Sound reflection of acoustic porous metasurfaces in uniform mean flow

Acoustic metasurfaces are two-dimensional (2D) structures with a sub-wavelength thickness that can realize some exotic properties such as non-trivial refraction, broadband, and low-frequency absorption. However, most relevant studies are conducted for a static medium, hindering their practical applications in aerospace engineering, where a background flow often exists. To fill this gap in our knowledge, the effects of mean flow on the acoustic performance of metasurfaces, which are designed based on the generalized Snell's law (GSL) to achieve anomalous reflections, are systematically studied. First, an extended analytical model of GSL taking the effect of background uniform mean flow into account is proposed, in which the wavenumbers of both incident and reflected waves are corrected. Then, the effectiveness of the derived model is validated by numerical simulations based on an acoustic porous metasurface. Results reveal that the reflected waves are deflected in the presence of a background flow. The critical incident angle, at which the incident sound wave is converted to a surface wave, decreases with the increasing oncoming flow velocity. The converted surface wave can only propagate along the metasurface, and only a small amount of acoustic energy is radiated into the far-field, which can be beneficial for noise attenuation applications.