A theory is developed for the interaction of an unsteady incident disturban
ce with an annular cascade in a mean swirling flow. IL is shown that the ce
ntrifugal and Coriolis forces caused by the mean swirl couple the acoustic
and vortical disturbance modes and can lead to the formation of a critical
layer. A normal mode analysis shows that, for moderate subsonic Mach number
s, the eigenmodes, which may not form a complete set, are segregated into p
ressure-dominated and vorticity-dominated modes. A complete description of
incident disturbances is proposed in terms of the pressure-dominated eigenm
odes and a spatially developing initial-value solution. For the special cas
e of a potential mean swirl, the vortical modes are uncoupled from the acou
stic modes, and their vorticity grows linearly as they propagate downstream
. For a general vortical mean swirl the vorticity-dominated spatially devel
oping disturbances may oscillate or amplify (decay) depending on the radial
distribution of the mean swirl. The analysis clearly indicates a significa
nt effect of the mean-How swirl on the evolution of the incident disturbanc
es and therefore on the blade upwash.