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FLOCON
Adaptive and Passive Flow Control for Fan Broadband Noise Reduction

State of the Art - Background

Tags: Air

Air traffic is predicted to grow by 5% per year in the short and medium term. Technology advances are required to facilitate this growth with acceptable 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 aero engines and the fact that in the past noise reduction efforts have been focused mainly on tone noise, today's 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 a turbo-fan engine with a bypass ratio (BPR) of 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 three noise certification conditions), the reduction of fan broadband noise has the maximum effect on aero engine 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 insufficient, thus new concepts involving flow control have to be developed.

Objectives

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 aero engines 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.

To achieve this, FLOCON will:

- design noise-reduction concepts and associated devices able to reduce fan broadband noise from aero engines;

- assess the noise reduction concepts by conducting lab-scale experiments;

- complement the experiments by numerical simulations that are assessing the capability of currently available numerical tools to design low broadband noise treatments and configurations;

- develop understanding of the mechanisms involved and extrapolate the results to the aero engine environment using state-of-the-art numerical methods;

- select the best concepts by balancing noise benefit and integration impact.

Description of Work

In FLOCON, a wide range of concepts will be considered and developed to Technology Readiness Level 4 (laboratory-scale validation):

- Rotor trailing edge blowing;

- Rotor tip vortex suction;

- Rotor overtip treatments;

- Rotor and stator leading and trailing-edge treatments;

- Partly lined stator vanes.

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 optimise the concepts for experimental validation and to extrapolate the results from laboratory scale to real-engine application.

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.

The potential benefit of each concept will be assessed, including any associated penalties (weight, complexity and 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.

Expected Results

FLOCON will provide the European aero-engine industry with demonstrated methods to reduce broadband noise at source. In doing so it will contribute towards achieving European aerospace industries' objectives for reduced noise from aircraft to meet society's needs for more environmentally friendly air transport, and 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 also applicable 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 contain 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.

Example result of an advanced large-eddy-simulation of broadband rotor-stator interaction noise
Example result of an advanced large-eddy-simulation of broadband rotor-stator interaction noise

DLR laboratory-scale fan rig for broadbandnoise investigations
DLR laboratory-scale fan rig for broadband noise investigations

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