Abstract: Abstract Change detection in remote-sensing images is used to detect changes during different time periods on the surface of the Earth. Because of the advantages of synthetic aperture radar (SAR), which is not affected by time, weather or other conditions, change-detection technology based on SAR images has important research value. At present, this technology has attracted the attention of increasingly more researchers, and has also been used extensively in diverse fields, such as urban planning, disaster assessment, and forest early warning systems. Our objective in this paper is to combine both the change detection of SAR images with the deep neural networks to compare its efficiency with fuzzy clustering method and deep belief network. Our experiments, conducted on real data sets and theoretical analysis, indicates the advantages of the proposed method. Our results appear that proposed deep-learning algorithms can further improve the change-detection process. PubDate: 2019-10-16
Abstract: Abstract This paper presents comparisons of rotor performance, blade pitch, rotor hub loads, and blade clearance between two sets of rotorcraft comprehensive analysis calculations and experimentally measured test data for a coaxial rotor system in forward flight conditions. The calculations are from the U.S. Army’s Rotorcraft Comprehensive Analysis System (RCAS) with one set calculated using a prescribed vortex wake model and another set calculated by coupling RCAS to a viscous vortex particle method. For rotor performance and blade pitch, the calculations and test data are presented as functions of lift offset for four different advance ratios and for two different test upper rotor collective pitch settings. Rotor hub loads and blade clearance are compared on a time-varying basis for test points that form a lift offset sweep, an advance ratio sweep, and a rotor-to-rotor phase sweep. The main conclusion is that both sets of calculations generally reflect the same trends as the test data, which provides confidence in the ability of RCAS at calculating the aeromechanics behaviors of lift-offset coaxial rotors. PubDate: 2019-10-15
Abstract: Abstract This paper addresses trajectory optimization in the mid-course phase of an air-to-ground missile, when the main objectives are (a) to ensure that the target is locked on in the center of the missile’s field-of-view at a specified flight path angle and (b) to attain maximum possible speed to allow for sufficient maneuverability in the terminal phase. The method presents as a second-order cone program (SOCP) formulation for this trajectory optimization, taking advantage of partial linearization and lossless convexification techniques that effectively handle underlying non-convex characteristics of the problem. A well-established SOCP solver can then be readily used to obtain the optimal solution to this convex program. The proposed approach is validated by (a) proving the losslessness of the convexification, and (b) numerically comparing the results with an existing pseudo-spectral method. PubDate: 2019-10-12
Abstract: Abstract Shock-wave/boundary-layer interactions (SWBLI) are of great importance in supersonic transport vehicles. The shock-induced separation and its unsteadiness may lead to harmful influences on the aerodynamic performance and fatigue life of supersonic air-intakes, turbo-machine cascades and supersonic nozzles. We particularly focus on a three-dimensional SWBLI in supersonic flow past a finite-span sharp wedge. Implicit large-eddy simulation is performed to investigate the flow features in the three-dimensional SWBLI. Results show that a bow-type side-edge shock wave is generated from the leading edge of the finite-span sharp wedge. The shock impinges on the turbulent boundary layer and causes additional turbulence fluctuations in the spanwise direction. Three-dimensional features dominate the shock impingement and reflection. A large-scale separation bubble is induced by the bow-type side-edge shock wave. Properties of this separation bubble are examined and qualitatively compared with a two-dimensional SWBLI case. PubDate: 2019-10-11
Abstract: Abstract In the present study, detached eddy simulations were conducted to investigate buffet characteristics of the vertical tail. Unsteady flow phenomena were observed to clarify the cause of buffet onset. The location of vortex breakdown was also predicted. Simulations were carried out for high-angle conditions of more than buffet onset. Finally, to attenuate buffet intensity, two design parameters involving the spanwise translation and the tilt angle of the vertical tail were considered. It was found that the wake-like vortex structure developed after the event of vortex breakdown was the main cause of buffet onset. It was also found that the location of vortex breakdown moves upstream with increase in an angle of attack, which accelerates dissipation of the vortex core near the vertical tail. Peak-to-peak excitation phenomenon was featured as buffet characteristics acting on the vertical tail. It was demonstrated that the factor to more excite buffet was vortex shedding originated from the trailing edge of the wing at the highest angle, in which the peak level was the largest. The most discernible effect of buffet attenuation was observed when two parameters were simultaneously applied, which are suggested as a design application under the development of aircrafts. PubDate: 2019-10-09
