Chinese Journal of Aeronautics最新文献

In Unmanned Aerial Vehicle (UAV)-assisted millimeter Wave (mmWave) systems, Channel State Information (CSI) feedback is critical for the selection of modulation schemes, resource management, beamforming, etc. However, traditional CSI feedback methods lead to significant feedback overhead and energy consumption of the UAV transmitter, therefore shortening the system operation time. To tackle these issues, inspired by superimposed feedback and Integrated Sensing and Communications (ISAC), a Line of Sight (LoS) sensing-based superimposed CSI feedback scheme is proposed. Specifically, on the UAV transmitter side, the Ground-to-UAV (G2U) CSI is superimposed on the UAV-to-Ground (U2G) data to feed back to the ground Base Station (gBS). At the gBS, the dedicated LoS Sensing Network (LoS-SenNet) is designed to sense the U2G CSI in LoS and NLoS scenarios. With the sensed result of LoS-SenNet, the determined G2U CSI from the initial feature extraction will work as the priori information to guide the subsequent operation. Specifically, for the G2U CSI in NLoS, a CSI Recovery Network (CSI-RecNet) and superimposed interference cancellation are developed to recover the G2U CSI and U2G data. As for the LoS scenario, a dedicated LoS Aid Network (LoS-AidNet) is embedded before the CSI-RecNet and the block of superimposed interference cancellation to highlight the feature of the G2U CSI. Compared with other methods of superimposed CSI feedback, simulation results demonstrate that the proposed feedback scheme effectively improves the recovery accuracy of the G2U CSI and U2G data. Besides, against parameter variations, the proposed feedback scheme presents its robustness.

Gas foil bearing faces severe and complex thermal-fluid–solid coupling issues when in ultra-high speed and miniaturized impeller machineries. In this study, a Thermo-Elasto-Hydrodynamic (TEHD) analysis of a specific multi-layer gas foil thrust bearing on the continuous loading process within a steady rotational speed is numerically investigated by a three-dimensional thermal-fluid–solid coupling method. Results indicate that the multi-layer foil exhibits nonlinear overall stiffness, with the thrust bottom foil serving as the primary elastic deformation structure, while the thrust top foil maintains a well-defined aerodynamic shape during a loading process, which helps reduce frictional damage and achieve an adequate loading capacity. For low loads, the fluctuation of the gas film is extremely sensitive, and it weakens dramatically as the load increases. The viscous heating and friction torque exhibit a linear relationship with an increasing bearing load after a rapid growth. Depending on the exact stacking sequence and contact position of the multi-layer gas foil, the overlapping configuration allows for efficient transfer of viscous-shearing heat accumulated at the smallest air film through thermal conduction while providing elastic support. Due to the strong inhomogeneity of the viscous heat under varying loads, the temperature distribution on the top foil surface shows pronounced variations, while the difference between the peak and average temperatures of the thrust plate and top foil surfaces widens substantially with an increasing load.

A method of star-tube combined segmented grain is proposed to improve the combustion performance of hybrid rocket motor. The star-tube combined segmented grain consists of a single-port star part and a single-port tube part. A mid-chamber forms between the fore-grain and the aft-grain for better mixing effect. The single-port feature gives hybrid rocket motor several advantages, such as simple structure, high reliability, and variable combinations. This paper is mainly aimed at studying the combustion characteristics of hybrid rocket motor with star-tube segmented grain through three-dimensional steady simulations. Combustion performance of the motors with different segmented grain combinations, including fore-tube/aft-tube, fore-tube/aft-star, fore-star/aft-star and fore-star/aft-tube, is contrastively analyzed. The motor in this paper adopts polyethylene and 90% hydrogen peroxide as the propellants. Simulations reveal that segmented grain with different-type grain combinations could greatly change the flow field in the second half of the combustion chamber. Transformation of the flow field is beneficial to the mixing between the fuel and the oxidizer, and it could increase the fuel regression rate and the combustion efficiency. The turbulence effect of tube aft-grain is better than that of star aft-grain. Among the four segmented grain combinations, the combination of star fore-grain and tube aft-grain is the preferred method with optimal overall performance. This grain configuration could increase the regression rate of tube aft-grain to surpass that of star aft-grain in other combinations. Besides, hybrid rocket motor with this grain configuration achieves the highest combustion efficiency.

