مجله مهندسی مکانیک
سال سی و نهم شماره 2 (پیاپی 58، پاییز و زمستان 1388)

Local wind is recognized as the principal driving force to simulate the trajectory of contaminantsover the sea. The wind field is determined by a Weibull probability distribution to adjust the prevailing wind verified by field data. To estimate the parameters of Weibull distribution، the least-squares approach is used. The wind field is combined with the water dynamic field to determine oil drift on the sea surface. The model is used to simulate the trajectory and fate of contaminantsin a simple and speedy procedure for the Persian Gulf using random walk technique. Comparison of the actual and simulated contaminantssuch as oil spill drift was found acceptable in the domain in order to generate large amount of results preparing risk map analysis.

Laminar two dimensional forced convective heat transfer of Al2O3-water nanofluids in a horizontal microchannel has been studied numerically، considering axial conduction effects in both solid and liquid regions. Thermal conductivity and dynamic viscosity of the nanofluid are considered variable with temperature and diameter of nanoparticles. The transport equations are solved numerically using the finite-volume approach and a collocated grid arrangement is utilized. The results show that using nanoparticles with higher thermal conductivities will intensify enhancement of heat transfer characteristics ofmicrchannels but on the other hand will slightly increase shear stress on the walls. The obtained results show more steep changes in Nusselt number for lower diameters and also higher values of Nusselt number by decreasing diameter of nanoparticles. Also، considering the effect of volume fraction on axial conduction using “Conduction number” showed that using nanofluid will strengthen this effect.

In present paper the torsion of prismatic bars has been investigated by the finite-volume method (FVM) for the first time. One of the distinguished features of this method is the ability for treatment of complex geometries. The Poisson equation has been solved on a cell-centered mesh with consistently derived boundary conditions. For this purpose a FORTRAN 90 code was written. The shear stresses at various locations of cross-section were determined. Also، thorough comparisons have been made with the conventional methods to verify the superiority of the present scheme in terms of convergence reduction and solutions accuracy.

The catalyst used for partial oxidation of ethylene to ethylene oxide is Ag/α-Al2O3 which its physicochemical properties، i. e.، surface area، pore volume، silver contents and their dispersion profoundly affect its catalytic performance. In this research، alpha alumina as catalyst support with surface area and pore volume of 0. 5- 1. 2 m2 g-1 and 0. 11– 0. 2 ml g-1، were prepared by physical mixing، grinding، extrusion and calcination. The deposition method of silver metal on catalyst support and the use of alkali metal during the catalyst preparation affect the silver particle dispersion on catalyst. In this work wetimpregnationmethod was used for catalyst preparation. Silver particles size، their percent dispersion and contents were determined using transmission electron microscope (TEM)، x-ray diffraction (XRD) and atomic absorption (AA). Results showed that the catalyst prepared in this work with the same silver content had higher silver percent dispersion compare tothe commercial type catalyst. Silver content، silver percent dispersion and silver particles size for catalyst prepared in this work and the commercial catalyst were in the range of، 5. 25-14. 30، 14. 3 wt%; 34. 40-19. 61، 4. 31 % and 67. 3-112. 1، 90. 3 nm.

The aim of this study is the necessity of applying optimal fabrication condition of Cu-20Zn alloy to increase the capability of controlling its properties، since physical and mechanical properties and microstructural changes of this alloy depend on trivial changes in sintering temperature. For this purpose، water atomized prealloyed brass powder، with averageparticle size of 40 to 100 μm، was compacted under pressure of 600 MPa and sintered at the temperature range of 900 to 950°C in Ar atmosphere. The results of the density and dimensional changes measurement، hardness and impact energy testing show that the optimum sintering temperature is 930°C. Moreover، microstructural and fractographic analysis is in a good agreement with obtained physical and mechanical. The other notable point in fracture surface images is settling of liquid to down part of samples and consequently different densification as a function of sample height. The amount of liquidphase، which depends on sintering temperature، results in different load bearing cross section areas at a specific height، also it affects the type of fracture morphology (transgranular or intergranular).

It is very important to implement correct boundary condition at the free surface of a flow field. A 2D cavity with movable bottom has been solved with a numerical method which can use two types of boundary conditions at the free surface. The numerical results show that in these types of flows، which non-hydrostatic pressure gradients have not dominant direction، and they are small in the whole of the flow field، even a little displacements of the free surface have very large effects on the flow field configuration. Therefore، ignoring the free surface displacements in the numerical solution leads to some errors in flow behavior predictions.

In this paper an analytical study of the inverse kinematics and dynamics of Stewart platform will be addressed. The dynamic equations are generated through the Newton-Euler approach and all the gravity، frictional، coriolis، inertial and external forces has been considered in the formulation. There have been number of previous researches in this subject which consist of simplifying assumptions for derivation of less complicated formulation. In this research، the kinematic and dynamic equations will be presented using kinematic model of universal joint، the rotational degree of freedom of the podsaround the axial direction. The intact relations for the inertia tensors of different elements of the manipulator and the justifiable direction of the reaction moment on the pods are taken into account in deriving the new formulation. The Dynamicequations of the new formulation are absolutely different from those introduced before.

In this paper، a new method to design self-adaptive PID controller is developed. This approach is used for replication of the adaptive feedback control systems with a reference model. This approach is based on the method «coupling of frequency” andutilized for synthesis of the adaptive control system for obtaining the answers of problem of objective replication. Application of this technique is shown by solving a design example for a nonlinear system and the results has been compared by PID linear and nonlinear controllers which have been reported by other researches.