Abstract: Abstract The present work reviews the new concept of operation that will be soon implemented in SESAR and NextGen (January 2020) using new state-of-the-art technology mainly based on onboard avionics: data link equipment, broadcast and surveillance systems, and Global Navigation Satellite System (GNSS) area augmentation. One of the main improvements of Performance-Based Navigation (PBN) is the use of satellites and more precise and accurate onboard instruments than current standard avionics related to ground-based navigation aids (VOR, NDB, DME, etc.). Air navigation systems have been without mayor updates for nearly 40 years, when most Very High Frequency Omnidirectional Range (VOR) and Tactical Air Navigation (TACAN) systems were implemented worldwide in the civil and military fields, respectively. These standard navigation systems lack new required performance navigation and required big deal of maintenance, especially in redundant systems. Recently, the development of new and precise GNSS and communication systems has allowed their use on different scenarios: Instrumental Flight Rules (IFR) departures, initial and final approaches, etc. Additionally, in several international airports, Ground Based Augmentation System (GBAS) approaches have been already successfully tested and implemented. Related to GBAS, the Automatic Dependent Surveillance Broadcast (ADS-B) is a cooperative technology that enhances pilots and controllers’ situation awareness, since ADS-B broadcasts own and other aircraft position. Controller Pilot Data Link Communications (CPDLC) may be another key element of the PBN concept of operations (CONOPS), since it provides air–ground data communications for the air traffic control (ATC) service and the aircrew, reducing risks associated to human factor: poor speaking, radio congestion, message confusion, standardization, etc. The main goal of this review is to present the PBN concept and the potentially supporting Communication, Navigation and Surveillance (CNS) systems, in the context of the NextGen and SESAR Air Traffic Management (ATM) modernization programmes. PubDate: 2019-10-03
Abstract: Abstract Coupled flow-thermal analysis is crucial for the performance evaluation and structural design of hypersonic vehicles. In this study, several strategies for computing the coupled flow-thermal response of air-breathing hypersonic flights in practical engineering are developed and compared. First, the basic direct correction method that amends wall heat flux via recovery temperature and wall temperature is proposed to approximate flow-thermal effects efficiently. Second, the improved DCM (IDCM) is further developed by interpolating the cold wall heat flux and the recovery temperature between adjacent trajectory points to improve computational accuracy. Third, the iteration solution method (ISM) that obtains solutions through mass and energy balances at a common interface by iterations between CFD analysis code and CSD analysis code is also presented. Thermal response and flow characteristics are compared through a test case of an air-breathing hypersonic vehicle. The results show that the thermal response tendencies are consistent by DCMs and ISM. However, for DCMs, the impact of hot wall on the flow characteristics is ignored, whereas it is fully considered in ISM; thus, the thickened boundary flow and complicated internal flow can be captured. However, while comparing computational efficiency, DCMs have a prominent advantage over ISM due to the decoupling algorithm and parallel strategy. Based on this, in the actual design process of an air-breathing hypersonic vehicle, the designers can select the proper flow-thermal analysis method according to the different design stages. PubDate: 2019-09-26
Abstract: Abstract This paper proposes a novel aerial system named as a ducted-fan flight array system. The distinct feature of the proposed system is a reconfigurable flight system by assembly or disassembly regarding its operational objectives. This array system comprises multiple ducted-fan unmanned aerial vehicles (UAVs) which have a favorable configuration to equip a connecting mechanism around its duct. For the proposed system, this paper discusses dynamics to take account of the change of physical properties due to its reconfigurable feature. Moreover, the dynamics deal with control effectors which comprise multiple flaps and rotors for the array configuration. Based on the derived dynamic model, performance analysis is conducted to consider total external force and moment, control effect, and mechanical connectivity between the vehicles with respect to various candidates for the array configuration. The analysis shows that dominant parameters are the control power and mechanical connectivity for the proposed flight array system. In contrast, the influence of the external force and moment is relatively small by configuration changes. Through this research, the suitable configuration of the array can be confirmed to be a close-set shape with a balanced control combination. PubDate: 2019-09-26
