Effect of fluid inserts on low order models of jet noise reduction

There have been several investigations of various methods to reduce noise in existing and next generation tactical jet aircraft engines. One such method is to use rows of injectors for steady blowing of bypass air at different locations in the divergent section of the nozzle. This results in the jet operating closer to an on-design condition and introduces streamwise vorticity that breaks the noise-producing large-scale turbulent structures of the jet. This fluid insert technology has been shown, at small and moderate scale, to effectively reduce noise up to 6dB in the peak noise radiation direction. Research has been performed in the past several years to demonstrate and improve upon this concept both in the form of experiments and computations. Some questions still remain about the behavior and working mechanisms of fluid inserts. Recently, an LES database was generated to provide further insight into the changes caused by the fluid inserts on the nozzle flow. This paper analyzes the LES database using Proper Orthogonal Decomposition of different flow variables as well as Doak’s Momentum Potential Theory to help understand the detailed mechanisms for the observed noise reductions.