نشریه مهندسی مکانیک مدرس
سال سیزدهم شماره 11 (بهمن 1392)

The main purpose of this study is to investigate nonlinear bending and buckling analysis of radially functionally graded annular plates subjected to uniform in-plane compressive loads by Dynamic Relaxation method. The mechanical properties of plates assumed to vary continuously along the radial direction by the Mori–Tanaka distribution. The nonlinear formulations are based on first order shear deformation theory (FSDT) and large deflection von Karman equations. The dynamic relaxation (DR) method combined with the finite difference discretization technique is employed to solve the equilibrium equations. Due to the lack of similar research for the bending and buckling of functionally graded annular plates with material variation in the radial direction، some results are compared with the ones obtained by the Abaqus finite element software. Furthermore، some comparison study is carried out to compare the current solution with the results reported in the literature for annular isotropic plates. The achieved good agreements between the results indicate the accuracy of the present numerical method. Finally، numerical results for the maximum displacement and critical buckling load for various boundary conditions، effects of grading index، thickness-to-radius ratio and inner radius -to-outer radius ratio are presented.

In order to analyze fatigue life of reinforced cylindrical shells، it is necessary to calculate stress and strain fields of the structure. The cost of three dimensional stress analysis of this structure is very high with respect to its geometric complexity. So the stress analysis of the problem is performed by shell-to-solid sub-modeling technique. For this purpose، the reinforced cylindrical shell is modeled using shell elements at first، which in this case all the bolts، rivets and spot welds are considered as bushing elements. Afterwards the candidate critical zones are modeled using 3D solid elements with the help of shell-to-solid sub-modeling technique and all the connections are also modeled using 3D elements. Then the fatigue life of the problem under multi-axial loading is estimated by Brown-Miller criterion. For this purpose a special script in ABAQUS software has been used.

Wind is one of the factors influencing the performance of the dry cooling tower. According to research، wind will decrease the efficiency of dry cooling tower about 20%. Therefore، many studies have been done to improve the performance of dry cooling towers. Although various numerical methods and wind tunnel studies have been conducted، but the data obtained has not been properly validated and therefore requires that the appropriate field research done. To study the effect of wind on the performance of dry cooling tower، it is appropriate to model airflow around the cooling tower and the entrance of the deltas. In this research، which is the field research، air flow pattern around the tower and the deltas of Montazar ghaem plant cooling tower has been evaluated. The survey results found that the flow around the cooling tower has no separation. The sectors are positioned in front and back of the wind have the most efficiency and the least efficient sectors that are tangential to the wind. There is a vortex flow pattern in the critical deltas.

The main goal in design of heating، cooling and air conditioning systems is to provide thermal comfort. In present study، performances of radiant cooling ceiling system and stratum ventilation have been studied separately and together to provide the overall and local thermal comfort conditions. Therefore، using computational fluid dynamics، indices PMV (predict mean vote) and PPD (predict percent dissatisfied) has been studied as two overall thermal comfort parameters. Also، vertical temperature gradient and draft risk has been studied as two indices dissatisfaction of local thermal comfort. According to the results of this study، combining stratum ventilation and radiant cooling ceiling caused a uniform distribution of overall thermal comfort conditions. Also vertical temperature gradient decreases and there is no draft risk. Therefore combining stratum ventilation and radiant cooling ceiling is introduced as the new Strategy and application ability to provide the proper conditions to achieve overall and local thermal comfort.

Graphene/Polypropylene nanocomposite is a new material and limited research is performed on mechanical properties of such material. A random distribution of the nano particles in the matrix has a special importance in having proper mechanical properties for Graphene/Polypropylene nanocomposites. To have a uniform distribution of the nano graphene in the polypropylene، a method developed by Kalaitzidou، et al. for distribution of exfoliated graphite in polypropylene is used in this research. In this paper، Polypropylene is coated with graphene and then the nanocomposite specimens are made using melt-blending and injection molding. Polypropylene reinforced with 0. 5، 1. 0 and 2. 0 wt% graphene sheets were prepared and their tensile properties are investigated. Good enhancement of Young’s modulus and yield stress at very low graphene contents are achieved. The results indicate that the mentioned method is suitable for fabrication of graphene/polypropylene nanocomposites، which yields a good dispersion of graphene in the polypropylene.

