hasan sajjadi; S. F. Hosseinizadeh; Y. Vazifehshenas
Volume 8, Issue 1 , March 2011, Pages 1-10
Abstract
Numerical investigation was performed on NACA 0015 which is a symmetric airfoil. Pressure distribution and then lift and drag forces are verified. Changing of ground clearance was a considerable point. Also the angle of attack was changed from 0° to 10°. Pressure coefficient reaches its higher amounts ...
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Numerical investigation was performed on NACA 0015 which is a symmetric airfoil. Pressure distribution and then lift and drag forces are verified. Changing of ground clearance was a considerable point. Also the angle of attack was changed from 0° to 10°. Pressure coefficient reaches its higher amounts on the wing lower surface when the ground clearance diminishes. Increment of the angle of attack leads to outspread of the high pressure domain over the lower surface and so leads to greater lift force. For higher angles of attack the ground clearance had no effect on the pressure distribution. Mostly for high angles of attack, the suction effect on the upper surface makes an adverse pressure gradient which can cause turbulent behavior and finally enhance drag force. To solve the problem SIMPLE algorithm is utilized. Moreover, QUICK algorithm and second-order upwind are used for momentum verification and turbulent viscosity, respectively. After all, the efficiency of the utilized model is checked. It is observed that k-e model cannot give acceptable results admitting experimental achievements in comparison with Spalart-Allmaras. This is depicted for drag and lift coefficient and also the pressure distribution on both surfaces of the airfoil.
Seyyed Amir Mahdi Ghannadpour; H.R. Ovesy; M. Nassirnia
Volume 8, Issue 1 , March 2011, Pages 11-19
Abstract
ABSTRACT This paper presents the theoretical developments of two finite strip methods (i.e. semi-analytical and full-analytical) for the post-buckling analysis of isotropic plates. In the semi-analytical finite strip approach, all the displacements are postulated by the appropriate shape functions while ...
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ABSTRACT This paper presents the theoretical developments of two finite strip methods (i.e. semi-analytical and full-analytical) for the post-buckling analysis of isotropic plates. In the semi-analytical finite strip approach, all the displacements are postulated by the appropriate shape functions while in the development process of the full-analytical approach, the Von-Karman’s equilibrium equation is solved exactly to obtain the buckling loads and the out-of-plane buckling deflection modes. The investigation of plates buckling behaviour is then extended to the post-buckling study with the assumption that the deflected form after the buckling is the combination of first, second and higher (if required) modes of buckling. Thus, the full-analytical post-buckling study is effectively a multi term analysis. In this method the Von-Karman compatibility equation is used together with a consideration of the total strain energy of the strut. Through the solution of the compatibility equation, the in-plane displacement functions which are themselves related to the Airy stress function are developed in terms of the unknown coefficients in the assumed out-of-plane deflection function. The in-plane and out-of-plane deflection functions are substituted in the total strain energy expressions and the theorem of minimum total potential energy is applied to solve for the unknown coefficients.
N. Fazli; S. Moammad Bag Malaek; Ali Abedian
Volume 8, Issue 1 , March 2011, Pages 21-28
Abstract
Here, we present a novel two-step approach for optimum design of cellular truss-beams based on desired natural frequencies. The proposed approach at- tempts to decrease the design complexities, based on the so called Axiomatic Design, before any e ort to solve the physical problem itself. It also serves ...
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Here, we present a novel two-step approach for optimum design of cellular truss-beams based on desired natural frequencies. The proposed approach at- tempts to decrease the design complexities, based on the so called Axiomatic Design, before any e ort to solve the physical problem itself. It also serves as a generic approach which nds its generality through dimensional analysis with its accuracy from nite element analysis and its optimality from the fact that it remains true for all similar frameworks. The applicability as well as strength of the method is highlighted by some numerical examples.
Seyed Hamid Jalali Naini
Volume 8, Issue 1 , March 2011, Pages 29-36
Abstract
In this paper, a closedloop strategy in the vertical plane is derived in order to determine the thrust direction of a launch vehicle in terms of velocitiestobe gained The two velocities-to-be-gained are utilized, here, for a given altitude and zero vertical speed in a specied nal time The formulation ...
