The development of aircraft transportation has brought an increase of air traffic volume and issues related to environmental aspects such as carbon and NOx emissions and noise pollution. The jet exhausting from aircraft engine is the responsible for pollutant discharge and one of the main sources of community and interior noise.
Jet noise is a challenging topic for the scientific community. Despite of many years of research, the mechanism by which turbulent structures generate noise is still unknown. The knowledge of near-field pressure helps to identify noise sources and to understand noise production mechanism aiming at the development of control devices. A microphone in the near-field senses fluctuations associated with non-radiating incompressible ‘pseudo-sound’ and propagating compressible ‘sound’ waves (Ffowcs-Williams, 1969). Hence the near-field comprises a ‘hydrodynamic’ pressure and an ‘acoustic’ component destined to reach the far-field (Tinney & Jordan, 2008).
The installation of the jet close to a surface such as the wing or the fuselage makes things even trickier. In the case of the wing, the surface modifies the turbulent mixing of the jet, noise is enhanced by diffraction and reflection phenomena and a new noise source, named jet-wing interaction, arises. In the case of the fuselage, the jet impinge on the panels causing stress and vibrations. Some of these vibrations are re-emitted in the aero-acoustic field as vibration noise, some of them are transmitted into the cockpit as interior noise. The knowledge of wall pressure field is the basis to set reliable prediction models for far-field noise (Amiet, 1975-1976).
The discussion above motivated the author to carry the research activity on two sections:
- Development of innovative hydrodynamic/acoustic separation techniques based on wavelet transforms
- Experimental analysis of jet-surface interaction using a flat-plate installed tangentially to the nozzle axis for different radial distances from the jet
Simultaneous near-field and far-field pressure measurements for jet Mach numbers 0.6 and 0.9 have been exploited for the development of 3 novel hydro/acoustic separation techniques. For what concerns the analysis of jet-surface interaction, simultaneous velocity (Hot Wire Anemometry) and wall pressure (Microphones) measurements have been performed in order to provide:
- Effect of the plate on mean and turbulent aerodynamic field;
- Characterization of the statistical and spectral content of wall pressure field aimed at laying the foundations for wall pressure modelling;
- Characterization of the cross-statistics between velocity and wall pressure fields;
- Detection of coherent structures responsible for high-energy wall pressure events