مجله مکانیک سازه ها و شاره ها
سال یکم شماره 3 (زمستان 1390)

In this paper fracture behavior of functionally graded materials under thermo-mechanical shocks is investigated. For this purpose classical coupled thermoelastic equations are used in the calculations. These equations are discretized with extended finite element method and then are solved by the Newmark method in the time domain. Micromechanical models for conventional composites are used to estimate properties of FG layers. The most general form of interaction integral is extracted for moving cracks in FGMs under thermo-mechanical loadings، and then it is employed to calculate the stress intensity factors at each time step. All stages of problem solution from mesh generation to obtaining results were implemented in MATLAB programming environment. Some examples are solved and obtained results are compared with existing analytical and numerical results in other papers. Good accordance between these results verifies presented method and written code in this paper. Finally the effects of material properties profile variation on dynamic stress intensity factors in a glass/epoxy beam under thermo-mechanical shocks are investigated. Also crack propagation path and speed are studied for this beam.

Bladed disks may be used in several engineering systems such as fans، impeller pumps، turbine generators and jet engines. Ideally، these 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، variations in material properties and wear in service. Each blade on a disk is almost always slightly different to the others. It can cause some unpredictable phenomena such as dramatic difference in forced vibration response. In this paper، effects of mistuning on nonlinear behavior of mistuned cyclical symmetric systems like bladed disk are investigated. A nonlinear، mistuned model based on the method of multiple scales is proposed and formulated in which nonlinearity and mistuning parameter is assumed to be small. Several mistuned system were considered and solved by the proposed technique. Numerical results show that only one jump will happen for the tuned system during the excitation and demonstrate that mistuning can lead to repeating and scattering of jump phenomena during the excitation frequency whereas in tuned cyclic system it occurs simultaneously.

In this research، the operational plan are prepared based on the standard requirement of flutter test for an aircraft. Therefore the analytic recognition algorithms of excitation frequency and damping estimations from response data is surveyed in different flight condition. The resultants is presented in condition of mid height and field elevation (FM); 5600 feet with usage of zero، 10 and 38 degrees flap angles. In this research the parametric data gathering to be considered with covering the Performance curves limitation. The purpose of the present work، was access to flutter airspeeds with application ability in type certification program (TC) and flight manual completion (FM) for a jet trainer. The results indicate that by increasing the angle of flap in critical limitations of flight envelope attention to leap of the dynamic frequency response due to structure excitation to infinite and entry to less rate of structural damping، flutter phenomenon occur in lower airspeed. Whereas by change in weight configurations from light to heavy in same flap angle، this decreased rate in flutter airspeed don''t observe significantly. in this article is imparted both method of flutter airspeed estimation in form of response-frequency diagrams and damping-airspeed in parallel and adaptively

The adsorbent particle diameter has a great influence on the performance of adsorption chillers since it directly affects the inter particle and intra particle mass transfer resistances. The chiller performance variation with particle diameter is dependent on various parameters، yet this study investigates the effects of most important ones such as fin height، fin spacing and thermal fluid heating and cooling temperatures on the optimum value of particle diameter which corresponds to the optimum value for the specific cooling power of the system. A plate fin-tube heat exchanger is employed and the fin spaces are filled with SWS-1L. The three dimensional heat and mass transfer equations of adsorbent bed are solved simultaneously with the heat transfer equations of thermal fluid، metal tube، fins and the chamber. The results show that the cycle time and the specific cooling power have an optimum value for a specified particle diameter. This diameter increases with an increase in fin height and a decrease in fin spacing. Moreover it has been found that the optimum particle diameter increases with an increase in heating and cooling temperatures of the thermal fluid.

In this paper، at first a computer code was developed to design a one-dimensional vaneless volute related to radial inflow turbine. Absolute mach number and direction of the rotor inlet flow are the bases of the design method. In this method، the one-dimensional compressible flow equations are solved considering dissipation of the fluid flow. Free vortex flow equations were used to model (simulate) rotational fluid flow inside vaneless volute، using experimental (correlation) relations of angular momentum dissipation، energy dissipation، and deviation of rotational flow. In the next step، the volute is generated using the computes code. Then، the volute geometry was modeled three-dimensionally and after mesh generation، the inside flow is analyzed numerically in three-dimensional، compressible and viscous flow. At the end، the numerical results were compared to the initial design values. The comparison showed the validation of the design method. Keywords: Volute; Radial inflow turbine; Vaneless; one-dimensional design.

In this paper، the steady flow of non newtonian fluid in axisymmetric sudden expansion with ratio 1:3 solved numerically. Finite difference method is used to discrete the governing equation on the staggered mesh. The non-newtonian model that used is the Carreau Yasuda five constants viscometric model. Using this model، non newtonian properties of the fluid can be simulated well. The range of the power law index is considered between 0. 6 and 1. 0. The results of this paper include the streamline diagrams، the axial velocity، viscosity and stress profiles for different values of the non newtonian property. These results show that length of vortex and fully developing region decreases، the pressure loss and the maximum axial velocity increases when the power law index (n) is decreases. It is shown that the magnitude of the velocity and viscosity in the center of the vortex are minimum and maximum respectively.

This paper discusses on improving the prediction of flow and heat transfer in jet impingement through the combination of low Reynolds-number second moment closure turbulence models. A slot impinging jet with the H/W=6 at three different Reynolds numbers 5200، 7800 and 10،400 are numerically analyzed. Three different explicit algebraic heat flux models are applied for predicting turbulent heat fluxes. Numerical results show that the turbulent heat flux model plays an important role in accurately predicting jet impinging heat transfer. The comparisons show that the model of Younis et al. is in reasonable agreement with the available experimental data for predicting the local Nusselt number. Also results show that the turbulent Prandtl can not be assumed as a constant prescribed value. It has been shown that this parameter is varied in the range of 0. 5-7. 0 from near of the wall to far from it، respectively. The jet Reynolds number affects on predicted turbulent number only in near wall region.

One of effective and important parameters on gas turbine output power is the ambient air temperature and increase of 1 degree centigrade of this parameter decreases output power of total cycle about 0. 6 to 0. 9 percent. Compressor inlet cooling is a useful way to increase of turbine power output. In this paper different ways of compressor inlet cooling such as evaporative cooling and refregerative cooling method are reviewed and at the end، direct jetting method (fogging method) is focused. Additionally thermodynamic modeling of cycle elements are studied and water jetting effects on operating parameters of GE - F5 gas turbine of Mashhad Power plant with consideration of cooling effect and humidity in combustion chamber and combustion modeling with humidity effect are presented. This study shows that 22 ℃ compressor inlet air cooling prevents power losing of total power plant cycle about 18. 79 percent in optimum case of operating.

Smart structures are mechanisms that react to changes of the physical and mechanical conditions of environment and are adaptable to their working conditions. There is a class of smart structures which do not have sensor systems and can be created using very simple mechanical mechanisms. In order to improve the overall performance of structures، their mechanical behavior is predefined to establish the best reaction to some conditions such as vibrations and impact forces. In this paper، in order to improve performance of bumper beam in collision، different mechanical behaviors has been employed such as two-linear smart behavior in the region where bumper and car are connected. LS-DYNA-V970 software has been used for simulating of collision and the most appropriate behavior has been identified. Results have shown that the most appropriate treatment for connections is the one which has higher initial stiffness where the structure becomes more flexible by increasing the collision force and its stiffness is reduced to prevent the failure of the bumper beam. Investigation of two-linear smart behavior in bumper connections and the displacement in which the stiffness changes showed that less displacement will result inbetter outcomes.