BU engineers, including (from left) undergraduate researcher Reka Sundem (ENG’27) and Associate Professor Sheryl Grace, are studying how water droplets break apart—and what that might mean for vehicles traveling at supersonic and hypersonic speeds.

High-Speed Vehicles

Tiny Water Droplets Can Ground Billion-Dollar Supersonic Vehicles. BU Engineers Are Working to Change That

Research into the physics of fluids could help keep future space and commercial high-speed flights in the air

Rockets, regularly blasted into space by companies such as SpaceX and agencies including NASA, are meticulously designed to withstand the harshest of environments. Engineers construct the bodies from heavy-duty aluminum-lithium and stainless steel alloys, but the equipment can be devastated by something as minuscule as pinpricks of water; as spacecraft sail through the atmosphere at supersonic speeds, a single droplet can hit with more force than a bullet, cutting directly through the metal or pitting it until it resembles the surface of the moon. 

“The higher the speed of the vehicle is, the worse damage you can get from a drop,” explains Sheryl Grace, a Boston University College of Engineering associate professor of mechanical engineering. “It can go right through the vehicle.”

The potential for that damage is estimated using complex computational models, which help certify supersonic vehicles for safe flight. But Grace and a group of colleagues started to suspect that the models were missing something essential: a phenomenon that helps explain how and when a drop breaks up, which can influence the force exerted on an object and how much damage it can inflict.

Those working in the field aim for their models to be as accurate as possible, in part because building—and repairing—these vehicles is expensive, says Grace. Her team is hunting for solutions, using supercomputers to model the drops and their explosive impact.

The BU team’s work “will help improve safety for space reentry vehicles and other high-speed vehicles,” says Rebecca Shannon (ENG’27), a PhD student in Grace’s lab. “The potentially catastrophic damage caused by atmospheric water drops could be minimized or avoided entirely.”

Bubbles, Droplets, and Shock Waves

The physical process they’ve homed in on is called cavitation, where the tiny bubbles—technically, trapped gas regions on the surface of a particle—that a water droplet forms around contract and expand like balloons amid different pressures.

“Cavitation has been modeled for more than a century,” says Grace, “but not as a potential breakup driver inside of a drop.”

Cavitation events can potentially increase the speed at which droplets break apart before and during their encounter with the shock waves emitted by vehicles traveling at extremely high speeds. The specific impact of the shock and whether cavitation plays a factor in droplets shattering in the air—and if so, how that happens—helps determine the force with which droplets splatter onto an airborne vehicle, says Grace.

But, adds undergraduate researcher Reka Sundem (ENG’27), who worked with Grace this summer as part of BU’s Undergraduate Research Opportunities Program, “We don’t understand fully how that broken-up droplet interacts with the vehicle.”

Past modeling of droplet disintegration has largely focused on surface modifications—how the outside of the liquid interacts with what surrounds it. To get inside the droplets coursing through our atmosphere, Grace’s team relies on computer models. Sundem used a supercomputer based at BU’s Shared Computing Cluster in Holyoke, Mass.—which is linked up to the BU campus using a pair of 10 gigabit ethernet connections—to run her experiments. But even with all that computing power, the basic elements of modeling these dynamics, including multiple fluids, different sized scales between bubbles and drops, and massive jumps in pressure, in a form that a computer can understand and solve, remains a serious challenge.

“While all computational fluids software aims to predict the same physical reality, different numerical approaches used by the developers can yield different results,” says Grace. “To develop a computational method that’s going to capture all of the physics and run on a supercomputer in our lifetime, it’s a hard thing.”

The team is still trying to find the best model to accurately simulate the interaction between the shock wave and the droplets. For years, Shannon has chipped away at modifying and testing one version; Sundem has been experimenting with a different model to see if it might be a better fit. Comparing results of the different computations, says Grace, can show “whether the predictions of droplet breakup and cavitation are robust findings or artifacts of particular numerical choices.”

From Aerodynamics Class to Undergraduate Research

Now in her fourth and final year at BU, Sundem jumped into the project after one semester in Grace’s aerodynamics class. She began with little experience in computational fluid dynamics, but says the trial and error has been a worthwhile challenge.

“When something I’m trying to run actually works, or the plot finally shows me what I want it to show me—that’s the most rewarding,” she says.

The experience will also inform her upcoming senior design project for mechanical engineering, which combines theory with hands-on manufacturing to optimize golf putter head performance. After graduation, Sundem envisions a career in improving existing technologies. She holds a particular interest in car racing, and hopes to focus on speedy vehicles that fly along a track rather than through the atmosphere.

Meanwhile, the researchers on Grace’s team will continue to concentrate on the fundamental physics of droplet breakup. Although vehicle design applications remain in the future, understanding the issue is an important first step, with the potential to shape billion-dollar commercial and government enterprises. A slate of private companies are interested in reintroducing supersonic air travel, and private rockets and satellites increasingly flood the skies. Government agencies could use the knowledge, too, for space flight and defense.

Usually, Grace says, the owners of these hypersonic and supersonic vehicles will choose not to fly if weather reports suggest a chance of hazardous conditions that could damage equipment. Reengineering aircraft to withstand these conditions could allow them to operate under more circumstances. “The issue is more the question of: How often are we not flying something that could fly because it’s going to get damaged?” she says.

But first, the team has to overcome the challenges of modeling the fundamental fluid physics. “We’re pushing limits here,” says Grace. “It’s a really interesting computational challenge.”

Grace’s research on cavitation and droplet breakup has been funded by the Air Force Office of Scientific Research. Shannon is supported by a National Defense Science and Engineering Graduate Fellowship.