Windbreak fences reduce airport pollution and noise

Simple runway fences can act as a ‘virtual chimney’ to funnel and disperse airport pollution and noise more effectively, according to academic researchers

Special ‘blast’ fences placed behind a runway, known as baffles, could help to tackle noise and air pollution from aircraft engines, according to academic research.

The baffles act as a ‘virtual chimney’ that can funnel nitrogen dioxide and other emissions from aircraft engines upwards so that the pollutants can disperse more effectively and reduce the environmental impact on residents close to airports.

The windbreak ‘baffle’ fences being tested at Cranfield Airport

The study also found that the baffles dampened engine noise downstream by a modest amount and were helpful in reducing jet blast on the airport perimeter.

A team of researchers from Manchester Metropolitan University (MMU), Cranfield University, Southampton University and the University of Cambridge carried out the tests with funding from the Engineering and Physical Sciences Research Council (EPSRC). The results were announced on the MMU website on Friday (January 1).

The fences are a cheap way of reducing the impacts of nitrogen dioxide and could be installed in UK airports in less than three years, the research team claim.

Long-term ground-level nitrogen dioxide concentrations around many major airports in Europe already exceed the legal limit enforced by the EU.

The first stage of the study was carried out by Cranfield University in Bedfordshire and involved preliminary wind tunnel testing of various prototype baffle shapes. This showed that using baffles of less than a man’s height and constructed from low-cost agricultural windbreak netting on lightweight frames could force an aircraft’s exhaust plume to leave the ground within the airport’s boundary fence.

‘Fifty per cent reduction’

Project leader Dr Mike Bennett, of MMU’s Research Centre for Aviation and the Environment, said: “Airfield surfaces are typically covered with grass, over which the wind can blow freely. An array of baffles makes the surface rough in an aerodynamic sense. This sucks the momentum out of the exhaust jet, allowing its natural buoyancy to come into play. By suitably angling the baffles, we can also give the exhaust an upwards push, encouraging it to rise away from the ground.”

He added: “The baffles we tested were tilted at angles between 40° and 60° in order to optimise this vertical flow — and to ensure the baffles didn’t blow over! Although the exhaust will still disperse to the ground eventually, it will do so at a lower concentration. We might hope to see a reduction in surface concentrations of around 50% at the perimeter fence behind the place where aircraft are taking off.”

The aim of the trials was to test the baffles’ aerodynamics. However, the temporary prototype installation was constructed differently from how a permanent installation might be made. According to the researchers, each baffle must be sufficiently robust to withstand the 80-90 knot blast from a jet engine, but flimsy enough to collapse harmlessly if an aircraft were to hit it.

This was achieved by restricting the prototype baffle widths to around two metres, although tests showed that in a permanent installation it would also be possible to make them much narrower. An area of baffles measuring around one thousand square metres would need to be installed behind a runway for permanent, full-scale use of the fences at an airport.

Dr Bennett said: “There’s no reason why baffles couldn’t start to be installed at airports within two or three years. From the point of view of local air quality, they represent a quick, cheap supplement to developing low-NOx (nitrogen dioxide) jet engines.”

A four-engined BAe146 aircraft was used in the tests at Cranfield Airport, which took off 12 times in total, on each occasion burning its engines for 5 -15 seconds at the end of the runway prior to take-off. The Natural Environmental Research Council’s (NERC) Facility for Airborne Atmospheric Measurements provided the aircraft.

The study, part of the three year project ‘A Study of Practical Abatement Techniques for Exhaust Jets from Commercial Aircraft’, concluded in the autumn of 2012 and received total EPSRC funding worth £413,000.

Air quality monitoring expertise for the field tests was provided by the University of Cambridge, while the Institute of Sound and Vibration Research in the University of Southampton carried out the acoustic studies.

At Cranfield Airport, laser scanning using lidar (the optical equivalent of radar) was used to test an array of three rows of baffles in the preliminary stages.

The research was carried out as part of the EPSRC-funded Airport Energy Technologies Network (AETN), which was established in 2008 to undertake aviation research. The EPSRC is a major UK agency for funding research in engineering and physical sciences.