The overall objective of this project was to reduce the noise from large-scale turbulent structures from high-speed turbulent jets. These flows and structures are responsible for the dominant noise from high performance aircraft. In particular, the large-scale coherent structures create the dominant noise in the downstream direction. This research explored a potential control system that measures the small amount of upstream radiating noise from large-scale structures for the use in a control system. The project team also explored the feasibility of controlling the large-scale structures through plasma actuation within the nozzle itself. The team focused on the core technology of statistical separation of mixing noise sources and potential integration into a control system operating on a supersonic off-design jet to lower both the near-field and far-field dominant noise source.

The project team explored reduction of large-scale mixing noise through a mechanism using actuators within the nozzle divergent section. The actuators would eventually be controlled by a decomposed signal consisting of the noise that propagates upstream from large-scale turbulent structures within the jet shear layer. The team used a combination of analytical methods to examine the source, decomposition approaches including the proper orthogonal decomposition, large-eddy simulation to study the turbulence, and an experimental implementation in the laboratory environment. Here, the researchers demonstrated elements of this noise reduction technology (in the laboratory environment) for the noise associated with large-scale coherent turbulence within high speed off-design jets.

The project showed that the large-scale coherent structures have high coherence between the upstream and downstream radiation directions. The large-scale structures also have associated statistics that are unique relative to fine-scale mixing noise and is related to the instability waves. This is useful information that was not previously explored. The fine-scale mixing noise for the cases studied is on the order of 10 dB higher than the large- scale mixing noise in the upstream direction in the near-field. This causes difficulty in extracting the signal to drive actuation and requires additional research. The experiments have been used to confirm the findings and predictions. Unfortunately, the plasma actuation technique was not as successful as the project team had desired. The researchers did not achieve control authority with actuators in the divergent section of the nozzle. This is puzzling because of the success previous investigators have had with actuation on the nozzle lip. Nonetheless, the measurement data has confirmed the statistics we found in the near-field upstream direction.

The project team have showed that there is upstream radiation from large-scale turbulent structures in the upstream direction, which was a question in the community. The team identified associated statistics and how low they are relative to other noise types. The researchers have also showed that the placement of plasma actuators within the divergent section of the nozzle and not the nozzle lip is unlikely an optimal place due to lack of authority. In the future, plasma actuators must be placed on the lip of the nozzle to be effective. In the future, a mathematical method that is fast and accurate should be developed to extract the noise from large-scale structures if an active control scheme is to ever be effective.