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Turbulence - the irregular motion of fluids like water or air - is all around us – in the surf, the wind or even the smoke from a candle.
But while we might know it when we see it (or feel it, if we’re on an aeroplane), scientists still don’t fully understand how turbulent processes work.
“Turbulent flows present one of the last great problems of classical physics,” says Professor Ivan Marusic, an international leader in fluid mechanics based at the University of Melbourne.
“Some of the world’s smartest minds have worked on this.”
One of the (many) reasons engineers like Professor Marusic dedicate their careers to understanding turbulent flows is that they cause drag when aeroplanes move through the sky, or ships through water. And overcoming that drag requires energy, which in turn means high fuel costs and lots of CO2 emissions.
In fact, around 10 per cent of all electricity produced every year is consumed by overcoming turbulence.
“The less power we need to fly the better off we will be, economically and environmentally,” says Professor Marusic.
“It’s scary when you look at how much fuel aeroplanes use at the moment.”
Professor Marusic and his team are investigating ways to suppress turbulence so that the flow around aeroplanes becomes ‘laminar’; that is, smooth and ordered. Their efforts have been focused at the layer immediately between the plane and the airflow, known as the ‘boundary layer’, where most skin friction drag originates.
They are in the early stages of a major leap forward, with ongoing preliminary work unearthing a new pathway to drag reduction.
Using their wind tunnel at the University’s Parkville campus, the team conducted a series of novel experiments that led to the discovery of superstructure features of turbulence. These are very-large coherent motions, of a size much larger than previously believed possible.
The team verified their results in 2007, describing experiments in a large wind tunnel as well as on the Utah salt flats, where the flat, uniform terrain allowed them to mimic boundary layer flow over a flat plate (like an aeroplane wing).
“We now know that under flight conditions the physics of turbulence has different pathways to drag reduction. It’s a new physical understanding as to how drag occurs,” explains Professor Marusic.
They are optimistic that this discovery will ultimately lead to new technologies that substantially reduce drag levels, with net power savings.
“The classical view of how drag occurs would never make those kinds of reductions feasible, but they become credible once we understand this alternate pathway,” Professor Marusic says.
But to learn more about this exciting alternate pathway, Professor Marusic and his team are going to need a bigger wind tunnel.
The new x-tunnel
In 2025, when the University’s new campus opens at Fishermans Bend just south of Melbourne’s CBD, it will be home to a unique wind tunnel that can simulate flight conditions on a commercial aircraft, but in a controlled laboratory setting.
Known as the X-tunnel for the extreme ‘Reynolds numbers’ it will be able to handle (the higher the number, the more turbulent the flow), the new tunnel will be 60 metres long and housed within a giant pressure vessel. Researchers will be able to pressurise the air inside up to 20 bar – similar to the turbulent flow experienced by passenger aircrafts in flight.
This new facility will allow Professor Marusic and his team to directly measure and investigate the new drag reduction pathway they discovered. It will mean their data is much more accurate, because they will no longer be forced to extrapolate from data gathered from their current, smaller tunnel or field experiments.
“Being able to do controlled experiments under these flight conditions in a laboratory is really exciting. It will give us access to flow regimes that we haven’t been able to test in the lab before,” says Professor Marusic.
The only other similar wind tunnel in the world is the European Transonic Wind Tunnel located in Germany, which is expensive to run because it requires large volumes of liquid nitrogen.
Because the X-tunnel will just use highly pressurised air, it will be a more cost-effective option.
And it’s not just aircraft travel that will benefit from the research that can be conducted in the X-Tunnel. It could also be used to address problems like noise from wind turbines and drag on ships. It could even be applied to many of the key issues related to supersonic flight.
“We do our research because we’d like to have an impact,” says Professor Marusic.
“We’re looking forward to working with people from a wide range of industries, from around the world, to help address their key challenges and problems.”
First published on 20 May 2022.
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