In civilian aeronautics, the increasing noise restrictions around airports are a challenging problem for aircraft and engine manufacturers. Despite significant progresses since the definition of ACARE objectives, jet noise remains the main source of noise for an aircraft at take-off and many works have been dedicated to its understanding and its reduction.
The first generation of noise reduction devices were passive (such as chevrons or mixer which enhance the mixing in the shear layers to reduce jet noise) but also decrease the engine performance during cruise. The second generation noise reduction concepts under development are active and can be switched off after take-off.
ORINOCO is a project based on the cooperation between Europe and Russia and deals with advanced engine noise control based on plasma actuators. Plasma technologies have initially been developed for flow control and their first applications for jet noise reduction were confronted with technical aspects far from acoustics. This use of plasma actuators is a novel concept that requires fundamental approaches to understand the interaction mechanisms with the main jet and the resulting radiated sound.
ORINOCO contains theoretical, numerical and experimental investigations for an efficient implementation of plasma actuators for jet noise reduction, especially on the basis of instability waves reduction. As a consequence of these fundamental considerations, most of the partners involved in ORINOCO (European and Russian) come from the Academic or Research community.
Several plasma actuators concepts will be studied and assessed in small scale sound absorbent facilities with an isothermal main jet. The most promising ones will be evaluated with a heated main jet. The work performed within the project will be monitored by an Expert Panel composed by industrial members from Europe and Russia. This proves their interest on this topic.
Quieter jet engines on demand
Scientists are developing the next generation of noise-reduction devices, allowing pilots to turn them on and off with a flip of the switch. Designed to attenuate engine noise during take-off, these silencers should provide the European aerospace industry with a competitive advantage.
Increasing noise restrictions around civilian airports continue to offer a challenging problem for aircraft and engine manufacturers. Research and development efforts over the years have resulted in significant reductions in the noise radiated by in-service aircraft engines. Despite such progress, jet noise still remains the main source of noise at take-off.
Jet noise reduction has been treated with passive devices that on the one hand reduce jet noise, but on the other hand decrease engine performance during cruise. Active devices that can be turned off after take-off will be the next-generation devices designed to meet the objectives of the Advisory Council for Aeronautics Research in Europe (ACARE) Vision 2020 for reducing external noise.
The http://www.orinoco-project.org/ (ORINOCO) project embodies the cooperation between Europe and Russia in the framework of advanced engine noise control based on plasma actuators.
Plasma actuators have been developed and improved with the support of theoretical investigations and numerical simulations. These were implemented on nozzles to generate coherent structures in the turbulent shear level of the jet.
To thoroughly understand jet noise mechanisms, an innovative tomographic particle image velocimetry technique was used. This enabled calculation of the 3D velocity field of the jet volume. Numerical simulations were performed to measure the near-field noise with or without the presence of actuators. Near-field measurements revealed that instability waves are strongly connected with the noise radiated in the far field.
To reduce jet noise, researchers placed the actuator inside the nozzle to assess if this would suppress the artificial instability wave generated by the loudspeaker.
Jet noise was successfully reduced in the far field by tuning the phase of the signal sent to the actuator. Investigators also placed six plasma actuators inside a circular nozzle to excite the most stable components of the flow. This reduced the amplification of the most unstable modes as well as associated radiated sound.
Final work will be focused on synthesising the acoustic performance of each actuator concept for jet noise reduction, and on outlining the specifications for extending the concepts to full-scale.