Just Plane Noisy

As the airline industry grows, and cities expand into the once-unpopulated areas that surround many airports, researchers are trying to figure out how to make aircraft quieter.

The airline industry is growing rapidly. A study by the World Bank estimated that the industry carried 4.2 billion (yes, billion) passengers in 2018. That's almost double the 2.2 billion who flew in 2008. And the industry is expected to grow further in the next decade. 

Though air travel has great benefits for people and economies, there are also tolls to having all those planes in the air. Perhaps the most obvious to anyone who has lived near an airport is the near-constant ear-splitting roar of jet engines. 

That din is not simply an annoyance — it can also lead to health and developmental problems for those who live along flight paths. Indeed, several studies have linked jet noise to decreased academic performance, high blood pressure, and difficulty sleeping.

Pankaj Rajput is working to solve this problem through innovative engineering that melds his background in the computational modeling of fluidic systems and his interest in aerospace.

Rajput, a recent graduate of the PhD program in the Mechanical and Aerospace Engineering Department at New York University and an NYU Abu Dhabi Global PhD Fellow, has devised an inventive way to dampen sound caused by jet engines that is both efficient and can be controlled in flight. He studied the topic during his doctoral research, and he has published several papers on it, culminating in a recently published paper with his PhD advisor, Sunil Kumar, NYUAD Vice Provost of Graduate and Postdoctoral Programs and Global Network Professor of Engineering. They also have a patent pending on the technique.

We want to reduce the noise that is being radiated towards the ground from an aircraft, and we are not concerned about the noise going above the aircraft towards space.

Most of the noise generated by an aircraft is caused by the difference in the velocity of the exhaust jet and the ambient, or surrounding, air. The marked difference between the two forms a shear layer, which is an extremely unstable, turbulent layer of air sandwiched between the exhaust jet and the surrounding air. This turbulence and mixing causes the ripping sound of an engine. The greater the difference between the velocity of the exhaust jet and the ambient air, the louder the aircraft.

But, as Rajput explained, it's possible to mix the jet and the surrounding air efficiently, "so that the velocity of the jet will be lower, and the strength of the sound will be lower as well," without affecting the aircraft. 

Rajput utilizes a method called fluid injection that consists of small jets of air, called microjets, that are placed at the nozzle of an aircraft's exhaust. When oriented in a particular way, these microjets generate a specific kind of mixing downstream from the exhaust called a counter rotating vortex pair, which helps to reduce the velocity of the exhaust and thus reduce the sound — without impacting the aircraft's stability or efficiency.

There are other ways to reduce the sound of a jet engine. For example, a mechanical design called chevron nozzles can be installed on aircraft to smooth the mixing between the exhaust and the ambient air. But these nozzles are permanent fixtures, and reduce the efficiency of the engine throughout the flight.

Fluid-injection-based noise reduction can be employed at specific times during flight — during takeoff and landing, for example — and turned off when an aircraft is flying over the ocean or some unpopulated expanse.  

What's more, with proper selection of microjets, fluid injection can reduce sound that emanates from the aircraft in a particular direction. "This basically means that using this technology you can selectively choose a direction and reduce the noise along that direction without affecting the noise along other directions," Rajput said. "We want to reduce the noise that is being radiated towards the ground from an aircraft, and we are not concerned about the noise going above the aircraft towards space." 

For those who live in the Abu Dhabi neighborhoods of Al Raha and Al Zeina, Rajput's research may one day prove to be a welcome innovation, making for quieter nights and sounder sleep.