Vol. 38, No. 18

2-27-2007
By: Adam King

'Shhh...' A flip of switch may one day quiet jet engines

Just as Mohammed Samimy rears back to serve at a local tennis court, the loud roar of a jet engine flying low disrupts his concentration.

The noise is annoying, but it may go away some day thanks to research by Samimy, a mechanical engineering professor at the Gas Dynamics and Turbulence Laboratory. He is developing technology that greatly mutes engine noise without adversely affecting an airplane’s power and fuel efficiency.

Samimy is already four years into his project, but his progress recently convinced NASA and the Air Force Office of Scientific Research to fund his work for another four years with two separate grants totaling $1.5 million.

“Current engines won’t satisfy environmental requirements even five years down the road, so we have to come up with some new technology,” he said.

The work of Samimy and his colleague, mechanical engineering Professor Igor Adamovich, centers on the use of silencer technology called plasma actuators.

The actuators, installed around the exit of the jet engine exhaust, dramatically change the airflow that passes through the exhaust using an electrical charge.

The engineers use eight actuators for lab testing on an engine 1/40th the scale of a 747’s. An actual jet engine will take about 200 or so actuators, depending on the engine’s size, Samimy said.

Exhaust air is what gives a jet its thrust, so it is loudest on takeoff. But Samimy’s actuators can be turned on or off by a pilot so a plane can be muted when passing over residential areas at the beginning and end of a flight.

“For the first 40 years of jet engines, to reduce the noise, you would have to reduce velocity,” Samimy said. “But you can’t drop beyond a certain velocity or you won’t get off the ground. So we’ve gone to larger engines, which have been the primary source of dropping noise because they drop velocity without losing momentum.”

The largest engines are already 9 feet tall and can’t go any bigger because current aircraft won’t support anything larger. The solution required different thinking, Samimy said.

In 1992, Samimy worked with a group of NASA researchers on developing chevrons, zigzag-shaped cutouts placed at the exhaust’s nozzle exit.

The chevrons have been used the last three years on engines, and while they help cut noise, they also lower fuel efficiency and performance. And because they’re permanent parts of the engine, they can’t be disengaged during flight when noise mitigation is no longer needed — not ideal for long distances, especially intercontinental travel.
The plasma actuators fix that problem, and the technology has led to advances in other areas.

When actuators are fired, they disperse an engine’s heat much faster, meaning military jets could be less visible on infrared radar and become less of a target to a heat-seeking missile. Samimy and electrical engineering Professor Andrea Serrani also are looking at how actuators can be applied to control the turbulence in military aircraft weapons bays.

For the next two years, Samimy, Adamovich and their staff will work to further develop and optimize the actuators’ frequency so they can apply their technology to larger engines.

“You want your idea to be as applicable as possible to as many practical applications as possible,” Samimy said. “It’s almost like hitting a jackpot — an excellent idea with lots of interest.”

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