Noise of Internally Mixed Exhaust Systems With External Plug For Supersonic Transport Applications
Internally mixed exhaust systems produce more noise than would be expected from an equivalent fully mixed flow. This ‘excess noise’ has never been fully explained and has not been researched recently as it is not as much of a problem for high bypass-ratio exhaust systems. Commercial supersonic vehicles, however, will use more moderate bypass ratio engines. Their exhaust systems will also likely feature external plugs to improve boat tail angle for cruise performance. Thus the problem of excess noise will need to be addressed along with the impacts of having an external plug. Recently, acoustic tests of dual-stream jet flows from both internal and external plug nozzles have been conducted. Measurements of noise from flows with an axisymmetric splitter and lobed forced mixer were made. These measurements show the baseline ‘excess noise’ previously found in forced mixer exhaust systems, and show how this noise source differs depending upon the plug geometry. Background-oriented schlieren measurements acquired simultaneously document the differences in shock structures over the transonic flow regime tested. The far-field acoustic measurements indicate that an external plug can slightly reduce the mixing noise of the jet plume when the flow is completely mixed, e.g. when the core and bypass streams have the same temperature and the flow is a single-stream jet. However, when the flow is not fully mixed, e.g. the usual case of an internally mixed exhaust system, the plug seems to amplify the ‘excess noise’ associated with the mixer. This additional noise source presents a challenge for nozzle designers wanting to use internally mixed exhaust systems with external plug nozzles.