M. R. Ghaffari; M. M. Hossainali
Volume 4, Issue 1 , July 2011, Pages 1-14
Abstract
The function based approach to three-dimensional tomographic modeling of Ionosphere is analyzed. Harmonic and empirical orthogonal functions are used as the base functions required in the modeling of the horizontal and vertical variations of the electron density. The instability of solution has been ...
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The function based approach to three-dimensional tomographic modeling of Ionosphere is analyzed. Harmonic and empirical orthogonal functions are used as the base functions required in the modeling of the horizontal and vertical variations of the electron density. The instability of solution has been numerically analyzed and the Tikhonov regularization technique has been used to regularize the solution. To come up with an optimum value for the regularization parameter, the direct measurements of the electron density obtained from the Tehran Ionosonde station, located at λ=51.640 and φ=35.870 , are used. The electron density model reconstructed in this paper has a maximum relative error of 36.44% and its minimum value is 0.8503. The maximum difference between the vertical total electron content (VTEC) obtained from the reconstructed model to that obtained from the corresponding IGS network Ionosphere product is 52.320 TECU and its minimum value is 1.268.
F. Nobakht-Orsi; M. A. Sharifi; A. R. Safari
Volume 4, Issue 1 , July 2011, Pages 15-22
Abstract
In this paper, Hilbert Huang transform (HHT) for time series analysis of the status of permanent GPS stations can be provided. Hilbert Huang transform and its, related spectrun is a new method for the analysis of nonlinear processes.This method represents not only a precision analysis in the time-frequency ...
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In this paper, Hilbert Huang transform (HHT) for time series analysis of the status of permanent GPS stations can be provided. Hilbert Huang transform and its, related spectrun is a new method for the analysis of nonlinear processes.This method represents not only a precision analysis in the time-frequency space, but also it declares the physics of the dynamic processes. This method has two speps. At the first step, the data is disintegrated into IMF elements by EMD method. At the second step Hilbert transform is applied for IMF elements and distribution of the time-frequency –Hilbert energy spectrum is then determined. In this spectrum, Location, frequency and energy defined by time events for the Hillbert transform will be reserved. So instantaneous frequency will have a major role in this method.
A. Ebrahimi; H. R. Fayazbakhsh
Volume 4, Issue 1 , July 2011, Pages 23-28
Abstract
Within operational life of satellites according to their mission, need to change their operational orbits. This change can includes change in semi-major axis, inclination or plane of orbit (RAAN angle). For this propose, we can use Electrical Propulsion (EP). With respect to light weight of EP in cases ...
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Within operational life of satellites according to their mission, need to change their operational orbits. This change can includes change in semi-major axis, inclination or plane of orbit (RAAN angle). For this propose, we can use Electrical Propulsion (EP). With respect to light weight of EP in cases which there is weight constraint, we use EP for transfer satellite to desired orbit. In this paper, we consider minimum-time transfer problem for these transfers. Initial and final orbits are circular and we assume thrust is continuous. Assuming constant acceleration, elements of orbit uniformly change and these changes analyze with Edelbaum Analysis
F. Ommi; S. Askari Mahdavi; S. M. Hosein- Alipour; E. Movahed-Nezhad
Volume 4, Issue 1 , July 2011, Pages 39-48
Abstract
A linear instability analysis of an annular liquid sheet emanating from an atomizer subjected to inner and outer air streams to investigate the liquid viscosity and swirl velocity on the maximum growth rate has been carried out. The dimensionless dispersion equation that governs the instability of a ...
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A linear instability analysis of an annular liquid sheet emanating from an atomizer subjected to inner and outer air streams to investigate the liquid viscosity and swirl velocity on the maximum growth rate has been carried out. The dimensionless dispersion equation that governs the instability of a viscous annular liquid sheet under air streams was derived with linear stability analysis. The dispersion equation solved by numerical method and investigated viscosity and swirl velocity effect on maximum growth rate and its corresponding unstable wave number. The results show that decrease in viscosity has positive effect on maximum growth rate and its corresponding unstable wave number. At low liquid swirl Weber number liquid swirl has a stabilizing effect and at high liquid swirl Weber number liquid swirl velocity has a destabilizing effect on the liquid sheet. The growth rate can be related to the breakup length of the liquid sheet and when the growth rate increase, breakup length was shorter. The drop diameter dependent to the wave number and decrease with increase on it that afford to improvement the combustion and decrease the specific fuel consumption.
