Journal of Aerospace Science and Technology
Volume:16 Issue: 1, Winter and Spring 2023

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.

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.

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.

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.

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.

This study aims to control a space robot's soft-landing trajectory on the asteroid EROS433 considering a weak, yet effective gravitational field. As the research innovation, the study employs a fast terminal sliding mode control (FTSMC) to manage the landing trajectory and enhance the dynamic tracking performance for the soft landing of the space robot on the asteroid. This controller can ensure that the system modes are positioned on the sliding surface within a limited time. As an advantage over the PD sliding mode controller, the proposed controller raises the speed and improves the accuracy of tracking the desired trajectory and enhances the robustness of the control system. The study further compares the results of simulations performed in MATLAB to evaluate the proposed controller design. The results show that the absolute error value for FTSMC is significantly lower than the PD sliding mode controller, and when the sign function is replaced by a hyperbolic tangent, it makes the system behavior smoother and reduces the oscillations.

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.

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.

The ever increasing demand for placing satellites in the geostationary orbit has caused the revision and change of the conventional mechanism of allocating orbital slots. Therefore, collocation approaches and station keeping of several satellites with a common position have been developed to improve the utilization of the capacity of the geostationary orbit. This, in turn, leads to an increase in the complexity and sensitivity of the modeling, guidance, and control processes. However, new restrictions are added to the problem of maintaining a common location, such as maintaining the minimum separation distance between satellites to prevent possible interference. Employing a collocation strategy is essential, especially for effective control of high-demand orbital regions that will lead to space congestion.Controlling the relative motion of satellites by maintaining a safe distance between them is the main rule in collocation. This article investigates the problem of the relative motion of satellites corresponding to collocation strategies. Then, the results are implemented and compared using a solution based on geometrical modeling of relative orbit and the concepts of spherical geometry. In this regard, the relative orbital elements of the two satellites are calculated using the presented relative motion modeling. Also, the relative position of the satellites is obtained. The case studies and evaluations confirmed that the inclination and eccentricity separation strategies are suitable options for meeting the fuel consumption requirements and providing more space for collocated satellites than other strategies.

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.

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.

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.