Time-periodic CFD simulations are widely used to investigate turbomachinery
components. The triple-plane pressure mode matching method (TPP) developed by
Ovenden and Rienstra extracts the acoustic part in such simulations. Experience
shows that this method is subject to significant errors when the amplitude of
pseudo-sound is high compared to sound. Pseudo-sound are unsteady pressure
fluctuations with a convective character. The presented extension to the TPP
improves the splitting between acoustics and the rest of the unsteady flow
field. The method is simple: i) the acoustic eigenmodes are analytically
determined for a uniform mean flow as in the original TPP; ii) the suggested
model for convective pressure perturbations uses the convective wavenumber as
axial wavenumber and the same orthogonal radial shape functions as for the
acoustic modes. The reliability is demonstrated on the simulation data of a
low-pressure fan. As acoustic and convective perturbations are separated, the
accuracy of the results increases close to sources, allowing a reduction of the
computational costs by shortening the simulation domain. The extended method is
as robust as the original one--giving the same results for the acoustic modes
in absence of convective perturbations.Comment: Accepted 15-05-11 by International Journal of Aeroacoustics to be
published in the special issue focusing on turbomachinery aeroacoustic
