A review on active control of jet noise

Kurzfassung

We review methods and technologies for active and passive control of jet noise. The reviewed technologies are placed on existing exhaust systems of turbofan engines and aim to improve the mixing of the jet flow to reduce its baseline acoustic emissions. Chevron nozzles are already flying on modern engines and nacelles, the challenge for new, possibly active, jet-silencing technologies is to increase the acoustic performance or to reduce the impact in terms of aerodynamic losses, compared to this benchmark technology. Active or vibrating chevrons or fluidic systems simulating the chevrons seem to be the first improvement in a step-by-step approach. Another conservative approach could be given by potentially active flow-control acoustic treatment of the nozzle walls. Active cancellation of convective flow structures seems instead a disruptive technology which is still far from application. The dynamics of convective flow structures needs to be further studied, in order to better comprehend their source mechanisms and potentially actively remove their noise contributions. All active methods will require control strategies with a certain level of automation: optimal variation setting with the help of acoustic transducers will play an important role if active control of jet noise will take off.