Diagnosing Noise Features of Internally Mixed, External Plug Exhaust Systems

Abstract

In internally mixed exhaust systems, a lobed or forced mixer is employed to mix the core and bypass streams before they exit the common nozzle. For moderate bypass-ratio engines this marginally improves thrust, and provides a significant acoustic benefit. Indeed, it has been thought that a fully mixed jet creates the lowest noise you can get from a two-stream engine. Typically, the forced mixer system produces a bit more noise than a fully mixed equivalent jet, noise that has been called “excess noise”. In recent testing, this excess noise seemed to be amplified when an external plug was employed in the exhaust system to reduce boattail drag. Identifying the mechanism of this excess noise and mitigating it is key to creating commercial supersonic vehicles that are acoustically acceptable around airports. This paper adds to that investigation, analyzing results from recent tests where far-field acoustic data were acquired on two sets of flows with matched fully mixed flow conditions. One set of flows had a heated core stream, making it a dual-stream jet. The other set had both streams heated to the same temperature, making a single-stream jet that matched the fully mixed temperature and velocity of the dual-stream jet. These flows were tested in two lobed-mixer exhaust systems with internal and external plugs. Analysis of flow and noise data from the nozzles with combinations of single- and dual-stream flows, and with and without flight streams, gives insight into how and why the noise of the dual-stream and single-stream jet differ. One additive source, present in nozzles both with and without external plugs, comes from inside the nozzle, likely the internal shear layer coming off the lobed mixer. This noise source is not impacted by a change in flight stream and has a distinctive spectral directivity that is amplified by the presence of the external plug. Finding the exact source mechanism of this new source will be key to making the internally mixed exhaust systems reach their lowest possible sound level.