TOWARDS WHOLE SPECTRUM JET NOISE PREDICTION

Driven by the rapid growth in air transport, the world's civil aircraft fleet is almost doubling in size every two decades. Major UK airports are under increasing pressure to add extra new runways. The impact of aircraft noise on the local environment is profound especially in the surrounding areas of an airport. In the worst case noise can be more than just annoying, potentially being a contributory factor towards illnesses such as hypertension. In the latest FlightPath 2050 noise emission targets, set by the Advisory Council for Aviation Research and innovation in Europe (ACARE), the perceived noise emission of flying aircraft is to be reduced by 65% relative to levels of 2000 by 2050. This is a huge ambition and tremendous challenge to the aerospace industry, potentially putting noise research to the forefront. Continuing reductions in permitted take-off noise levels are also making the existing technology and more established plane/engine models obsolete giving the strong economic implications for the proposed work.

Large Eddy Simulation (LES) based numerical prediction of jet noise has become increasingly promising in both research communities and industry. Despite LES prediction being generally more affordable compared to acoustic experiments, it is still time consuming and costly, often requiring significant High Performance Computing (HPC) resources, in order to capture both low- and high-frequency noise spectra. This is known as the scale disparity, and clearly is preventing LES based noise predictive methods from being further applied to more complex and realistic industrial jet configurations. To circumvent this obstacle of scale disparity, the work proposed aims to explore the potential advantages of a novel approach of combining noise spectra (in the spectral space) obtained from LES solutions with different grid resolutions in the physical space. The current work extends the methodology developed in the recently completed and highly successful EPSRC project GR/T06629/01, in which the applicant being the lead researcher, and the ongoing Rolls-Royce/TSB funded SILOET programme jet noise project. Results obtained from this work will be benchmarked against NASA experimental data as well as results from a full length sampling fine-grid single LES solution. The work will also investigate turbulent energy flux directed from smaller eddies to larger ones by a low-pass filtering analysis of the resolved flow field, the evaluation of velocity structure function and turbulent kinetic energy spectra.

With the project's nature being industrially inspired, the potential benefits and impacts to society are high. Air transport growth is at a fast pace, almost doubling every two decades. This is evidenced by the fact that major UK airports are under the pressure to add extra new runways. It is becoming urgent to take steps to reduce the environmental impact with respect to noise emissions giving strong strategic importance to the current work's context. In the meantime, continual reductions in permitted take-off noise levels make existing technologies and established plane/engine models no longer commercially viable giving strong economic implications of the current work.

We expect the current work will have strong technological impacts in the aerospace industry, especially for Rolls-Royce plc. With Rolls-Royce being a project partner and the applicant being a member of the Rolls-Royce University Technology Centre (UTC), strong engagement with Rolls-Royce plc is expected from the start. This will naturally take place through regular interactions between the project team at the UTC and Rolls-Royce engineers. The methods developed in the proposed work will help ensure that the latest regulatory noise emissions targets can be met, and the potential to manufacture quieter and more efficient engines offers tremendous competitive business advantages. This will have the subsequent impact on creating greater national wealth and securing employment in manufacturing, hence the UK economy.

With regards to people and skills, the project will help produce two highly trained aerodynamicists (CFD experts), who will benefit from regular supervision and technical advice from the investigator as well as their own interactions with Rolls-Royce engineers. This training is valuable to the UK science/technology base, and there is a well-established recruitment route leading such researchers to Rolls-Royce and other key UK aerospace companies. The new efficient computational and mathematical modelling technology will offer the potential for substantially reduced design and wind tunnel/acoustic test costs. The project aims for disseminating the developed computational procedures to Rolls-Royce plc as well as the wider research and development community.

In summary, the proposed work has great environmental importance to the society. It has commercial importance to the UK economy, potentially securing UK jobs in a high technology area and also producing highly trained skills. This is because greater physical understanding, better mathematical models, and novel methods for acoustic prediction will be created. The spectral relay technique based on LES and acoustic analogy devised during the project along with the physical insights on turbulence energy flux should thus all have substantial scientific impacts. The work also has wide potential application beyond jet acoustics since a generic predictive approach will be developed as well as insights into turbulence physics.