Nigel Peake – Sound and Silence: Understanding Aircraft Noise

If I had to summarise my research in a single sentence then I would say that I’m interested in waves of all kinds: how they are generated, how they propagate and how they interact with other objects.

Over the years this interest has led me to delve into a wide variety of areas, including the way polar sea ice breaks up because of the action of the ocean waves, precisely why it is that aircraft vapour trails hang around so long in the sky, and even on one occasion how vibrations from a cruise ship propeller could be reduced so as to improve the dining experience of the paying passengers! However, most of my time has been spent working on sound waves, and especially on the sound waves generated by aircraft, which are a cause of such annoyance for communities living round our busiest airports. Emmanuel has a fine tradition in this area, but more of that later. Of course, noise pollution is not the sole preserve of airports, and the noise from large on-shore wind farms is a subject of growing controversy (and no doubt academic investigation).

Aeroacoustics is the science of sound in air, with the aim of calculating numerical values for the decibel noise levels inflicted upon the observer. In my experience this involves attempting (and often failing) to solve mathematical equations of ever greater complexity. As an academic discipline, aeroacoustics was founded by Sir James Lighthill in 1952 who, in two papers which neither contained, nor required, reference to any previous published work, described how to calculate the noise generated by a turbulent jet of air. This was a subject of huge concern at the time, because of the advent of commercial jet aircraft with very noisy engines.

Lighthill’s analysis represented a major step forward, but his interests soon moved into other areas and the person who did more than anyone else subsequently to develop the subject of aeroacoustics was our great former Master, Shôn Ffowcs Williams. The Ffowcs Williams & Hawkings (FWH) equation (published with Shôn’s PhD student David Hawkings at Imperial College) extended Lighthill’s work so that it could be applied to a vast range of situations, not just to jet noise, and is now the starting point for anyone who wants to predict the noise produced by an aircraft, a wind turbine, a submarine, or even an owl ...

What FWH shows is that sound is generated by three physical processes, which are, in descending order of efficiency: sudden air expansion (like a balloon bursting, or bubbles breaking the surface in a ‘babbling’ brook); applied force (such as the force on aircraft landing gear when the plane comes in to land); and Lighthill’s mechanism of turbulent fluid motion. However, an equally important Ffowcs Williams contribution (with another Imperial PhD student, L H Hall) came with his realisation that Lighthill’s turbulent noise source would become significantly louder if placed in the vicinity of a sharp edge. Indeed, the Ffowcs Williams & Hall effect is so strong that much of the noise you can hear from aircraft coming in to land comes not from the aircraft engines at all, but from the air flowing over the trailing edges of the wings!

So how does an owl help with aircraft noise? The point is that many owls can hunt in acoustic stealth, producing much less sound than one might expect in the audible range of their prey. To do this they have to overcome the Ffowcs Williams & Hall effect, of course, but precisely how this is done has remained something of a mystery. Recently, Dr Justin Jaworski and I in Cambridge, in collaboration with experimentalists at Virginia Tech in the States, have been able to shed a little light on what might be going on. The owl has two unique features, which are not found on any other bird. One is that the microscopic structure of the feathers on the upper wing surface is exceedingly complex, with layer upon layer of interleaved barbs and hairs. We believe that this has the effect of damping out a lot of the noise-generating turbulence present in the flow very close to the wing before it can ever get to the wing trailing edge. And the second is that at the wing trailing edge itself the feathers possess a small flexible and semi-porous comb, which we have been able to show negates the Ffowcs Williams & Hall effect for that part of the turbulent flow which has not already been damped out by the feathers upstream.

So could these features from the owl be useful in engineering practice? Wind turbines with serrated rigid trailing edges already exist, but to get the same sort of noise reductions seen in nature more flexible materials, and perhaps metal foams, will be needed. Mimicking the complex structure of the feathers is much more of an engineering challenge, but it’s not beyond the realms of possibility that carefully designed and placed roughness elements might achieve something similar. Whatever else happens, what is certain is that Shôn Ffowcs Williams’s original insights will continue to take centre stage, in what we hope will be a less noisy future.