، after deriving the equation in details، two simulations are performed. In the first case، determining the maximum load without considering the vibration is solved، in the second simulation، optimal trajectory with maximum load and minimum vibration is obtained. Finally discussions on the obtained results are presented.

Turbulent wind flow over buildings occurs due to the complexity like sharp corners، ground effect and different vortexes is one of the best choices to evaluate turbulence methods. DES and DDES are hybrid RANS-LES models for simulating turbulent flow which for their characteristic treat near wall as RANS and farther the wall act as LES model. Consequently computational time will decrease compared to traditional LES models. In this article to evaluate DES and DDES models، turbulent incompressible flow in Re = 22000 over 3D building is simulated using parallel processing facilities. For verification purpose other investigators experiment results are used. Also the mentioned models are compared with classic RANS and LES models، like k-ε and LES-Smagorinsky to depict their performance. Our results illustrate DES model with fine grid has good precision for simulating turbulent incompressible wind flow over building and decline of 26 percentage of computational time compared to LES-Smagorinsky model.

The incompressible Newtonian and non-Newtonian fluid flow in a tube with disk insert is studied numerically using finite volume and boundary fitted coordinate method. The non-Newtonian fluid is time independent purely viscous that is simulated by the power law model. The effects of power law index، thickness، aspect ratio، Prandtl number and the distance between insert tubes on heat transfer، pressure drop and overall enhancement ratio (OER) are investigated for the Reynolds numbers 500، 1000 and 1500. The results show that the effect of power law index on pressure drop and overall enhancement ratio is more than the other parameters.

A novel design of multi-generation package based on the PEM fuel cell and Maisotsenko cycle is proposed and analyzed in detail. This package consists of the proton exchange membrane (PEM) fuel cell stack، novel dew point indirect evaporative cooling system (Maisotsenko cycle)، heating coil، heat storage tank and the backup boiler. This package is capable of producing electricity، water and space heating as well as indirect evaporative cooling. The system performance is evaluated through the steady-state mathematical models and thermodynamic laws. Using the Matlab and EES software the results are presented in the form of Tables and Figures. Energy and exergy analyses revealed the fuel cell stack efficiency، heating and cooling cogeneration efficiency and the package multi-generation efficiency. The results indicate that، the actual output power and voltage from the PEM fuel cell stack are 3307 W and 0. 6787 V، respectively.

In this paper، nonlinear bending of rectangular nanoplates of Graphene subjected to a transverse uniform load، with incorporation of the nonlocal effect of Eringen based on the first-order shear deformation theory (FSDT) of orthotropic plates and Von Karman nonlinear strains is investigated using differential quadrature method (DQM). In order to validate of the solution accuracy، the simplified results have been compared with results of two developed numerical solution methods and other available results. Comparisons show an excellent agreement between the results. Finally، effects of small scale parameter، aspect ratio، thickness of plate، load value، boundary conditions and efficacy of large deflection، on the maximum deflection and different deflections ratio for nonlocal theory of thin plate and nonlocal FSDT are investigated. Results reveal that among the considered parameters، just aspect of plate is the parameter of difference between two employed nonlocal theories and the small scale parameter has not any effect on the mentioned difference. Also، it is found that the small scale parameter has a noticeable effect on the decrease of deflection of nonlinear solution; so that، unlike the larger values of mechanical load، this parameter has less effect for long length of square plate.

In this study، the radial flow turbine of a cooling turbine is investigated numerically and then compared with the experimental results at some operation conditions. Performance characteristics of the compressor are obtained experimentally by measurements of rotor speed and flow parameters. In this investigation، the turbine performance curve is obtained and three dimensional flow field in the turbine is analyzed. The rotor and casting geometry are modeled in BLADE GEN and CATIA softwares respectively. The TURBO GRID software is used for grid generation of rotor while the ANSYS MESH software is applied for grid generation of casting. Finally، 3D numerical solution of fluid flow in the turbine is solved by CFX flow solver. In this approach، compressible flow equations are solved according to the pressure based method with SST turbulence model. To ensure the numerical results، the grid independency is studied. Finally، the performance characteristics of the turbine are obtained numerically which are then compared to the experimental results. The comparison shows good agreement between numerical and experimental results.

In this research، the hot deformation behavior of API X70 steel was investigated by hot compression tests. A temperature range between 950 and 1150 °C was used for experiments with different strain rates of 0. 01، 0. 1 and 1 s-1. The work hardening rate versus stress curves were used to reveal if dynamic recrystallization (DRX) occurred. The application of constitutive equations to determine the hot working constants for the tested steel was discussed. Using regression analysis، the stress multiplier (α)، the apparent stress exponent (n)، and the activation energy (Qd) for DRX were calculated as 0. 016 and 4. 420، and 382 kJ/mol، respectively. Furthermore، the effect of Zener–Hollomon parameter (Z) on the characteristic points of flow curves was investigated using the obtained relations. The dynamic recrystallization (DRX) kinetics of API X70 steel was also studied and its governing equation was derived.

