Scientia Iranica
Volume:16 Issue: 6, 2009

Nonlinearity characteristics, such as backlash, in various mechanisms, limit the performance of feedback systems by causing delays, undesired oscillations and inaccuracy. Backlash in uence analysis and modeling is necessary to design a precision controller for this nonlinearity. Backlash between meshing gears in an electromechanical system is modeled by the use of di erential equations and a nonlinear spring-damper. According to this model, the paper shows that oscillations and delays cannot be compensated by a state feedback controller. Therefore, an adaptive algorithm is designed, based on di erent regions of the system angular position error. Since this controller needs an estimation of the backlash value, it is estimated by a learning unit in the adaptive controller. Simulations show that the presented adaptive controller can eliminate backlash oscillations properly, in accordance with previous works. Keywords: Backlash nonlinearity; Backlash estimation and compensation; Adaptive controller.

In many applications, convection heat transfer is coupled with conduction and radiation heat transfer, which generate temperature gradients along the walls and may greatly a ect natural convection heat transfer. The main objective of this study is to calculate the heat-transfer characteristics for natural convection from a non-isothermal vertical at plate into a supercritical uid. The in uence of the non-uniformity of wall temperature on the heat transfer by natural convection along a vertical plate, having a linearly distributed temperature (characterized by the slope S) is also investigated. The thermal expansion coecient is considered as a function of the temperature, the pressure, the van der Waals constants and the compressibility factor. The trends of the curves obtained with this equation and with values from tables of thermodynamic properties were similar and diverged at a critical point. These features con rmed the validity of this equation. Then, the governing systems of partial di erential equations are solved numerically using the nite di erence method. The local Nusselt number was then calculated and plotted as a function of the local Rayleigh number. It was observed that a positive slope of temperature distribution increases the heat transfer rate and a negative slope decreases it. Keywords: Finite di erence method; Natural convection; Nusselt number; Rayleigh number; Supercritical Fluids.

In this article, a new systematic method for deriving the dynamic equations of motion for exible robotic manipulators is developed by using the Gibbs-Appell assumed modes method. The proposed method can be applied to the dynamic simulation and control system design of exible robotic manipulators. In the proposed method, the link de ection is described by a truncated modal expansion. All the mathematical operations are done by only 33 and 31 matrices. Also, all dynamic expressions of a link are expressed in the same link local coordinate system. Based on the developed formulation, an algorithm is proposed that recursively and systematically derives the equation of motion, then this method is compared with the recursive Lagrangian method. As shown, this method is computationally simpler and more ecient and it reduces a large amount of computational complexity. Finally, a computational simulation for a manipulator with two elastic links is presented to verify the proposed method. Keywords: Manipulator; Flexible link; Recursive; Gibbs-Appell; Complexity.

In this paper, a complete four axle rail vehicle model with 70 Degrees Of Freedom (DOFs) is addressed, which includes; a carbody, two bogies and four axles. In order to include track irregularity e ects on vehicle behavior, a simpli ed track model for a straight line is proposed. As the performance of the suspension components, especially for air springs, has signi cant e ects on rail-vehicle dynamics and the ride comfort of passengers, a complete nonlinear thermo-dynamical air spring model which is a combination of two di erent models is introduced and then implemented in the complete rail-vehicle dynamics. By implementing the Presthus formulation [1], the thermo-dynamical parameters of an air spring are estimated and then tuned, based on the experimental data. E ects of air reservoir volume and connecting pipe length and diameter on system performances are investigated. For improving passenger comfort during their trips, air suspension parameters of the modeled rail vehicle are tuned to minimize the Sperling ride comfort index. Results showed that by modi cation of air suspension parameters, passenger comfort is improved and the ride comfort index is reduced by about 10%. The Genetic Algorithm (GA) optimization method is also used to optimize air suspension parameters. Results showed that improved air suspension con gurations are more practical, compared to optimized ones. Keywords: Rail vehicle dynamics; Air spring model; Air suspension parameters; GA; Optimization.

