Impact of continuously extending/retracting wall on Mack-mode evolution in hypersonic boundary layers

Structural morphing is regarded as a potential solution for future space transportation systems, necessitating aircraft capable of extensive altitude ranges, wide speed ranges, and multi-regime operations. The investigation of laminar-turbulent transition incorporating morphing effects aims to compensate for the lack of physical mechanisms in aircraft designs. This paper particularly focuses on the impact of a continuously extending or retracting wall in the chordwise direction on the evolution of inviscid Mack modes in a Mach 5.92 hypersonic boundary layer. Using the high-Reynolds-number asymptotic technique, a model is developed to quantify the morphing effect by an amplification factor, which includes both the scattering effect at the rigid-morphing junction and the successive modification of the Mack growth rate by the morphing velocity in the downstream region. Such a model enables us to conduct a comprehensive investigation across a wide parameter space defined by morphing speed and flow parameters. A critical frequency is identified near the most unstable second-mode frequency. For an extending wall, the oncoming Mack modes are suppressed for frequencies above this critical frequency and enhanced for frequencies below it. Conversely, the retracting-wall effect exhibits an inverse impact on the Mack modes. To validate the accuracy of the asymptotic predictions, the harmonic linearized Navier–Stokes calculations are performed, resulting in favorable agreements. This research sheds light on the complex interplay between wall morphing and the Mack-mode evolution, offering informative contributions to the understanding of the aerodynamic behaviors in hypersonic boundary layers.