Work of friction

Researchers say controlling airflow to reduce drag could cut aircraft fuel burn and pollution

Drag caused by airflow on an aircraft’s body, also known as skin-friction drag, can increase the level of fuel burn and pollution produced.

Airbus aims to reduce fuel burn per passenger/km by at least 50 per cent by 2020, which researchers at Warwick University have said is not possible without a 30 to 50 per cent reduction in this drag. The researchers are therefore developing a method to control airflow and ultimately reduce this drag using micro-scale Helmholtz resonators.

‘The only way we are going to get that degree of reduction is partly due to aircraft weight, or by improving engine or combustion efficiency. But those two things alone won’t get the 50 per cent reduction,’ said Dr Duncan Lockerby, one of the fluid dynamicists working on the project that has been funded equally by Airbus and the EPSRC.

‘The Holy Grail in aerodynamic control is to reduce the very large contribution of skin friction drag that comes from turbulence near the wings’ and the fuselage surface,’ added Lockerby.

He said that although turbulence is non-deterministic and chaotic, which makes it difficult to control, research has shown that in the area closest to the surface of the aircraft, known as the viscous sub layer, there is a deterministic element in the form of coherent flow structures. Lockerby refers to the flow structures as streaks because of their long, thin shape.

‘Although it is only a theory, it is widely accepted that these streaks are critical to the regeneration of turbulence. They cause the turbulence outside of the viscous sub layer, which in turn potentially generates more streaks, so it’s a cyclical process,’ he said.

‘If you can stop those streaks generating or cancel them in some way, you might be able to get rid of the turbulence and reduce drag.’

According to Lockerby, passive devices are necessary because first of all, the streaks are very finely distributed and in cruise conditions, the 1,000 microns long streaks are separated by a distance of 100 microns. Second, there is an estimated one billion streaks on an aircraft fuselage at any given time.

‘Imagine the kind of control scheme needed to detect where these streaks were and tell a device to puff some air up to cancel them, or something similar to that. That’s a suggestion that has been around for some time, but not put into practice because it is so ambitious,’ he said.

Warwick is looking at a number of ways to control the streaks using a combination of passive techniques involving Helmholtz resonators, riblets and spanwise oscillation.

Helmholtz resonance is illustrated when you blow over the top of a bottle and the large flow of air going in and out of the top creates a tone.

‘Envisage shrinking those bottles to micron scale and randomly distributing them underneath a wing so that the bottle top is flush with the surface. The idea is that as the streaks travel over these little cavities sunk beneath the surface of the wing some kind of Helmholtz resonance will occur, which might be enough to disrupt the development of the streaks,’ said Lockerby.

The whole aircraft surface would be perforated by small cavities with necked exit orifices which would be manufactured using Micro-Electro-Mechanical Systems (MEMS) technology.

Another technique the researchers will study is putting riblets — aligned grooves — on the surface of the wing. Riblets have been proved to reduce drag by six per cent in lab-based experiments, but by only 1.5 per cent in flight tests.

‘We’re proposing to look at different configurations of riblets, such as using wavy grooves, to improve the efficiency,’ he said. This technique is based on studies of spanwise oscillation devices, which work from left to right rather than up and down.

‘If you look over the wing from a passenger window, part of its surface would be coming towards you and moving away from you, along the span. This affects the streaks and has proven to reduce drag, but it is quite difficult to create a surface of the wing that oscillates like that.

‘So, if the flow travels over the wavy riblets and follows them, in a way it is mimicking the spanwise oscillation, so the streaks are being bent back and forth. At this stage, we don’t know that wavy riblets will be better than straight ones, but we have the software to test it,’ said Lockerby.