Hypersonic objects are those that travel at more than five times the speed of sound, or 3,806 mph, fast enough to fly from London to New York in less than an hour. For missiles — such as those recently deployed by Russia against Ukraine or those tested by the United States — reaching such speeds can allow them to evade current air defenses and anti-ballistic missile systems. It also makes them more adept at penetrating heavily armored structures and capable of destroying targets by means of kinetic energy alone, without even considering a payload of high explosives. However, the ability to move and maneuver at hypersonic speeds presents significant and varied technical challenges.
When a missile or aircraft breaks the sound barrier, it begins to generate a shock wave that is hotter, denser and higher in pressure than the surrounding air.
And in the hypersonic regime, air friction reaches such magnitude that it would begin to melt parts of a conventional commercial aircraft.
On top of all this, aerospace engineers must consider not only how the air flows around the craft or weapon in question, but also how it behaves as it moves through the engines and interacts with the fuel. .
Conventional air-breathing jet engines, such as those seen in large airliners, actively draw in and compress oxygen to allow them to burn fuel during their flight, for example through fan blades made of spin.
Above three times the speed of sound, however, this becomes useless, as passing the jet or weapon through the air achieves this on its own.
So-called ramjet and scramjet engines that take advantage of this principle can achieve levels of fuel efficiency that, for comparison, rockets cannot achieve.
However, the fluid dynamics models needed to develop such motors by predicting how they will react to fluid forces around and within them are inherently tricky.
Mechanical engineer Dr Sibendu Som, from the Center for Advanced Propulsion and Power Research at the US Department of Energy’s Argonne National Laboratory, said: “The interactions between chemistry and turbulence are so complex in these engines.
“Scientists had to develop advanced combustion models and computational fluid dynamics codes to accurately and efficiently describe the physics of combustion.”
NASA, for example, has developed a hypersonic computational fluid dynamics code dubbed VULCAN-CFD, named after the Roman god of fire, which simulates the behavior of combustion in turbulent engine airflows at speeds under, super and hypersonic.
The software works by representing the burning fuel in massive, multi-dimensional arrays, where each input stores a single one-dimensional snapshot of the flame called “flamelets”.
The challenge with this approach, however, is that the sheer size of these datasets means that they require an enormous amount of computer memory to process.
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Argonne team member Dr. Sinan Demir said, “Working with NASA has given us the opportunity to integrate our new developments into state-of-the-art computational fluid dynamics code.”
This, he added, will help “further improve developments for more efficient design and optimization of hypersonic jets”.
Dr. Demir added, “The partnership between Argonne and NASA is valuable because our models and software can be effectively applied to theirs.”
“It’s a different way of doing high-speed propulsion computational fluid dynamics simulations.”
The full study results were presented at the American Institute of Aeronautics and Astronautics SciTech Forum and Expo earlier this year.