NASA Experts Break Ground in Simulations for Urban Air Mobility Safety
Researchers in the NASA Advanced Supercomputing (NAS) Division have developed new simulations that incorporate ground effect into their studies on the aerodynamic flow around urban air mobility vehicles (UAMs). The ground effect—where the ground surface is added to the computational model—is an important factor in making sure that real-world products are safe and efficient when released by industry manufacturers in the future.
The simulations play a key role in NASA collaborations supporting development of the UAM market, which is part of the Federal Aviation Administration’s Advanced Air Mobility (AAM) concept for a new era of air transportation to operate safely and efficiently in urban and rural environments.
In the team’s most recent study, two generic quadcopter design simulations that incorporate ground effect were analyzed: the first vehicle configuration is designed with all four rotors mounted at the same height; the second has the rear rotors offset and mounted higher than the front rotors. Through these simulations, the NAS team was able to identify the “cautionary” areas around the vehicle where it is too dangerous for people to stand due to high winds and large gusts created by the whirling rotors. Their simulations showed that the cautionary region in front of the second vehicle is wider, but the wind speeds are lower, suggesting that its design affects the flow patterns in between the rotors.
“Since this relatively small design adjustment to the rotor height causes such a significant change in the cautionary region, we expect a completely different topology of the flow for other, non-quadrotor concepts,” said Denis-Gabriel Caprace, a Fellow of the NASA Postdoctoral Program whose project focuses on ground effect. “Currently, the strength of the outwash—that is, the wind produced by the spinning rotors—is usually not accounted for when designing a new vehicle, but it could later impose stringent limitations on its operation. This is why it’s crucial to develop outwash modeling capabilities so that ground effect safety can be factored in as soon as possible in the design process,” he explained.
Where the Rotor Wake Meets the Ground
Incorporating a ground surface makes simulations more challenging, according to Patricia Ventura Diaz, an aerospace engineer in the NAS Division’s Computational Physics Branch, which focuses on developing models, numerical methods, and simulation tools that support NASA programs. “It’s very exciting to work on new concept vehicles that do not follow the conventional structure of helicopters and to study the aerodynamics and behavior for different rotor configurations,” she said.
“When the wake from the rotors reaches the ground, it generates the outwash region, which needs to be captured correctly by having a refined grid size to accurately calculate the behavior of the flow in this region and around the vehicle,” said Ventura Diaz. The vehicles, ground, and surrounding air were modeled using hundreds of millions of grid points. Running on the NAS facility’s Pleiades, Electra, and Aitken supercomputers took about 40 days on 2,000 processors to complete a just a single-rotor case.
Simulation results come to life with help from the NAS Division’s visualization and data analysis experts, who turn the researchers’ computational results into images and videos that depict the complex flow patterns. “Combining visualizations with numerical diagnostics allows us to better understand and predict the interactions between the rotor blades, vehicle, and surroundings,” said Caprace.
Getting Ready for Vertiports
Another important aspect of the team’s ground effect studies supports the FAA’s design standards on vertiports—facilities that will support operations of electrically powered AAM aircraft that take off and land vertically. While some vertiports will be built on the ground, others will be located on building rooftops or using terraces on their sides.
“There are a lot of possible vertiport configurations, and each of them will generate site-specific aerodynamic interactions,” said Ventura Diaz. “So far, we have compared a flat ground case and a rooftop case. With the latter, we looked at what happens when the vehicle enters in ground effect from the side. Our simulations confirmed that the flight dynamics during takeoff and landing at the vertiport, with the ground effect included, are hard to predict. The vehicle will experience large variations in aerodynamic forces and moments which could cause a risk for safety. It is important to take that into account when designing the departure and arrival procedures for a given site,” she said.
The ground effect studies are a continuation of the NAS Computational Physics Branch work on AAM vehicle aerodynamics using high-fidelity CFD for a variety of concept vehicles over the past several years. During that time, they have studied designs for drones and air taxis that will deliver people and goods across cities. The team continues working to help ensure that the vehicles are safe, quiet, and efficient—and look forward to the day when their contributions come to fruition in real-world UAMs that fly the skies.
—Jill Dunbar, NASA Ames Research Center
