Computational investigation of plane wave reflections at the open end of subsonic intakes

Abstract

The reflection of plane waves at the open end of intakes carrying subsonic mean flows are considered in this work. The investigations are conducted with a three-dimensional numerical scheme based on the lattice Boltzmann method. Two different geometry profiles were considered to represent the intake, including a cylindrical bell mouth and a Borda tube. The comparison between numerical and analytical results provided by the low-frequency theory suggested two different traits. For the cylindrical bell mouth, the incoming flow remains potential and the wave reflection is only influenced by the phase velocity shift caused by convection. In such a situation, the available low-frequency theory holds for the entire frequency range of plane wave propagation. Conversely, for sharp geometries, such as the Borda tube, flow separates at the duct's entrance and the wave reflection becomes mostly influenced by the exchange of energy between the flow and the acoustic field. In this case, the numerical results deviate considerably from the theory, even in the low-frequency limit. The mechanisms involving the transfer of rotational kinetic energy from the flow to the acoustic field are discussed and an empirical formula is proposed in terms of reflection coefficient for the entire range of plane wave propagation.