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Ben Brumfield


He Quieted Deafening Jets

Taming the world’s loudest manufactured noise through engineering and perseverance

Published: Wednesday, August 7, 2019 - 12:02

For decades, Krishan Ahuja tamed jet noise, for which the National Academy of Engineering elected him as a new member this year. Today, Ahuja is an esteemed researcher at the Georgia Institute of Technology, but he got his start more than 50 years ago as an engineering apprentice in Rolls Royce’s aero-engine division, eventually landing in its jet noise research department.

“In those days, if jets went over your house and you were outside, you’d feel like you needed to put your hands over your ears. Not today,” says Ahuja, who is a Regents Researcher at the Georgia Tech Research Institute (GTRI) and Regents Professor in Georgia Tech’s Daniel Guggenheim School of Aerospace Engineering.

Cyclists watching a jet soar overhead, circa 1960s. Credit: National Archives, Records of the Environmental Protection Agency.

Jingling bangles for FMEA

To tackle jet racket more than 40 years ago, engineers had to manipulate acoustics physics that were hardly understood. But Ahuja’s childhood had prepared him well for it, when hardship led him to hone the ability to connect sound with structure and motion in what he likes to call the real beginning of his career.

The family business in the women’s bangles industry in Firozabad, India, had failed, and Ahuja’s father left for Calcutta to make money for the family. Ahuja, then a small boy, his mother, and six siblings went to work to scrape together the rest of their living.

Bangles were made of glass and had to be tested for cracks before applying gold color to them, and Ahuja knew how to proof large numbers of them quickly by pinpointing odd noises in their otherwise pristine ringing sounds. He went from one bangle maker to the next offering his auditory skills for a fee.

“You took 10 bangles in each hand and held them together in a loop made with your thumb and forefinger, and you shook them,” Ahuja recalls. “You could figure out that they were all intact if they had one clean pitch. If there was even a single crack, the whole group would make an off sound.”

Taming the loudest noises

Ahuja has spent his adult life sorting out structures behind the jet’s roar, one of the loudest noises humans have produced. Its biggest root cause by far has been turbulence, a phenomenon that physics still grapples to solve today.

In this case, turbulence does not refer to weather-induced swirls in the sky that jostle airplanes, but to what happens in the plume that blasts out of the back of a jet engine. It is seething, and it shears and mixes violently with slow, cool outside air, grinding out howling turbulence. The real source of racket is not in the engine but behind it, in the plume. 

Turbulence coming from a jet in an experiment by Georgia Tech’s Philip Roberts and Ozeair Abessi

But taming it through engineering posed the risk of also diminishing travel speed.

“Velocity of the plume exiting the jet is a main factor in turbulence, which produces jet noise,” says Ahuja, who also administers aeronautic acoustics operations at GTRI, where his research facility is located. “If you lower velocity, you lower turbulence and noise, but that also reduces thrust.”

He latched onto the problem early in his career, which paired the pursuit of advanced degrees with pragmatic industry research. Notably, back then Ahuja experimentally verified a 20-year-old mathematical theory that calculated the cumulative acoustic power of jet noise. Ahuja would go on to help change thinking about jet turbulence.

“When I started out, people thought about turbulence only as chaos in flow dynamics,” Ahuja says. “We began seeing order in the chaos.”

A black-and-white picture on the wall of Ahuja’s office shows two shots of the same jet, one a normal photo of chaotic turbulence, the other from a film that he exposed multiple times to reveal an orderly swirl. It was a piece of turbulence anatomy today called coherent large-scale turbulence.

Krish Ahuja in a sound-insulated chamber. During his early research, he remained in the chamber to observe the jet from a position just below the wire grid he is standing on. Credit: Branden Camp/GTRI. 

Ducking under roaring beasts

For years, Ahuja observed plumes blasting out of experimental jet nozzles from about 12 feet away. He ducked under them in sound-absorbing chambers the size of a large conference room as the jets he helped build cranked up to takeoff force with a thunderous roar. 

“We didn’t have the amazing digital high-speed camera setups we do today, so I climbed down to the bottom of the chamber with ear protection to watch what the plume did,” Ahuja says. 

Turbulence not only creates sound, but sound can also shape turbulence. In these experiments during the 1980s, Ahuja used a gentle whistling sound directed through a tube to alter turbulence so it would give a wing additional lift or produce other desirable dynamics.

Through the years, he and his team connected what they analyzed in the plume with the noise they recorded over multiple microphones. Subtle solutions, such as inserting screwdrivers at the top and the bottom of the nozzle, reshaped turbulence to tame its growl.

