Scientia Iranica
Volume:16 Issue: 1, 2009

Abstract. Based on the instability of magnetic behavior through the concept of local exchange and hybridization, a new class of heavy fermion is constructed with a stable local magnetic ion, \Gd". The lattice constant, DC/AC- magnetic susceptibility, (T), and electrical resistivity, (T), measurement in the magnetic unstable intermetallic compounds of Gd2AxB1􀀀x, show that (I) The hidden magnetic internal energy is manifested by shape and eld dependence is strongly at x = 0:4, (II) The lattice parameter of a crystal and magnetic structure, as well as high transition temperature, \Tc", strongly depends on the conductive electron concentration. Both the functional change of the lattice parameter (non-Vegard behavior) and magnetic character with electron concentration suggest that the free electrons are not completely free and are somehow participating in the band functions and magnetic character, and that (III) There is a coexistence of Kondo behavior and magnetic ordering \re-entrant antiferromagnet" for x = 0:4 in the range of temperature 30 < Tk < 90 K with TN = Tmax = 30 K, so that (IV) Finally, the metal insulator-like behavior with a complete quench of magnetic ordering occurs antiferromagnetically, named superparamagnet, at a certain conductive electron, \x = 0:3", where the sample is eld dependent (on which, it it is suggested, P"ijJij = 0). Keyword: Possibility of Kondo e ect on \Gd".

Abstract. In this study, the free vibration of moderately thick rectangular plates with several internal line supports was analyzed; the plates having twenty one possible boundary conditions (a combination of clamped, simply supported and free classical boundary conditions). The dimensionless equations of the strain (potential) and kinetic energy were derived, based on the Mindlin plate theory, to study the transverse vibration of moderately thick rectangular plates (in terms of the resultant stress, with consideration of transverse shear deformation and rotatory inertia). The Rayleigh-Ritz method, assuming two dimensional polynomial functions as admissible displacement functions, was applied. Numerical results were presented for a wide range of aspect ratios and thickness to length ratios. The in uence of line support location and thickness to length ratio on the frequency parameters was shown graphically. Keywords: Free vibration; Line support; Rectangular Mindlin plate; Rayleigh-Ritz.

Abstract. A series of low speed wind tunnel tests were conducted to study the unsteady aerodynamic behavior of an airfoil sinusoidally oscillating in plunge. The experiments included measuring the surface pressure distribution over a range of reduced frequencies, k = 0:03 􀀀 0:06. In addition, steady state data were acquired and were used to furnish a baseline for further analysis and comparison. The model was oscillated with amplitude of 15 cm and at three di erent mean angles of attack of 0, 10 and 18. The unsteady aerodynamic loads were calculated from the surface pressure measurements, 64 ports, along the chord for both upper and lower surfaces. The plunging displacements were transformed into the equivalent angle of attack. Variations of the pressure coecients and aerodynamic loads with the equivalent angle of attack showed strong sensitivity to the reduced frequency and mean angles of attack. Keywords: Plunging; Unsteady aerodynamics; Airfoil; Wind tunnel; Reduced frequency; Hysteresis loop. INTRODUCTION Unsteady ows and dynamic stall prediction methods used by the industry are largely based on empirical or semi-empirical approaches that are fast and relatively accurate; where non-linear e ects are not too great. Increased development in aircraft and wind turbine aerodynamics has created a demand for more detailed information of non-linear unsteady loads, the dynamic response and aero elastic stability caused by dynamic motions, including dynamic stall e ects [1]. Wind turbine or helicopter rotor blade sections encounter large time dependent variations in angle of attack, as a result of control input angles, blade apping, structural response and wake in ow. In addition, the blade sections encounter substantial periodic variations in local velocity and sweep angle. Thus, the unsteady aerodynamic behavior of the blade sections must be properly understood to enable accurate pre- 1. Department of Aerospace Engineering, Sharif University of Technology, P.O. Box 11155-8639, Tehran, Iran. *. Corresponding author. E-mail: msoltani@sharif.edu Received 24 July 2006; received in revised form 17 September 2007; accepted 23 December 2007 dictions of the air loads and aero elastic response of the rotor system [2]. One underlying assumption in most aerodynamic models is that the e ects of all blade motions and wake in ow variations can be adequately represented by an equivalent angle of attack. However, some studies [3- 5] have postulated that fundamental di erences exist in the air loads when di erent modes of motion are imposed (i.e., pitching versus plunging displacements). Most angle of attack changes that the rotor blades encounter are, in fact, due to variations in the apping and elastic bending of the blade, i.e., a plunging forcing type [6]. Virtually all of the available unsteady air load data are for pitching motions and information about the aerodynamic behavior of a model in a plunging motion is rare. This study addresses some of the most important aspects of the unsteady aerodynamic behavior of an airfoil oscillating in plunge at a subsonic regime. The experiments were conducted at a freestream velocity of 30 m/sec, corresponding to the Reynolds number of 0:42  106, and at an oscillation amplitude of 15 cm. This investigation involves the e ect of reduced frequency on the pressure, hence, aerodynamic, coecients of the airfoil at low to moderate angles of

Abstract. The accuracy of Reynolds Averaged Navier-Stokes (RANS) turbulence models to predict the behavior of 2-D density currents has been examined. In this work, a steady density current is simulated by the k 􀀀 ", k 􀀀 " RNG, two-layer k 􀀀 " and modi ed 2 􀀀 f model, all of which are compared with the experimental data. Density currents, with a uniform velocity and concentration, enter a channel via a sluice gate into a lighter ambient uid and move forward down-slope. The eddy-viscosity concept cannot accurately simulate this ow because of two stress production structures found within it. Results show that all isotropic models have a weak outcome on this current, but by improving the ability of the models, the results will improve. Keywords: Density current; Turbulence models; k 􀀀 " RNG; Two-layer k 􀀀 "; Modi ed 2 􀀀 f.

