Jason Rabinovitch Awarded $1.1 Million Office of Naval Research Grant to Predict High-Speed, Shock-Induced Droplet Breakup and Surface Damage
In collaboration with Cornell University, the assistant professor of mechanical engineering will combine fluid dynamics with structural analysis to develop a novel approach to understanding the mechanics, behavior and impact of rain droplets when they encounter vehicles traveling at or above supersonic speeds
Imagine the sound of raindrops tapping against the window as you drive through a rain cloud on a high mountain pass, or looking out at a storm as you fly across the country in a commercial airliner.
How worried do you feel? How much damage do you think each water droplet can cause?
Now imagine you are traveling faster than the speed of sound — “so fast,” explained Stevens Institute of Technology Assistant Professor of Mechanical Engineering Jason Rabinovitch, “that those little droplets could break through your windshield.”
When a high-speed vehicle collides with such atmospheric conditions, those seemingly innocuous droplets of liquid water suspended in the air can effectively turn into tiny projectiles, capable of eroding a vehicle’s outer coating, damaging its thermal protection system — even punching a hole straight through its surface. Damage caused by this interaction between extreme speed and adverse weather can cause a vehicle to perform poorly, veer off course or lead to catastrophic failure or explosion.
To better understand how droplets behave and impact high-speed vehicles, the U.S. Office of Naval Research has awarded Rabinovitch a $1,067,782 grant for his project, “Advancing Understanding of and Predictive Modeling Capabilities for High-Speed Shock-Induced Droplet Aerobreakup and Surface Damage.”
In partnership with Olivier Desjardins, a professor at Cornell University’s Sibley School of Mechanical and Aerospace Engineering, the four-year research project will leverage advanced computational fluid dynamics tools to explore both how droplets morph and break up when they encounter the shock wave that is produced by a vehicle traveling at or above supersonic speeds and the resulting surface damage to the vehicle they ultimately produce upon impact.
This research will contribute to the design and development of cost-effective, high-speed vehicles that are safer, faster and more capable of withstanding the unique conditions that occur when traveling far beyond the sound barrier.
Extreme conditions
One of the toughest challenges of modeling high-speed flight, said Rabinovitch, is figuring out what happens to a rain droplet after it passes through a bow shock, the curved shock wave of air pressure that forms in front of a high-speed vehicle when it travels faster than sound. Acting as a semi-invisible wall of air, this sudden rise in pressure can alter the size, shape and density of droplets before they collide with the vehicle, affecting how much and what kind of damage they will inflict.
“If a big droplet breaks up into smaller ones before it hits the surface, you’re less concerned because there's not much water left to hit the vehicle,” he explained. “But if it stays cohesive and just changes shape a little bit, then hits the vehicle, that could cause a lot of damage.”
The very high temperatures and pressures under which this interaction occurs may additionally affect how the droplet breaks up: namely, whether it remains a liquid or has started to turn into a gas.
“In these really extreme conditions, we don't necessarily even know which physics are the most relevant or most important yet,” Rabinovitch said. “So we want to make sure we can simulate all of these conditions.”
But to develop simulations capable of predicting how a droplet acts under such conditions, one must first decide where exactly a droplet begins and ends.
A sharp approach
In fluid dynamics, both air and water are considered fluids, but at different phases: water being a liquid, and air being a gas. Both are modeled in similar ways, but Rabinovitch believes that how the boundary, or interface, between these phases is represented in a computer simulation will influence the accuracy of the droplet break-up predictions.
“When modeling multiphase flows, what you do computationally at that interface right where you have the edge of the liquid droplet and it's touching the air is very challenging,” Rabinovitch said.
Most researchers use what’s called a diffuse interface method, representing the boundary between air and water as a gradient where one phase gradually transitions into the other.
“It kind of smooths the interface so you have mainly air, then a little bit more water, smoothly leading up all the way toward 100% water,” Rabinovitch explained.
Rabinovitch’s team, however, is taking a more unequivocal approach. Employing the sharp interface method, either a fluid is a liquid or a gas, but never both.
“It's a different numerical technique, so we expect to see different results than researchers using diffuse interface methods. This modeling approach is one of the biggest differences between what we're doing and what other people in the field are doing,” he said.
A unique tool set
Rabinovitch’s research will leverage an open-source computational fluid dynamics framework for applying this unique approach called NGA2.
Developed by co-PI Desjardins, NGA2 is capable of modeling the entire process of what happens before and after a droplet hits a high-speed vehicle.
“It's very rare to find a tool that can do both of those problems at the same time. Not only do we need to model a Mach 5 flow around a vehicle body, but we also need to model what is happening right at that liquid and air interface around the water droplet,” Rabinovitch said. “Through NGA2, we will be able to efficiently couple both the fluid side and the structural side all in one code.”
Desjardin’s framework allows the researchers to “hit the ground running,” Rabinovitch said, while enabling them to computationally investigate phenomena that are still too difficult to recreate experimentally.
Rabinovitch referenced Mechanical Engineering Professor Nicholaus Parziale’s experimental high-speed vehicle research in this regard.
“The work that Professor Parziale is doing is very cutting edge, and the data he’s collecting is one of a kind. It’s also super hard to do,” Rabinovitch said.
In contrast, Rabinovitch’s computational approach can not only model the experimental conditions investigated by Parziale: it can also explore new flight-relevant geometries or configurations with his simulations with just a few keystrokes.
Together the different approaches complement one another, Rabinovitch said, enabling his team to validate the accuracy of their computational results against experimental data like Parziale’s, while also pushing the investigative envelope beyond Earth-bound physical limitations to explore scenarios that are currently impossible to replicate in a lab.