Reducing Jet Noise: Research in Focus

Photo: Plane over houses

Dr Karthik Depuru-Mohan, Junior Research Fellow in Engineering, gave Homerton’s inaugural Lunchtime Fellow’s Research Seminar on “Jet Noise and its Reduction”.

Aircraft noise is an environmental problem of significant importance, as the associated stress can result in health problems from sleep disturbance to blood clotting and cardiovascular disease (effects thankfully yet to be seen in Homerton, despite the College’s position under the busy Cambridge air corridor).

Hopefully scenes like this will soon be much quieter in the College!

Scenes such as this may soon be much quieter...

Karthik aims to develop a quick and accurate jet noise prediction model, in order to develop quieter nozzles for future civil aircraft.

Aircraft noise has a number of contributory components, including noise from the landing gear and wing flaps, but the most significant originates from the jet engines themselves. This arises due to a significant speed gradient between the flow of air exiting the fan of the engine and the exhaust nozzle. This gradient creates shear between the two streams of air, which results in noise. Noise levels roughly scale with velocity, so the greater the gradient between the two streams, the greater the noise emitted.

Aircraft noise is made up of several components, with the biggest coming from engine exhaust noise.

Karthik’s work is a two-step process. As physical tests are time consuming and expensive, accurate computational models must first be developed, which allow for quick, cheap, and variable computational experiments. By tweaking the parameters of these experiments, you can then work out the theoretically optimum arrangement for reducing aircraft noise, and produce physical nozzles to test in a wind tunnel.

There are two principal models used in the field of acoustic modelling; Reynolds-Averaged Navier-Stokes equations (RANS), and Large Eddy Simulations (LES). These equations model how the airflows in the jet exhaust behave and interact, and what level of noise will be produced. Although the RANS model is much quicker than the LES, it is less accurate in describing real-world conditions. Karthik’s research has allowed him to work out mathematical constants linking the results of the two models, which can significantly increase the accuracy of the RANS model, whilst preserving its efficiency.

After optimising the computational models, Karthik could now turn to practical solutions of the jet noise problem. There are two major solutions preferred by engineers, both of which interfere with the airflows to reduce the level of shear. One idea is to install a series of microjets around the rim of the exhaust, which would inject small streams of air to disturb the main jet flow.

Photo: Microjets feeding into exhaust nozzle Credits: Castelain et al. (2008) and Airbus (2013)

photo: Chevroned aircraft nozzle Credits: Castelain et al. (2008) and Airbus (2013)

Top: Microjets feeding into exhaust nozzle. Bottom: Chevroned aircraft nozzle. Credits: Castelain et al. (2008) and Airbus (2013)

The alternative, and the focus of Karthik’s current research, is modifying the shape of the exhaust nozzle to incorporate serrations, or chevrons. These break up the flow of air directly, decreasing the shear gradient and therefore the noise levels. Both computational and physical experiments investigating the results of these chevrons have been positive, with reductions of up to 6 decibels being recorded.

Now, Karthik has been able to modify the model to predict which arrangements of chevrons might be optimal. The graph below shows that, at a chevron angle of 20 degrees, six appears to be the optimum number of chevrons to give the greatest noise reduction.

A graph showing noise level predictions for nozzles with varying numbers of chevrons, each at 20 degrees to the nozzle rim. Credit: Dr K Depuru-Mohan.

A graph showing noise level predictions for nozzles with varying numbers of chevrons, each at 20 degrees to the nozzle rim. Credit: Dr K Depuru-Mohan.

You can read more about Karthik's work on his personal profile. Karthik was awarded a Junior Research Fellowship at Homerton after completing his PhD at St. John's College, Cambridge, and has undertaken secondments to NASA's Langley Research Centre and the Department for Business, Innovation, and Skills.