@article { author = {Heydari, Ali and Dr. Pourtakdoust, Seid H.}, title = {TIME OPTIMAL SLEW MANEUVER OF MAGNETICALLY ACTUATED LEO SATELLITES}, journal = {Journal of Aerospace Science and Technology}, volume = {5}, number = {2}, pages = {51-56}, year = {2008}, publisher = {Iranian Aerospace Society}, issn = {1735-2134}, eissn = {2345-3648}, doi = {}, abstract = {Conventional magnetic attitude control methodologies require several orbital periods to accomplish the required attitude maneuvers due to the existence of an uncontrollable axis, namely the local Earth’s magnetic field vector. Since in some attitude maneuver missions the elapsed time is of critical importance, those time-consuming controllers are not satisfactory and we need a much faster controller to achieve the maneuver in a fraction of an orbit. In this research the attitude slew maneuver using magnetic torquers is formulated as a time optimal problem and solved through the calculus of variations. The resulted controller is shown to be very fast in forcing the attitude to converge to the desired condition.}, keywords = {}, url = {https://jast.ias.ir/article_51554.html}, eprint = {https://jast.ias.ir/article_51554_a8853e0a675cfca956d3916e841f9661.pdf} } @article { author = {Dr. Taghavi, Reza}, title = {Numerical Simulation of Separation Bubble on Elliptic Cylinders Using Three-equation k-? Turbulence Model}, journal = {Journal of Aerospace Science and Technology}, volume = {5}, number = {2}, pages = {57-66}, year = {2008}, publisher = {Iranian Aerospace Society}, issn = {1735-2134}, eissn = {2345-3648}, doi = {}, abstract = {Occurrence of laminar separation bubbles on solid walls of an elliptic cylinder has been simulated using a recently developed transitional model for boundary layer flows. Computational method is based on the solution of the Reynolds averaged Navier-Stokes (RANS) equations and the eddy-viscosity concept. Transitional model tries to simulate streamwise fluctuations, induced by freestream turbulence, in pre-transitional boundary layer flows by introducing an additional transport equation for laminar kinetic energy term. It includes three transport equations of turbulent kinetic energy, KT, laminar kinetic energy, KL, and dissipation rate frequency, ?. Numerical results show that the model is capable of predicting each of subcritical (final separation in laminar), critical (existence of bubble/bubbles) and supercritical (final separation in turbulent) flow regimes. Laminar separation bubble is simulated precisely, due to the modeling of transition from laminar to turbulent in separated free shear layer. Separation bubble region and its characteristics were detected by inspection on the distributions of surface pressure and skin friction, and also by streamlines pattern. Excellent agreements were observed between results obtained through current mathematical modeling with available experimental data for all the flow regimes. In addition, some of the results of the present numerical method are compared to those obtained through application of conventional fully laminar and fully turbulent standard k-? models.}, keywords = {Boundary Layer,Laminar separation bubble,Eddy-viscosity model,Critical flow}, url = {https://jast.ias.ir/article_51555.html}, eprint = {https://jast.ias.ir/article_51555_68162ae580c82c22b23eef3c9d53a0aa.pdf} } @article { author = {Ganjaei, A. A. and Dr. Noor Azar, S. Salman}, title = {Development of a novel method in TRMC for a Binary Gas Flow Inside a Rotating Cylinder}, journal = {Journal of Aerospace Science and Technology}, volume = {5}, number = {2}, pages = {67-79}, year = {2008}, publisher = {Iranian Aerospace Society}, issn = {1735-2134}, eissn = {2345-3648}, doi = {}, abstract = {A new approach to calculate the axially symmetric binary gas flow is proposed Dalton’s law for partial pressures contributed by each species of a binary gas mixture (argon and helium) is incorporated into numerical simulation of rarefied axially symmetric flow inside a rotating cylinder using the time relaxed Monte-Carlo (TRMC) scheme and the direct simulation Monte-Carlo (DSMC) method. The results of flow simulations are compared with the analytical solution and results obtained by Bird, 1994. The results of the flow simulations show better agreements than the results obtained by Bird, 1994 in comparison with the analytical solutions. However, the results of the flow simulations using the TRMC scheme show better agreement than those obtained using the DSMC method in comparison with the analytical solutions.}, keywords = {Boltzmann equation,TRMC,DSMC,rotating cylinder}, url = {https://jast.ias.ir/article_51556.html}, eprint = {https://jast.ias.ir/article_51556_5b307569612ce483099448e50cda4eec.pdf} } @article { author = {Bahrainian, Seyed Saied}, title = {Construction of Hexahedral Block Topology and its Decomposition to Generate Initial Tetrahedral Grids for Aerodynamic Applications}, journal = {Journal of Aerospace Science and Technology}, volume = {5}, number = {2}, pages = {81-90}, year = {2008}, publisher = {Iranian Aerospace Society}, issn = {1735-2134}, eissn = {2345-3648}, doi = {}, abstract = {Making an initial tetrahedral grid for complex geometry can be a tedious and time consuming task. This paper describes a novel procedure for generation of starting tetrahedral cells using hexahedral block topology. Hexahedral blocks are arranged around an aerodynamic body to form a flow domain. Each of the hexahedral blocks is then decomposed into six tetrahedral elements to obtain an initial tetrahedral grid around the same aerodynamic body. This resulted in an algorithm that enables users to produce starting tetrahedral grids for variety of aerodynamic bodies. To construct an initial starting tetrahedral grid suitable for computational flow simulations, representing a solid surface geometry (fuselage or a wing section) attached to a plane-of-symmetry, a topology containing at least 5 hexahedral blocks is required. The resulting initial starting grid consists of 30 tetrahedral cells with 74 faces and 16 vertices, which is the same number of vertices as for the hexahedral blocks. A face-based global data structure is then produced for the tetrahedral cells. To represent multiple surface definitions a topology containing nine hexahedral blocks is required. When decomposed, the nine hexahedral blocks, produce a tetrahedral grid consisting of 54 cells and 24 vertices.}, keywords = {}, url = {https://jast.ias.ir/article_51557.html}, eprint = {https://jast.ias.ir/article_51557_461f09758297e3b7bf1f37843a5e5116.pdf} } @article { author = {Alashti, R. A.}, title = {Parametric Study of Plastic Load of Cylindrical Shells With Opening Subject to Combined Loading}, journal = {Journal of Aerospace Science and Technology}, volume = {5}, number = {2}, pages = {91-98}, year = {2008}, publisher = {Iranian Aerospace Society}, issn = {1735-2134}, eissn = {2345-3648}, doi = {}, abstract = {In this paper, the plastic limit load of cylindrical shells with opening subject to combined bending moment and axial force are found by finite element method using ANSYS suite of program. The effect of various parameters such as the geometrical ratios of shell and the shape, size, axial and angular position of the opening on the limit load of cylindrical shells under various loading conditions are studied. The plastic limit loads of cylindrical shells with circular openings of different opening ratios and combinations of dimensionless bending moment and axial force are calculated by finite element elastic-plastic method and their interaction curves are presented.}, keywords = {Cylindrical Shell,Plastic Analysis,Limit Load,Combined Loading}, url = {https://jast.ias.ir/article_51558.html}, eprint = {https://jast.ias.ir/article_51558_28934c7c6797e9d35d0d4a53413b2aa1.pdf} }