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Sky taxis

Speed read
  • City streets clogged with pedestrians and cars can make speedy transport difficult
  • NASA Urban Air Mobility Grand Challenge hopes to solve congestion with air transport systems
  • Designing these vehicles for urban use is going to take a lot of computing power

There are few activities that drain your faith in humanity like driving through a crowded city. Whether its other cars cutting you off or jaywalking pedestrians staring at their phones, navigating a metropolitan area in a vehicle is stressful.

Wouldn’t it be nice if you could just fly above those packed streets and quickly reach your destination?

Side-by-side rotors. In this visualization of NASA’s side-by-side, intermeshing-rotor air taxi concept, researchers are working to understand complex air flow interactions using high-fidelity computational fluid dynamics (CFD) methods. Courtesy NASA’s Ames Research Center.

It might seem far-fetched, but it’s a serious topic of discussion for Patricia Ventura Díaz, an aerodynamics engineer at NASA Ames Research Center in Silicon Valley. We sat down with her to discuss the Urban Air Mobility (UAM) Grand Challenge, a NASA-driven mission to help create a system to utilize flying vehicles for city transport.

“UAM is envisioned to be a safe and efficient air transportation system,” says Ventura. “It would be for everything like small packages being delivered by drones to big passenger taxis. We will operate in cities and overpopulated areas.”

Bringing flying machines to densely-populated regions will be no small feat. If we want to do this right, researchers like Ventura will need some powerful tools.

Simulating the future  

To understand some of the challenges that may face an urban air transport system, Ventura uses computational fluid dynamics (CFD) to better understand airflows.

<strong>Complex interactions.</strong> Modified design of a complete DJI Phantom 3 quadcopter configuration in hover. The vortex wake is colored red for high vorticity and blue for low. Simulations reveal the complex motions of air due to interactions between the rotors and the airframe. Courtesy Patricia Ventura Diaz, NASA/Ames.“Airflows are very complex,” says Ventura. “Before our work, there was nothing to visualize what the vortex from the rotors looked like or how the interactions were working. Thanks to CFD, we're able to design much better vehicles and understand why and where we need to make changes.”

One major concern for UAM is the noise these flying machines would produce. For example, think about how loud a helicopter is, even when it’s far away.

<strong>Simulating rotor placement.</strong> Side-by-side concept rotorcraft for urban air mobility in forward flight. The intermeshing rotors can generate more thrust than two non-overlapping rotors with a more compact configuration. Courtesy Patricia Ventura Diaz, NASA/AMES. “You can hear a helicopter flying over you because of the tap-tap-tap-tap-tap – that noise is coming from blade-vortex interaction (BVI),” says Ventura. “At the end of each blade, at the tip, there’s a vortex. When that vortex hits the next blade, that’s what makes the noise. That's on the big helicopters with one rotor. But, with smaller drones, you can hear this mmmmmm. That's also coming from BVI, but from a much higher frequency on a much smaller blade.”

It might seem like a small detail, but acoustics in a city can have a huge impact on how the public perceives a new technology. Ventura explains that alterations to the vehicle’s design based on CFD simulations will be needed to keep the operating acoustics to within the acceptable level of background noise.

Big ideas, big computer

Modeling the airflow around a flying vehicle’s rotors is difficult to say the least, and requires a powerful computer to do the heavy lifting. Thankfully, Ventura can rely on NASA’s Pleiades and Electra supercomputers. With 240,640 total cores, 893 TB of memory, and just shy of 6 PetaFLOPS of performance, Pleiades is a force to be reckoned with. The environmentally-friendly Electra provides additional support.

<strong>Designing the future.</strong> Aerodynamics engineer Patricia Ventura Diaz uses NASA supercomputers to model the airflow around flying vehicle rotors, helping to bring a future of urban air mobility to reality. Courtesy NASA/Ames.“We typically use from one thousand to two thousand processors and the computations can take from two to eight days,” says Ventura. “That wouldn't even be possible on a powerful workstation—it would take months and might not even be successful. One of my last simulations was around 40 terabytes just to save a few revolutions to visualize the flow.” 

However, the obstacles presented by this kind of project aren’t stopping Ventura. In fact, they inspire her.

“I really enjoy working on challenging and very difficult problems,” says Ventura. “With airplanes and helicopters, the designs have been developed and well established over the 20th century. But for UAM, we're talking about completely new designs. We still don't know what design will be the best option. Is it going to be a quadcopter, a six copter, or an octorotor? A tilt-wing, a tilt-duct, or a vehicle with fixed open rotors? ”

While we may not know exactly what the future holds, it’s reassuring that scientists like Ventura are being given the resources they need to find ways to improve our lives. Flying chariots whizzing around a gleaming metropolis may have been a science fiction dream, but some hard work from smart people could make it a reality sooner than you think.

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