Coherent Noise Sources in Subsonic Lobed Jets

In order to combat heightened levels of landing/takeoff (LTO) jet noise, future supersonic civil transport (SST) aircraft are likely to employ low-to-moderate bypass, internally mixed exhaust systems. Lobed internal mixer nozzles have been shown to reduce this noise by improving the mixing efficiency between the core and bypass streams; however, a detailed understanding of the noise features associated with lobed nozzles remains elusive. Through the use of a simple experimental coherence-based technique, the component of jet noise attributable to large-scale turbulent structures is isolated and compared between a model-scale axisymmetric nozzle and two lobed mixer nozzles of different penetrations. The increase in high-frequency jet noise from lobed jets relative to the axisymmetric baseline is shown to be highly coherent indicating that it is not simply the result of elevated levels of fine-scale turbulence. A double-peaked behavior is observed in the coherent output power spectra taken from the lobed jets, indicative of two distinct large-scale turbulent mechanisms in lobed flowfields.