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Visualizing the flow of noise

Noise from airplanes flying overhead may sound like a mild distraction. But for people living under airport flight paths, aircraft noise can be a continuous torment.

This image shows the unsteady vorticity - tendency for elements of a fluid to spin - field surrounding the nose gear of a Gulfstream 6550 jet, showing where  flow structures are created and how they interact with the gear subcomponents. Insert shows corresponding two-dimensional perspective of moving plane.Time steps used in simulation are five microseconds (millionth of a second). In the movie, approximately 0.0363 seconds of real time data is shown in 60 seconds of animation.Image courtesy NASA.

NASA scientists working towards a solution for these communities have identified the fluid mechanics of this sound problem by using the recently announced seventh fastest high-performance computer in the world, the Pleiades. This high-performance computer is comprised of 84,992 cores and is based at the NASA Advanced Supercomputing (NAS) Division of NASA Ames Research Center in California, USA.

Aerospace technologist Mehdi Khorrami, the lead project investigator at NASA Langley Research Center in Virginia, and colleagues Veer Vatsa and David Lockard simulated the complex pressure fluctuations of sound on aircraft landing gear, underside fuselages and ground surfaces. NASA’s high-performance computer provided, “high speed and reliable interconnectivity for efficient communications between the processors,” said Khorrami.

The scientists’ calculations were run simultaneously on 1,200 cores and took 1.7 million processor hours - or two months - to complete. The computer itself has over 70 times more cores than what was used by the researchers, but they determined that 1,200 was the optimal number needed for efficient completion of the task.

The data volumes involved were not trivial either. “We used 64 TBs of data to generate the animations and we collected 225 TBs for performing full analysis,” Khorrami said.

All to do with gear noise

“Aerodynamic sound is generated by unsteady motion of gas and its interaction with surrounding surfaces. These interactions produce minute disturbances in the local pressure field (waves) that propagate outward from the source. It is well known that, in addition to engine noise, sound generated by the airframe constitutes a major component of total aircraft noise during approach and landing,” the researchers wrote in Scientific Computing magazine.

Khorrami and the team were able to determine the hot spots of intense pressure fluctuations or loudest noises on an aircraft and the causes of these noises were visualized by complex flow interactions. They found that the loudest noise sources were caused when upstream landing gear components generated complex flow structures that interacted with each other. These interactions were then convected or transferred downstream and impacted other gear components. 

Now, these simulations will be incorporated with experimental flight test and wind tunnel data. “The current landing gear simulations serve two distinct purposes … further advancement of state-of-the-art airframe noise prediction methodologies and allowing us to gain a fundamental understanding of airframe noise generation mechanisms to develop viable concepts to mitigate noise.

“Simulations could be used to come up with a modified gear design that strategically repositions some of the gear subcomponents in order to mitigate most of the flow interaction that occurs,” said Khorrami.

This means that future designs of subsonic aircraft will be quieter during landing and takeoff, especially for those living underneath their wake.

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