Influence of mean flow gradients on fan exhaust noise predictions

Tester, B.J., Gabard, G. and Ozyoruk, Y. (2008) Influence of mean flow gradients on fan exhaust noise predictions. Proceedings of the 14th AIAA/CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), Vancouver, Canada. 04 - 06 May 2008. 13 pp .

Abstract

Aft fan noise is becoming a more dominant source as engine bypass ratio is
increased n this paper an assessment of the effect of the mean flow gradients on
fan exhaust noise propagation is carried out using both analytical models for
simplified problems and numerical methods for realistic configurations. Fan
exhaust noise can be significantly refracted by the mean flow gradients in the jet
mixing layer, especially at high operating conditions (i.e. during take off). The
refraction effect is predicted using either Lilley’s equation or the linearized
Euler equations. For parallel base flows, an issue with these linear models is the
presence of Kelvin-Helmholtz instabilities whose unlimited exponential growth
is unphysical and problematic for computational methods. This problem is less
critical for developing mixing layer for instance where the growth of the
vorticity thickness reduces the growth of the instability waves [1]. Various
techniques have been used for suppressing the instability; these include adding
non-linear terms to saturate the growth of the instability [2], using frequency
domain analysis [3], or removing the mean flow gradient terms [4]. It is the last
approach, termed Gradient Term Suppression (GTS), which is investigated in
the present work.

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