S. Shahmirzai Jashoghani; M. Nosratollahi
Volume 4, Issue 1 , July 2011, Pages 49-60
Abstract
In this research optimal trajectory of lunch vehicle based on maximizing payload is being attended. At first, motion of missile concluding modeling of environment, atmosphere, gravity, mass, motion equations and aerodynamic coefficients would be simulated. Then procedure of an optimized design by using ...
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In this research optimal trajectory of lunch vehicle based on maximizing payload is being attended. At first, motion of missile concluding modeling of environment, atmosphere, gravity, mass, motion equations and aerodynamic coefficients would be simulated. Then procedure of an optimized design by using optimal control theory would be described. Applying variational calculus and mathematical modeling of optimization problems would lead project to a two point boundary condition problem which would be solved by numerical solutions such as steepest descent. At last a code would be generated in which optimal trajectory of missile calculated by using indirect optimal control and steepest descent numerical solution. An interesting point in this article is that some variables are used both as state and control variables. Hence state control variables here are divided to two groups, slow state variables concluding ones which are only state variables, and fast state variables concluding ones which are both state variables and control variables simultaneously. Solution for such control problems is described here.
M. Naghikhani; H. R. Ali Mohammadi
Volume 4, Issue 1 , July 2011, Pages 61-67
Abstract
Selection of the optimal tolerance for the components is one of the most important and complex issues in the design of the parts and products especially in sensitive and accurate industries (like as aerospace). Generally the design engineers tend to consider close tolerances in terms of product performances ...
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Selection of the optimal tolerance for the components is one of the most important and complex issues in the design of the parts and products especially in sensitive and accurate industries (like as aerospace). Generally the design engineers tend to consider close tolerances in terms of product performances and high reliability, on the contrary to manufacturing engineers that used to open tolerances because of easier, cheaper and practicality of manufacturing processes. Study the balance between tolerance and other factors affecting it, such as quality, cost and production, is the most important issue in this case. In this regard, there are numerous articles and methods. In this paper we use the tools of statistics and mathematics as" response surface method", first method of diagnosis of the critical tolerances describes and then tolerance of the optimal allocation - in case the cost of production or quality characteristic is optimized- has been examined.
A. Mohammadi; E. Taheri; J. Roshanian; M. Bahrami
Volume 4, Issue 1 , July 2011, Pages 69-78
Abstract
In this research a perturbation based guidance method is developed for non-Keplerian problem. Problem is linearized in the presence non-gravitational forces like aerodynamic, so it can improve the performance of C* guidance for reentry problems. In this study developed method is used for a reentry guidance ...
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In this research a perturbation based guidance method is developed for non-Keplerian problem. Problem is linearized in the presence non-gravitational forces like aerodynamic, so it can improve the performance of C* guidance for reentry problems. In this study developed method is used for a reentry guidance accuracy and performance. Results showed significant increases in accuracy compared to Keplerian approaches.
A. A. Jafari; A. Mozafari; M. Maghsoudi Mehrabani
Volume 4, Issue 1 , July 2011
Abstract
The effect of rotating speed and stiffeners on vibrations for composite rotors is investigated using Sander's shell theory. The frequency equation is derived implementing the Rayleigh-Ritz procedure based on energy method. The effects of initial hoop tension, centrifugal and Coriolis forces due to the ...
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The effect of rotating speed and stiffeners on vibrations for composite rotors is investigated using Sander's shell theory. The frequency equation is derived implementing the Rayleigh-Ritz procedure based on energy method. The effects of initial hoop tension, centrifugal and Coriolis forces due to the rotation are considered to derive governing equation. The displacement functions satisfying the both ends simply supported boundary conditions are assumed to be trigonometric expressions. By using simple shell theory like as Sander's shell theory the amount of equations and time expenditure are considerably reduced and provides feasible analysis, solution and design especially for composite materials optimization.UD composite materials are used for stiffeners. The effects of these stiffeners are evaluated by an averaging method. Some of stiffeners shapes are considered to optimize the ratio of natural frequency to weight.
