Modal axial group velocity in the swirling flows of aeroengine interstage

What is it about?

The paper considers the modal axial group velocity in ducted swirling flows, typical of the aeroengine interstage. The axial group velocity has so far been an analytical construct in the duct acoustics literature and not much use has been made of it to study the modal propagation. In this paper, we develop the physical interpretation of the concept of axial group velocity in the swirling flows using the idealized rigid body swirl model, whose analytical framework enables the derivation of simple utilitarian expressions, the accuracy and suitability of which on the realistic interstage swirling flows have been verified via more thorough and exact numerical computations. It has been shown in this paper that the axial group velocity is a sufficiently definite modal parameter for the co-rotating spinning modes across the broad range of flow Mach numbers encountered during engine testing. On the other hand, for the contra-rotating spinning modes, the physical significance of the axial group velocity is reasonably conserved up to duct Mach numbers not exceeding 0.35, beyond which the parameter becomes ambiguous in its meaning and usage. The work has also been extended to clarify previously published remarks on the concept of the modal cut-on ratio and the modal sound powers in a swirling flow. The challenges intrinsic to measuring the axial group velocities in a swirling flow have been discussed. The paper subsequently presents a modified beamformer approach to measure the broadband duct modes in terms of their axial group velocities.

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Photo by Jakob Braun on Unsplash

Photo by Jakob Braun on Unsplash

Why is it important?

The work presents a formal definition of a new modal parameter in duct acoustics - the axial group velocity - which has been less explored in the study of modal propagation. The physical interpretation of the axial group velocity has been developed, and its definiteness in the swirling flows has been investigated via more thorough and exact numerical calculations. A phased array technique is also presented to measure this axial group velocity in a broadband sound field. The future application of the concept of axial group velocity in the swirling flows includes the design of interstage wall-liners by correlating the modal optimum wall impedances and attenuation in terms of the axial group velocity.

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