سال:1389 | دوره: | شماره: |تعداد مقالات:9

This paper presents a novel model to simulate the flow-current characteristic curve of an electro-hydraulic servo valve in steady state condition. This characteristic curve has three major zones: dead zone, linear zone and saturation zone. By using the presented approach, we can simulate the behavior of all types of valves including under lapped, critical center (zero lapped) and over lapped valves. A hydraulic tester has been designed and constructed for validation of the results. It can test the performance of flow-current and some other properties of the valve. Comparison of experimental and simulated curves describes that the model has an acceptable accuracy in determining the four important valve characteristics: flow gain, saturation flow, saturation current and dead zone. As a result, the model can be used for other studies such as geometrical sensitivity analysis (the ultimate objective of this modeling) or design optimization. Since this model is presented in a computer based algorithm, it can be easily used to observe and measure the effects of change in the parameters of interest (particularly geometrical parameters of the valve) on the flow-current curve.

In this paper, a compressible, one-dimensional and two-phase flow is analytically modeled by using a homogeneous classical nucleation rate equation in a Laval nozzle. For droplet growth calculations, the heat transfer between the droplets and the surrounding steam is modeled by a free molecular flow model and a semi-empirical two-layer model which is deemed to be valid over a wide range of Knudsen numbers. In order to reduce the condensation shock strength, the water droplets are injected at the inlet, and just after the throat of the Laval nozzle. According to the results, the nucleation rate is considerably suppressed due to the small droplet injection, and, therefore, the condensation shock nearly disappears, particularly for the inlet injection of water droplets.

This paper presents a model to analyze contact phenomenon in Microsystems actuated by ramp voltages, which has applications in frequency sweeping. First-order shear deformation theory is used to model dynamical system using finite element method, while finite difference method is applied to model squeeze film damping. The model is validated by static pull-in results. The presented hybrid FEMFDM model is utilized to compute values of contact time and dynamic behavior. Considering this model, effects of different geometrical and mechanical parameters on contact time are studied. The incense of imposing the additional reverse voltage on dynamic characteristics of the system is also investigated. It is shown that magnitude and position of applying the reverse voltage is very important in preventing pull-in instability.

Cardiovascular diseases are one of the major causes of death in the world; atherosclerosis being one aspect. Carotid bifurcation is one of the sites that are vulnerable to this disease. Wall Shear Stress (WSS) is known to be responsible for the process of atherogenesis. In this study, we have simulated the blood flow for Newtonian and non-Newtonian, steady and unsteady, flow conditions in three idealistic and five realistic geometries. A risk factor has been presented based on the results of wall shear stress and, then, a relation was found between geometrical features and the wall shear stress risk factor. Our main conclusions are: 1) The non-Newtonian behavior of blood elevates the value of wall shear stress, however, the wall shear stress pattern is similar, 2) The bifurcation angle is not the main cause of atherosclerosis and cannot be considered a predictor for atherosclerosis disease, and 3) The ratio of sinus diameter to the internal carotid artery diameter is more important than other geometrical factors, and the WSS pattern is inuenced by this factor.

Free vibration analysis of microtubules (MTs) is presented based on the Euler-Bernoulli beam theory. The size effect is taken into consideration using the Eringen's non-local elasticity theory. The governing differential equations for MT vibrations are being solved using the Differential Quadrature (DQ) method. Numerical results are presented to show the effect of nonlocal behavior on the frequencies of MTs. It is hoped that the results in the manuscript may present a benchmark in the study of vibration for microtubules.

Decoupling of dynamic equations in robotic mechanisms has attracted many researchers in the recent years. This kind of decoupling can be achieved by modifying the original kinematic structure through the use of counterweights attached to the moving links. Therefore, the robotic control becomes easier due to reducing the coupling disturbances. In this paper, different methods of decoupling including the static balancing, Coriolis and centripetal force eliminating and dynamic balancing are introduced and their concepts are described based on Lagrange-Euler equations. The systematic adaptive approach is proposed for any open-chain planar robots whose links are connected by revolute joints. The method is tested for two-link and three-link manipulator. The results indicate that the system is fully-decoupled and simple classical approach is sufficient to control it.

One major disadvantage of many arc welding processes is welding-induced residual stresses and distortions. The non-uniform heating and cooling during arc welding result in non-uniform expansion and contraction of the weld and surrounding base material, which produces undesirable residual stresses and deformations in the welded joint. A number of methods can be used to control welding-induced distortions. The methods used for controlling welding distortions affect residual stresses and vice versa. One practical method for minimizing welding angular distortions is the use of clamping. In this paper, the effect of clamping and clamp releasing time on welding residual stresses and distortions in the single-pass butt welding of 304 stainless steel plates are investigated. Cases with and without clamping have been studied, and residual stresses and angular distortions have been predicted by three-dimensional finite element simulation. Moreover, experiments have been carried out to measure temperature histories, angular distortions and residual stresses for the unclamped case to verify the numerical model. The results of this study revealed that clamping and clamp release time have a great inuence on the distribution of residual stresses and final angular distortions. Using clamping during welding and releasing after cooling to ambient temperature can significantly reduce the amount of final angular distortions.

Aerodynamic behavior of an airfoil with circle, right and inverse triangle shaped damage was numerically investigated. The flow through the damage was driven by the pressure differential between the upper and lower wing surfaces. For all damage shapes, the results showed that the flow could be categorized as weak or strong jets. The jet exited the rear of the damage; its size was determined by the width of the rear part of the hole and was dependent on the shape of the damage. Generally, when compared with an undamaged model, increasing the angle of attack for a damaged model resulted in increased loss of lift coefficient, increased drag coefficient and more negative pitching moment coefficient. This effect was more severe for the right triangle case. Furthermore, the numerical results were compared with experimental data to study the effiect of a wall on the flow field.

This paper describes a design methodology for ducted propellers operating in a non-uniform wake field. An improved lifting line model is used to represent propeller blades where more realistic slipstream deformation behind the propeller is considered, while the duct and hub are modeled by ring vortex distributions. Duct and hub geometries are assumed to be known and their effects on the propeller are calculated during propeller design stage. At the same time, mutual interaction between duct/hub geometry and propeller is taken into consideration based on an iterative procedure. For numerical application of the present method, a design example is carried out and the effect of slipstream deformation is investigated in detail. Another application based on experimental work is also taken into account to present the accuracy of the calculated results by present method and Computational Fluid Dynamics (CFD) code. Finally, propulsive characteristics of ducted propellers obtained from present method are compared to those of CFD code and experimental results.