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NASA is using Exa's software to simulate full-scale aircraft acoustics. Image shows the radiated sound field from a business jet with flaps and main landing gear deployed.

NASA embraces CFD to replace cancelled flight tests

NASA has expanded its use of Exa's PowerFLOW software for the simulation of concepts to reduce aircraft noise within NASA's Environmentally Responsible Aviation (ERA) project.

“The goal of NASA’s ERA project is to reduce the noise footprint of aircraft during landing,” said Dr. Swen Noelting, Vice President, Aerospace, Exa Corp. in an email to Aerospace Engineering. “With engine jets having become much quieter over the last two decades, the noise from high-lift wings and landing gears (generally called ‘airframe noise’) has become the main contributor to noise heard on the ground.”

The reduction of noise generated by landing gear and high-lift wings is critical to meet the lower community noise targets of the future.

PowerFLOW predictions were successfully validated against high-fidelity wind-tunnel tests in 2013, and now, after the cancellation of a planned flight test due to budget constraints, NASA is using PowerFLOW simulations of a full-scale aircraft with flap and landing gear noise treatments to reach the original airframe noise objectives of the project.

“The simulations of the full-scale aircraft with noise-reduction concepts will help recover a significant part of the loss of the flight test. In addition, the simulations provide valuable insight into the sources of airframe noise," said Mehdi Khorrami, Senior Research Engineer at NASA Langley Research Center.

“The simulations capture every relevant detail of the fuselage, wing with extended flaps, and landing gears of the full-scale business jet, down to components of less than 1 mm in size on an aircraft with 28-m wingspan,” said Noelting.

Using simulation to test realistic noise-reduction concepts is an extremely challenging task for CFD, Noelting added. It is difficult to accurately predict the noise heard on the ground. The entire aircraft has to be simulated because of complex interactions between landing gear, wings, and other parts of the aircraft. In addition, the highly complex geometry of the landing gear has to be represented exactly in the simulation since even small geometry details can have significant impact on community noise.

"With these simulations—some of which are running on more than 10,000 cores—we are truly pushing the boundaries of CFD for industrial applications," said Noelting. “These are unsteady simulations, unlike typical CFD simulations that produce only steady-state (averaged) flow results. This enables us to directly capture flow fluctuations that are the cause of noise.

“From the fluctuating flow close to the aircraft and the resulting noise, we can calculate the noise heard on the ground from an airplane flying hundreds of meters overhead, enabling us to assess the impact and efficiency of noise-reduction concepts mounted on the wing and landing gear.”

The accuracy of the previous simulations during an extensive wind tunnel campaign at NASA Langley occurred during "a completely blind test (in fact, the simulations were carried out many months before the wind tunnel test)," said Noelting.

“The simulations use up to 8 billion cells—a world record for commercial CFD tools (to the best of my knowledge),” said Noelting. “They are being executed on NASA’s Pleiades Cluster (currently number 11 on the top 500 list of the world’s fastest computers) on up to 18,000 cores. PowerFLOW is the only software (commercial or otherwise) that can execute productive, industrial simulations on a cluster of this size despite of claims from other codes to have run on 30,000 cores. These are clearly one-off tests and not productive simulations producing real results.”

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