Adaptive and Passive Flow Control for Fan Broadband Noise Reduction
Air traffic is predicted to grow by 5% per year in the short and medium term. Technology advances are required to facilitate this growth without unacceptable levels of noise. FLOCON addresses this requirement by delivering the technology to reduce fan noise at source through the development of innovative concepts based on flow control technologies.
Due to the continuously increased bypass ratios of aeroengines and the fact that in the past noise reduction efforts have been focussed mainly on tone noise, todays engines are generally designed in a way that the tone noise does not significantly emerge from the broadband noise floor. A quantitative assessment shows that if all tones are removed from the total engine noise spectrum, the resulting EPNL (Effective Perceived Noise Level) at approach is reduced by only 2.2 EPNdB for an turbofan engine with BPR (bypass ratio) 5 and by 1.5 EPNdB for a geared fan engine with an extremely high BPR of 16. As most of this broadband noise is generated by the fan (summed up over the 3 noise certification conditions), the reduction of fan broadband noise has the maximum effect on aeroengine noise reduction. To fulfil future demands in aircraft noise reduction, the reduction of fan broadband noise by design (e.g. tip speed or blade shape) is expected to be not sufficient; thus new concepts involving flow control have to be developed.
Previous attempts at reducing broadband noise have been inhibited by a limited understanding of the dominant mechanisms and by a lack of high-fidelity numerical models. These issues are addressed in the ongoing PROBAND FP6 project. In FP7, FLOCON moves beyond the scope of PROBAND to the development of specific concepts for reducing broadband noise in aero-engine fan stages.
FLOCON will demonstrate methods capable of reducing fan broadband noise from aeroengines at source by 5dB at approach and takeoff conditions, contributing to the European objective of reducing aircraft external noise per operation by 10dB by 2020.
The impact of scaling from lab- to engine-relevant operating conditions will be assessed, as well as the side/complementary effect of broadband noise reduction features on fan tone noise. Generally speaking, FLOCON will increase the understanding of the flow physics and broadband noise generation and control mechanisms.
Description of work
In FLOCON, a wide range of concepts will be considered and developed to Technology Readiness Level 4 (laboratory scale validation):
Experiments will be performed on two rotating rigs, supported where possible by more detailed measurements on a single airfoil and a cascade. Numerical methods will be used to optimize the concepts for experimental validation and to extrapolate the results from laboratory scale to real engine application.
The potential benefit of each concept will be assessed, including any associated penalties (weight, complexity, aerodynamic performance). Recommendations will be made as to which concepts could be integrated into new engine designs and which will require further validation at industrial rig or full engine-scale. Any developments required in enabling technologies will also be identified.
FLOCON will provide the European aero-engine industry with demonstrated methods to reduce broadband noise at source. In doing so, it will contribute to achieve European Aerospace industries’ objectives for lower-noise aircraft to meet society’s needs for more environmentally friendly air transport, and to enhance European aeronautics’ global competitiveness.
The broadband noise reduction concepts developed in FLOCON will be broadly applicable to the fan stage of all new aero-engine designs. A subset of the methods (to be determined within the programme) will be applicable also to core compressor designs. FLOCON itself will bring each concept up to Technology Readiness Level 4 (validation at laboratory scale) and recommend a subset for development to engine-ready level.
Recommendations will be produced which contains all the necessary information for further development and exploitation of the recommended noise reduction methods. In particular, the experimentally determined efficacy of the method together with an extrapolation to expected performance at full engine scale will be given, in addition to an initial assessment of any penalties related to weight, aerodynamic performance, stress or mechanical complexity.