The wave modes in ducted swirling flows

The small amplitude wave modes inside a ducted inviscid, compressible swirling flow are investigated. In order to avoid possible mathematical ambiguities arising from the use of an inviscid flow model, the wave modes are cast as the solution of an initial boundary value problem. Two families of propagating waves are found. The acoustic modes are supported by the compressibility effect of the flow. The rotational modes are sustained by the centrifugal force field associated with the mean flow rotation. Two cases, one with a free vortex swirl and the other with a rigid body swirl, are investigated in some depth. Numerical results are provided. The effect of swirl on the propagation characteristics of the acoustic modes and the effect of compressibility on those of the rotational modes are found to be relatively insignificant.

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