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Our previous research has demonstrated that a single forward-facing step (FFS) could delay the hypersonic boundary layer transition on a cone [Xu et al., "Influences of steps on the hypersonic boundary-layer transition on a cone," AIAA J. 59, 439–446 (2021)]. This paper aims to further this study by investigating the control effects of FFS arrays on the hypersonic boundary layer transition on a 7° half-angle sharp cone. Experiments are conducted in a Mach 6 wind tunnel using nano-tracer-based planar laser scattering techniques and high-frequency pressure sensors. Cases with smooth surfaces, single FFS, or FFS arrays with different spacing and step heights are studied for comparison. The results show that FFS arrays present a better performance on stabilizing the second mode wave and delaying the transition than a single FFS. Notably, the spacing and height between and of the steps also play an important role in the delay effect. For the cases studied, a better control effect can be achieved using FFS arrays spaced at a larger distance or with step height increasing along the flow direction. Moreover, FFS arrays could restabilize the second mode wave that has been amplified by the upstream backward-facing step. [ABSTRACT FROM AUTHOR]

In a previous research on the hypersonic boundary layer transition over a sharp cone at 0° angle-of-attack (AoA), we concluded the different effects of the forward-facing step (FFS) and the backward-facing step (BFS) on the transition [Xu et al., AIAA J. 59, 439–446 (2021)]. This further study intends to examine if the conclusion still maintains after changing the nose bluntness and the angle-of-attack of the cone. Experiments are conducted in a Mach 6 wind tunnel using nano-tracer-based planar laser scattering techniques, temperature sensitive paints, and high-frequency pressure sensors. The results show that although the FFS delays the boundary layer transition while the BFS promotes the transition at AoA = 0°, a completely different pattern is observed at AoA = 10°, the FFS significantly promotes the transition, while the effect of the BFS appears only weakly to advocate the transition. Noteworthy, the nose bluntness will not change the effects of the BFS/FFS on the transition. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]