Reducing noise pollution for air traffic

Aircraft noise has an impact on people living under flight paths. With air traffic set to increase further over coming years this is an issue which must be dealt with, says Dr Fulvio Scarano of the FLOVIST project, an ERC-backed initiative researching flow diagnostics and experimental aero-acoustics.

The development of the Tomographic Particle Image Velocimetry technique, along with the non-intrusive pressure field characterisation method, has opened up previously unforeseen perspectives in the area of unsteady flow diagnostics and experimental aero-acoustics. As a result it is now possible not only to quantify complex turbulent flows in their three-dimensional structure, but also to extract quantities such as pressure, allowing researchers to analyse aero-acoustics in unprecedented depth in order to devise new flow control strategies.

“These techniques enable us to measure, quantify and study the air flow around parts of an aircraft. They also have applications in ground transportation – for example cars and trains – as well as in the developing field of wind energy,” says Prof Fulvio Scarano, the Principal Investigator of the FLOVIST project. Bringing together academic and industrial partners from across Europe, FLOVIST aims to fully describe and quantify the flow pattern and related acoustic production at its origin, research which Prof Scarano says is central to efforts to minimise aircraft noise.

“One of the main sources of noise from a landing aircraft is its airframe, which is not really produced by the airframe itself, rather it originates by the turbulent flow around the wings – the trailing edges and flaps in particular – the undercarriage and of course the jet engines.

In a turbulent flow, the velocity and pressure fluctuations interact with the structure also producing aerodynamic noise, which propagates efficiently in the atmosphere down to the ground,” he explains. “As things stand the quantification of aero-acoustic noise relies on the use of microphones arrays. Such devices are used for field tests; meaning around an airport to measure the noise of approaching aircraft. Wind tunnel testing is the most effective approach for the aircraft industry R&D cycle. However, the far-field sound levels do not provide the designers with direct information on the causes of noise production. Such gap requires to be bridged by skilled aerodynamicists and by flow visualisation and extensive computer simulations to understand the dynamic behaviour of the flow”

Current methods of measuring aero-acoustics are not ideally suited for this purpose. Anechoic wind tunnels, for accurate acoustic measurements, are sparse and highly expensive in Europe. The FLOVIST projects uses Particle Image Velocimetry (PIV) for detailed analysis of the source of aero-acoustic noise. “We approach aero-acoustic problems from the source of sound. We use state-of-the-art 3-D flow imaging based on high-speed digital cameras and solid-state lasers; the principle is a blend of tomography and quantitative flow visualisation by injection of micrometric droplets in the flow. The illuminated particles’ position is recorded in time simultaneously by several viewing directions and the computerised analysis returns the velocity field. The resulting technique is Tomographic PIV, which allows a 3-D time-resolved (4-D) quantification of the flow. Further data analysis can yield the instantaneous pressure field and the localize the sound source at high-resolution,” outlines Prof Scarano. This ability to measure air-flow in three dimensions forms a key part of the project’s experiments.

“We have already preformed preliminary experiments on the transition of jets showing the full potential of the FLOVIST approach to spot the regions in the flow that are responsible for sound production. Now we are investigating the most popular approaches to reduce the acoustic noise – chevrons and synthetic jets for instance – and try to explain why in some circumstances they work and in others fail,” says Prof Scarano. “Moreover, we also concentrate on the trailing edge of wings and some experiments are planned early in 2010 that should shed light on the mechanism responsible for sound production. The trailing edge is the part where most airframe noise emanates from; there the flow from both sides of the wing merge and the turbulent structures, are not in equilibrium with each other; their interaction produces so-called trailing-edge noise.”

“However, the situation for an aircraft in landing configuration is more complex; the high-lift devices are multiple flaps deployed to produce extra lift at lower speed. The complex flow within these elements involves not only trailing edge noise but also unsteady separation and strong tip vortices at the edges of the flaps; all these elements require to be carefully assessed for an optimized aero-acoustic design of a more silent aircraft.”

“One of the expected advantages of this approach, if proven feasible,” continues Prof Scarano, “is the possibility to perform aero-acoustic research in conventional wind tunnels, because this flow-visualisation method does not require anechoic environment to detect and quantify the sources of sound emissions. This can give a tremendous impetus to experimental aero-acoustics so far limited to the research labs equipped with anechoic facilities”.

Research in this area also has environmental implications, although the link between noise emissions and the overall efficiency of an aircraft is still unclear. “In principle an efficient aircraft should also produce less noise. Although the energy of acoustic emissions is a very small fraction of the overall energy in the air-flow, a loud aircraft indicates the presence of a strongly turbulent flow around the vehicle, which is representative, to a degree, of how much energy is being wasted,” he outlines. “Noise is a diagnostic element to see if there are important inefficiencies in the aircraft. Our main focus at the moment is on identifying the main physical processes leading to noise emissions in order to aid the development of a greener aircraft in line with the European objectives for 2020.”

This development work is very much in line with the wider environmental agenda. The airline sector is under pressure to reduce emission, but safety and performance is also paramount. “Safe flight is the highest priority. Often because the effect of a modified design is not well known in terms of flow stability – for instance sudden and uncontrolled flow separation - we may renounce to improving the aircraft performance,” says Prof Scarano. The project’s sophisticated, visualisation-based approach allows them to pursue more rigorous research in these areas, and study simultaneously the aerodynamic behaviour and acoustic emissions seeking for unexplored margins in performance optimization with no detriment to safety.

“We need to find first what is to blame for noise production, only then we can really analyse the phenomenon in greater depth. We expect that these phenomena will have important relevance in terms of aerodynamics too.”

“We have an extra opportunity now, not only to hear the noise emitted by the flow, but also to look at it while being generated. By this we can interpret the phenomena on the acoustic noise side, as well as on the energy efficiency side, and pursue drag reduction,” he outlines. “After we introduced the Tomographic PIV technique, only four years ago, many groups worldwide have initiated research projects following this direction”, states Prof Scarano, “we wish that this project will be the driver of a similar success in experimental aero-acoustics, by the opportunity we want to open for wind tunnel laboratories where flow visualisation tools are well developed and anechoic facilities are not at reach.”

For more information on the project, contact Dr Fulvio Scarano at aerolab@lr.tudelft.nl.

Published: Monday, 1st February 2010 by Tom Freeman

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