فصلنامه مکانیک هوافضا
سال سوم شماره 2 (پیاپی 8، تابستان 1386)

In most literature on lubrication theory, the assumption of is considered to solve the Navier-Stokes equations in which inertial forces are neglected in comparison with the viscous ones. In problems of bearing with high speed motion, neglecting the inertial forces can not be utilized. In this study, the Navier-Stokes equations for a slider bearing are solved, considering both the inertial and the viscous forces. Then, the effective forces were computed and analyzed. Finally, the results were compared with the available approximate ones, which showed close agreements.

In this paper, an optimal controller for a mobile manipulator pushing a cart on a specified path by using time-based and event-base control method is designed. Therefore, first the kinematics of the robot and the cart is studied and their kinematic equations on are derived. The kinetic of the robot and the cart is then studied and the related equations are obtained. Next, after optimizing the robot, an event-based path planner and the force/position controller is designed for the robot, using a hybrid control of force/position type. Finally, the results of the event-based and the time-based control methods are presented and compared and the advantages of event-based control method are discussed.

In this paper, the effect of cutting parameters and tool geometry on the burr dimensional properties in face milling operation was considered. Many experimental tests in different machining conditions have been performed and the amounts of burr height and thickness have been measured. Further, by statistical method, a mathematical model was developed to predict burr height and thickness where the type of work piece material was St37. Then, the model was used to define optimum cutting parameters to reach at minimum burr height.

A computational scheme is developed to study flow separation in a two-dimensional, converging-diverging nozzle with consideration of jet and external flow interactions. Different chamber to ambient pressure ratios are considered. The exhaust flow pressure is adapted to the ambient pressure by shocks emanating into the flow field in the over expanded nozzles. In general, the boundary layer and the shock interaction will cause boundary layer separation in the nozzle. The code developed in this work employs an implicit finite volume lower-upper symmetric successive overreaction scheme (LU-SSOR) with Van Leer flux vector splitting scheme, using MUSCLE technique. The Baldwin-Lomax algebraic model is used to close the Reynolds averaged equations. The position of the separation point and the oblique shock angle is compared with the experimental data and shows good agreements.

A numerical methodology for two-phase gas-solid flow is developed, using parallel computing. The particles are assumed elliptic and their translation and rotation are both considered. The drag, weight, and hydrodynamic torques are taken into account. Due to high computational time, a parallel is used. The results are obtained for particles ranging from 2 to 30 microns with aspect ratios of 1-10. The effects of this ratio on particle deposition are examined. The results are compared with previous experimental and numerical benchmark data, which show acceptable agreements.

In this paper, the hydrodynamics and the thermal behavior of the flow in a rotating U-shape channel and also the effective parameters are studied. This kind of flow is most applicable in internal cooling of gas turbine blades. In this study, because of the complication of geometry, it has been divided into distinct zones and using different relative coordinate systems, the flow has been analyzed. Also, the grid is staggered and the governing equations are formulated explicitly, using finite difference method. In this paper, the hydrodynamics and the thermal behaviors of coolant fluid are simulated by numerical modeling at rotary and stationary blade and the effective parameters, such as turbine rotation radius, radius of internal arc curvature, blade staggered angle, and rotation number are studied at two- and three-dimensional conditions. The recent considerable result of this research is the study of the manner of secondary flows interaction that formed due to curvature and rotation at different staggered angle and effect of this interaction in heat transfer.

Computations are performed to determine the steady 3-D viscous fluid flow forces acting on an impenetrable rotating spherical suspended particle at low and moderate Reynolds numbers in the range of. In order to extend the capabilities of the finite volume method, the boundary (body) fitted coordinates (BFC) method is used. Transformation of the governing partial differential equation to algebraic relations is based on the finite volume method and collocated variables arrangement. For solving the algebraic relations, the TDMA in a periodic state is used. To approximate the convective fluxes, the differencing scheme of Van Leer is used and the SIMPLEC algorithm handles the linkage between velocity and the pressure. Rotation increases the drag and lift forces exerted by flow at the surface of on the sphere. Using velocity components in Cartesian coordinates causes slight decrease in the run time of program with respect to using it in contra-variant and covariant coordinates. The flow patterns are changed with increasing rotation at x-y plane, but flow at x-y plane remains symmetric the present numerical results are in complete accord with other results of flow around a rotating sphere.

In this paper, a new method is introduced for recognition, location, and severity of damage in engineering stochastic structures, based on Autoregressive Moving Average (ARMA) parametric model and fuzzy classification. The important aspect of the proposed method is the fuzzy viewpoint on stochastic structural damage diagnosis, which uses estimated ARMA parameters as feature vector. Moreover, the proposed method eliminates the optimization stage in finding membership functions parameters of fuzzy system by substituting the variances of estimated ARMA parameters directly as tuning parameters in membership functions. Another important aspect of the proposed method is the inessentiality to measure the excitation input force applied to the structure. A finite element model of a frame for diagnosing damage, wherein the damage is modeled by different stiffness reduction and location, is considered as a case study. After obtaining satisfactory results from numerical simulations, the proposed method is applied to a simply supported beam as an experimental laboratory structure, where the spring connected to the structure in different locations with different stiffness is considered as the damaged object. the results are considerably satisfactory.

In this paper, a semi-analytical finite element method is presented to study the thermal buckling behavior of moderate-thick cylindrical shells. The theory is formulated based on first shear deformation assumptions. Simplified Sanders theory and non simplified theory are employed for calculation of geometrical stiffness matrix. The results obtained from these two theories are compared which shows that the critical thermal buckling temperature based on non-simplified theory is higher than that of the simplified Sanders theory. The material is high strength carbon and the boundary condition is clamped-clamped. The influence of fiber angle orientations is also studied.