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In this paper, a closedloop strategy in the vertical plane is derived in order to determine the thrust direction of a launch vehicle in terms of velocitiestobe gained The two velocities-to-be-gained are utilized, here, for a given altitude and zero vertical speed in a specied nal time The formulation is obtained for constant gravity assumption, but it works when the velocitiesto-be-gained are obtained for a spherical-Earth model via explicit or implicit relations-
Mohammad Reza Seif; mohammad ali sharifi; M. Najafi Alamdari
Volume 8, Issue 1 , March 2011, Pages 37-44
Abstract
Linearization of the nonlinear equations and iterative solution is the most well-known scheme in many engineering problems. For geodetic applications of the LEO satellites, e.g. the Earth’s gravity field recovery, one needs to provide an initial guess of the satellite location or the so-called reference ...
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Linearization of the nonlinear equations and iterative solution is the most well-known scheme in many engineering problems. For geodetic applications of the LEO satellites, e.g. the Earth’s gravity field recovery, one needs to provide an initial guess of the satellite location or the so-called reference orbit. Numerical integration can be utilized for generating the reference orbit if a satellite’s state vector i.e., position and velocity is known at a reference epoch. However, the numerically integrated orbit deviated from the real orbit due to imperfect force models. The more accurate reference orbit the less linearization error occurs. The deviation between the reference and real orbit can be minimized using the least squares method. Different analytical and numerical techniques have been developed for calculation of the design matrix of the least squares method. Herein, we have generalized the idea of the Lagrange coefficients for the determination of the design matrix’s entries in the gravitational field an attracting inhomogeneous mass body. Numerical implementation of the proposed method shows its high performance.
Jalal Karimi; Seid H. Dr. Pourtakdoust; Hadi Nobahari
Volume 8, Issue 1 , March 2011, Pages 45-56
Abstract
Performance characteristic of an Unmanned Air Vehicle (UAV) is investigated using a newly developed heuristic approach. Almost all flight phases of any air vehicle can be categorized into trim and maneuvering flights. In this paper, a new envelope called trim-ability envelope, is introduced and sketched ...
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Performance characteristic of an Unmanned Air Vehicle (UAV) is investigated using a newly developed heuristic approach. Almost all flight phases of any air vehicle can be categorized into trim and maneuvering flights. In this paper, a new envelope called trim-ability envelope, is introduced and sketched within the conventional flight envelope for a small UAV. Optimal maneuverability of the intended UAV is evaluated for minimum time pull-up and turn maneuvers. For both the trim and the maneuver problems, the nonlinear 6DOF dynamic models as well as the vehicle constraints are considered. A heuristic based constrained optimization approach is developed to solve both the trim and maneuver problems. Several interesting performance characteristics are extracted. The results are indicative of a good potential for the proposed algorithm to handle complex constrained optimization problems in aerospace engineering .
mostafa ghayour; Ahmad Sedaghat; Mohsen Mohammadi
Volume 8, Issue 1 , March 2011, Pages 57-67
Abstract
Multi-layer orthotropic finite cylindrical shells with a viscoelastic core in contact with fluids are gaining increasing importance in engineering. Vibrational control of these structures is essential at higher modes. In this study, an extended version of the wave propagation approach using first-order ...
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Multi-layer orthotropic finite cylindrical shells with a viscoelastic core in contact with fluids are gaining increasing importance in engineering. Vibrational control of these structures is essential at higher modes. In this study, an extended version of the wave propagation approach using first-order shear deformation theory of shell motion is employed to examine the free vibration of damped finite cylindrical shells in vacuum or in contact with interior or exterior dense acoustic media. For this purpose, a one-layered viscoelastic finite cylindrical shell and a three-layered orthotropic finite cylindrical shell with a viscoelastic core layer were used. Complex natural frequencies have been extracted and the effects of fluid coupling on real and imaginary parts of natural frequencies have been examined. The results reveal that the fluid reduces the imaginary part as much as the real part of the damped natural frequency but that the proportion of the imaginary to the real part (loss factor) remains rather unchanged. Another aspect of the study involves the investigation of the effect of shell parameter, m, when the circumferential mode number, n, increases on both entities of damped natural frequencies. It is found that by increasing n, the real part of the natural frequency follows a u-shape trend; however, the imaginary part reduces and levels off for higher circumferential numbers. The loss factors remain almost constant for these higher modes. The results of the current approach are finally compared with ABAQUS solutions showing superiority of current approach.
