مجله مکانیک سازه ها و شاره ها
سال یکم شماره 2 (پاییز 1390)

An extensive review on the topic of linear and nonlinear behavior of axisymmetric shells, laminated composites and smart materials is done by the authors. It is evident from the open literature that only a few studies on the geometrically nonlinear analysis of smart axisymmetric laminated shells by using higher-order shear deformation theory have been performed to the best of the author's knowledge. In this paper, geometrically nonlinear analysis of axisymmetric laminated shell with the piezoelectric layer is presented. Two types of higher-order shape functions are used to approximate better the transverse shear strain field across the thickness direction. To obtain more accurate solution, two degrees of freedom are added to the degenerated one-dimensional shell element. Total Lagrangian formulations along with Newton-Raphson technique are employed. The validity of this geometrically nonlinear method is illustrated through some numerical examples. The results not only demonstrate the effectiveness of the proposed approach, but also indicate much more precise than what has been shown before.

It is now a well-recognized fact that bearings play a vital role in the performance of any rotor bearing system. Though the plain journal bearing is the oldest and a general purpose bearing configuration, it does not suit the requirements of modern high speed rotating systems and precision machine tools. In such specialized applications, the plain journal bearing has mostly been replaced by noncircular two and three lobe bearings which exhibit superior dynamic performance. In contrast to the plain circular bearing, noncircular journal bearing may be oriented in various ways with respect to a given direction of external load. This orientation may be described in terms of an angle, which in the present work has been referred to as the mount and tilt angles. In the present work applying GDQ method, analyses of micropolar lubricated circular and noncircular two and three-lobe journal bearings, are presented. Effects of micropolarity characteristics of lubricant, design and assemble parameters on the performance parameters of these bearings have been investigated. In general, results show that micropolar characteristic parameters of the lubricant improve the performance of the circular and noncircular bearings. Also it is observed that in the noncircular bearing the effect of mount and tilt angles is generally marginal. of the two types of bearings considered, the two lobe bearings are most sensitive to mount and tilt angles. However, the mount and tilt angles are assemble and design parameters, but sometimes they can be selected just to meet some specific operation goals.

In practical application, creation of cutout is inevitable. Besides on the other hand, the cutouts reduce the load carrying capability of structures. These cutouts cause stress concentration in the structure which is important in design problems. Since the stress distribution is not uniform due to the presence of cutout, for plane stress problems, the selection of appropriate cutout bluntness can reduce stress concentration factor (SCF). The analytical solution which is based on the Lekhnitskii theory is used for determining the stress distribution in composite and isotropic materials containing a central cut out and subjected to tensional load. Only circular and elliptical cut outs were solved for anisotropic materials by Lekhnitskii. In this paper, by using the complex variable theory, the Lekhnitskii method is expanded to study of the effect of cutout bluntness as effective parameter to reduce SCF on stress distribution around different cutouts. results based on analytical solution are compared with the results obtained using finite element method. The results obtained clearly demonstrate the effect of cutout bluntness on stress concentration in perforated plates subjected to uni-axial tensile load.

Ideally, bladed disk systems are tuned and all blades are identical but, in practice there always exist small, random differences among the blades. Mistuning, imperfections in cyclical symmetry of bladed disks is an inevitable and perilous occurrence due to many factors including manufacturing tolerances and wear in service. It can cause some unpredictable phenomena such as dramatic difference in forced vibration response. In this paper first a finite element model of bladed disk system with 24 blades were created in ANSYS. The model is then used to calculate the frequency response of the blades for the tuned system. Next, two hundred experiments, with different density for each blade, were selected in a specified range. For each test case calculations were performed and the maximum response was obtained. Then, by integrating neural networks and genetic algorithm the worst frequency response of the mistuned bladed-disk system was calculated. The problem of finding the worst specification is formulated as an optimization problem subjected to constraints such as the manufacturing tolerances. Based on the calculated parameters, a new model was created and the maximum response of the mistuned system was calculated. The results indicate that the responses obtained from the neural network and genetic algorithm have reasonable accuracy and are in good agreement with responses obtained from the ANSYS and shows the efficiency of the method

