Spectral Leading Edge Serrations for the Reduction of Aerofoil-Turbulence Interaction Noise

The Department for Transport forecasts that by 2020 the number of passengers using UK airports will be around 400 million, compared to 200 million today. Aviation noise represents a major obstacle to the future expansion of many existing airports and thus the growth in the capacity of the air transport system. In 2001 the Advisory Council for Aeronautics Research in Europe (ACARE) set out a target to reduce perceived aviation noise to one half of the current level by 2020. To achieve the ACARE target by the year 2020 a "technology breakthrough" is urgently needed. Wind turbine manufacturers also require new technology for the significant reduction of aerodynamic noise in order to make wind turbines more acceptable to communities, especially concerned with onshore wind farms. Such a technology breakthrough can only be achieved through a fundamental evaluation and re-design of aerofoils, particularly the "leading edges" (LE), since upstream turbulent flows impinging on the LE of an aerofoil is believed to be the dominant source mechanism of broadband noise in turbofan engines (rotor wakes scattered by the outlet guide vanes - OGV) and wind farms (upstream rotor wakes scattered by the downstream turbine blades). In turbofan engines, it is envisaged that new LE design would be applied to the OGV since noise reductions can only be achieved by modifying the OGV response or the rotor wake turbulence (much more difficult).

The proposed 30-month research project aims to develop and investigate new aerofoil LE designs for the reduction of the broadband noise generated by the interaction between the aerofoil's LE and impinging turbulent flows, whilst minimising its impact on aerodynamic performance. The new aerofoil LE designs will be constructed by combining "smooth" spectral (wavy) serrations with multiple wavelengths, which has never before been attempted. In this project, a coordinated aeroacoustic and aerodynamic study of this new LE topology is proposed, particularly focused on the effects of smaller wavelengths (comparable to the impinging turbulence length scale), which are expected to be effective in reducing noise without making a significant impact on aerodynamic performance. The proposed project will take full advantage of the experimental and computational expertise of the two investigators. The successful outcome of this project will lead to a new aerofoil LE design that offers maximum noise reduction and minimum aerodynamic penalty. The commercial and academic impact of this work is potentially substantial.

The proposed research programme will be largely split and managed in four stages: 1) testing baseline aerofoil models for calibration and validation purposes; 2) identifying the most effective Fourier modes of the proposed LE serrations with respect to noise reduction; 3) combining the identified individual Fourier modes into an integrated spectral LE design (8 models in total) and testing the aerodynamic performance as well as the overall noise reduction; and 4) further understanding and improving the most favourable design found in Stage 3 via detailed numerical simulations. The experimental measurements will be performed in our AWT (anechoic wind tunnel) facilities. The numerical simulations will be carried out by using CAA (computational aeroacoustics) techniques. The CAA and AWT activities are closely coordinated and mutually supportive to ensure maximum value to the project. The proposed study will be based on a NACA65(1)-210 aerofoil with the Reynolds number up to 1.1x10^6 and the Mach number of 0.3 to 0.6. The length scales of impinging free-stream turbulence will be determined and generated in accordance with the guidelines from the industrial partners representing the aero-engine and wind turbine industries.

A reduction in aerofoil noise will have benefit in a variety of applications including aircraft engines, high-lift devices and wind turbines, which in turn will make a significant impact to environmental noise. The proposed research will benefit the general public, particularly those in areas surrounding airports or wind farms, suffering from physical/mental health implications due to long-term exposure to environmental noise. The proposed work is intended to deliver a practically viable aerofoil leading edge design concept that can be adopted in the near future by the industrial partners to achieve significant reductions in environmental noise levels.

Aviation noise represents a major obstacle to the future expansion of many existing airports and thus the growth in the capacity of the air transport system. Regulations in aviation noise are becoming increasingly stricter in response to which aircraft manufactures strive to develop quieter aircraft. Although steep approach operation is becoming widespread to reduce noise emissions in urban airports, a technology breakthrough is urgently required in order to achieve genuine noise reductions at source. Recent advances in novel low-noise engine technologies such as ultra-high bypass ratio and low-speed fans have led to significant reductions in jet noise and fan tone noise. However, fan broadband noise particularly due to outlet guide vanes (OGV) is not significantly affected by these technologies and is now one of the major noise sources in a modern aero-engine. Its attenuation is the primary objective of this proposal.

Another principal application of the proposed technology is wind turbines. The implication of wind turbine noise is highlighted in a recent white paper, "Wind Turbine Acoustic Noise" by Rogers, Manwell & Wright. They measured a striking increase in noise levels by as much as 13dB(A) under certain conditions when measured at 180 meters from the base of a 10kW wind turbine at Rockport, MA, US. They argued that a buffer zone of almost half a kilometre would be required to meet Massachusetts noise regulations. Clearly, any reduction in broadband noise levels, which this proposal aims to achieve, would alleviate such local planning problems, leading to increased viability of power stations sited near populated areas.

Rolls-Royce and Vestas Technology UK have highlighted the importance of this proposal in their letters of support and have suggested interactive partnership throughout the lifetime of the project. Collaboration with Rolls-Royce will be developed through the two University Rolls-Royce Technology Centres (UTCs) at Southampton. The impact of the proposed research on the industrial partners will be maximised by delivering bi-annual progress reports and organising annual full-day mini-conferences in which they will provide confidential aerodynamic/aeroacoustic data and feedback for us to act upon. Vestas offers an opportunity for field testing of the proposed technology worth £30,000.

The impact of this work will be brought to the attention of the general public through the media towards the end of the second year when the field testing with Vestas is undertaken to verify the new design concept of the aerofoil leading edges developed for the reduction of environmental noise. The applicants of this project will arrange meetings with science journalists to publicise the field testing results via BBC News and local newspapers as well as the official website of the University of Southampton. In view of the sensitivity of wind turbine noise at the present time, dissemination of the results from this test will be of significant interest to the media. Successful dissemination of the outcome will certainly improve the general public's perception and acceptance of future wind farm planning and operation. We also plan to utilise the University's Open Days to showcase our interesting aerofoil designs to inspire and recruit the best young minds in this country.