In this paper، thermal effect on deflection، critical buckling load and vibration of nonlocal Euler-Bernoulli beam on Pasternak foundation using Ritz method is proposed. Equations of motion Euler-Bernoulli beam on Pasternak elastic foundation under thermal load is achieved by using energy method. Ritz method is used to solve the governing equations of motion. By this method، mass، stiffness and hardness buckling matrices are obtained. In this study، the effects of thermal، various boundary conditions، Winkler-type spring constant، Pasternak-type shear constant، non-local parameter on dimensionless deflection، critical buckling load، and natural frequency of Euler-Bernoulli beam theory are assessed. The obtained results indicate that with an increase of Winkler and Pasternak constants، the dimensionless natural frequency and critical buckling load increase، while the dimensionless deflection decreases. However، with increasing the temperature change in nonlocal Euler-Bernoulli beam on Winkler–Pasternak elastic foundation، the dimensionless natural frequency and critical buckling load decrease، while the dimensionless deflection increases. Moreover، with considering Winkler and Pasternak constants، the lower mode shape are removed and replaced with higher mode shapes.

Jet engine Fuel Control Unit (FCU) has been manufactured recently in the SSAC laboratory of Iran University of science and technology، and is being tested now using HIL testing. Regarding there is no possibility utilizing a jet engine in this test set; an induction motor was used as an actuator to transit rotation acquired using jet engine real time simulation to the FCU pump. In this paper، the induction motor’s velocity control is presented considering condition for the test set. To do so، system identification method is used to model the components employed in induction motor velocity control system. Then، the model is evaluated using experimental test results. Afterward، it is used to design ANFIS controller، and the controller parameters is adjusted employing IWO optimization algorithm as a strong tool for seeking in vast disorder spaces. Subsequently، the controller designed is implemented on a real system. Results gained using simulation and the designed controller implementation show that ANFIS controller designed using IWO algorithm works appropriately.

In the present study، the motion and deformation of a red blood cell in the incompressible viscous flow is simulated using the lattice Boltzmann method combined with the immersed boundary method. The lattice Boltzmann method is used to solve the flow field، whereas the immersed boundary method is used to simulate the dynamics of the red blood cell. The red blood cell is considered as an elastic boundary immersed in the fluid domain. The main advantage of the lattice Boltzmann method is that it solves only an algebraic equation. In the immersed boundary method the fluid domain is descretized using a regular Eulerian grid، while the immersed boundary is represented in the Lagrangian coordinates. The Eulerian grid points would not necessarily coincide with the Lagrangian points. The fluid- immersed boundary interaction is modeled using an appropriate form of delta function. The effects of the no-slip condition are taken into account via a forcing term added to the Navier-Stokes Equations (here the lattice Boltzmann equation). In the present study، the tank-treading motion of a red blood cell in the viscous shear flow is simulated. The results are found to be in good agreement with the available experimental and numerical ones.

The stability analysis of a curved sandwich beam with isotropic skins and flexible core is investigated in this research. Derivation of equations for face sheets is accomplished via the classical theory of curved beam، whereas for the flexible core، the elasticity equations in polar coordinates are implemented. The beam construction consists of two skins (not necessarily identical)، metallic or composite laminated symmetric، and a soft core made of foam or a low-strength honeycomb that is flexible in the vertical direction. Employing the von-Karman type geometrical non-linearity in strain-displacement relations، nonlinear governing equations are resulted. Linear pre-buckling analysis is performed neglecting the rotation effects in pre-buckling state. Stability equations are concluded based on the adjacent equilibrium criterion. Considering the movable simply supported type of boundary conditions، suitable trigonometric solutions are adopted which satisfy the assumed edge conditions. The critical uniform load of the beam is obtained as a closed-form solution. Numerical results cover the effects of various parameters on the critical buckling load of the beam. To the best of the authors'' knowledge، this research is investigated here in for the first time.

