Abstract:
This PhD dissertation investigates the noise radiation produced by a rotor inside a
duct, which is convected by a swirling-translating mean
flow. The study is based
on an extension of Gennaretti's and Morino's boundary element method to the
frequency domain for scattering problems in conjunction with a spinning rotor source
model in the presence of a swirling-translating
flow. Firstly, two different source
models of the rotor are analyzed in absence of mean
flow. The parametric study of
the two dipole components distributed over a ring or a disc shows that the source
radius is a crucial parameter. The scattered pressure directivity patterns of the ring
and disc source models are in perfect agreement when a particular ratio between the
two model radii is adopted. Therefore, the present analysis justifies the preference
for the ring source model due to its simplicity. The proposed formulation is validated
by means of exact solutions and used to investigate the effects of the translating
flow
Mach number and swirling
flow angular velocity on noise radiation both in the far
and in the near field. The scattered sound is highly affected by the convecting
mean
flow. The modal content of the scattered field increases when increasing the
translating
flow Mach number, while a swirling
flow leads to a reduction of the mode
propagation, if co-rotating with respect to the azimuthal order of the spinning source,
or an increase of the modal content, if counter-rotating with respect to the source.
This is clearly confirmed by the scattered pressure patterns and levels both in the far
and in the near field for all the source frequencies. In general, the mean translating
flow moves the main lobes of the directivity patterns downstream, whereas in some
cases the mean swirling
flow appears to neglect this effect and the downstream
lobe is completely shifted. However, the investigation on the in-duct propagation
shows that the main effect of the convecting mean
flow is to change the modal
duct characteristics, more than the pattern itself. This results in turn in the strong
modification of the patterns noted in the far field.
