Wind turbines, helicopters, weather and why turbulence matters

If you’d like to earn yourself $1 million, perhaps take a look at the Millennium Problems. Established by the Clay Mathematics Institute in Boston, the problems are seven mathematical Everests that so far, no-one has been able to climb. 

Conquer any one of them, and $1 million is yours. But to really impress people, solve the problem relating to turbulence which is called the Navier Stokes Equation. In the 200 years since it was first posited, the equation has been insurmountable. 

While it can confidently predict turbulence up to a point, making it useful for designing anything from oil pipelines to cars, no-one has been able to determine an analytical solution for the Navier-Stokes equation in turbulent flows. This is because turbulence itself, like flows around a car, a plane, or inside a star, can be staggeringly complex. 

For example, a gently flowing river might be almost turbulence free, but when the water encounters a bridge pylon, the result is a turbulence event across a huge range of scales from the obvious, churning waves to countless smaller turbulence points so miniscule they are measured in micrometres, all busily transferring energy between each other.

Yet for all the ubiquity of turbulence, a single model of it has been surprisingly resistant to the giants of mathematics, physics and engineering. Even the mathematics colossus, Werner Heisenberg, is said to have remarked that if he could ask questions of god, one would be 'why turbulence?'

This question takes on added urgency when you’re in a passenger plane being tossed around the sky like it’s made of balsa wood. On that score, allow yourself to be reassured by Associate Dean, Research Professor Ben Thornber of the School of Aerospace, Mechanical and Mechatronic Engineering and the Sydney Fluid Dynamics Research Group.

"An aircraft can take a hell a lot more of a beating than you would expect," says Thornber who came to Australia via Burnley, in Lancashire, UK, and various international organisations that used his skills in aerodynamics and space propulsion. "If you look at the extreme structural testing that is done to passenger plane wings, they can be bent like a bow without failing."

As a keen jogger and cyclist, Thornber has created quite a bit of turbulence in his time, especially when you consider that a simple wave of your hand in the air generates invisible patterns of turbulence so complex that they are beyond fully describing or predicting. 

Even more problematic is the turbulence in rivers, weather, the ionised gasses swirling through deep space and the blood surging around our bodies.

When you consider that an estimated 25 percent of the energy consumed by global industry is used to move fluids or move objects through fluids, a fuller understanding of turbulence would have profound economic benefits. It could also lead to dramatic advances in engineering, medical device design, all kinds of vehicles, weather prediction and understanding climate change.

But even as we constantly hear about rapid advances in technology, they’re not rapid enough for turbulence researchers like Thornber.

"Even using the world's fastest supercomputers, we can only simulate the turbulence around a few centimetres of a civil aircraft wing," says Thornber. "It will be around 2050 before we have the computing power to simulate in full, the flow over a whole wing."

Another example of the challenges is the turbulence generated by scramjets. Thornber was involved in what became the world’s largest calculation of turbulence relevant to these supersonic propulsion systems. It ran for the equivalent of 228 years on one computer. 

Rather than wait for technology to make these calculations happen faster, Thornber and the team in the University’s Fluid Dynamics Research Group have focussed on developing software algorithms and models of turbulence that can allow current computers to squeeze out the sort of information that future computers should be capable of. 

Demonstrating that idea is the backdrop image that Thornber is using as SAM talks with him online. It looks like a dramatic piece of chaotic contemporary art. It is, in fact, a rendering of the turbulence patterns of an experimental high speed Sikorsky helicopter and the focus of one of Thornber’s projects.

"The Sikorsky X2 has two coaxial rotor blades, meaning the two blades share the same shaft but rotate in opposite directions. This makes them pretty complicated to simulate," he says. "Standard methods might need to run on 2000 processors for few weeks to do it. We have a research program aimed at making that doable in a day on a computer that could sit under your desk."

Thornber's team has also investigated not just one helicopter taking off or landing, but many at once. "It's about establishing if the downwash of one helicopter could be dangerous for another helicopter, so reducing the risks of shipboard or urban operations. Not so long ago, these were nearly impossible questions to answer." 

The helicopter projects started for Thornber nearly a decade ago when he was approached by the Australian Navy to help their pilots land and take off safely from ships and, if bad weather makes it necessary, the occasional iceberg. It's work that is right at home among his broader interest in anything that generates relatively high-speed turbulence – speeding cars, aircraft, rotating detonation engines (which are a story for another time), and the Ingenuity helicopter currently on Mars having just made the first controlled flight by an aircraft on a non-Earth planet.

Though Thornber admits he didn’t really apply himself in high school ("I loved sports too much."), he has certainly made-up ground. His childhood fascination with the rivers near his home eventually became a Masters in Mechanical Engineering which then sparked a passion for aerospace engineering. A Masters in Space Studies from the International Space University soon followed, leading to work with plasma propulsion systems at the NASA Jet Propulsion Laboratory and another masters, this one in Computational Fluid Dynamics.

"I think my parents gave me an excitement for this stuff," says Thornber. "Their generation saw aviation evolving, the first rocket go into space, then just 12 years later, the moon landing. And my dad loved the visionary science fiction writers like Arthur C Clarke and Asimov.

"The race to break new frontiers in space travel might have slowed down but what has changed dramatically is the efficiency and cost of access to space – falling by more than a factor of ten in just a decade."

The future figures prominently in another element of Thornber's work which is understanding how turbulence affects wind turbines. As climate change threatens the human liveability of our planet, wind turbines are a cornerstone of the renewable energy response, and Australia has some of the best locations in the world for using wind to generate power, particularly coastal Western Australia and southern Australia around Bass Strait.

"This is something we’re really excited about actually," says Thornber."We’ve put an expert team together that covers data sciences, computer software and hardware, structural design and optimisation and people from the Australian Centre for Field Robotics for real time asset management."

Even if you remove natural wind turbulence from the equation, the turbulence generated in the wake of an upwind wind turbine enormously affects what happens to any turbines downstream. As wind passes through the forward turbine, it slows down and takes on strong turbulent eddies. 

For the downstream turbines this means a 5 to 20 percent drop in efficiency from the slowdown and greater structural fatigue from the increased turbulence. Some wind farms will even turn off turbines in certain circumstances to prevent this fatigue damage. Add together the lost power production and fatigue damage repair and the cost to the wind farm industry is around a quarter of a billion dollars every year in Australia alone.

"We’re spending a lot of time on this, and applying what we’ve learned from helicopters,” says Thornber. “The goal is to develop a digital twin of the wind farm and feed in real-time data from the actual wind farm. That way we can tune the model and simultaneously use it to optimise the performance of the real wind farm."

The team is in place, the need for this approach is clear, and there is no shortage of ideas. "We are co-developing our digital twins with the world number one producer of wind power, Iberdrola."

"Australia is so well set up for a renewable energy future. It’s an area we have to go into," says Thornber. "And I know the team here can make a big contribution."

Written by George Dodd for Sydney Alumni Magazine. Photography by Louise M Cooper.