In this paper، viscoelastic insulators are employed into an automotive brake system to improve the vibration stability. Thus، the system stability has been considered with hypothesis of couple modes. Therefore، the originality of the paper includes the complex eigenvalue analysis of viscoelastic model in brake squeal phenomenon. Accordingly، the brake system is simulated in a FEM code and then، the viscoelastic materials are applied using Negami-Havriliak model. Comparing the eigenvalue results in both cases، in which the viscoelastic material is treated as an absorber at the first case and without treatment for another case، indicates an improvement in instability mode at 12 kHz. In addition، applying these absorbers has no significant effects in low frequency. Furthermore، comparison of the results presented here with experimental ones done by other author، indicates the reliability of the presented model. Finally، with applying the strain energy analysis، the location of absorber treatment as well as its optimum thickness is concluded.

In this research، interaction of a shock wave with incoming turbulent flow is investigated. To this end، different turbulent flows with different intensities and integral length scales are generated and impact of these turbulent flows on a shock wave are examined. In this study، two-dimensional Navier-Stokes equation is numerically solved using a high order spatial-temporal discretization. For spatial discretization، two different methods are implemented. In stream wise direction، i. e. perpendicular to the shock wave، a sixth-order accurate essentially non-oscillatory method (ENO) has been used which is able to capture the shock wave. In spanwise direction، i. e. parallel to the shock wave، a sixth-order Pade scheme has been used which is able to accurately capture small scale flow field structures. Time integration is performed using a third-order Runge-Kutta method. Overall، it has been observed that the turbulent kinetic energy increases across the shock wave. Fluctuations with larger integral length scale show higher turbulent kinetic energy increase across the shock wave. Further، it has been observed that although the integral length scale of the upstream fluctuations does not influence the location of the shock، the intensity of the upstream fluctuations have a profound effect on the shock wave location.

In this paper, the process of joining Ti-3Al-2.5V titanium alloy thin sheets by means of micro-plasma arc welding (MPAW) is reported. An experimental set-up was developed using traditional gas tungsten arc welding apparatus and a home-built torch for butt welding of coupon specimens. The specimens were welded under controlled welding parameters, such as voltage, current, travel speed and shielding gas flow rate. An appropriate set of parameters for MPAW process was examined by mechanical properties tests and microstructure characterization. Mechanical tests including tensile test, bending test and micro-hardness evaluation across the weld line generally show that if suitable welding parameters are used, the tensile strength of the welded specimen is well comparable with that of the base metal while its hardness increased at the fusion zone (FZ). The bending test revealed that using appropriate welding parameters, no crack or notch appeared at the welded joint. Fractography, X-ray diffraction and metallograpghy were also performed to study the microstructure evolution. SEM images of the fracture surface presented characteristics of ductile rupture. Studies on microstructure morphology of the specimens at the FZ and HAZ reveal occurrence of phase transformation from high temperature phase to acicular phase

In this paper, LQG/LTR controller is designed for attitude control of the geostationary satellite at nominal mode. Usage actuator in this paper is the reaction wheel and control torque is determined by the LQR regulator. Usage sensors in this article are sun and earth sensors and EKF are used for estimation of noisy states. LQR controller signal has good performance, if all system's states are considered in system output feedback. But this method is ideal and does not include model noise and sensors noise. Therefore, LQG and LQG/LTR controllers are designed based on the estimated states, and are compared with LQR controller. Controllers gain coefficients are obtained based on linearization about working point. It caused to robustness and similarity of LQG and LQG/LTR response. The results show that control overshoot of LQR is greater than the others.

In this paper the effect of pressure ratios on the performance characteristics of a radial twin entry turbine is investigated using computer aided design (CAD) and computational fluid dynamics (CFD). First, geometric models of the turbine flow passages are constructed by simultaneous use of measuring tools and computer aided design software. Because of geometrical complexity of flow passages, tetrahedral cells are used to generate unstructured grid in the computational domain. Three dimensional flow of steady, viscous, and compressible nature is solved by Multiple Reference Frame (MRF) technique. Characteristic curves of the turbine are obtained by post processing flow simulation results. Mass flow parameter, delivered torque parameter and total to static turbine efficiency are plotted against different pressure ratios. Results show that at constant rotational speed, increasing pressure ratio leads to increase in mass flow parameter until choke limit while the total to static efficiency decreases and delivered torque increases.