Abstract: Abstract To examine the high-angle-of-attack (AOA) aerodynamics, the conventional lift and drag were measured in one revolution AOA by the dynamic load cell in the wind tunnel for rectangular wings of the NACA0012 section with four different aspect ratios, 3, 4, 5, and 6, at a Reynolds number of 1.0 × 105. The results were analyzed in the normal and reverse modes of the airfoil. It was found that the reverse airfoil is disadvantageous to the lifting device because of the earlier stall than the normal and the substantial drag increases before the stall. In the entire AOA range, Prandtl’s lifting line theory seems to be applicable in general, but the profiles of the lift coefficient are not linear anymore. It was also found that the drag coefficient of the normal airfoil mode is affected by the delta wing-type vortex wrap as well as the downwash, and that the downwash effect was dominated between the deep stall and the second peak. Using the expanding scales, which have an exponential decay rate with the aspect ratio, the polar plots of the four different wings overlap in one circle with a radius of 1.0 at the same origin. PubDate: 2019-09-13
Abstract: Abstract It is well known that the selection of a frequency for signal generation and data acquisition significantly affects the quality of ultrasonic or laser ultrasonic nondestructive testing (NDT) results, and the analog filter is essential to prevent the aliasing effect. This paper presents a systematic approach using a hierarchical inspection scheme for automatic inspection processes. A frequency band divider (FBD) with a single-input–multiple-output (SIMO) system was newly created to examine quickly various frequency ranges at single scanning using laser-based broadband excitation. The FBD was implemented into a pulse-echo ultrasonic propagation imaging (PE UPI) system capable of fully noncontact laser ultrasonic inspection. Only two scans are required to determine the optimal frequency range for unknown specimens. This tremendously reduces the number of scanning trials for the investigation of complete frequency ranges. PubDate: 2019-09-12
Abstract: Abstract Lateral thrust jets exhibit better maneuverability performance than control surfaces such as conventional fins for attitude control or the orbital transfer of guided weapons. In the supersonic region, however, jet interaction phenomena occur due to the lateral thrust jet during flight and a complicated flow structure is exhibited by the interaction of the shock wave, boundary layer flow, and vortex flow. In particular, hit-to-kill interceptors require precise control and maneuvering; therefore, it is necessary to analyze the effect of the jet interaction flow. A number of conventional jet interaction analyses have been performed under low-altitude conditions; however, there are not many cases with respect to medium-altitude conditions. Unlike low-altitude conditions, jet interaction flows at medium-altitude conditions have different flow characteristics. In this study, a jet interaction flow analysis is performed on a lateral thrust jet controlled interceptor operating at medium altitudes. Based on the results, the characteristics of the flow structure and the changes in the aerodynamic coefficients are analyzed. PubDate: 2019-09-11
Abstract: Abstract An experimental study was conducted to investigate the effect of fuel boiling point on the hydraulic characteristics of high-temperature liquid jets, simulating injection of fuel used as coolant in the active cooling systems of a hypersonic flight vehicle. Two hydrocarbon fuels were specially created to have higher boiling points than conventional aviation fuels. The fuels were heated to close to 573 K (300 °C) using an induction heater at an upstream pressure of up to 1.1 MPa, and discharged to atmospheric downstream pressure conditions through a plain orifice nozzle of diameter 0.7 mm. The fundamental hydraulic characteristics represented in Cd with respect to Tinj at three injection pressure conditions for the three fuels show that the temperatures at which Cd begins to decrease are very close to each fuel’s boiling or bubble point and remain almost constant for each fuel even when ∆P is varied. In the relationship between Cd and Re, the discharge coefficients, which are almost identical regardless of fuel and ΔP conditions in relatively low ranges of Re, begin to deviate and decrease sharply as Re increases, due to the collapse of the mass flow rate induced by the choked cavitation. The present results also confirm that the effect of fuel boiling point on thermal cavitation at temperatures above the boiling point is well correlated with the relationship between Cd and cavitation number, and the degree of choked cavitation as quantified by the cavitation numbers collapses to almost the same line, even for fuels with different boiling or bubble points. PubDate: 2019-09-11
