“If you fly through the air at very high speeds you heat the air around the vehicle. If you go fast enough you can actually change the chemistry of the air… heat will get dumped into the aircraft, so the aircraft can get very hot.”
Professor Andrew J Neely from UNSW Canberra’s School of Engineering and Information Technology works with objects travelling at extremely high speeds—hypersonic speeds, in fact.
“The standard airliner is a subsonic aircraft but it’s approaching the speed of sound… when you fly faster than the speed of sound you become supersonic, and all sorts of things happen: you get shockwaves, they create drag, you get sonic booms,” Professor Neely explained.
“There’s no actual cut-off for when speed becomes hypersonic—the effects that start to occur happen at about five times the speed of sound… the physics that characterise supersonic are all still there, but in addition to that you start to see some new effects come in.”
When it comes to designing aircraft that can operate safely and efficiently at hypersonic speeds, it is vital to understand exactly what happens to the aircraft under those conditions. For example, most aircraft are made from aluminium because it is lightweight and durable, at least until it gets hot.
“If you fly through the air at very high speeds you heat the air around the vehicle,” said Professor Neely.
“If you go fast enough you can actually change the chemistry of the air… but the other thing that can happen is that heat will get dumped into the aircraft, so the aircraft can get very hot.”
But as the aircraft heats up and its structure changes, those changes can in turn change the way the aircraft interacts with the air around it, creating a highly complex system that is not only difficult to measure and understand, but that can permanently damage the aircraft.
“Even if it doesn’t happen to destruction, you still want to understand how it’s happening on an aircraft because it can affect the lift, the drag, et cetera,” said Professor Neely.
Much of the work of understanding the effects of hypersonic speed happens in complex physics simulations. However, the systems involved are so complex that they are difficult to accurately simulate—it’s only recently that computers have been powerful enough to calculate the interactions involved.
Ordinarily the simulations would be validated against experimental data, but in the case of hypersonics there isn’t much data available. Of the few hypersonic wind tunnels around the world, most have run-times that are too short to be suitable for these experiments.
Researchers are a long way from putting a whole vehicle into a hypersonic wind tunnel, but this is where Professor Neely’s work comes in. He tests the wind tunnel on what are called “simple geometries”, reducing the complex structure of an aircraft down to its most basic shapes.
The data from those experiments is then fed back into the simulations to validate their results, which allows the researchers to simulate more complex systems with greater levels of confidence.
Professor Neely explained that understanding hypersonics has a wide range of applications.
“In terms of defence—they’re obviously interested in high-speed aircraft,” he said.
“There’s a whole range of military applications and threats, everything from transport to surveillance to strike—everything from missiles to unmanned to, eventually, manned aircraft.”