External aircraft noise has become an important political issue. With a 5% estimated annual growth rate of air traffic volume, substantial reductions of aircraft noise are required in order to keep air travel environmentally sustainable. The ACARE Strategic Research and Innovation Agenda (SRIA) ‘Flightpath 2050’ calls for, among other targets, a reduction of the perceived noise of flying aircraft by 65% (which is equivalent to a reduction by 15 EPNdB) relative to typical new aircraft in the year 2000.
For large passenger aircraft, engines with high bypass ratios (HBR) offer a substantially improved fuel efficiency. However, these large engines are mounted closer to the wing, which leads to additional installation-noise sources. Ultra-high by-pass ratio (UHBR) engines are expected to generate lower isolated engine jet noise levels than HBR turbofans due to a further decrease of nozzle exhaust velocities. But the generation of jet-airframe interaction noise represents a major risk.
For business jets, where the engines are mounted at the rear part of the fuselage above the wing, reflections of jet noise on the tail-plane or fuselage also increase aircraft noise levels. As an example, an innovative Falcon jet aircraft configuration was designed to reduce rearward radiation of engine noise and jet-noise installation effects via a U-shaped tail located below the engine. Although this configuration is promising in terms of aircraft noise reduction, the proximity of the jet exhaust generates higher unsteady pressure loads on the aircraft surfaces compared to conventional afterbodies.
Up to now, comprehensive work on jet-wing interaction noise with representative engine, pylon and wing configurations at relevant flight conditions remains very limited. In the EU project JERONIMO, fundamental studies on noise source mechanisms and prediction methodologies were conducted up to frequencies corresponding to a non-dimensional jet Strouhal number around 3. Initial low-noise technology ideas were proposed using hybrid RANS-LES methods (HRLM) for design validation. Similar work was carried out recently in the USA by NASA and in Japan by JAXA. However, all these initiatives are based on simplified nozzle-wing geometries and flight conditions and could only provide a first and basic, thus insufficient, understanding of future complex installation noise challenges. Hence, the DJINN project aims to close the existing gap between this basic understanding and the interaction noise for fully representative configurations (wing, pylon, nacelle) and various flight regimes.
The ability to understand, model and predict jet noise and its installation effects is a key requirement in the design of efficient and environmentally acceptable aircraft systems. Against this background, the DJINN project sets out a highly ambitious and innovative programme of work conceived to enable optimised aircraft-engine designs that take full advantage of UHBR turbofan technologies.