Aerospace Science and Technology
Shahrokh Zohrabzadeh Bozorgi; Abolghasem Naghash
Abstract
In this paper, a few hybrid satellite constellations including combinations of LEO and GEO satellites for providing satellite navigation and positioning services for users in Iran have been designed and proposed. The performance of the constellations has been analyzed based on DOP values variations. ...
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In this paper, a few hybrid satellite constellations including combinations of LEO and GEO satellites for providing satellite navigation and positioning services for users in Iran have been designed and proposed. The performance of the constellations has been analyzed based on DOP values variations. It is shown that theoretically, it is possible to provide satellite positioning and navigation service with acceptable DOP values based on the introduced hybrid pattern including three GEO satellites and a constellation of about 30 to 60 LEO satellites in 3 or 4 orbit planes. The design has been performed based on studying the skyplot of the Iranian territory considering the GEO satellites as fixed points, and then determining the effect of the instantaneous position of the LEO satellites on the DOP values. A few LEO constellations have been designed to provide best DOP values based on the skyplot analysis results. Then, scenarios including similar GEO satellites and different patterns for LEO satellites have been simulated for half a sidereal day. The performance of the hybrid constellations provides satisfactory results with the average PDOP values of less than 4 which is acceptable. Optimizing the resulted pattern can lead to more desirable performance. In addition to navigation mission, hybrid constellations can perform other missions. Therefore, the proposed constellations can be operated as multi-mission space platforms.
Aerospace Science and Technology
Alireza Sekhavat Banis; Reza Aghaei Togh
Abstract
The compressor blade is responsible for increasing the flow pressure. By adding a blade behind the main blade, the compressor performance can be improved by increasing the pressure ratio and reducing the weight. The tandem improves the performance and increases the compressor absorption coefficient by ...
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The compressor blade is responsible for increasing the flow pressure. By adding a blade behind the main blade, the compressor performance can be improved by increasing the pressure ratio and reducing the weight. The tandem improves the performance and increases the compressor absorption coefficient by increasing the pressure ratio, preventing flow separation and controlling the boundary layer. This has led compressor designers to seek to reduce weight, increase pressure ratio and increase efficiency by using tandem. The geometry of the compressor blade and stage along with its tandem has been obtained from previous valid sources and has been drawn in three dimensions and numerically analyzed. Then the various parameters for the blade and the tandem are examined separately and the pressure and velocity vectors are plotted to show the control of the vortices, which results in improved compressor performance. The characteristic curve of the compressor and the pressure ratio for this particular tandem are also plotted at the end. Calculations show that by using the tandem and removing the excess vortex after the main blade, we will see a 28.5% increase in total pressure, a 15% decrease in relative mach number and a 1.5% decrease in entropy.
Aerospace Science and Technology
Ali Cheraghi; Reza Ebrahimi
Abstract
Feed pumps play a crucial role in the dynamics of hydraulic systems. The surge phenomenon is a common type of instability in pumps and compressors. This phenomenon is a systematic instability and is influenced by the dynamics of all components of a hydraulic system, including tank, valves, suction pipes, ...
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Feed pumps play a crucial role in the dynamics of hydraulic systems. The surge phenomenon is a common type of instability in pumps and compressors. This phenomenon is a systematic instability and is influenced by the dynamics of all components of a hydraulic system, including tank, valves, suction pipes, impeller and the turbomachine itself. Surge emerges when a pump is operating with a positive slope of head and flow curve. The coincidence of the surge phenomenon with cavitation results in a damaging phenomenon called "auto-oscillation." Thus, predicting a pump's behavior outside the design points is of great importance particularly in low flow rates. In this paper, the characteristic curve of a high-speed centrifugal pump is extracted using CFD analysis to determine the stable operating range of the pump. The studied pump consists of an inducer, impeller and volute. The simulation in the pump was carried out three-dimensionally due to the asymmetry of geometry. The simulations are performed over a wide range of flow rates and the characteristic curve of the pump (head coefficient in terms of mass flow rate coefficient) is extracted. Finally, the range of stable operation of the pump is determined using its characteristic curve.
