The overall noise radiated by modern aircraft has two sources which are quite balanced at approach: the engine and the airframe. The airframe noise (AFN) has a broadband character and is mainly due to the interaction of the turbulent airflow with the high lift devices (slats and flaps) and landing gears, and to a second extent to cavities, spoilers and to boundary layers developing along the fuselage.
One of the major ACARE objectives is the reduction of perceived noise level of fixed-wing aircraft by 50% by 2020 compared to 2001. In achieving this required breakthrough towards quieter aircraft, reduction of AFN is very important already today and will become even more important in the future, especially for large aircraft, due to the already anticipated development of quieter engines.
With the clearly expressed tendency of modern airframe industry towards virtual prototyping and the increasing reliability of the design cycles on numerical simulations with the experimental verifications performed only at later stages of the design cycle, it is of utmost importance to increase the trust in noise predictions. However, the complexity and diversity of broadband turbulent AFN sources makes that prediction and subsequent reduction with present numerical tools extremely challenging and far from mature.