نشریه دانش و فناوری هوافضا
سال یکم شماره 1 (بهار و تابستان 1391)

In this research، effects of blowing ratio (velocity ratio) difference between the controlling small jets and the main jet in the compound triple jets arrangement on the film cooling effectiveness and the flow hydrodynamics are investigated. This blowing ratio difference was selected such that the total mass flow rate of the cold flow becomes equal to that of the simple compound triple jets. The triple jets are inclined normally into the cross flow and the cooling jets to the hot cross flow temperature ratio and the jets Reynolds number are 0. 5 and 4700، respectively. The numerical approach is based on the finite volume and the SIMPLE algorithm over a multi-block، staggered، structured and non-uniform grid arrangement was applied. The numerical simulations are performed for three controlling jets to main jet velocity ratios. The obtained results show that when the velocity ratio is higher than unity، the film cooling effectiveness increases، while it decreases when the velocity ratio is less than unity. Due to high complication of the flow (results from interaction of three jets with the cross flow) and time dependency of the introduced vortex structures، the large eddy simulation approach was used to investigated the flow more accurately.

In this study we introduce and validate an algorithm for simulating of flow field over moving boundaries where there are transverse cylinder and flapping airfoil. The governing equations are presented in arbitrary Lagrangian-Eulerian approach. We use finite element shape functions to approximate pressure and diffusion fluxes and finite volume to satisfy geometric and flow conservation laws. Our results show a good accuracy with large time steps and coarse meshes. The changing in oscillation parameters causes considerable effects on aerodynamic coefficients.

In this paper، a proportional-derivative attitude control system will be designed for large- angle maneuvers of a three-axis stabilized satellite using four reaction wheels and its stability is proven using Lyapanov theory. In this method، the nonlinear terms of satellite dynamic model are effective because of large angle rotational maneuvers and will be considered in designing the control law. Since the complex structure of the reaction wheels and their performance are effective on the stability and accuracy of satellite، therefore، in order to evaluate the effectiveness of the designed control system، the control accuracy and stability of the satellite will be evaluated using simulation in presence of Reaction Wheels Constraints with High Fidelity Model.

Nowadays in view of increasingly rise in the number of air trips، the sufficient spaces for air traffics are reduced. The main part of these researches is to increase the accuracy of airplane altitude measuring systems in order for «Reduced Vertical Separation Minimum» (RVSM) and increasing the usable flying spaces above 29000 ft. In this paper، necessary systems and equipments in airplane are investigated to change in RVSM condition. Also، after consideration the effective error in altimitery system، uncertainty analysis is done. This analysis is carried out using the pressure tester and also different flights testing. The results show that، the error of altimetry system is about 22 ft which is in good agreement with the allowable value for this airplane in RVSM conditions.

In this paper an active point-mass filter is applied to match the radar observations with terrain reference map utilized in aircraft navigation. In the proposed matching method, based on an recursive Bayesian estimation approach, the point-mass filter is modified in order to become robust against the suddenly errors of radar system and Inertial Navigation System (INS) as well as to avoid instability of the estimation process when the aircraft passes from very smooth regions. The point-mass filter considers a network of points around the estimation point and assigns separated probabilities to them. The probabilities show uncertainties of the aircraft location. The probabilities are similar initially; however, they converge to true values gradually through a recursive estimation based on Bayesian approach. The simulation results demonstrate the robustness and efficiency of the proposed approach in terrain navigation systems and put it in the category of developing and operational systems.

Nonlinear vibration analysis of cross-ply laminated composite shallow circular cylindrical shell was performed. This study applies the Hamilton''s principle and Galerkin''s method to establish the governing equations of motion and obtain the spatial mode shapes. The geometric non-linear strains are of the von Karman type، boundary condition is clamped-clamped and the shell is subjected to radial dynamic excitation. The modal expansion، considered as a summation of modes، is directly substituted into the non-linear equation of motion for the shell based on the von Karman non-linear strain، and the Galerkin’s method is applied to obtain the governing non-linear equations of motion in generalized coordinates. The resulting ordinary differential equations are solved by the Runge-Kutta method. Using the presented model، the effects of lamination sequence and material properties on the vibration characteristics of the shell are studied and some conclusions are drawn. The results are compared with previous studies and finite element analysis.

In this paper stability of thin steel cylindrical shells of various lengths including unilateral and bilateral cutouts with various shapes (square، rectangular، circular and elliptical shapes) subjected to axial compression were systematically studied with both experimental and finite element methods. Buckling tests were performed using an INSTRON 8802 servo hydraulic machine and the results of experimental tests were compared to numerical results. A very good correlation was observed between numerical simulations and experimental results. Results show that circumferential extension of various cutouts with identical area and stress concentration are effective parameters on buckling load of these structures.