Aerospace Science and Technology最新文献

Both bio-helical toughening and fiber hybridization provide significant strengthening effects on the penetration properties of composite, and their combination may lead to even greater breakthroughs. Hence, this study explores the synergistic strengthening mechanism of bio-helical unidirectional-basalt fiber composite (BFRP) and weave-carbon fiber composite (CFRP) hybrid composite laminates (HFRP) subjected to penetration load. Three kinds of HFRP are designed and fabricated, and the quasi-static penetration tests and finite element simulations are conducted to evaluate the mechanical properties. Results show that the bio-helical HFRP samples present a significant improvement in anti-penetration property and energy-absorption as compared to other traditional samples. The penetration behaviors of each laminate present a good consistency between experiment and simulation. It is indicated that as the helical angle θ increases, the strengthening effect gradually increases, but when θ continues to increase, it tends to be stable. The penetration failure mechanism and energy-absorbing mechanism are analyzed by experiment and simulation. Finally, it is revealed that bio-helical toughening with hybrid effect from weave-CFRP can form a synergistic strengthening mechanism. This study provides an important reference for the anti-penetration design of aerospace composite with multi-mechanism cooperation.

The environment around asteroids is of significant uncertainties, which could cause great challenges of exploration missions to asteroids. This paper presents a novel approach to stochastic optimal problem in path-constrained trajectory optimization near asteroids based on convex approach. The path constraints that prevent the spacecraft from colliding with asteroid in different scenarios are composed of keep-out zone constraint and glide-slope constraint, which are formulated in chance-constraint forms due to the uncertain state. To convexify these constraints, the first-order Taylor expansion is applied for linearization and the nonconvexity is tackled with the introduction of additional intermediate variable and virtual buffer. Thereafter, a convex optimization based method is proposed to solve stochastic optimal problem. The effectiveness of the proposed method is validated through simulations of soft landing and hopping transfer scenarios near irregular-shaped asteroid 1996HW1. The results indicate that the proposed method can generate path-constrained trajectories under uncertainties using convex optimization.

The corrugated sandwich structure has been widely used in the field of engineering due to its excellent static and dynamic performance. However, current research on its vibration behavior mainly focuses on low-frequency regimes, limiting its applicability in extreme environments. To address this deficiency, a theoretical model based on the dynamic equivalent method (DEM) is developed to study the vibration performance of corrugated sandwich structure in a broad frequency range. The wave finite element method (WFEM) is used to solve the dispersion relation between frequency and wavenumber of the unit cell of the periodic corrugated sandwich structure. A modified Timoshenko beam model is constructed to calculated the dynamic equivalent parameters by matching the propagation and evanescent bending wave numbers of the corrugated sandwich beam. The vibration differential equation of the corrugated sandwich structure is established using the dynamic equivalent parameters. The analytical solution for the vibration response of the corrugated sandwich beam is obtained by solving the differential equation established using Laplace transformation, and the analytical results are compared with finite element method (FEM). The results reveal that the method presented in this paper is highly accurate compared with static equivalent method (SEM) and could be confidently used as a reference for further study of corrugated sandwich structure.

Unmanned aerial vehicle (UAV) flights in urban environments are challenging due to the complex flow structures and elevated turbulence around buildings. Consequently, research has shifted towards investigating the impact of gusts on the aerodynamic stability and control of UAVs. This study focuses on enhancing gust numerical modelling capabilities to understand the aerodynamic response, specifically exploring gust mitigation strategies for UAVs operating in turbulent urban environments. The split-velocity method, originally designed for two-dimensional compressible inviscid flows, where the velocity components were decomposed into a prescribed gust velocity and the remaining velocity components, is extended to three-dimensional incompressible viscous flows. To facilitate effective gust mitigation techniques, a radial basis function is applied to the modified split-velocity method to numerically model wings in pitching motions under gust encounters. A novel strategy is proposed to correct the discretized gust velocities and ensure gust flux conservation, showing effective improvement to the numerical predictions. The computed results agreed well with the experimental data available in the public domain, confirming that wing pitch motion can effectively mitigate the effects of gusts.

A solid fairing and a wire-mesh fairing consisting of very fine wires and pores are numerically and experimentally investigated for the mitigation of landing gear noise. A slightly modified LAGOON landing gear and two configurations, one equipped with a solid fairing and the other with a wire-mesh fairing, are numerically simulated using the Improved Delayed Detached-Eddy Simulation (IDDES) in combination with the Ffowcs Williams and Hawkings (FW-H) analogy. Instead of resolving the detailed flow features through the wire mesh, a recently proposed numerical model is used to represent the effect of the wire-mesh fairing. The simulated flow fields and the far-field noise spectra are validated against the experiments conducted in an anechoic wind tunnel. The superiority of the recently proposed wire-mesh model over a classical wire-mesh model in modelling both the aerodynamic and aeroacoustic effects of the wire mesh is demonstrated. Results also show that the dense wire-mesh fairing functions very similarly to the solid fairing and that significant noise can be reduced through the installation of a solid fairing or a wire-mesh fairing upstream of the landing gears. For the baseline landing gear, the torque link and the brakes are identified noise sources. With the aerodynamic penalty of a 50% increase in drag, both fairings mitigate the pressure fluctuation on the torque link and brakes, resulting in the reduction of surface noise sources. The noise directivity shows that a solid fairing or a dense wire-mesh fairing contributes to a noise reduction of 4-6 dB in all radial directions. The findings in this study pave the way for the low-noise design of aircraft landing gears.