Abstract: Abstract A technique was developed to estimate the energy required for the complete deployment of folded fins installed on a projectile launched under gust, based on an integral equation of the fin deployment dynamics. One key parameter, ideal angular speed of deployment, defined as an angular speed of the fin at the final position when only torque acts without any other external moments, was introduced to stand for energy stored in or work done by torque generator, such as spring and to be a main parameter for the estimation. Following the previous study on the fin unfolding motion simulation, aerodynamic load consisted of static moment and dynamic effects of fin rotation motion, which was proportional to the angular speed, so that it was difficult to evaluate the work done by the dynamic load due to its dependence on the angular speed. Integration of the dynamic load could be completed by presumed angular speed profile, the peak value of which is determined by the ideal angular speed of deployment. Combined with ideal angular speed of deployment and angular speed profile model, an integral equation of unfolding motion was converted to a quadratic algebraic equation, the solution of which is the ideal angular speed of deployment for the minimum energy required for the complete deployment. For demonstration, this estimation technique was applied to a folded fin system and estimation of minimum energy corresponding to given requirements of gust condition. Fin unfolding motion simulations using a previously developed technique were carried out for verification and simulation results, allowable gust speed, were shown to meet all the requirements. PubDate: 2019-09-11
Abstract: Abstract In the present study, turbulent flow at submerged inlets is numerically studied. Five different geometries including the standard inlet developed by National Advisory Committee for Aeronautics (NACA), divergent inlet, parallel inlet and two new proposed geometries are compared. The effects of the boundary layer thickness ratio (0.31, 0.8 and 2.56) defined as the ratio between the boundary layer thickness ( \( \delta \) ) and the entrance depth (d) and velocity ratio defined as the ratio between the flow velocity at the duct entrance and the free stream (0.2–1.6) on their performance are further investigated. Moreover, the \( \,K - \omega \; \) model is used to simulate the three-dimensional, incompressible and turbulent flow of interest. NACA inlet is validated with experimental data. Results showed that in all studied cases, increasing the boundary layer thickness has a negative effect on the Ram recovery ratio (or efficiency); however, the effect of the velocity ratio on the Ram recovery ratio was different with an initial increase followed by a decrease in the efficiency. It is shown that our two proposed inlets present the same efficiency as the NACA inlet, while the efficiency for the divergent and parallel inlets was lower. Interestingly, the proposed inlets show a lower drag force compared to the NACA inlet at the high velocity ratios. PubDate: 2019-09-10
Abstract: Abstract Aerial refueling technology has been widely applied in various fields and it is one of the hotspots but difficulties for the aeronautical technologies. DLR-F6 WBNP model is used as a tanker and a fighter model is used as a receiver. The flow field of Probe–Drogue refueling and Flying Boom refueling is numerically simulated using the Reynolds-averaged Navier–Stokes equations, and the effects of the jet flow and the aerodynamic characteristics of the receiver are taken into consideration. The results indicate that the effect of downwash of the tanker reduces the lift coefficient and decreases the pitching moment coefficient of the receiver. The jet flow of tanker increases the dynamic pressure while decreases the local angle of attack, which increases the pressure difference between the upper and lower surfaces of receiver. Compared with the results without jet, the jet flow can increase the lift and the drag of the receiver and reduces the pitching moment, and even cause the change of rolling moment direction. Therefore, engine jet is an important factor when simulating aerial refueling. PubDate: 2019-09-09
Abstract: Abstract This paper analyzes the effect of spanwise distribution of twist angle on the forces generated by flapping wings as well as the power requirements. We consider four sample profiles of the twist angle as a function of spanwise location, and compute the forces and power requirements under non-accelerating level flight conditions. We investigate three different wing geometries, with varying wingspan and aspect ratios. It has been found that for planforms with moderate to high wingspans, a quadratic profile performs better than the constant and linear ones; whereas for planforms with smaller wingspans, a linear profile performs better than the rest. The analysis presented in this paper can be used to identify the most suitable wing twist profile as a function of the flight parameters and can be used as the basis for wing morphing. PubDate: 2019-09-01