Aerospace Science and Technology
Mahsa Azadmanesh; Jafar Roshanian; Mostafa Hassanalian
Abstract
This paper employs the fast terminal sliding mode control with the sign and the saturation function to track the landing trajectory of a probe on an asteroid and to further improve the dynamic tracking performance. Then the controller is enhanced by adding the fuzzy control to both fast terminals. To ...
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This paper employs the fast terminal sliding mode control with the sign and the saturation function to track the landing trajectory of a probe on an asteroid and to further improve the dynamic tracking performance. Then the controller is enhanced by adding the fuzzy control to both fast terminals. To make fair judgments on the performance of the suggested method, the proportional derivative sliding mode control with both the sign function and the saturation function is simulated as well. The two-point barycentric gravitational model is used to describe the weak gravity around the asteroid. The proposed fuzzy fast terminal method raises the convergence speed, improves the desired trajectory tracking accuracy and ensures that the system modes are placed on the sliding surface in a short, limited time. The absolute errors for the proportional derivative sliding mode controller, fast terminal sliding mode controller and improved fast terminal sliding mode controller are about 244, 139 and 113. The trajectories along all three coordinate axes in the proportional derivative sliding mode controller, fast terminal sliding mode controller and improved fast terminal sliding mode controller were tracked in 8 seconds, 5 seconds and 4 seconds. The results show how the fuzzy-fast terminal sliding mode control with the saturation function is the better choice of controller and how the fuzzy system is able to adapt to the momentary fluctuations and cover them successfully.
Aerospace Science and Technology
Mana Ghanifar; Milad Kamzan; Morteza Tayefi
Abstract
This paper investigates different intelligent methods of tuning feedback-linearization control coefficients. Feedback-linearization technique is an effective method of controlling nonlinear systems. The most critical part of designing this controller is tuning the gains, especially if the plant has complex ...
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This paper investigates different intelligent methods of tuning feedback-linearization control coefficients. Feedback-linearization technique is an effective method of controlling nonlinear systems. The most critical part of designing this controller is tuning the gains, especially if the plant has complex nonlinear dynamics. In this research, to improve the performance of the overall closed-loop system, the feedback linearization method has been integrated with the conventional proportional-integral-derivative (PID) controller. Also, a quadratic performance index was used to compare the functionality of the controllers tuned by the proposed intelligent methods. These intelligent methods include Genetic Algorithms (GA), Particle Swarm Optimization (PSO), Fuzzy Logic, and Neural Network tuning algorithms. A quadrotor aircraft is used as the plant under study in order to evaluate the performance of the controllers tunned in this research. Finally, MATLAB simulation tests demonstrate the effectiveness of the presented algorithms. According to the results, it is demonstrated that the class of online algorithms performs better, even with the specified perturbation.
Aerospace Science and Technology
Amir Akbari; Hossein Khaleghi
Abstract
The use of unshrouded turbine rotor blades can considerably reduce the weight of an aero engine. However, in an unshrouded high-pressure turbine, the tip leakage flow generates about 30% of the turbine total loss. Another factor which affects the loss in axial-flow turbines, is the axial distance between ...
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The use of unshrouded turbine rotor blades can considerably reduce the weight of an aero engine. However, in an unshrouded high-pressure turbine, the tip leakage flow generates about 30% of the turbine total loss. Another factor which affects the loss in axial-flow turbines, is the axial distance between the rotor and stator. The purpose of the current work is to investigate the impacts of the blade tip clearance and the axial distance between rotor and stator on the performance of a high-pressure axial turbine, by using three-dimensional numerical simulations. Comparing the numerical results to the experimental data shows that the numerical simulations can predict the turbine performance fairly accurately. Results reveal that increasing the tip clearance and the axial distance between the rotor and stator reduce the turbine efficiency. The effects of tip clearance and rotor-stator axial distance on the performance and endwall flow field of the studied turbine stage have been presented and discussed.
