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Invention makes jet engines quieter

With nearly seven million passengers moving through Port Columbus International airport every year, the jarring sound of airplane noise may be painfully familiar. NASA expects U.S. air traffic to double or triple by 2025, a projection that has engineers working hard to silence the roar. Engineers at Ohio State University have applied for a patent on a technology called plasma actuators, that could reduce jet noise.

Mohammad Samimy and Igor Adamovich, both professors of Mechanical Engineering at Ohio State University, have an invention that could make jet exhaust quieter.

At takeoff, the exhaust from a jet engine is the most important source of airplane noise. The exhaust is loud because it's turbulent. Just like a whitewater river, the air leaving a jet engine forms disorganized eddies and pockets, and other structures that make a lot of racket.

"Some of those structures, dynamics of those structures, will produce noise and some dynamics will not," Samimy said. "When they are interacting with each other, breaking or merging, all of these processes in the jet generate noise."

Keep those turbulent air structures from interacting with each other, and the jet gets quieter.

"Whatever the dynamics of the structures in the flow that's generating noise-- what I need to do is to make these structures benign, to make them less active," he said.

Samimy already helped invent one way of making turbulent structures less active. He found that if the trailing edge of the jet engine has serrated edges, called chevrons, the exhaust is quieter. But that silencing comes at a cost. Even though noise is most severe only at takeoff and landing, chevrons are in place during the entire flight, draining about 1% of the plane's power. Exact figures are elusive, but this adds up to a lot of extra fuel.

So, after the invention of chevrons, Samimy kept thinking. He wanted a way to calm turbulence that wouldn't waste so much energy.

And so he and Adamovich conceived of plasma actuators. The actuators are a series of electrodes applied around the jet exhaust. And the flip of a switch turns them on and off. When they're on, they make electric arcs that ionize the gas around them, generating pockets of plasma in the jet exhaust.

But it's not the mere presence of plasma in the gas in the exhaust that calms the turbulence. The actuators have to turn on and off at a certain frequency, calculated for the specific jet. The exhaust has properties called instabilities, and when the actuator frequency matches the right instability, the silencing occurs.

"We are trying to take away energy from those structures that will radiate noise and give to those that will not generate noise," Samimy said.

Right now, Samimy operates the actuators on a scale model jet exhaust. He uses a compressor and a heater to force hot air through the exhaust, into a chamber.

Because the model jet is only one inch in diameter, the actuators alter its exhaust turbulence in a way that human ears can't hear. But Samimy says the process scales up. It's worked on a bigger model at NASA. When it's applied to a real exhaust, the jet will be noticeably quieter.

The OSU Office of Technology Licensing and Commercialization manages the technology, which is patent-pending.

Ed Envia, an expert on engine noise mitigation with NASA, says the most important place for the actuators would be as a corrective measure on so-called current generation engines the engine designs that are now in use.

"For the current generation engines jet noise is a significant component so applying plasma actuators to reduce jet noise would make sense," Envia said. "But to really achieve aggressive goals you really need to look into not just noise reduction technologies, but is there a way to change the engine architecture."

NASA is currently developing a new generation of engines that have a quieter exhaust to begin with. But in the mean time, with more and more planes taking off and landing, every little bit helps.

David Wall manages the Columbus Regional Airport Authority Noise Compatibility Program. He's excited to hear about any technology that could help keep peace and quiet for airport neighbors.

"We look forward to it-- anything we can do to lower the noise levels is great," he said.

He says the Airport Authority has spent 34 million dollars on noise abatement projects since 1987.