"Put a Nozzle on It," Feature Article, November 2006

put a nozzle on it

Teeth-like tabs are turning down the volume on jet cacophony.

by Michael Abrams, Contributing Editor

it's a noisy world we live in. Automobile traffic may be the biggest offender, but if you're tired of listening to the roar of the highway, you can always buy a house with a longer driveway.

The second largest contributor to noise pollution is harder to escape. Aircraft drown out conversation whenever they land and take off. At cruising altitude they're somewhat quieter—thanks only to the additional distance from the earth—but can be just as distracting for the more rural neighborhoods they pass over. "Dead zones" around airports remain undeveloped and people who already own homes near them have seen their property values plummet as air traffic increases. A recent report by the Federal Interagency Committee on Aviation Noise showed that high school students do significantly better on standardized tests when they hear less jet noise. Earplug sales are on the rise.

The earplug companies won't be glad to hear that, after years of research, something called a chevron nozzle is turning down the whine of airplanes by several notches. Instead of the smooth circle around the exhaust exit of the engine, the rim is serrated, or scalloped.

Most jet noise comes from the turbulence of the air directly behind a jet's engines. On internally mixed engines a lobed mixer can reduce the velocity of the exhaust, but the mixing itself causes high-frequency noise as bad as the low-frequency noise it eliminates. For separate flow exhausts this tradeoff isn't even an option. Where the core, high-velocity air stream meets the lower-velocity air coming out of the fan bypass duct, a layer of turbulence is created called the shear layer.

"At that intersection there will be a cylindrical sheet that's going downstream," said Dennis Huff, chief of the acoustics branch at the NASA Glenn Research Center in Cleveland, who spearheaded the development of the chevron nozzle. "The chevron nozzles create vortices in that sheet. They mix the two flow streams together more rapidly than if they were not there."

In essence, the serrated teeth of the nozzle make the shear layer disappear.

Take a bite out of noise pollution. Chevrons may be effective on the core nozzle, as is the case with the CF-34 engine on this America West CFJ, or there and on the bypass duct, too (below).

The idea of mixing jet exhaust for the purpose of noise reduction first appeared in the 1980s, when the U.S. Air Force started looking for ways to reduce fighters' infrared signature. NASA subsequently noticed that those same nozzles had an effect on noise emissions as well. That's when Huff began looking into the use of tabs and chevrons. At that time, researchers had no computer models to predict the results and had to go on "experimental intuition." They had little idea whether or not they would run into the same problem of a lobe mixer by trading low-frequency noise for high.

"To be honest, we were not quite sure we were really going to be successful with this," Huff said. "It was high risk."
But Huff found that by making the angle smaller by which the chevrons penetrate the exhaust stream, he could obtain a low-frequency noise reduction without the high-frequency penalty.

Loss of Thrust

There was another penalty, though, and a more prohibitive one: thrust loss. Initially, this loss was as great as five percent when noise was reduced by about 3 EPNdB. That's not a trade airlines would be clamoring to make. (EPN, or effective perceived noise, refers to the Federal Aviation Administration's method of measuring aircraft noise. The rules are spelled out in Federal Aviation Regulations Part 36.)

"We started to quantify that a little bit," said Huff, "If we could get something that gives you the noise reduction with less than a 0.25 percent thrust loss, we'd get enough people interested in applying it." Huff's team has recently made great strides in an experimental technique that has given them those prediction tools they once lacked. And they've managed to bring thrust loss down to 0.25.

PIV POV: Particle image velocimetry lets researchers take snapshots of jet turbulence in several planes, in whatever orientation they need. Pockets of amplified turbulence can be pinpointed and subsequently squelched.

In the past, it had been difficult to make probes that would survive the jet blast they were meant to measure. But their new particle image velocimetry system sends two sheets of light into a jet stream seeded with fine particles, while high-resolution cameras snap at a rate of 10 kilohertz (and they're about to roll out a new system that captures images at 50 kHz). The sheets of light can be oriented perpendicular or parallel to the flow or in any other direction.

"Typically, you look for where you think the interesting flow field is and then you zoom in on that with a finer resolution," said Huff. "You end up having to take a fairly large survey because a lot of times the noise is being generated 12 or 15 nozzle diameters downstream in the plume." With PIV they were able to find exactly where pockets of turbulence build up. Those hot spots—of turbulence reinforcing turbulence—act as speakers, amplifying and broadcasting the broadband noise to the world. They could now target them by refining the teeth of the nozzle to get down to that 0.25 percent thrust loss.

Wings and Chevrons

In theory, though, chevrons shouldn't hinder thrust at all—they should actually help performance. The trailing edge of a bird's wing is serrated with the tips of its feathers not for the sake of noise reduction (though one guesses a peregrine falcon doesn't want its pigeon prey to hear it stoop), but to achieve laminar flow and lower drag. The difference between a bird's wing and a chevron nozzle is that the bird's wing can change in midflight. That difference, though, may soon be eliminated as the newest generation of chevrons is being made to morph.

Using shape memory alloys, the chevrons will be able to bend in and out of the flow exactly as needed. Activated by temperature, the tabs can be told when to change shape based on the atmospheric conditions at takeoff and cruise altitude. Already they've been tested on a GE-90 engine on a Boeing 777, the first commercial application of a morphing structure.

But even with that pre-morphing thrust loss, many airlines are signing on. Canadair Regional Jets that use a CF-34 engine already use a chevron nozzle as will new 787s. "As we go traveling around the country, we see these parked at the airports and that is exciting," Huff said. It's exciting to see them, but it's even more exciting not to hear them.