Dr. Ganesh Raman proposes a paradigm shift to deal with this issue. Instead of the current passive flow control techniques, like lobed nozzles and chevrons, which are unable to adapt to changing working loads of the aircraft, he plans to develop an active High Frequency Flow Control (HFFC). Large air vortices produced by the engine are responsible for a significant percentage of total jet noise. Powered Resonance Tubes (PRTs) produce a high intensity acoustic tone that disrupts the air vortices, reducing the jet noise; however, they create their own noise. Dr. Raman plans to make the PRT produce high frequency sound instead, which is inaudible to the human ear, but still interferes with the air vortices to reduce the engine noise. An added benefit to this technique is that the HFFC can be turned on during take-offs and landings, when aircraft are noisiest, and off during cruising time, to retain maximum fuel economy. In addition, the high frequency PRT has no moving parts, making the operation simple and highly reliable.
Dr. Raman has has some very good results from his research project. He made a presentation about this work at the American Institute of Aeronautics and Astronautics at the 2010 Aerospace Sciences Conference in January in Orlando, Florida. Click here to download a PDF of his paper.