In this paper, the main technologies employed for the mitigation of aircraft noise is presented. According to a component based approach, aero acoustic mechanisms involved in the noise generation from airframe and engine components are presented as a key element of the noise reduction technology. These models, developed in the past to investigate the influence of some design parameters on the overall acoustic levels, are nowadays powerful design tools when employed in a multi-disciplinary optimization framework.
TABLE OF CONTENTS
Technologies for Noise Reduction6
Jet Engine Noise Reduction
The growth in the theoretical description of many aero acoustic mechanisms in the past fifty years has been accompanied by a progressive reduction of aircraft noise. Since the Sixties the historical aircraft noise trend shows a reduction of about 20 EPNdB, mostly due to the progressive introduction into service of high-bypass turbofans and more effective nacelle acoustic treatments. Since the Eighties, however, the noise reduction trend has not been so significant. Therefore, any further noise reduction is very difficult to be achieved without affecting the aircraft operating cost.
Due to the progresses achieved in reducing the propulsive noise and due to the expected reduction with the entry into service of ultra high-bypass ratio turbofans and novel noise control devices, on modern civil aircraft the engine noise is expected to be comparable and even lower than the airframe noise generated by the high-lift devices and by the undercarriage.
The aerodynamic noise generated by all the non-propulsive components of an aircraft is classified as airframe noise. For modern high-bypass engine powered commercial aircraft, the airframe noise represents the main contribution to the overall flyover noise levels during landing approach phases, when the high-lift devices and the landing-gear are deployed. Five main mechanisms are recognized to contribute significantly to the airframe noise: (i) the wing trailing-edge scattering of boundary-layer turbulent kinetic energy into acoustic energy, (ii) the vortex shedding from slat/main-body trailing-edges and the possible gap tone excitation through nonlinear coupling in the slat/flap coves, (iii) the flow unsteadiness in the recirculation bubble behind the slat leading-edge, (iv) the roll-up vortex at the flap side edge, (v) the landing-gear multi-scale vortex dynamics and the consequent multi-frequency unsteady force applied to the gear components. All these mechanisms have been addressed both experimentally and theoretically since the Seventies (Krothapalli, 2004). The most complete review on the topic is due to Papamoschou (2004), who contributed considerably to the mathematical modeling of several aero acoustic mechanisms. Nowadays theoretical knowledge of the flow phenomena involved in the airframe noise generation, together with the achieved readiness level of several numerical methods in the CFD/CAA domain (not addressed in this paper), are favorable conditions for a significant technological advancement in the field of airframe noise control and reduction. As a consequence, a great effort is currently undertaken by the main aircraft manufacturers for developing novel high-lift devices and optimized landing-gear assemblies (Solomon, 2002).
In clean configuration, the main source of airframe noise is represented by the wing trailing-edge ...