This paper proposes a closed-loop detection and guidance method for optimizing the detection effectiveness of Radio Frequency (RF) stealth targets by terminal-guided hypersonic glide vehicles (HGV) during the terminal-guidance phase. This method divides the guidance process into two stages based on the detectability of the target. In the search stage, the Twin Delayed Deep Deterministic Policy Gradient (TD3) coupled with Successive Convex Optimization (SCO) guidance method is employed. This approach separates the detection performance from the numerical solution process of the optimization problem, enabling online adjustment of performance indicator functions to refine trajectory configurations. In the tracking phase, the dynamic characteristics of the target's Radar Cross Section (RCS) are convexified and discretized, incorporated into the objective function of the optimization problem as performance indicators. The Higher-Order Soft-Trust-Region Sequential Convex Programming (HSSCP) method is employed to improve the convergence of the algorithm. Additionally, the online trajectory planning problem is integrated into the Model Predictive Control (MPC) framework to achieve closed-loop guidance. Simulation results indicate that this method can improve the detection performance of RF stealthy targets during the terminal guidance phase, the cumulative detection information for the target increased by approximately 20.6%, and the maximum instantaneous detection probability improved by about 14.3%. Moreover, it can achieve convergence of complex multi-constraint problems within a limited number of iterations.

The oblique detonation engine (ODE) faces issues such as no-ignition, difficulty in combustion, and unstable combustion, some proactive auxiliary ignition measures should be implemented. This paper explores the application of radical-assisted combustion technology in the ODE and studies the effects of radicals on the oblique detonation wave (ODW) initiation characteristics. The two-dimensional Navier-Stokes equations which take into account the elementary reaction of acetylene/ethylene-air mixtures are solved, including chain reaction initiation, branching, and termination. The results demonstrate that the addition of radicals yields significant improvements in accelerating the initiation of ODW. The introduction of H, O, or OH radicals decrease the characteristic length of the induction zone with the mole fraction increases. Conversely, the inclusion of CHO radicals leads to an increase in the characteristic length of the induction zone with increasing mole fraction, primarily due to dominant chemical reaction kinetic effects. Additionally, the paper conducts an ignition sensitivity analysis of the radicals on the chemical reaction mechanisms and their influence on the initiation characteristics. Finally, a comprehensive analysis is performed by comparing the numerical results with the constant volume combustion theory, focusing on two initiation patterns: wave-controlled and chemical kinetics-controlled configurations. It is observed that varying the particle parameters results in changing the boundary lines between these two initiation patterns, and increasing the mole fraction shifts the initiation pattern from wave-controlled to chemical kinetics-controlled.

In this paper, an appointed time sliding mode control scheme is proposed for trajectory tracking control of a tilt-rotor electric vertical takeoff and landing (eVTOL) aircraft. First, the error dynamic model of helicopter mode of the tilt-rotor eVTOL aircraft is presented. Second, an appointed time prescribed performance function (ATPPF) is designed as the expected value of the error, which guides the error to converge to zero at the appointed time. Then, a predefined time controller ensures that the actual error tracks the expected error before the settling time set by the ATPPF. By setting the time parameters of the ATPPF and the predefined time controller to the same value, the actual error will converge to zero vicinity at the appointed time. Finally, the predefined time convergence of the closed-loop system is proved via Lyapunov analysis. The effectiveness of the appointed time control strategy is demonstrated in the simulation results in the presence of external disturbances, dynamic uncertainties and control input constraints.

The flight environment of aerodynamically controlled missiles is full of complexity and uncertainty. To cope with the uncertainty more effectively and enhance the convergence performance in trajectory optimization problems for aerodynamically controlled missiles simultaneously, the chance-constrained sequential convex programming (CC-SCP) algorithm is proposed in this paper. The uncertainty is regarded as the chance constraint, and a smooth and differential approximation function is designed to transform this chance constraint into the constraint that the convex optimization method can handle. Subsequently, the originally non-convex trajectory optimization problem is reformulated into a series of convex optimization subproblems, in which an initial reference trajectory guess generation strategy is proposed, and a theoretical proof of the exact convex relaxation is given to enhance the algorithm's convergence performance and theoretical value, respectively. Numerical simulations are provided to verify the convergence and effectiveness of the CC-SCP algorithm, and the advantages of using the CC-SCP algorithm to cope with the uncertainty are illustrated. Furthermore, comparative simulation examples show that the proposed algorithm possesses a low conservatism, which means the proposed algorithm can obtain a bigger convergence region and a better solution than other current methods when handling the same chance constraints. Finally, the robustness of the algorithm is discussed.

The effect of local equivalent ratio on spark ignition characteristics is investigated in this article. By adapting combined injection, the fuel-lean and fuel-rich local equivalent ratio are formed inside ignition cavity, and the processes of initial flame generation and local flame self-sustain have been analyzed. The experimental result shows that, the characteristic of spark ignition is distinct in fuel-lean and fuel-rich local equivalence ratio. When the local equivalence ratio is relatively low, few initial flame kernels can be generated after the spark discharges, resulting in potential ignition delays or failures. Alternatively, when the local equivalence ratio is relatively high, initial flame can be generated frequently after spark discharge. However, the fuel-rich environment inside ignition cavity hinders flame stabilization, causing rapid flame extinguishment and continuous pressure oscillations inside ignition cavity. The spatial distribution of the fuel spray significantly impacts the characteristics and intensity of local flame. During the experiments with double injections, the spray exhibits a dense concentration near the shear layer of cavity T1. This dense spray weakens the local flame, suppressing the formation of a tail flame and preventing its propagation. In contrast, the experiments utilizing single injection demonstrate a sparse distribution of spray near the shear layer of cavity T1. Consequently, the tail flame can be formed at the rear edge of cavity T1 more frequently. These local flames possess the potential to develop into downstream flames through the spreading of the tail flame.