SOUND PROPAGATION IN AN ANNULAR DUCT WITH MEAN POTENTIAL SWIRLING FLOW

The propagation of pressure disturbances is investigated in an annular duct with mean potential axial and swirling flow. The swirling mean motion causes refraction of the acoustic waves as they propagate along the duct. A normal mode analysis leads to a non-linear eigenvalue problem. The numerical solutions are validated with an asymptotic analysis in the limit of large radius. The results show that while for a uniform flow the evanescent modes propagate at the same speed only upstream, for a swirling flow such modes propagate at different speeds both upstream and downstream. Moreover, most non-axisymmetric acoustic modes propagate opposite to the mean flow swirl.