Impedance-Near-Zero Acoustic Metasurface for Hypersonic Boundary-Layer Flow Stabilization

Hypersonic boundary-layer transition induced by the Mack second mode is a fundamental issue in fluid mechanics and hypersonic vehicle design, whose physics are not yet fully understood. Nevertheless, given the acoustic nature of the Mack second mode, ultrasonic absorptive coatings have been proposed to dissipate the wave energy and thus stabilize the hypersonic boundary-layer flow. We, however, show that even with little damping, the Mack second mode can be greatly suppressed by introducing an artificial boundary of near-zero surface acoustic impedance. This phenomenon can be attributed to the minimized acoustic pressure perturbation at the antinode of the Mack second mode, which prevents the surface-wavelike mode from being effectively excited. As a practical realization, we present a grooved acoustic metasurface and numerically verify its feasibility. Results reveal that the out-of-phase behavior between the incident and reflected waves at the resonant frequency minimizes the near-surface acoustic pressure, largely inhibiting the growth of the Mack second mode. Our study sheds light on the physical mechanism of the Mack second mode and opens up alternative possibilities toward full control of hypersonic boundary-layer transition with acoustic metasurfaces.