The use of the classical Boussinesq approximation is a straightforward strategy for taking into account the buoyancy effect in incompressible solvers. This strategy is highly effective if density variation is low. Whenever the density variation is high, this can cause considerable deviation from the correct prediction of fluid flow behavior and the accurate estimation of heat transfer rate. In this study, an incompressible algorithm is suitably extended to solve high-density-variation fields caused by strong natural-convection with mixing of oxygen and nitrogen in unsteady laminar compressible flow in a cavity. The continuity, momentum, energy and species equations are discritized based on finite volume methods and then numerically solved with extended algorithm with SIMPEL method. This new algorithm is capable of solving both Boussinesq and non-Boussinesq regimes. The fluid is assumed to be calorically an ideal gas and its thermodynamic properties depend on temperature and pressure. The extended algorithm is then verified by solving the benchmark convecting cavity problem at Rayleigh 106 and a temperature range of ε = 0.01–0.6. The results show that the method can vigorously solve unsteady mixing flow fields with extreme density variation.

One of the most common facilities utilized in supersonic wind tunnels is multi-stage ejector system. In this system high speed air blowing with specified Mach numbers into the wind tunnel at predetermined points, helps to obtain the desired Mach number in the test section. In this study, according to characteristics of test section including Mach number, static temperature and stagnation pressure, relevant prominent features of the single-stage, two-stage and three-stage ejectors such as mass flow, velocity and stagnation pressure are computed. Two different aspects are implemented in solving governing equations of ejectors; first one is to assume the same Mach number for all ejectors and second one is based on assumption of the same stagnation pressure for the ejectors. It will be revealed that the later one is more optimal in comparison with the former criteria. Validation of the developed computational code results is performed via comparison with the experimental data of a wind tunnel operating at Mach number 2, which resulted in a difference of 5 to 7 percent. At the earlier section, design parameters of single ejector versus different mach numbers of the test section are presented and then effect of outlet static pressure reduction as a means of vacuum generation at the exhaust of the wind tunnel is discussed. Furthermore design of two-stage ejector according to both methods of equal Mach number and the same stagnation pressure is completed and in the last section three-stage ejector is studied base on the same stagnation pressure for comprising ejectors.

In this study, the three dimensional flow and heat transfer of viscoelastic fluid has been modeled with the Giesekus constitutive equation. In the most previous research, they focused on the fully developed region of flow which is due to the lack of comprehensive research in this field, the studding of the viscoelastic flow in the developing region seems be essential. The governing equation that should be solved are: mass, momentum and energy conservation with the Giesekus constitutive equation that are descried with the finite difference method and solved by the artificial compressibility method on the staggered grid. It should be noted that, with this constitutive equation, the second normal stress difference is non-zero and therefore the secondary flow in the cross section are formed and will be visible. The dependency of the fluid property to temperature is one of the advantages of this research. Due to the dominant group of the viscoelastic fluid are melt polymers and in this situation, the variation of the temperature is high, therefore this assumption seems be necessary. The results in the fully developed zone are in good agreement with the other reported results.

In this paper, the thermo-hydrodynamic analysis of incompressible fluid flow in conjunction with cell-centered finite volume-lattice Boltzmann method is developed. To demonstrate the temperature field, the double distribution function model was used. Since, instability is the most severe problem of the thermal lattice Boltzmann methods to handle flows; a stable and accurate cell-centered scheme is presented. For this purpose, the pressure and temperature based upwind biasing factors are used as flux correctors. Also, additional lattices at the edge of each boundary cell are introduced, which allow a much better description of the actual geometrical shape. The unknown energy distribution at the boundary cells were decomposed into its equilibrium and non-equilibrium parts. This treatment enlarges the domain stability and leads to faster convergence. In addition of calculating numerical viscosity, The method is applied to two dimensional incompressible thermal plane duct flow. The results show a very good accuracy and agreement with the exact solution and previous numerical results.

In case of studying and modeling gas flow in the pipeline network, there are two types of flow: steady flow and transient flow. The natural gas pipeline network consists of pipelines, pressure compressor stations, pressure reduction stations, line break valves and storage fields. The pipeline is the key element in determining the overall network dynamic behavior. Dynamic behavior of a gas pipeline due to changes of gas flow rate for isothermal transient status has been modeled and investigated in this study. The modeling of isothermal transient flow is resulted in differential equations using mass conservation equations, momentum and an equation of state. By solving these equations according to time and place, the behavior of gas pipeline will be obtained. Three cases of the operating conditions for gas transmission pipelines in transient status have been studied. The modeling results of each case have been shown and interpreted in curves of flow rate and pressure variations along pipeline in different time spots.