نشریه دانش و فناوری هوافضا
سال سوم شماره 2 (پاییز و زمستان 1393)

This paper describes an analytical solution for calculating the aerodynamic coefficients and forces that are depending on the satellites angle of attack in LEO orbit. Aerodynamics forces are one of the perturbing forces which are government forces in LEO orbit and they can reduce satellite life time. Unfortunately these forces are function of geometric parameter، density of atmosphere، temperature، time، velocity and force coefficient so simulation of these kinds of forces are too hard and most of method which use for modeling have low Accuracy، because of this we could not predict satellite lifespan correctly. In this paper we produce new approach for solving this issues which improve Accuracy by solving equations as a function of angle of attack. We calculate satellite ballistic lifetime with numerical method (Cowell method) and aerodynamics torque simulates as function of angle of attack. At the end our simulation validated with STK8 software which shows good similarity of our method with STK8 software.

Steady flows over a 60º delta wing with sharp leading edge are computationally studied at different angles of attack and Mach numbers. Flow patterns over the upper surface of a delta wing are classified into six types based on the component of angle of attack normal to the leading edge and component of Mach number normal to the leading edge MN. Flow patterns over the delta wing studied in this research are compared to known patterns of previous studies and their variations with free-stream Mach number and angle of attack are investigated. Visualization results obtained by numerical simulations show that a leading edge vortex is formed on the upper side of the wing with or without the presence of the secondary separation which gradually expands and becomes closer to the wing surface with increasing Mach number. At Mach numbers higher than 1. 2، the leading edge becomes supersonic and the expansion wave emanating from the leading edge accelerates the flow. At this condition، shock waves are formed on the wing which interact the vortices. With increasing angle of attack، the vortex gets away from the wing and closer to the shock wave. The effects of free-stream Mach number and angle of attack on the location of vortex breakdown are also studied.

This paper investigates the use of the Hilbert-Huang transform to identify airplane flight modes and their characteristics. This study shows that the Hilbert-Huang transform، as a new powerful tool in the signal analysis field، has good potential capabilities to improve the airplane flying quality analysis and to overcome some drawbacks of the classical method in flight dynamics. To utilize these capabilities، some improvements such as online implementation of the empirical mode decomposition algorithm are presented. The new online-local algorithm can estimate the signal trend by the Savitzky-Golay filter and eliminate it from the signal in the EMD algorithm. A performance comparison of the new and traditional algorithms is also presented. Then، a new method is proposed based on the online-local EMD algorithm and Hilbert transform to determine the airplane modes and their characteristics. The new method is able to extract some airplane modes، including natural and non-standard modes، directly from measurements of flight parameters during the flight tests in the time domain. The results indicate the ability of the proposed method to extract the airplane modes with small damping ratios. Also، the consistency of the results obtained from the simulated output signals of the linear perturbed model verifies the new method performance. Finally، an example of applying the proposed method to the real flight test data is presented. It reveals the existence of some non-standard modes with small damping ratios at nonlinear flight regions and confirms the new method performance.

Buckling phenomena in different loading conditions، will lead to structural instability. Critical buckling load is dependent to factors such as geometry، size، load type، and boundary conditions. The aim of this paper is to study of the structure effect on the buckling behavior of carbon nanotubes (CNTs). In order to investigate the effect of chiral angle independent from size effects، all structures are used with the same dimensions but different chiralities. To simulate the chemical bonds between carbon atoms، carbon-carbon covalent bond energy is modeled using molecular mechanics theory and beam element. Coordinates of the nodes are determined using a simple algorithm. Then، chirality effect on axial and torsional buckling load is analyzed using finite element method for different structures. The results of this research show that the chiral angle has no significant effect on critical axial buckling load. However، CNT''s structure has considerable influence on the stability. Chiral structures can be weaker or stronger against torsional buckling than symmetric structures.

The purpose of this paper is to analyze an internal longitudinal semi-circular crack in a hollow cylinder subjected to thermo-mechanical loading. The inner surface of the cylinder was subjected to both internal pressure and forced convective heat transfer loading where the outer one only was subjected to external pressure loading. The temperature at the outer surface has been assumed to be constant. In order to solve the problem، a three-dimensional finite element model with 20-node singular elements around the crack front was employed. Transient thermal stress intensity factors were obtained for some points of the crack front. Transient thermal stress intensity factor variations along the crack front were calculated for different relative depths and Biot numbers which indicate the type of forced convective heat transfer in different times. In the special cases of loading، the results show to be in accordance with those cited in the literature. The results show that the prescribed temperature at the inner surface of the cylinder is the conservative thermal boundary condition and the corresponding stress intensity factors are the maximum values in any Biot numbers. It is observed that the steady state analysis is the most critical one and possesses the highest value of stress intensity factor along the crack front.

In the first part of this paper، the gas tungsten-arc welding process of a thin cylinder made of Al6061-T6 alloy is simulated using 3-D finite element model in the ABAQUS code and the distribution of residual stress is obtained. Temperature dependent thermo-mechanical properties are considered for the aluminum alloy and for simulating the heat source of tungsten arc welding، Goldak''s double ellipsoid model is also employed. Based on the finite element results، the value of residual stress is considerably positive around the weld line and HAZ area which can increase the risk of crack initiation and propagation in the weldment zone. Hence، in the second part of the numerical analyses of this research a longitudinal semi elliptical crack is considered in the wall of internally pressurized aluminum cylinder and its mode I stress intensity factor (KI) is determined numerically for different crack geometries. Finally the influence of both residual stress field and internal pressure is taken into account on the value of KI. It is observed that the effects of combined internal pressure and tensile residual stresses around the crack can facilitate required conditions for crack propagation in the analyzed cracked thin cylinder.

Simple designing، low maintenance and build cost، and high ratio of thrust to weight، also new methods of numerical simulating and solution causes to develop the Pulse Jet engine usage in non-military fields. Impact of geometrical parameters of valve less pulse jet engine on thrust has been studied in this article. First، study concentrate on main geometrical parameters like diameter and length، and transform those parameters to dimension less parameters (length to mean diameter ratio) for each section of the engine. Then the allowable range for parameters is achieved based upon experimental researches of Lockwood. Ten new geometries for valve less pulse jet engine has been defined and numerical solution of two-dimension flow field of inside engine has been presented with Fluent Code. New method has been presented (without solving the combustion) with considering combustion as initial condition. To ensure that the accuracy of solution is obtained، validation has been done with a valve pulse jet engine that shows excellent results with less than 5 percent error. Eventually the main result shows that amount of parameters that has most impact on thrust is 29 for exhaust pipe، 1. 25 for combustion chamber and 3. 5 for intake pipe. The volume of combustion chamber should not exceed the limits otherwise it will be caused non-uniform pressure distribution and will be effect on engine performance.