Computational fluid dynamics (CFD) simulations have been used in the aerospace industry for decades to create lightweight designs, improve aerodynamics, reduce friction during high velocity scenarios like reentry and much more. But the experts at NASA and their partners are always pushing the envelope to see how CFD can expand state-of-the-art technology and even help us explore further into the solar system.
Here is a look at three unique examples—some big and some small—all of which make meaningful change in the aerospace industry using CFD.
Big Ass Fans in Space
What works on Earth is not guaranteed to work in space, and that includes airflow models. The Kentucky fan maker, Big Ass Fans, uses CFD models in Ansys to perfect its blade shape and fan designs, as well as simulate airflow in its clients’ spaces.
But when the company had the chance to test its fans and models in a NASA environment, it jumped at the opportunity. In 2019 Big Ass Fans launched a scaled-down study that ran fans on the International Space Station (ISS). The goal was to test whether CFD simulations of airflow were impacted by microgravity, or if these simulations could accurately be used for airflow conditions on future space flight missions.
“Our ultimate goal in the project started out as a call to action within the company,” says Mike Smith, prototype engineer on the project, to engineering.com. “What came out of it was, let us do a CFD experiment with Fanny [the company’s mascot] in space and collect the data and then run that against our CFD simulations. We compared and contrasted the data that we collected in microgravity at the space station versus how we would see it calculated using our traditional CFD methods.”
Smith handled the design and placement of the project’s wind tunnel and assembly of the prototype, and chose how to mount and locate the sensors in the 1-foot-by-1-foot container. The team did not expect microgravity to cause substantially different airflows because the container was a sealed and controlled box, but were excited to launch the study to find this out.
“It came out like we expected, but the exciting aspect of proving that we had the capability, whether it was here on Earth or in space, to be able to give accurate simulations was a big part of it for us,” Smith said.
This data and results were given to Space Tango, the company that helped package and integrate this study for launch, as well as others in the microgravity research community. Having a better understanding of the accuracy of airflow CFD simulations in space will benefit the scientific community for years to come.
While Smith says Big Ass Fans is not planning to launch any more fans to space anytime soon, he is still seeing the company’s CFD simulations make an impact in crucial space facilities back here on the ground.
“Terrestrially here on Earth, a lot of the NASA facilities and a lot of Air Force facilities do have our products on site,” Smith said. “So, while we may not be sending one of our fans directly to space, we are helping get things to space.”
Monitoring Fuel Slosh
Spacecraft must carry fuel for launch and their long journey ahead. This means that engineers have fought the forces caused by fluid motion since the early days of space travel. Until very recently, the full impact of that propellant slosh in low gravity situations was not well understood, meaning increased uncertainty and risk for payloads.
The SPHERES-Slosh study conducted aboard the ISS helped change that, providing the groundwork to propel new space slosh research.
“Before we launched this, there really wasn’t any data of low gravity slosh like this for the purposes of validating CFD,” says Brandon Marsell, fluids and CFD engineer at the Launch Services Program of the NASA Kennedy Space Center, to engineering.com. “That is what we built this for, and so getting that data was a really big deal. It is the only set of data in the world for this.”
After seeing launches from international partners delayed for slosh issues, a team led by principal investigator Paul Schallhorn in NASA’s Launch Services Program saw the need to verify that their microgravity CFD models were accurate. His group teamed up with the Florida Institute of Technology and Massachusetts Institute of Technology to provide a slosh dataset for the whole industry.
The setup involved a tank that was attached to a Synchronized Position Hold Engage Reorient Experimental Satellites (SPHERES) within the station. These free-floating robots are instrumented platforms that were attached to the experiment’s clear tanks, which contained green water.
Teams on the ground ran simulations for how they expected the fluid to perform under specific forces in OpenFOAM, STAR-CCM+ and other CFD software. The SPHERES then moved to create these forces. Astronauts were asked to move the tanks at faster speeds as well. While the CFD simulations made by the team matched adequately in some scenarios, the experiments showcased where the models based on existing data fell short.
“It wasn’t terrible. Often, especially if we could get a good initial condition, they would track fairly well at the beginning,” says Jacob Roth, SPHERES-Slosh study coinvestigator, to engineering.com. “It is a standard situation where you have a ton of forces that are small but build up over time and things start to deviate. And then there are some things that CFD simply could not capture.”
Those things that could not be reflected in models included the interactions of air bubbles and even some surface tension interactions in microgravity.
The data collected from this experiment is already being used in slosh calculations for current spacecraft. Marsell was working on the Magnetospheric Multiscale (MMS) Mission, which had strict requirements and small margins.
“We ended up using this CFD code that was validated by the ISS data and coupled it to a controls code,” Marsell says. “We ran the mission that way to get more accurate numbers on what sort of delta-v we can expect at payload separation.”
In the future, as refueling stations are set up in space for spaceships and satellites, this knowledge will be even more important. Fluids and our understanding of them are what will get us to the Moon, Mars and beyond.
CFD Studies to Go Back to the Moon
NASA plans to return to the Moon in the next few years by launching humans aboard the Space Launch System (SLS) rocket for the first time. The Artemis II mission, planned to launch in late 2024 to orbit the Moon, will have a crew of four: NASA astronauts Reid Wiseman, Victor Glover, and Christina Hammock Koch, and Canadian Space Agency astronaut Jeremy Hansen.
Putting people on board a new rocket comes with a certain degree of risk, but teams are using CFD tools to make things a bit safer.
SLS has two solid rocket boosters mounted on its side for the initial liftoff. Around 2 minutes into flight, they separate and fly away from the rocket thanks to 16 separation motors. This period of flight is extremely hard to model but is a crucial time to examine to ensure these boosters do not hit the main core of the rocket.
Engineers are using two main tools to simulate this period of flight: FUN3D and OVERFLOW.
FUN3D, or Fully Unstructured Nacier-Stokes, is a custom CFD suite of fluid flow modeling tools developed at NASA for use in aeronautics, space, technology and exploration. OVERFLOW is a NASA-developed CFD flow solver. All CFD simulations for this project were run on the Pleiades and Electra supercomputers at the NASA Advanced Supercomputing facility. See a magnetic field simulation that was also run on the Pleiades supercomputer here.
According to NASA, the aerodynamic data for these booster simulations is “a function of 13 independent variables: six describing position (three for translation and three for rotation), four describing free-stream conditions (angles of attack/sideslip, Mach number and air density) and three describing thrust conditions (core, booster, and booster separation motors).”
“Ultimately, the results of these simulations will be critical in reducing the risk to the crew of Artemis II, the mission’s first crewed flight test, during booster separation,” said Jamie Meeroff, acting deputy chief of NASA’s Computational Aerosciences Branch at Ames Research Center, in an article on NASA.gov.
This information feeds into the creation of a database that can then be used by the Guidance, Navigation and Control team at Marshall Space Flight Center.