Abstract: Abstract In this paper, mission planning for an earth observation satellite is studied. Generally, the mission planning problem is known as a highly combinational problem. While exact methods can provide optimal scheduling solutions, they require large computation time so the exact methods may not be adequate for timely operations. In this paper, a simple yet effective heuristic method for mission planning is proposed. An additional degree of freedom in pitch axis is taken into account, which can significantly increase the number of images compared to the roll-only observations. Also, possibility of reverse order observation is considered with a simple objective function. The proposed method is applied to short-horizon mission planning in low earth orbit. The exact brute-force search is utilized as a counterpart to analyze optimality and time-effectiveness of the proposed method. Numerical results show that the proposed method offers a slightly degraded solution but runs very fast due to its simplicity. PubDate: 2019-09-01
Abstract: Abstract An adaptive control approach is presented for time-varying formation control of multiple UAVs with nonholonomic constraints and input quantization. The UAVs are described by nonholonomic kinematic model and autopilot model with uncertainties. A transverse function is designed to release the nonholonomic constraints. To avoid chattering, an enhanced hysteretic quantizer is utilized to process the input signals. The quantized signals are analyzed by a new decomposition method to release some restrictions. Based on Lyapunov stability theory, the adaptive backstepping controller is proposed for the formation tracking of multiple UAVs. Tuning functions are devised to make estimations of the unknown parameters and disturbances. A transformation function is applied to the control inputs to eliminate quantization effect. Stability analysis proves that the tracking errors can converge to the origin asymptotically, and all the signals in the closed-loop system are globally bounded. A simulation example is provided to illustrate the effectiveness of the proposed approach. Based on the control approach, the multi-UAV system can track the reference trajectory while forming and maintaining the predefined formation shape. PubDate: 2019-09-01
Abstract: Abstract This study investigated the small drone propeller noise, particularly the discrete blade passing frequency (BPF) tone and its harmonics at low frequencies less than 1000 Hz. The unsteady Reynolds-averaged Navier–Stokes equations were solved to investigate the steady and unsteady loading noise sources around the blades with a radius of 17 cm rotating at 5000 rpm (a blade tip Mach number is 0.264). The uRANS computations showed eccentric ellipsoidal isobaric surfaces on the upper and lower blade surfaces identified as a steady loading noise source of the drone propeller. A simple mathematical model of an ellipsoidal steady loading noise with the lattice Boltzmann method predicted the BPF tone and even-number harmonics comparable to NASA’s SPL measurement of two different APC 1147 SF and DJI 9443 CF drone propellers. The decaying rate of \(- 6\) in the SPL spectrum was quite closely matched for the first two discrete tones. The transient pressure fluctuation characteristics on the upper surface of the rotating blades revealed that the unsteady loading noise by blade–vortex interactions is found most closely related to the third and sixth harmonics of the round per second noise. An unexpected fifth component also arose in some other rotational speeds because of the random nature of the phase difference of pressure fluctuations, even at the same positions of the two propeller blades. PubDate: 2019-09-01
Abstract: Abstract The aim of this work is to develop a new numerical computing program to design a new form of a dual bell axisymmetric supersonic minimum length nozzle contour, adapted to two different altitudes, usually at sea level and space, giving a supersonic uniform and parallel flow to the exit section. The first bell has a uniform and parallel sonic inlet and a low-altitude adaptation, with a reduced supersonic Mach number, while the second bell has a supersonic inlet and uniform and parallel flow and high altitude adaptation with high supersonic Mach number. The two bells are attached to an inflection point at the exit of the first bell. The passage from low altitude to high altitude is done without any mechanical activation. The purpose of this type of nozzle is the possibility of supersonic flight in two different regimes adapted into two different altitudes, under the assumption of a calorically and thermally perfect gas. This type of nozzle has an inflection point at the attachment point of the second bell to the first bell. The design is done using the method of characteristics. Solving the equations is done numerically by the predictor corrector algorithm. The validation of the results is controlled by the convergence of the critical sections ratio, calculated numerically, to that given by the theory. In this case, all the design parameters automatically converge towards the desired solution. PubDate: 2019-09-01
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