A contribution to the understanding and prediction of jet noise generation in forced mixers: Part II Flight Effects

The work described in this paper constitutes a further contribution to the on-going efforts by the aircraft industry worldwide to understand, predict and control jet noise generation by the modern high bypass aero-engine. In the present work the ‘two-source’ model, previously described by Tester et al [3] and Garrison et al [4] for buried nozzle configurations has been extended to include the effects of flight. The model is based on the hypothesis that, while the shear-induced turbulence in the mixing layer is reduced by flight, the additional turbulence driven by the mixer induced vortices remains unchanged by flight. The latter follows from the fact that these vortices depend on the velocity difference across the buried mixer (which does not change with flight). Formalisation of these concepts leads to an expression for the ratio of the intensity in flight to that observed statically for 90 to the jet axis. To predict the in-flight directivity, this basic source is attributed the directivity of a static jet of velocity (VJ – Vflight ) and a corresponding Doppler frequency shift based on the flight speed Vflight, since such a shear layer will reproduce, approximately, the effects of convective amplification and acoustic-mean flow interactions of the in-flight shear layer. Comparison of data with prediction includes an annular mixer together with low and medium penetration lobed mixers. Good agreement between data and prediction is observed in all cases, except where a nozzle based source, termed the high Mach number lift (HML), is significant at the high power condition. Garrison et al [5] are conducting an extensive CFD study, using an efficient and affordable RANS model, of the mixer flows outlined above. This is providing a vital key to rendering the approach more generally applicable and avoiding expensive aerodynamic measurement programmes. In the companion paper, their CFD calculations are benchmarked against measurements and are used to justify the noise modelling. Their CFD results also support our hypothesis that the HML source is associated with the interaction of the mixer vortices with localised patches of supersonic flow at and near the final nozzle. The CFD results show that these supersonic patches can occur due to flow turning even at sub-critical nozzle pressure ratios. The main purpose of this paper is to demonstrate how the simple prediction methods of the original four source model have been extended to cover forced mixers both statically and in-flight. The work also shows clearly the doubly deleterious effect of strong upstream mixing. First the high frequencies are increased statically; second, they demonstrate a reduced static to flight noise decrease. We also believe that the type of approach described here, when combined with the efficient CFD approach described in the companion paper, can provide a useful way forward in jet noise prediction and optimisation for complex nozzle geometries.