In aviation, the generation of thrust always meant acting on the air surrounding the aircraft in order to generate a reacting propulsive force: a backwards acceleration of air generates a forward directed thrust. The obvious outcome of aviation propulsion systems is then a higher-speed air flow, or jet, trailing the propulsion systems and directed opposite to the flight direction. Less obvious is the generation of acoustic noise, due purely to the presence of this jet and its mixing in the surrounding atmosphere. The noise generated by jets has been studied since the advent of turbojet propulsion, when it became part of humans'life as the dominant component of aircraft noise. Following over half a century of aeroacoustic research and development of propulsion systems, jet noise lost part of its impact as an aircraft-noise source, especially for transport aviation. It still represents the main source of noise during take-off, for aircraft powered by modern turbo-fan engines. Its importance as a component of environmental noise pollution in areas near airports means that research on jet-noise estimation, control and reduction is still a very active subject in the aeroacoustics community.
The engine acoustics department is active in jet-noise estimation and measurement.
The estimation of jet noise can be done by using measurement-scaling methods, semi-empirical acoustic-analogy approaches or by performing direct estimates, after jet-flow simulations. In partnership with engine manufacturers, we perform jet flow large-eddy simulations, by using an industrial software. The LES field is then extrapolated to the far field by means of a source-term surface integral across a surface enclosing the high-intensity turbulence field, in a standard direct simulation. A further activity is the development of an in-house measurement-scaling prediction tool. In the development of semi-empirical acoustic-analogy approaches, we use LES data bases with the aim of improving acoustic-analogy source modeling. This, we expect, will reduce the empiricism of these approaches and give information regarding their accuracy. It will possibly give new evidence about the most relevant mechanisms of acoustic-disturbance generation and propagation, by supplying a parallel approach to the standard direct simulation. The first step in this direction has been made by processing an available turbulent-jet LES field (the figure reports an instantaneous snapshot of this jet flow), in order to extract flow statistics which are relevant in the source description for the Lilley acoustic analogy.
The ongoing jet-noise measurements are made by using microphone arrays. Most of the measurements are done on real engines, either in engine-test facilities or in fly-over experiments. Some measurements are also conducted in large jet-noise facilities, on jets issuing from reduced-scale models of engine nozzles. A nozzle-design campaign has been conducted in collaboration with Penn State University. Here a number of minimally invasive nozzle-wall treatments (see figure on the left) have been tested, in order to determine their effectiveness in reducing jet noise in the far field of a model scale jet. A rig for the generation of single-stream unheated air jets is available in our laboratory. The rig is used to perform flow measurements in high subsonic regime.