The so-called parabolic equation (PE) has proved its capability to deal with the long range sound propagation as an alternative to the ray model. It was shown that the High-Order Parabolic Equation (HOPE), based on a Padé expansion, significantly increases the aperture angle of propagation, compared to the standard PE and the wideangle PE. As a result, for the in-duct propagation it allows us an accurate prediction close to the cut-off frequency.

Within the context of the two major European Research Projects, NACRE and OPENAIR, the potential of acoustic installation effects on the aft fan noise radiated by innovative installations of coaxial turbofans are evaluated. Three different installation concepts are considered: a semi-buried engine, a rear-fuselage nacelle and, finally, a scarfed nozzle.

This article focuses on advanced noise prediction methodologies, in regard to aircraft noise mitigation. More precisely, the so-called aeroacoustic hybrid methodology is first recalled here, before illustrating its potentialities through several examples of application to realistic aircraft noise problems. Among other things, this paper highlights how Onera has contributed to the development of reliable computational methodologies over the last decade, which can now help in solving aircraft noise issues.

SScreech and broadband shock-associated noise linked to the presence of a shockcell structure in supersonic jets are reviewed in this paper. Only underexpanded supersonic circular jets issued from a convergent nozzle are considered here. An overview of the flow and of these two noise components is presented, based on recent experimental and numerical work. Flight effects on broadband shock-associated noise are also introduced, within the framework of aeronautical applications.

Combustion noise has recently been the subject of attention of both the aeroacoustic and the combustion research communities. Over the last decades, engine manufacturershave made important efforts to significantly reduce fan and jet noise, which increased the relative importance of combustion noise.

Although the acoustic comfort in helicopter cabins is not subject to European Recommendations regarding aeronautic environmental noise (ACARE 2020), helicopter manufacturers use many resources to improve internal acoustic comfort. This task is particularly difficult because, on the one hand, passengers are in close proximity to the disturbing sources and, on the other hand, the noise frequency range is located in the domain of high sensitivity of the human ear (500-5000 Hz).

This paper investigates the effect of a flow control device on turbofan sound generation, applied to a low-speed axial compressor model in a laboratory test rig. This treatment consists in a secondary mass flow ejected through the trailing edge of the rotor blades, designed to fill the velocity defect behind the rotor and to decrease the turbulent kinetic energy related to the wakes, so that broadband interaction noise should be reduced.

The reduction of noise emission has become a key commercial argument for helicopter manufacturers, such as Airbus Helicopters. For years, Airbus helicopters has placed emphasis on the good acoustic behavior of its helicopters, as proven by its communication on the fenestron concept, the acceptance of its aircraft for the Grand Canyon tours and the presentation of its recent Bluecopter™ Technology. Thus, Airbus helicopters has become one of leaders in the manufacture of low noise helicopters.

The aim of this article is to provide a broad overview of current and future noise reduction technologies used in aircraft industries. It starts by recalling the regulation framework and the European incentives that have triggered efforts in this domain, as well as the major dedicated EU research programs. Then, technologies are introduced in four parts: engine nacelle, fan, jet and exhaust technologies and finally the airframe noise.

This issue of Aerospace Lab is dedicated to Aeroacoustics, the field of
science that deals with sound generated by air flows and possible interaction
with solid bodies. As noise issues became a major environmental
challenge over the past decades, the aerospace industries have paid great
attention to Aeroacoustics for the design of aircraft, as well as helicopters.
An environmentally efficient air vehicle offers a competitive advantage.