In 1969, the roar of a passing jet airliner broke a bone in Carolyn Brobrek’s inner ear, as she sat in the living room of her East Boston home. Many flights took off too close to rooftops then, but even at a distance, jet engines were a notorious source of permanent hearing loss.
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.
Jet-setters had been a rare elite, but early in Ahuja’s career in the 1970s, air travel went mainstream, connecting the globe. The number of flights multiplied over the years, and jet engine thrust grew stronger, but remarkably, human exposure to passenger jet noise in the same time period plummeted to a fraction of what it had once been, according to the Federal Aviation Administration.
Ahuja not only had a major hand in it, but he also has felt the transition himself.
“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,” said 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.
Jingling bangles for cracks
To tackle jet racket over 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 said. “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 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.
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,” said 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 help change thinking about jet turbulence.
“When I started out, people thought about turbulence only as chaos in flow dynamics,” Ahuja said. “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.
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 said.
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.
“With tabs, I can alter the noise big time. I can even cool the flow from 1,000 to 600 degrees Celsius quickly, but you get thrust loss,” Ahuja said. “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 said. “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.
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, including Georgia Tech civil and environmental engineering professor John Koon. This year’s induction ceremony will be held on October 6 in Washington, D.C.
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. 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,” Ahuja said.
Lowering supersonic jets’ booms
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 that 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 they are 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 said as he walked between half a dozen experimental setups with a spring in his step.
Source: Georgia Tech