Breaking Down Barriers for the Future of Spacecraft

As space vehicles reach hypersonic speeds, five times the speed of sound, how can we protect them from the extreme heat and instability they experience?

The issue centers around the boundary layer of these vehicles, a thin layer of fluid like air or water that forms around a moving object. When a boundary layer transitions from a laminar, or smooth, predictable flow to a turbulent and chaotic flow, heat and drag increase at an extremely high rate. The problem is so difficult to solve that most engineers opt to design around it. Even major programs at NASA and other agencies use an empirical correlation to guess where the transition is going to occur. They then apply a factor of safety, a "fudge factor" to make sure each part is stronger than it needs to be in order to ensure mission success.

PhD student Carter Vu’s research could help solve the complicated problem.

“I'm hoping to build cheaper boundary layer transition models that can capture 80% of the physics at 20% of the cost, which would hopefully make this something that even entry-level engineers can simulate, rather than just designing around it,” says Vu.

Vu is using the latest high-fidelity methods in computational fluid dynamics to simulate all the length scales involved in the problem of these complex flight vehicles. He aims to compare his findings against flight data to show that these simulations are capable of handling these tough problems and bring these academic techniques to the aerospace industry.

“The key obstacle is the expense of these simulations. They're very complicated and they require a lot of computing resources to run. Computing resources are becoming more widely available, and these high performance supercomputers continue to get faster and faster,” explained Vu. “It makes doing my type of simulations much easier and more affordable, not just for academics, but for everyone in the aerospace world to be able to contribute to these complicated problems.”

On the other hand, Vu is also hoping that he can start using less expensive models instead of using complicated direct numerical simulations, which resolve every single tiny feature in the flow field. Vu and the rest of his research team are looking at equations to capture the bulk of the problem at a substantially reduced cost. Beyond that, Vu wants to build better empirical correlations that are more straightforward, allowing engineers to take a very simple boundary layer profile inside of a wind tunnel and find characteristics that inform where the flow is transitioning. In short, making the problem easier to approach.

“If we can build something that's better than what's out there already, we can make this problem much more accessible and much easier for everyone to solve,” says Vu.

So far, Vu has found that they can reproduce flight results using these methods and computing capabilities. Although it still requires a lot of expense, time, and expertise, Vu and his research group are one step closer to solving the puzzle. Upon completion of his PhD, Vu has his mind set on working at a national laboratory. 

“I would love to go somewhere where I can still do research and solve hard problems, but at a more applied level,” said Vu. 

Whatever he decides to pursue, Vu is sure to bring with him the skills he’s learned from dealing with one of the most complicated problems in modern aerospace history.