نشریه دانش و فناوری هوافضا
سال چهارم شماره 1 (بهار و تابستان 1394)

Vibration control of clustered launch vehicles has been significantly encountered with many challenges as a result of multi degree of freedom. The control system must cover all degrees of freedom to approach desired performances. In this regards, frequency response methods play an important role in vibration control of multi body dynamic systems. In this view, vibration control is implemented via dominant frequency estimation and filtering. In several activities used full band adaptive filters, the filtering system cannot apply to complicated vehicles. Powerful properties of signal processing toolboxes lead to design practical and useful vibration control systems. In this paper, vibration reduction is implemented in subband frequency and this is one the main contributions of this study. In this way, high delay made from the subband filtering may lead to degradation of the control system. In this paper, a method is presented for reducing the delay based on the Smith predictor and model reference adaptive approaches. The results of numerical simulation show the proposed approach can satisfactory compensate the problem of delay in the online sub-band filtering. This performance is carried out for a clustered launch vehicle with convenient responses

In this paper, the guidance and control system of an aerial robot for tracking a reference trajectory is designed. The proposed algorithm uses the tracking errors to derive the guidance commands. These errors are in the form of acceleration command along inertial coordinate and the obtained commands are mapped to body fixed coordinated system. Then, using a new analytical approach the commands are converted to suitable inputs for the control system in the form of linear velocity, roll and pith angles. The proposed approach does not use the polar conversion, which in turn does produce nonphysical singularity defects. In addition, the aerodynamic and performance capability of aerial robots and corresponding limitations are considered. Using an aerial robot model with six-DOF, a control system is designed to track the designated guidance commands. Simulation results of a fixed wing aerial robot using six-DOF model reveal that the proposed guidance and control approaches significantly follow the guidance commands. In fact, the aerial robot tracks the desired trajectory with much higher accuracy than previous methods.

In this paper, we introduce a new system design process for a cube satellite system which in turn, the design structure matrix method as a powerful design tool is employed for the system analysis. The cube satellite is a type of Pico satellites and the proposed systematic design process is applied to a typical version of these satellites type by illustrating, optimizing the dependency of design parameters driving the satellite conceptual design phase. Based on the systematic decision making logic including in the DSM approach, system design framework could be decomposed into smaller components and parts in design matrix which could show the dependency inherently defined between the actual satellite designs parameters. This fact is demonstrated, based on the supporting theory and considered case study, which the design parameters should influence the selection of each component at multiple levels of design requirements, mission, system and subsystem specified. Finally, the design matrix is developed and should be analyzed for a basic student cube satellite and then design structure matrix could support the enhancing the design formation parameters. In this case, 135 systematic driving and design parameters of cube satellite are identified based on a conceptual design phase activity and then these parameters passing through mathematical clustering procedures to form design process partitions and also design recursive loops.

Low speed aerial vehicles which fly at low Reynolds number have been designed like Ornithopter birds to increase the range and reduce fuel consumption. The researches show that although the harmonic oscillating airfoils in upstroke generate more lift than fixed airfoils but sometimes reduction of power at downstroke is enormous. The aim of this study is aerodynamic improvement of low speed simple harmonic oscillation airfoils by using of gurney flap which present better aerodynamic efficiency in upstroke and downstroke than common oscillating airfoils. For this purpose, the pressure based finite volume element numerical method is used on a moving gird. In the present work, the location and height of gurney flaps as the two most important parameters have been studied. The results show, whatever the flaps be closer to trailing edge, the aerodynamic efficiency increases. However, the height should be limited to a certain extent. Generated Vortices by the flap Gurney play a major role in improving aerodynamic efficiency. These vortices by changing the pressure distribution and their effects on flow separation on the upper surface of the airfoil increase aerodynamic efficiency.

In present investigation, the flow field passing through a highly loaded low pressure (LP) turbine cascade is numerically studied at design and off-design conditions. The Field Operation and Manipulation (OpenFOAM) platform is used as the Computational Fluid Dynamics (CFD) tool. Firstly, the influences of grid resolution on the results of RANS k-ε and k-ω models and LES utilizing Spalart-Allmaras subgrid scale model (LES-SA) are investigated and compared with those of the experimental measurements. Numerical results show that a pressure under-shoot is obtained near the end of blade pressure surface which is highly sensitive to grid resolution and flow turbulence modeling. Secondly, it is shown that the LES-SA model is able to resolve separation on the coarse and fine grid resolutions. The LES-SA results show the separation about S/C=0.85 whereas the experiments measure it about S/C=0.75.Thirdly, the off-design flow condition is modeled by imposing negative and positive inflow incidence angles. The numerical results of LES-SA model show that a separation bubble is generated on blade pressure side. The calculation of total pressure drop at incidence angles between -20 and +8 degrees illustrates that the k-ω and LES-SA models could estimate the minimum total pressure drop at the design point.

Longitudinally seam welded pipes are frequently used in the oil and gas industries. Failure of such pipes may be occurred due to the crack growth initiated in the weld zone. At service conditions, cracks existing in these pipes usually experience complex tensile-shear deformations. For estimating the onset of fracture in the cracked pipes during their service life it is necessary to obtain the stress intensity factors. Hence in this paper, several 3D semi elliptical cracks initiated longitudinally along the weld line in the outer and inner wall of a welded pipe with different aspect ratios ranging from 0.5 to 1 are analyzed using ABAQUS software. It is shown that the contribution of all three modes (KI, KII and KIII) may affect significantly the onset of fracture in the investigated pipes. However, the effect of mode I deformation (KI) is more pronounced than the shear mode deformations (KII and KIII). It was also shown that the value of equivalent stress intensity factor in the inner wall is greater than the outer wall of cracked pipe.

As the main parts of rotor of aero-gas turbine engine, due to arduous working conditions, the design of turbine as well as compressor disks is of importance. These disks are loaded under centrifugal and thermal forces which gets higher with increasing rotating speed, and the gas’s pressure and temperature. To improve the rotor dynamic behavior, decrease the bearing’s load and motor’s weight, the weight of the rotor, as the main part of the motor, should be minimized. Rotating speed, high temperature working condition, high temperature gradient, and the demand for minimum weight, imply serious restrictions on the design of rotor, especially turbine’s rotor. On the other hand, the strength analysis is prior to the study of rotor’s life because, to assess the cyclic loading, first the static one should be investigated. The aim of this study is to optimum the weight of a turbine integrally bladed disk (blisk) of a mini-turbojet engine. Aero-thermodynamic design parameters such as geometry, blades’ number and location, aerodynamic loads’ distribution, temperature and pressure distribution on the rotor, and rotating speed are the input parameters of the optimum design problem of the rotor’s disk under strength and geometrical constraints. To do so, numerical programs for design, analysis and optimization of the disk is developed and the obtained results are validated through previous ones in the literature. In addition, the structure of a special mini-turbojet engine is designed in an optimum manner.