Effect of mean entropy on unsteady disturbance propagation in a slowly varying duct with mean swirling flow

The propagation of acoustic disturbances in a slowly-varying duct with mean swirling flow and non-uniform mean entropy is considered. This is representative of the region of swirling flow downstream of the rotor in aeroengine applications where variations in mean entropy can be significant. The duct is assumed to vary slowly in axial cross section and a consistent multiple-scales solution is derived. The variation in axial wavenumber and amplitude of the duct modes is determined as part of the solution.Comparisons are made between the cases of isentropic and non-isentropic flow. The cut-on/cut-off characteristics of a given mode are altered by any form of non-uniform mean entropy, with modes pushed closer to cut-off resulting in larger variations in amplitude. The greatest departure from the baseline isentropic result occurs for a positive entropy gradient. Increasing the swirl velocity at the duct inlet leads to increasing levels of mean entropy.