Aerospace Science and Technology
Shayan Dehkhoda; Mohammad-Ali Amiri Atashgah
Abstract
This paper is dedicated to the optimal path-planning of a quadrotor to deliver the goods in the form of a round-trip mission. At first, quadrotor modeling is performed by the Newton-Euler method and then the problem is formulated as an optimal control effort problem. Then, by discretization of the equations ...
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This paper is dedicated to the optimal path-planning of a quadrotor to deliver the goods in the form of a round-trip mission. At first, quadrotor modeling is performed by the Newton-Euler method and then the problem is formulated as an optimal control effort problem. Then, by discretization of the equations using the direct colocation method, the problem becomes a nonlinear programming system that can be solved by available optimization methods. This discretization helps to make the derivative values in the equations of motion as simple algebraic expressions and the path optimization problem becomes a standard form of nonlinear programming problem (NLP). In this method, instead of obtaining state and control functions, state and control values are obtained at the beginning and endpoints of smaller time intervals. This method is one of the most explicit methods for the numerical solution of differential equations. It should be noted that in this research, safe areas around urban obstacles are considered fixed cylinders. Extensive simulations are evidence of the usefulness of this method, while the vehicle realizes all geometric, dynamic, and kinematic constraints.
Aerospace Science and Technology
Seyedeh sepideh Madani; mohammad ali shahi ashtiyani
Abstract
Nowadays, operational usage of the unmanned aerial vehicles (UAVs) in various missions is on the increase considering their capabilities. Provided that there is coordination between the UAV, navigation and control system, operational capability of the UAVs increases. Since there is no pilot in UAVs, ...
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Nowadays, operational usage of the unmanned aerial vehicles (UAVs) in various missions is on the increase considering their capabilities. Provided that there is coordination between the UAV, navigation and control system, operational capability of the UAVs increases. Since there is no pilot in UAVs, the task of guidance and control of the UAV for carrying out the mission depends on the ability of the autopilot and guidance system. This paper regards the control and the guidance as two separate entities in way point tracking problem. To do so, backstepping controller design for inner loop to track the commands is generated by the outer loop. The outer loop is designed based upon fuzzy logic. The proposed system uses standard Mamdani fuzzy controllers that provide speed, heading, and flight path angle references for the autopilots. Nonlinear six-degree-of-freedom equations of motion are used to model the vehicle dynamics. Simulations were carried out to verify the performance of the system. The results indicate the ability of way point tracking system to track the desired set of waypoints.
Aerospace Science and Technology
Jafar Jafari; hamid Parhizkar; Sajad Ghasemlooy
Abstract
This research investigates the effect of optimization of blade cross-section on the performance of the Darrieus wind turbine. The fluid flow around a Darrieus wind turbine is simulated by URANS (Unsteady Reynolds Averaged Navier Stokes) method. And blade cross-section was modeled by the Bezier curve ...
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This research investigates the effect of optimization of blade cross-section on the performance of the Darrieus wind turbine. The fluid flow around a Darrieus wind turbine is simulated by URANS (Unsteady Reynolds Averaged Navier Stokes) method. And blade cross-section was modeled by the Bezier curve and optimized to increase the average torque of the wind turbine. We used a novel, simple way for remeshing new design points in the optimization process. The Nelder-Mead simplex method was used for optimization, which enhanced the Turbine's performance by 33.7 percent. Results show that optimization of the blade cross-section is effective for increasing the performance of a VAWT (Vertical Axis Wind Turbine), and Nelder-Mead simplex is a proper and fast optimization method to be used in this case. Finally, the optimized airfoil was analyzed and compared with the initial one to understand optimization effects. It was concluded that optimization was more effective in azimuth positions of 90 to 160 degrees. And it decreased the performance in some regions according to changing nature of flow around each blade because of rotational motion. Analyzes show that optimization increased the Turbine's performance by increasing lift force of airfoil in some positions or affecting interaction flow -even accompanied by decreasing lift force of airfoil- in other positions, and it decreased performance in some other azimuth points.
Aerospace Science and Technology
Sahar Noori; Armin Sheidani; Djavad Kamari
Abstract
In this study the effect of different configurations of three plates located in an air-filled container, which included vertical, horizontal and tilted, on coupled radiation and natural convection heat transfer has been numerically investigated. The side walls of the cavity were kept a constant temperature, ...
