Stabilization mechanisms of various acoustic metasurfaces on the second mode in hypersonic boundary-layer flows

Abstract

Acoustic metasurfaces have been shown to stabilize the Mack second mode in the hypersonic boundary layer through various acoustic wave manipulations, but the stabilization mechanisms still lack unified clarification. In the present work, momentum potential theory is used to develop a physics-based analysis of the perturbance flow field above three kinds of acoustic metasurfaces: the absorptive, impedance-near-zero, and reflection-controlled metasurfaces. It found the thermal-acoustic source term Pta contributes the most to the instability, and the main differences in the stabilization mechanisms of the various metasurfaces can be derived from the distributions of Pta. The absorptive metasurface largely restrains the negative Pta term near the surface and slightly attenuates it near the critical layer. The impedance-near-zero metasurface generates a positive contour under the critical layer, while the reflection-controlled metasurface induces additional positive Pta intertwining along the critical layer. In addition, a uniform macroslit surface without particular acoustic characteristic is verified to stabilize the Mack second mode because the recirculation zones inside the macroslits attenuate the near-surface negative Pta. By deflecting the reflective waves, additional larger positive Pta could be produced and wrapped along the critical layer and that achieves a more prominent stabilization performance by designing a reflection-controlled macroslit surface.