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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:524170 |
| Date | January 2009 |
| Creators | Quaranta, Erika |
| Contributors | Drikakis, Dimitris |
| Publisher | Cranfield University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://dspace.lib.cranfield.ac.uk/handle/1826/4577 |
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