The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow. Gas-liquid flow field was studied at different divergent angles and injection schemes. It is found that complex wave structures exist in the divergent cavity-based combustor. The spray field can be divided into three distinct zones: surface wave-dominated breakup zone, rapid atomization zone and cavity mixing zone. A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively. As a result, it is revealed that for the large divergent angle cavity, the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution, where the injection scheme near the cavity has a more spray distribution. For the small divergent angle cavity, the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution. The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity. Finally, a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.

Most Level 3 airports around the world suffer severe congestion and flight delays. Airlines have to obtain airport slots in order to schedule flights at such airports. The main way for airlines to acquire slots is primary slot allocation, in which a slot coordinator distributes slots to airlines according to certain rules and regulations. Due to excessive demand for slots and restrictions on allocation rules, it is difficult for some airlines to obtain the desired slots in this manner. Another way for airlines to obtain slots is through secondary slot trading, in which slots can be redistributed among airlines without being returned to slot pool. The secondary trading of airport slots has played a positive role in promoting the efficient utilization of slot resources, but systematic studies are insufficient. This paper discusses the reasons for the existence of slot secondary trading, sorts out the main policies and rules governing the mechanism, investigates its impacts on the industry and society, and points out the major problems and challenges. The paper provides a reference for subsequent research and practical application of airport slot secondary trading in the future.

It is difficult to simulate the strong interference and serious flow separation of Fenestron by the CFD method based on the widely used RANS equation, and the detailed experimental data, which could be used to validate the aerodynamic and noise numerical methods, is unavailable. The experimental investigation on the aerodynamic and noise characteristics of Fenestron is carried out. In view of the complex internal flow field in duct and the relative motion between the stationary duct and the rotating rotor, a comprehensive aerodynamics and pressure measurement scheme is designed based on the bottom support rig. In this measurement scheme, the thrust generated by the rotating rotor can be measured by the rotating shaft balance, the thrusts from Fenestron are measured by the external balance and the pressures on duct inner wall are monitored by a pressure measuring system. To fully capture the noise directionality of Fenestron, a series of noise observers located at an arc array are arranged. In terms of the Fenestron test models, the baseline model, the performance improvement model based on the high-performance tail rotor and the noise reduction model based on the non-uniform blade distribution are designed respectively. By the designed measurement scheme, aerodynamic forces and pressure distributions and noise were measured for the three different Fenestron models. The results show that the aerodynamic thrusts of the tail rotor and duct increase greatly and the noise increases slightly for the performance improvement model because of the larger aerodynamics. The rotor aerodynamic performance of the noise reduction model is reduced, but the modulation effect of the tail rotor improves the forces of the duct. The noise radiated by the noise reduction model is reduced and a good noise reduction result is obtained in frequency domain.

Aimed at improving the real-time performance of guidance instruction generation, an analytical hypersonic reentry guidance framework is presented. The key steps of the novel guidance framework are the parameterization of reentry guidance problems and the optimization of parameters. First, a quintic polynomial function of energy was designed to describe the altitude profile. Then, according to the altitude-energy profile, the altitude, velocity, flight path angle, and bank angle were obtained analytically, which naturally met the terminal constraints. In addition, the angle of the attack profile was determined using the velocity parameter. The swarm intelligent optimization algorithms were used to optimize the parameters. The path constraints were enforced by the penalty function method. Finally, extensive simulations were carried out in both nominal and dispersed cases, and the simulation results showed that the proposed guidance framework was effective, high-precision, and robust in different scenarios.

In this paper, a missile terminal guidance law based on a new Deep Deterministic Policy Gradient (DDPG) algorithm is proposed to intercept a maneuvering target equipped with an infrared decoy. First, to deal with the issue that the missile cannot accurately distinguish the target from the decoy, the energy center method is employed to obtain the equivalent energy center (called virtual target) of the target and decoy, and the model for the missile and the virtual decoy is established. Then, an improved DDPG algorithm is proposed based on a trusted-search strategy, which significantly increases the train efficiency of the previous DDPG algorithm. Furthermore, combining the established model, the network obtained by the improved DDPG algorithm and the reward function, an intelligent missile terminal guidance scheme is proposed. Specifically, a heuristic reward function is designed for training and learning in combat scenarios. Finally, the effectiveness and robustness of the proposed guidance law are verified by Monte Carlo tests, and the simulation results obtained by the proposed scheme and other methods are compared to further demonstrate its superior performance.