The screwdrivers led to the development of “tabs,” also known as chevrons, which are small protrusions into the exiting plume.

Ahuja placed “tabs” on jet nozzles to reduce noise, which has inspired the addition of chevrons, highlighted in blue, to commercial jet engines. Credit: NASA.gov.

“With tabs, I can alter the noise big time,” he says. “I can even cool the flow from 1,000 to 600 degrees Celsius quickly, but you get thrust loss. Now an idea is to make tabs retractable because you only need the noise reduction at takeoff and landing when jet noise can harm people.”

The tabs would retract when the plane is high in the sky, restoring the lost thrust.

Taking more sound from the fury

Ahuja also has lowered noise with moisture. He has added sounds via loudspeakers to shape turbulence, devised means to adjust the combustion inside of jet engines, tweaked fan and turbine dynamics, and added sound-absorbing liners. In the aircraft engine industry, an innovation to increase thrust fortuitously also throttled noise.

“High bypass ratio engines increase the diameter of the jet significantly and put a fan on the front of it to add a slow-moving flow around the outside of the engine’s combustion chamber and turbines,” Ahuja explains. “Those kinds of engines move a lot of air mass to add a lot of thrust but do it at a lower velocity. Less velocity means less turbulence, which means less noise.”

The outer layer of fan-generated airflow also buffers the contact between the rushing, hot jet plume and the slow, cold outside air, easing shear flow.

Image courtesy of Wikimedia Commons under CC license/Richard Wheeler

When Lockheed Martin’s research program left Smyrna, Georgia, for California in 1989, Ahuja, who had been a Lockheed employee, stayed back, and he and his lab joined Georgia Tech and GTRI. Ahuja had already gained attention for his studies on jet noise, but in 2007 he co-published a pivotal study encompassing the jet’s roar, The Sources of Jet Noise: Experimental Evidence. 

The NAE’s citation for Ahuja’s election reads: For the development of quieter aerosystems and contributions to aeroacoustics research, literature, and education.”

Election into the NAE is a rare and extremely high honor. There are fewer than 2,600 living members. In 2019, the Academy elected 104 new members. This year’s induction ceremony will be held on Oct. 6, 2019, in Washington, D.C.

Image credit: Federal Aviation Administration

Although jet plume noise is now much tamer, Ahuja’s work is far from done. When engineers make jets quieter, industry takes advantage of the opportunity to make them more powerful, so acoustics engineers have to go back to make jets quieter again. And after turbulence outside the jet became calmer, other noises coming from inside the jet grew audible to people on the ground and required hushing, too.

“Inside the engine, you measure noise as well,” says Ahuja. “Combustion could cause noise. Flow on the inside of the engine could be hitting all kinds of things that produce their own turbulence and noise.”

A supersonic fighter jet flying faster than the speed of sound. A stark drop in air pressure and temperature around the tail forms a white cloud. Credit: United States Navy.

Today, digital photography and tracking particles infused into the plume allow for observation in high resolution and at high speed. Computation helps to pinpoint noise sources, and Ahuja and his team are in the middle of a much bigger problem: supersonic jet noise. He has to solve it without putting a dent in the thrust and speed of fighter jets.

Supersonic flight over land is prohibited because the noise is much worse than that of commercial jets. But many people happen to work very close to supersonic jets.

“People on aircraft carriers have F-15s and F-18s flying right past them using an afterburner (burst of fuel in the nozzle) to thrust off the end of the carrier,” Ahuja said. “Every time you double the velocity, the noise goes up 24 decibels, which is a huge, huge jump.”

Supersonic jets add an entire arsenal of ruckus: booms, screeches, shocks, and a louder roar. The plane breaks the sound barrier, sending whiplashing booms down to people on the ground, and the noise is powerful enough to sever small branches from treetops if a jet is flying low enough.

Then eddies, or tight turbulent spirals, shoot out the back of the engines and also break the sound barrier, adding further annoyance to the already noise-packed plume.

“With the tools we have today and this great team I have at Georgia Tech, I feel confident we will find solutions,” Ahuja says as he walks between half a dozen experimental setups with a spring in his step.

First published June 10, 2019, on Georgia Tech Research Horizons.


About The Author

Ben Brumfield’s picture

Ben Brumfield

Ben Brumfield is the senior science writer at Georgia Institute of Technology, and vice chair of the Georgia Cyber Academy.