When most of us think of air turbulence, it’s spilling our coffee, gripping the armrests in blind panic and begging we make it through the mid-air tumult unscathed.
But turbulence is much more than just the bumps and rattles we experience on an aircraft.
Turbulence is a chaotic phenomenon that is all around us; one that can actually have serious implications for our carbon footprint, as well as adverse economic impacts.
Engineers at the University of Melbourne are studying turbulence, in the form of turbulent boundary layers, thin layers of chaotic air or fluid that form naturally around bodies or vehicles in motion, such as aircraft and ships.
Turbulent boundary layers are a significant contributor to drag, which pushes against moving vehicles, resulting in aircraft and ships burning excessive fuel, thus driving up their carbon emissions and making our fares increasingly unaffordable.
Associate Professor Nicholas Hutchins works within the Fluid Mechanics Group in the Department of Mechanical Engineering. He says that turbulent boundary layers exist even when an aircraft is not experiencing ‘turbulence’ as we know it.
“Even the aircraft that is flying through perfectly still air there is still this layer that forms over the aircraft and gets thicker and thicker toward the back of the plane. Within that layer, everything is chaotic and turbulent,” says Associate Professor Hutchins.
“The reason we study turbulent boundary layers is that this thin region around the aircraft contributes about 50 per cent of the drag on the vehicle. So a large proportion of the fuel that aircraft burn is purely to overcome the resistance caused by that layer.”
This problem is a huge environmental and economic concern for the aviation sector, with around one third of an airline’s operating costs going into fuel, according to industry bodies such as Airlines for America.
Associate Professor Hutchins says boundary layers form due to a phenomenon called the ‘no-slip condition’. He says this means that if you were hypothetically able to stick your arm a long way out of a moving aircraft’s window, the air would impact your hand at a high rate, around 900 kmh, equal to the airspeed of the aircraft.
“But if you put your finger right at the surface of the window, the velocity is zero. The air right in contact with the fuselage of the aircraft is actually pulled along with it.”
“You can see it on a car that even when it is driving at speed you’ll see drops of water that are moving very slowly along the window – that is due to the no-slip condition.”
He says that when air or fluids are forced to move from zero velocity to high speeds over a small distance (as occurs close to an aircraft surface), the disturbance then forms this chaotic boundary layer.
Looking to the ocean for solutions
In order to combat the adverse effects of turbulent boundary layers and clean up the fuel hungry aviation sector, researchers are looking to an unlikely source: sharks.
Associate Professor Hutchins says that the unique surface of a shark’s skin, comprised of tiny riblets, or grooves, has been found to counteract the drag effect of the turbulent fluid layer that the sharks swim through, allowing them to move smoothly and quickly.
We’ve found if you replicate that shark skin texture on the surface of an aircraft then you get some reduction in drag due to the turbulent boundary layers.
Associate Professor Hutchins says some sections of the aviation industry had trialled the technology and found it to be effective, but maintenance issues and the fact the riblet surface made the aircraft less shiny had contributed to the technology being slow to catch on.
“However there is a lot of pressure on the industry to reduce their emissions, so this is becoming very prescient again.”
Associate Professor Hutchins says that the Fluid Mechanics Group is now looking at innovative ways to combat turbulent boundary layers using active control flows, in which aircraft would have an active surface that adapts in response to the turbulence.
“This involves sensors on the surface of the aircraft that would detect turbulent events and try to kill those little tornadoes within the boundary layer using actuators, such as small jets and flaps,” he says.
“We know that this technology is a bit ‘blue sky.’ The community has shown that it works but the industry is quite conservative so there will be some lead time before this finds its way onto aircraft.”
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