In this article، fracture toughness of austenitic–martensitic functionally graded steels fabricated by electroslag remelting with crack arrester layers is investigated by experimental and analytical methods. The material contains austenite phase in addition to martensite layer. The Young’s modulus and the Poisson’s ratio have been assumed to be constant، while other mechanical properties like the yield strength and the strain hardening exponent vary exponentially along the specimen width. In analytical case، unloading compliance method is modified to calculate the critical value of J-integral for cracked three point bend specimens while standard specimens with different crack lengths are tested in experiments. The effect of crack length on the fracture toughness has been studied. It is observed that، as the crack tip goes toward a martensite layer، fracture toughness of the specimen decreases considerably. The obtained results from the proposed model are in good agreement with the experimental results.

In this article، a combined GAX-ejector absorption refrigeration cycle is proposed and its performance is compared with those of combined single effect-ejector، simple GAX and single effect absorption refrigeration cycles. For the ejector، based on the Keenan''s theory، a new model is developed and validated and then is combined with the developed models in the EES software (for simulating the processes in the cycles). After obtaining the optimum critical area ratio for the ejector، three different ejectors، with the mentioned specification، are selected for the combined GAX-ejector and combined single effect-ejector cycles. The performance of these cycles is investigated through changing their evaporator and generator temperatures. Results indicate that، at identical conditions، the COP and second law efficiency of the combined GAX-ejector cycle are around 25% and 16% higher than those of the combined single effect-ejector absorption refrigeration cycle. In addition، it is observed that as the generator temperature increases from 140 oC to170oC، the COP and second law efficiency of combined GAX-ejector cycle are maximized at a particular generator temperature. However، at similar condition، an increase in generator temperature results in a decrease of the COP and second law efficiency of combined single effect-ejector refrigeration cycle.

Centrifugal pumps performance is highly affected by working fluid viscosity. So، optimization of such pumps for pumping of viscose fluids is very important. In the present paper، multi-objective optimization of the centrifugal pumps is performed to obtain optimum impellers for pumping fluids with various viscosities at different volumetric flow rates. In this way، theoretical head and impeller hydraulic losses are considered as objective functions. Design variables defined in this optimization problem are passage width of impeller and outlet angle of blade. Diagrams of Pareto fronts and Pareto sets are extracted for different viscosities and different volumetric flow rates. Some trade-off optimum design points are selected from all non-dominated points using three different methods namely break point، TOPSIS and near to ideal point. Such methods are defined completely and employed to achieve compromising point successfully. Obtained optimum points contain interesting results which cannot be achieve without using proposed multi-objective optimization approach.

In the present work the dynamic response of laminated composite beam reinforced with nano-particles has been investigated. Most of the existing works on the effects of nano-particles on stiffness of the composite structures are limited to very low weight fraction، around 3% to 5%. This work studies the effect of higher percent of Nano particles (up to 10%) on the dynamic behavior of the composite structures via experimental tests. Adding Nano clay up to 3% of weight fraction increases the natural frequency، beyond that up to 5 % the natural frequency slightly decreases and at 10% a sharp reduction in natural frequency is observed. Another feature of importance is the increasing damping coefficient of laminated beam when the amount of nano-particles reaches to 10%.

Increasing the pressure in gas turbine cycle is done by Compressor which is one of the most important components of the Gas Turbines. Due to positive pressure gradient، the nature of the flow inside the compressor is complicated and for this reason and because of the duty of the blade for transferring energy to the flow، precise design of the compressor’s blades from the aerodynamic view of point is very important In this study، effects of changes the rotor blade stagger angle on a transonic axial compressor performance curves including efficiency and pressure ratio has been studied. To simulate the complicated three-dimensional flow field in axial compressors، a numerical code is used to solve Reynolds Average Navier-Stokes (RANS) equations. comparison between numerical results and experimental data shows a good agreement. When numerical code are verified. Then the rotor blade twist changes on axial compressor performance have been studied. The results show that the rotor blades twist leads to decrease in compressor efficiency and pressure ratio.