Tooling Up for Cleaner Skies, Quieter Aircraft
Let's see a show of hands: Who wants to breathe in more jet fumes and hear the thunder and boom of planes taking off and landing?
What, no raised hands? No surprise there. Everyone who has spent any amount of time near an airport puts up with the sound of airplane noise, but may brush it off as just an everyday annoyance. Yet studies abound with evidence that exposure to such noise leads to increased stress, possible hearing loss, and other negative health effects. To create cleaner, quieter aircraft now and in the future, NASA is working with aviation industry partners to develop green aviation tools and technologies that will improve fuel efficiency, reduce harmful carbon emissions, and meet noise restriction standards for people living around airports and for passengers in airplane cabins.
One concept being studied under NASA's Environmentally Responsible Aviation and Advanced Air Transport Technology (AATT) projects is the contra-rotating open rotor propulsion system, which has two ultra-thin blades spinning in opposite directions on the same shaft—picture two rows of blades on a giant kitchen blender. These contra-rotating blades rotate around the outside of a turbofan jet engine. Some advantages of this design include more efficient airflow through the turbofan blades to improve flight performance and reduce carbon emissions. But like blenders, the blade rotations are noisy.
To support the AATT project's goal to develop feasible hybrid gas-electric propulsion systems for commercial aircraft, aeronautics researchers at the NASA Advanced Supercomputing (NAS) facility at Ames Research Center have produced unique aeroacoustic simulations using their in-house Launch Ascent and Vehicle Aerodynamics (LAVA) software framework to reliably predict noise sources for contra-rotating open rotors. The simulations and results generated by LAVA are helping the AATT project assess the agency’s modeling and simulation tools to support the design and development of new propulsion systems.
"We took on the challenge of simulating the complex aerodynamic flows in the open rotor design with LAVA to see if we could help the aeronautics community better understand the noise-generation process," said Cetin Kiris, chief of the NAS Division's Computational Aerosciences Branch and leader of the LAVA development team.
Kiris explained that one of LAVA's key features is a Cartesian immersed boundary flow solver that takes advantage of traditional computer-aided design (CAD) files, giving NASA scientists the ability to bypass the tedious grid-generation process and start their number crunching right away on their open-rotor (and similar) computational problems.
"One of our most important findings is that acoustic tones predicted by the Cartesian immersed boundary and traditional body-fitted methods compared well with wind tunnel experiments," Kiris said.
While similar CFD tools have been developed in the past, this is the first time the so-called higher-order immersed boundary method has been used to predict noise generated by an open rotor system.
To verify the accuracy of the LAVA CFD methods, the NAS team compared their simulated sound pressure level ranges with extensive wind tunnel test data from Glenn Research Center in Cleveland and from GE. The LAVA results from two different computational grid models matched closely with the test results for individual tones produced when the rotor blades rotate up to 6,848 RPM—humans can hear frequencies up to 20,000 RPM.
These highly accurate, detailed simulations are made possible through the use of the supercomputing resources and visualization services provided at the NAS facility.
The LAVA team ran low- and high-speed open-rotor aeroacoustic simulations around the clock for three weeks on NASA's most powerful supercomputer, Pleiades. The 350-million grid point simulations required between 500 and 2,000 processor cores. The 211,872-core Pleiades system, which runs at a sustained performance of 4.09 petaflops (just over 4 quadrillion floating-point operations per second), is currently ranked as the 7th most powerful system in the U.S., and 13th in the world on the TOP500 list.
While results from the LAVA immersed boundary layer method keep NASA moving forward in understanding the noise-generation process of contra-rotating systems, the analysis still requires a massive amount of computing power and time. The team is working to improve LAVA's algorithms and optimization methods to speed up the simulation and analysis process and cut down on computing resources needed.
"Due to the transient nature of the flows, these problems are more complex and computationally challenging than ever before,” Kiris said. “Our work is not done yet."
— Jill Dunbar, NASA Ames Research Center
