Roughness effect of an acoustic metasurface on supersonic boundary layer transition

Roughness is a key property of an acoustic metasurface in determining the laminar-to-turbulent transition of a supersonic boundary layer, and the investigation of its effects requires consideration of the real microstructures inside the metasurface without any simplification. This work experimentally studies the transition behavior of the boundary layers over four different metasurfaces in a supersonic low-noise wind tunnel. Using a nanoparticle-based planar laser scattering technique and an intermittency factor analysis, it is found that the rough metasurfaces considered here tend to increase the probability of boundary layer transition more than a smooth flat plate. It is also found that the roughness of a metasurface as represented by its longitudinal scale is positively correlated with the turbulence onset probability and exhibits an interesting linear trend.