In laminar hypersonic boundary layers, it is known that secondary instability plays a crucial role in transition to turbulence. The secondary instability usually includes the fundamental mode, the subharmonic mode, and the detuned mode. Considerable research exists on the secondary instability mechanism in hypersonic boundary layers with the smooth wall condition. The topic of using micro-porous surfaces for disturbance stabilization has recently drawn interest. The stabilization and, thus, a possible delay in the transition arise due to a reduction in the growth rate of the primary Mack mode by the porous surface. This paper focuses on investigating whether the secondary instability mechanism of Mack modes can also be affected by a surface porosity condition. It is known that the primary Mack mode linear disturbances are changed significantly on the porous surface, and how it subsequently influences the secondary instability of the modified time varying basic flow is our concern. The analysis demonstrates that on the porous surface, as the amplitude of the primary Mack mode increases, the fundamental mode is not stable. Instead, the fundamental mode amplifies rapidly with increasing primary amplitudes. At larger secondary instability spanwise wavenumbers, when the primary amplitude exceeds a certain threshold value, the fundamental modes surpass the subharmonic modes and dominate the secondary instability. However, when the spanwise wavenumber is relatively small, especially at the spanwise wavenumber corresponding to the maximum growth rate of the subharmonic mode, the fundamental modes are weakened and lose their dominant position. We find that corresponding to different amplitudes of primary Mack mode disturbances affected by the porosity parameters, there are no strongly preferred interaction modes that dominate the secondary instability; this contrasts with smooth wall findings. We further find that the larger the pore size or porosity, the more severe the suppression of the fundamental mode.

Aerodynamic design optimization is a key aspect in aircraft design. The further evolution of advanced aircraft derivatives requires a powerful optimization toolbox. Reinforcement learning (RL) is a powerful optimization tool but has rarely been utilized in the aerodynamic design. It can potentially obtain results similar to those of a human designer, by accumulating experience from training. In this work, a popular RL method called proximal policy optimization (PPO) is proposed to investigate multi-object aerodynamic design optimization. By observing the aerodynamic performances of different airfoils, the PPO updates a reasonable policy to generate the optimal airfoils in a single step. In a Pareto optimization problem with constraints, the PPO requires only 15% of the computational time of the non-dominated sorted genetic algorithm (II) to achieve the same accuracy. The results from testing show that the agent learns a policy that can achieve ∼4.3%–10.1% improvements of the aerodynamic performance compared with the results of baseline.

In the framework of twistors, a saturated adaptive six-degree-of-freedom (6-DOF) control law for asteroid landing, which avoids the collision between the spacecraft and asteroid, is proposed with both the inertial parameters of the spacecraft and the bounds of the uncertainties unknown. First, the 6-DOF dynamics of the spacecraft relative to the desired coordinate frame is represented in a unified way by the application of twistors, and the collision avoidance constraint is established via separating the spacecraft and asteroid with an elaborately designed safety surface. Then, a saturated controller for the landing is proposed with the collision avoidance constraint considered by combining the backstepping method and artificial potential field technique. Further, the inertial parameters and the upper bounds of the magnitude of the uncertainties in the controller are substituted with their estimates obtained from the designed adaptive laws. Finally, the previous design is adjusted slightly to complete the adaptive control scheme. The stability of the closed-loop system is proven via Lyapunov theory, and numerical simulations are presented to demonstrate the effectiveness of the control scheme. The proposed adaptive control law can fully consider the orbit-attitude coupling and effectively enforce the collision avoidance constraint in the presence of unknowns in asteroid landing missions.

CO₂ adsorption amount is enhanced with below 0.1 ppm humidity, and water molecule partial charge is a dominant factor in adsorption.