Second-mode instability plays an important role in the transition of a hypersonic boundary layer. The second-mode wave on a flared cone at Mach 6 has been investigated experimentally through the Nanoparticle-tracer based Planar Laser Scattering (NPLS) technique in this paper. Different behaviors of the second-mode wave ranging from linear growth to deformation are visualized under different unit Reynolds numbers. The propagation velocity of the second-mode wave is quantitatively calculated, and the spectral properties of the second-mode wave have been analyzed by the power spectrum density. The characteristic frequency of the second-mode wave calculated from the NPLS images agrees well with the results measured by the transducers. Evolution of the wall-normal distribution of disturbance shows that high frequency harmonics are generated during the growth process of the second-mode wave and its normal distribution range shrinks with the increase in its frequency. The bispectral analysis indicates that the high degree of phase coupling may be the reason for the deformation of the second-mode wave. The nonlinear interaction between the disturbance waves is strong during the deformation stage of the second-mode waves.

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. When an aircraft flies at high speed in a dense atmosphere, the airflow outside the optical window interacts to form a complex flow field structure with irregular and nonuniform refractive index distribution, which makes it difficult to accurately calculate the ray propagation path based on the refractive index field. To solve this challenge, three ray tracing implementation methods with fourth-order accuracy are proposed. By comparing with the spiral ray resolution results, it is found that the Adams method has the smallest computational error of 1.2  ×  10  −  11 compared with the fourth-order Runge–Kutta method and the Richardson extrapolation method. The Adams method is the most computationally efficient in terms of computational time cost, followed by the Richardson extrapolation method, and finally the fourth-order Runge–Kutta method. The results show that the Adams method is the fastest and most accurate for different step sizes and grid volumes.

Remarkable progress has been made in satellite-based quantum key distribution (QKD), which can effectively provide QKD service even at the intercontinental scale and construct an ultralong-distance global quantum network. But there are still some places where terrestrial fiber and ground stations cannot be constructed, like harsh mountainous areas and air space above the sea. So the airborne platform is expected to replace the ground station and provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the randomly distributed boundary layer that surrounds the surface of the aircraft when the flight speed is larger than 0.3 Ma. Previous research of airborne QKD with boundary layer effects is mainly under the air-to-ground scenario in which the aircraft is a transmitter, while the satellite-to-aircraft scenario is rarely reported. In this article, we propose a performance evaluation scheme of satellite-to-aircraft QKD with boundary layer effects in which the aircraft is the receiver. With common experimental settings, the boundary layer would introduce a ∼31 dB loss to the transmitted photons, decrease ∼47% of the quantum communication time, and decrease ∼51% of the secure key rate, which shows that the aero-optical effects caused by the boundary layer cannot be ignored. Our study can be performed in future airborne quantum communication designs.

In the development process of high-speed aircraft, the head of the aircraft is subject to high temperatures and high speed flows, supporting the maximum heat flow and thus requiring a reliable cooling system. A new type of head cooling system is proposed herein. An internal flow channel model of the heat transfer in a ball head made from high-temperature alloy steel is constructed, then an experimental platform is built to carry out relevant experiments on the performance of this cooling system. Firstly, the influence of different experimental conditions on the cooling efficiency of the ball head is studied. For given liquid-nitrogen supply pressure, a higher heating heat flux density on the outer surface of the ball head corresponds to higher cooling efficiency. Then, the vaporisation effect under different experimental conditions is evaluated using temperature sensors at the inlet and outlet of the ball head heat exchange channel in combination with images of the visualised glass tube. It is found that liquid nitrogen can vaporise completely when flowing through the heat exchange channel. The characteristics of the heating effect and liquid nitrogen injection for the ball head were evaluated using an infrared camera. Finally, under different experimental conditions of liquid-nitrogen supply pressure, it is found that liquid nitrogen can vaporise completely in each case, and the total temperature of the vaporised nitrogen is about 300K. It can thus be collected as a secondary gas source.