On the production and absorption of sound by lossless liners in the presence of mean flow

Abstract

The interaction of sound with the leading and trailing edges of a nominally lossless acoustic liner is examined in the presence of mean flow. It is argued that acoustic energy is conserved only if the whole flow is irrotational. In practice the sound induces vorticity production at the liner, and in particular that generated at the edges of the liner leads to a net transfer of energy between the acoustic field and the mean flow. Analytical results are given for a “pressure release” liner in a rigid plane wall, it being concluded that there is a net production of acoustic energy at a trailing edge for all subsonic mean flow Mach numbers M. At a leading edge there is a net production or absorption of sound, depending on both the direction of propagation of the incident wave and the value of M.