Frequency-domain model of tonal blade thickness and loading noise

Abstract

A tonal thickness noise and loading noise model of rotating blades has been developed as an extension of the exact frequency-domain solutions for rotating monopole and dipole point sources. The present model has two advantages over the previous methods and models for noise prediction. The first is the unified expression for sources in subsonic and supersonic rotation even at rest. The second is that the present model has no limit on the location of the observer and no interpolation error. Two test cases are carried out to validate the present model and emphasize its advantage at the noise prediction for sources in supersonic rotation. Moreover, as a specified application of the present model for the rotating blades whose tip radius is acoustically compact, acoustic energy distribution at different frequencies and in different directions is analyzed. Result shows that the acoustic energy of acoustically compact rotating blades is mainly concentrated at the source frequency while propagating along the axial direction, leaving the rest propagating along the radial direction at the other frequencies.

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“…32 It is worth noting that formulation 1A of the FWH equation gives a singular value on the axis of rotation, while the trailing-edge noise model has its singularity on the plane of rotation. The singularity in the axis of rotation has also been reported by Lowson 33 and Mao et al 34 Steady-loading noise (tonal noise) has zero efficiency on the rotation axis.…”

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“…On the other hand, loading noise is generated by an acoustic disturbance induced by unsteady fluid forces. 6 The phase velocity of these unsteady blade forces is often significantly larger than the corresponding tip speed and may even exceed the speed of sound. 7 As a result, the unsteady blade forces have very high acoustic radiation efficiency, leading to the creation of tonal noise at the blade passing frequency (BPF) and its harmonics.…”