A prediction method for sound attenuation in acoustically lined
circular ducts of uniform cross section containing radially sheared
fluid flow is developed. The flow is regarded as consisting of a core
of uniform flow surrounded by a thin layer near the duct wall in which
the flow is sheared. The method is based on the solutions (acoustic
modes) of an eigenfunction derived from the governing wave equation
and the boundary conditions. Sound attenuation is computed by assuming
equal distribution of the acoustic energy among the propagating acoustic
modes at the reference plane. Amplitudes of modes reflected from the
duct termination are considered negligible.
To test the method, predictions are compared with measured data
from circular section flow duct absorber test facilities and from liner
tests in the intake duct of the RB 211 turbofan engine. The agreement
between the measured and the predicted attenuation spectra is very good
when sound propagation is upstream. In the downstream propagation case,
equal modal energies lead to slight over prediction of sound attenuation
at higher frequencies. An empirical solution to this problem is found by
restricting the computation to the modes of the first four circumferential
A numerical technique is developed which uses only a fraction of the
modal solutions to compute attenuations without loss of accuracy, thus considerably saving computing time and costs.
A detailed study is carried out to highlight the effects on modal and
total attenuations of changes in the values of the principal parameters
and a number of design guidelines are deduced. Procedures for the
prediction of sound attenuation in engine ducts are discussed and an
approach to design the most effective liners is suggested.
A comprehensive set of computer programs is developed to assist the
Noise Engineer in the predictions and design optimisations of acoustic
liners in project engines.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.