The Holy Grail of Aircraft Noise Reduction

As the US economy recovers from the economic downturn of 2008, aviation and air passenger demands continue their slow and steady growth. The Federal Aviation Administration (FAA) forecasts an average growth in US carrier passenger flights of 2.2% per year between 2013 and 2033, with slightly higher than average growth in the first five years. As takeoff and landing cycles increase, so do noise complaints in communities around US airports. The number one issue reported to FAA public liaisons is aircraft noise.

"With growth comes adverse effects," says Mehdi Khorrami, who leads NASA's effort to reduce airframe noise emissions within the Environmentally Responsible Aviation (ERA) project under the Aeronautics Research Mission Directorate. "To maintain the role of the air transportation system within the US and global economies, we face an enormous challenge."

During landing, noise generated by a commercial aircraft airframe is as loud, or even louder, than propulsion noise. The primary sources of airframe noise are the landing gear and high-lift devices such as wing flaps and slats. To successfully mitigate airframe noise, says Khorrami, contributions from all major sources must be reduced concurrently. NASA is using large-scale, high-fidelity simulations to develop and evaluate novel noise reduction concepts.

These simulations, performed on the Pleiades supercomputer, model the complex interactions between airflow and airframe components that generate noise. "Extracting every possible decibel out of a noise mitigation concept is an extremely challenging process," says Khorrami. "When you reduce noise by three or four decibels, effectively you've cut it in half."

NASA's aircraft noise reduction goal for next generation civil transports, which will enter into service between 2020 and 2025, is 32 decibels below the current FAA Stage 4 standard. For aircraft that enter service between 2025 and 2030, the target is 42 decibels below Stage 4. "But the holy grail," says Khorrami, "is Stage 4 minus 71 decibels. When you get to that stage, you've essentially confined aircraft noise to within airport boundaries."

NASA has already partnered with Gulfstream on airframe noise research simulations involving wing flaps, and more recently added the main landing gear components to the same 18% scale, semi-span model aircraft. "As the incoming airflow interacts with wing flaps and landing gear, it creates turbulent vortex filaments that eventually merge and become stronger," explains Khorrami. The filaments impinge on the surrounding solid surfaces, creating pressure fluctuations that become the noise we hear.

Using the Gulfstream model, NASA simulated the noise sources associated with deployed landing gear, flaps, and interactions between the two. The simulations focused on capturing the dominant noise-generating mechanisms at the flap tips and main landing gear for the baseline configuration. "We've been able to evaluate the effectiveness of several viable noise-reduction technologies applied to the flap tips and main landing gear," says Khorrami.

One such technology, Flap Edge Noise Reduction Fins (FENoReFins), reduces the noise generated at the flap tips by decreasing the amplitude of the pressure fluctuations that result in noise. "We've already assessed the effectiveness of this concept, and have validated it via simulation before wind tunnel testing. This is a first," Khorrami notes. "It is [an] extremely complex geometry, and we wanted to simulate the airflow as it makes its way around thousands of those little fins."

"Because of the extreme geometric complexity, between two and five billion cells were used to define the space surrounding the aircraft surfaces," says Khorrami. The time-dependent simulations were performed using the PowerFLOW Lattice-Boltzmann flow solver. The calculations used several thousand processors on Pleiades for weeks, and sometimes months, at a time. These efforts represent a major step toward predicting the radiated noise field from a full aircraft in landing configuration. -- by Amber Harmon, © i SGTW