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Improvement of Fan Broadband Noise Prediction: Experimental Investigation and Computational Modelling


Fan broadband noise is a major aircraft noise challenge now and will be even more important in the future. Novel low-noise engine architectures, such as ultra-high-bypass-ratio engines and lower-speed fans, can help address jet noise and fan tone noise, but they are unlikely to reduce fan broadband noise significantly. The accurate prediction and control of fan broadband noise is therefore essential if aircraft noise is to be reduced.

The most significant broadband noise sources are believed to be generated by the different interaction mechanisms between:

1. the blade tip vortex of the rotor fan and the turbulent boundary layer on the inlet-duct (rotor boundary layer interaction noise)

2. turbulent eddies convected in the rotor boundary layer past the rotor trailing edge (rotor self noise)

3. the impingement of the rotor wake onto the downstream outlet guide vanes (OGV interaction noise)

4. turbulent eddies convected in the vane boundary layer and the vane trailing edge (OGV self noise).

These four mechanisms each generate a whole spectrum of frequencies, making it difficult to use conventional noise measurements to isolate the contribution of each mechanism. Furthermore, the broadband noise generation process is very complex to model, requiring representation of the fine length scales involved in turbulence generation and propagation. Consequently, current methods for industrial broadband noise prediction are almost exclusively semi-analytic in nature. They rely largely on correlating measured noise levels to a few relevant aerodynamic and geometric parameters, but are unable to predict the effects of different blade geometries. The advances in purely numerical methods, which have revolutionised tone noise prediction, have yet to make an equivalent impact on broadband noise prediction.

Project objectives

The objective of PROBAND is to develop methods to allow the design of a fan system that will generate sufficiently low broadband noise to meet the EU noise level targets. This will be achieved by:

1. developing a better understanding of broadband noise generation mechanisms using advanced experimental and computational techniques

2. developing and validating improved prediction methods using conventional computational fluid dynamics, and integrating them into industrial codes

3. exploring new prediction strategies using advanced computational techniques

4. developing low broadband fan noise concepts.

Description of the work

The main goals of the PROBAND research programme are to be achieved by:

  • developing coupled RANS/semi-analytic models for fan stage broadband noise sources and validating these models against representative fan rig measurements
  • developing and evaluating the application of advanced CFD methods based on LES and DES and demonstrating their potential application for industrial fan stage noise assessment
  • promoting an increased understanding of turbulence-driven, broadband noise generation in aero-engine fan stages through detailed measurement of turbulence structure and noise on representative configurations
  • developing concepts for low-broadband noise fan stage configurations by exploiting the project numerical and experimental results.

Expected results

PROBAND will enable improved physical understanding of the source mechanisms of self-noise, interaction noise, and tip clearance noise. The fundamental experiments will provide, in conjunction with advanced CFD, a deeper insight into the flow physics in the source regions. PROBAND will develop new tools allowing large scale advanced CFD, and will validate them in a realistic experimental environment. PROBAND will deliver an improved prediction capability for broadband noise that will be exploited by the European engine industry to develop low broadband noise fan concepts. The final goal of PROBAND is to develop the methods that will enable the design of a fan with sufficiently low broadband noise to allow the EU Sixth Framework Programme’s short-term and long-term objectives of reducing aircraft external noise by 4-5 dB and by 10 dB respectively to be achieved.