A semi-empirical jet-surface interaction noise model

Abstract

The bypass ratio of modern turbofan engines continues to increase in order to improve propulsive efficiency and to reduce the amount of fuel burned by aeroplanes. However, the increase in bypass ratio has reduced the distance between jet and wing, increasing jet-surface interaction noise. It is, therefore, important for the next generation of ‘Ultra-High-Bypass’ turbofan engines that the level of jet-surface interaction noise can be predicted and mitigated during the preliminary design process. In this thesis, a semi-analytical jet-surface interaction noise model has been created. The model scales a database of experimentally measured isolated jet near-field pressure spectra with jet velocity, flight velocity, core nozzle area and secondary nozzle area. The near-field pressure is then propagated onto the surface using cylindrical harmonics, whereupon Amiet’s theory is used to calculate the far-field noise scattered by the surface trailing edge. The model has been validated against small scale laboratory measurements of installed jet noise for flight Mach numbers less than 0.2. The scattering solution has been further extended with back-scattering theory, improving the prediction of the spectral shape for surfaces with chord to jet nozzle diameter ratios of 2.5 or less. However, at these chord to diameter ratios the amplitude of the laboratory measurements are overpredicted at mid and rear polar angles, most noticeably for jet Mach number of 0.6 or greater. Strip theory has also been used to model the cranked planforms of modern airliners, accounting for the swept and unswept portions of more realistic wing trailing edges. Finally, comparisons to measurements of large model-scale installed jets have demonstrated that the model can scale to larger jet diameters.

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