Abstract. Volume Of Fluid (VOF) is one of the most powerful methods to resolve free surface ows. In this study, a new algorithm is developed in a curvilinear coordinate system, which implements an implicit pressure based method (SIMPLE) with a staggered grid and a Lagrangian propagation of the interface, using the VOF method in the computational domain. Based on this algorithm, a computer code is generated and two test cases of dam-breaking problems, both in curvilinear and Cartesian grid systems, are examined and, then, two applications of this method, including ow through a curved gate under a dam and the impact problem of a circular cylinder, are presented. The results show good agreement with experimental and other computational results. Keywords: Free-surface; Volume-of- uid; Front tracking; Curvilinear coordinate system; Dambreaking

Abstract. In this paper, the e ect of shaft rotation on its natural frequency is investigated. Considering rotary inertia and gyroscopic momentum, the fourth order di erential equation representing the exural vibration of the shaft is solved by the new analytical method. The Distributed Lumped Modeling Technique (DLMT) is applied to obtain the transfer matrix for the distributed elements using the proposed method and for the lumped elements consisting of rotary inertia and gyroscopic e ects. The results obtained by this method are compared and veri ed with the results of two other methods. The e ects of shaft diameter, shaft length and disk inertia on the natural frequency are discussed for various speeds. It is shown that, while the new method brings highly accurate results, its simplicity and accuracy provide proper application for use in industrial systems. Keywords: Distributed; Lumped; Modeling; Rotary inertia; Gyroscopic e ect.

Abstract. The Plasma Electrolytic Saturation (PES) technique is a relatively new process, which uses a high DC voltage power supply with a related organic electrolyte. Within the phenomenological model, the mechanism of the nanocrystalline coating formation processes has been developed to clarify the controlling variations of the procedure. Within the informative model, a feedback parameter for the formed surface state estimation was established. This parameter is the power spectral density of the current. The voltage versus current characteristics of the treatment within the plasma region depend on the main electrolysis conditions, such as current density, electrolyte concentration and agitation rate. Application of the control algorithm of the parameters could increase the eciency of the plasma electrolytic saturation and nanocrystalline coating formation. In this process, the electric eld strength, \E", within the surface region reached a value of between 106 and 108 V/m, which was sucient for initiation of ionization processes in the vapor envelope. The ionization phenomena initially appeared as a quick sparking in scattered gaseous bubbles and then transformed into a uniform glow distributed throughout the vapor plasma envelope. Because of the hydrodynamic stabilization of the vapor envelope in the region of 180- 210 V, the current dropped and the glow discharge transformed into intensive arcing, accompanied by a characteristic low-frequency acoustic emission. Keywords: Plasma treatment; Glow discharges; Nanostructure; Dielectrics; Di usion.

Abstract. Certain strati ed charge divided chamber engines have a very small pre-chamber, equipped with a spark plug and a main chamber connected to the pre-chamber through nozzles. A theoretical model is presented in this research to predict ignition delay and initiation of combustion in the pre-chamber. It considers ame progress in the pre-chamber up to the point where the ame penetrates the main chamber through the connecting nozzles. Step by step calculations then continue in the main chamber and the mass fraction burned and the energy release rate are calculated. The process continues to the point where all the fuel is burned. At each step, due to a one degree rotation of the crank shaft, there is a change in the cylinder volume, due to the movement of the piston and, also, a change in the mole fraction burned, due to the burning of a fraction of the mixture. Considering heat transfer from the cylinder contents to its surrounding area, some important operating parameters, such as indicated power, indicated thermal eciency, indicated speci c fuel consumption, indicated mean e ective pressure and volumetric eciency, are predicted. Stepwise calculations also provide in-cylinder pressure-volume and pressure-crank angle diagrams, as well as the in-cylinder contents temperature variation with the crank position and concentration of species existing in the combustion products. Predicted values obtained by the present model are compared with corresponding experimental values available in the literature to evaluate the accuracy of the model. The comparison shows reasonable agreement between theoretical and measured values. Keywords: Divided chamber; Strati ed charge; Modelling; Engine performance.

Abstract. In this paper, a formulation is developed for obtaining the optimal trajectory of robot manipulators to maximize the load carrying capacity for a given point-to-point task. The presented method is based on open loop optimal control. The indirect approach is employed to derive optimality conditions based on Pontryagin''s Minimum Principle. The obtained necessary conditions for optimality lead to a two-point boundary-value problem solved via a multiple shooting method with the BVP4C command in MATLAB r. Since the carrying payload is one of the system parameters, a computational algorithm is developed, which provides the capability of calculating the maximum payload for a point-to-point task. The main advantage of this method is obtaining various optimal trajectories with di erent maximum payloads and path characteristics by changing the penalty matrices values. To demonstrate the eciency of the proposed method and algorithm in obtaining the maximum payload trajectory, simulation is performed on a two-link manipulator. Keywords: Robot manipulator; Maximum payload; Optimal trajectory; Optimal control.