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In this study the effect of different configurations of three plates located in an air-filled container, which included vertical, horizontal and tilted, on coupled radiation and natural convection heat transfer has been numerically investigated. The side walls of the cavity were kept a constant temperature, while the upper and the lower walls were thermally insulated. In addition, non-uniform temperature distribution was applied to each of the plates. Moreover, in this study the effect of coupled heat transfer on flow separation and local Nu number was studied. The flow separation on the heated plates due to the thermal gradients was captured and the subsequent the effects were discussed. Also, the results reveal there are two main flow patterns known as separation of the convective flow and stretching of the CW vortex which are created by combined heat transfer. It was also demonstrated that these flow patterns are the main responsible for variations in the heat transfer.
Aerospace Science and Technology
Hamzeh Eshraghi
Abstract
In the current article, using results of previous researches, a guideline has been developed to select a proper value for solidity of a tandem blade row in an axial flow compressor stage. Next, using this guideline, a highly loaded tandem compressor stage has been designed. To verify the selected solidity ...
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In the current article, using results of previous researches, a guideline has been developed to select a proper value for solidity of a tandem blade row in an axial flow compressor stage. Next, using this guideline, a highly loaded tandem compressor stage has been designed. To verify the selected solidity value, some other cases have been designed with different solidity values. Other geometrical parameters have been selected similarly in all cases. At the next stage, a three dimensional numerical model is developed to predict the characteristic performance of each tandem stage. The model is validated with the experimental results of NASA Stage and Rotor 37, and the level of the accuracy of the model is presented. Using a similar model, the performance of all cases has been derived and the effect of solidity variation on the overall performance of machine has been discussed. Lastly, the effect of solidity variation on the tip leakage flow structure near peak efficiency point is discussed for all cases.
Aerospace Science and Technology
Amir Moghtadaei Rad
Abstract
In this article, a complete model including cross-coupling of azimuth and elevation axes, the effect of axis friction, non-perpendicularity and imbalance of axes was implemented for the platform with two degrees of freedom. Since this model includes 3 loops of current, stability and tracking from the ...
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In this article, a complete model including cross-coupling of azimuth and elevation axes, the effect of axis friction, non-perpendicularity and imbalance of axes was implemented for the platform with two degrees of freedom. Since this model includes 3 loops of current, stability and tracking from the inside to the outside, it was necessary to design a suitable controller for each loop separately from the inside to the outside after linearizing the obtained model. Also, due to the presence of two channels, azimuth and elevation, it was necessary to repeat and design 3 controllers for both channels separately. Since the purpose of this article is to compare the performance of different controllers, PID, Fuzzy, Fuzzy PID and Fuzzy self-tuning controllers for both channels and all loops, their design and performance in time and frequency domains were analyzed. At the end, relative advantages of each controller according to different parameters of the system were presented in a comparative table.
Aerospace Science and Technology
کامران رئیسی چرمکانی; غلامرضا مرادی
Abstract
Knowledge of aircraft last position and trajectory is of the utmost importance for search and rescue in aviation today. In case of an accident, this information is necessary for rapid response.
The existing method of reporting the whereabouts depend upon pilot reports during flight or signals from special ...
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Knowledge of aircraft last position and trajectory is of the utmost importance for search and rescue in aviation today. In case of an accident, this information is necessary for rapid response.
The existing method of reporting the whereabouts depend upon pilot reports during flight or signals from special equipment such as emergency location transmission. Future renovation to Air Traffic Control system has been planned which requires the installation of automatic Dependent Surveillances – Broadcast. However, it will take some time to implement such systems in all aircrafts and providing the necessary infrastructure. This paper proposes Geo-tagging the pilot communication using Code Division Multiple Access (CDMA) method. It is an intermediate, yet inexpensive, solution for worldwide application in aviation which can be implemented quickly. It takes advantage of the existing equipment both in the air as well as on the ground. The simulations presented show the applicability and efficiency of the proposed routine.
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