In this work, the rising of a single bubble in a quiescent liquid under microgravity condition was simulated. In addition to general studies of microgravity e ects, the initiation of hydrodynamic convection, solely due to the variations of interface curvature (surface tension force) and thus the generation of shearing forces at the interfaces, was also studied. Then, the variation of surface tension due to the temperature gradient (Marangoni convection), which can initiate the onset of convection even in the absence of buoyancy, was studied. The related unsteady incompressible full Navier-Stokes equations were solved using a nite di erence method with a structured staggered grid. The interface was tracked explicitly by connected marker points via a hybrid front capturing and tracking method. A one eld approximation was used where one set of governing equations is only solved in the entire domain and di erent phases are treated as one uid with variable physical properties, while the interfacial e ects are accounted for by adding appropriate source terms to the governing equations. Also, a Multi-grid technique, in the context of the projection method, improved convergences and computational sti ness. The results show that the bubble moves in a straight path under microgravity condition, compared to the zigzag motion of bubbles in the presence of gravity. Also, in the absence of gravity, the variation of surface tension force due to interface curvature or temperature gradient can still cause the upward motion of the bubble. This phenomenon was explicitly shown in the results of this paper. Keywords: Marangoni convection; Microgravity condition; Hybrid front capturing and tracking method; Rising bubble; Multi-grid method.

Cavitating ow is investigated around marine propellers, experimentally and numerically. Two di erent types of conventional model propellers are used for the study. The rst one is a four bladed model propeller, so called model A, and the second one is a three bladed propeller, model B. Model A is tested in di erent cavitation regimes in a K23 cavitation tunnel. The results are presented in characteristic curves and related pictures. Finally, the results are discussed. Model B is investigated based on existing experimental results. In addition, model B is used for validation of the numerical solution prior to the testing of model A. The cavitation phenomenon is predicted numerically on a two dimensional hydrofoil, NACA0015, as well as propeller models A and B. The cavitation prediction on a hydrofoil is carried out in both steady and unsteady states. The results show good agreement in comparison with available experimental data. Propeller models are simulated according to cavitation tunnel conditions and comparisons are made with the experimental results, quantitatively and qualitatively. The results show good agreement with experimental data under both cavitating and noncavitating conditions. Furthermore, propeller cavitation breakdown is well reproduced in the proceeding. The overall results suggest that the present approach is a practicable tool for predicting probable cavitation on propellers during design processes. Keywords: Cavitation; Marine propeller; Cavitation tunnel; Experimental; CFD.

In this work, a supersonic turbulent ow over a long axisymmetric body was investigated, both experimentally and computationally. The experimental study consisted of a series of wind tunnel tests for the ow over an ogive-cylinder body at a Mach number of 1.6 and at a Reynolds number of 8106, at angles of attack between -2 and 6 degrees. It included the surface static pressure and the boundary layer pro le measurements. Further, the ow around the model was visualized using a Schlieren technique. All tests were conducted in the trisonic wind tunnel of the Qadr Research Center (QRC). Also, the same ow at zero angle of attack was computationally simulated using a multi-block grid (with patched method around the block interfaces) to solve the thin layer Navier-Stokes (TLNS) equations. The numerical scheme used was implicit Beam and Warming central di erencing, while a Baldwin-Lomax turbulence model was used to close the Reynolds Averaged Navier-Stokes (RANS) equations. The static surface pressure results show that the circumferential pressure at di erent nose sections varies signi cantly with angle of attack (in contrast to the circumferential pressure signatures along the cylindrical part of the body), while the total pressure measurements in the boundary layer vary signi cantly both radially and longitudinally. Two belts with various leading edge angles were installed at di erent locations along the cylindrical portion of the model. The computational results obtained were compared with some experimental ones (found by these authors), showing considerably close agreements. Keywords: Supersonic ow; Pressure distribution; Boundary layer; Long axisymmetric body; Multiblock; TLNS equations.