jet engine

Jet engine noise could be dramatically reduced using new simulations

Jet engine noise is being accurately simulated and modelled by a team working for the US Department of Energy (DOE) in an attempt to develop engineering solutions to make them quieter.

Joe Nichols, an assistant professor at the University of Minnesota, is working with the DOE’s Argonne National Laboratory, to create high-fidelity computer simulations to determine how jet turbulence produces noise.

Jet engines are the loudest sources of human-made noise that exist, Nicols said, significantly eclipsing the decibels produced by even stadium rock bands.

He hopes the simulations will lead to novel engineering designs that reduce noise over commercial flight paths which can be a constant irritation to inhabitants living below.

“His project leverages computational data with what he calls input-output analysis, which reveals the origins of jet noise that are otherwise hidden in direct run-of-the-mill forward simulations, or even experiments,” said Ramesh Balakrishnan, an Argonne computational scientist.

Jet engines produce noise in different ways, but mainly it comes from the high-speed exhaust stream that leaves the nozzle at the rear of the engine.

In addition, planes are loudest when they move slowly, such as at takeoff or at landing. As the exhaust stream meets relatively still air, it creates tremendous shear that quickly becomes unstable. The turbulence produced from this instability becomes the roar of the engine.

Aeronautic engineers already include some solutions to the problem such as incorporating chevrons, broken eggshell-shaped patterns, into exhaust nozzle designs to change the shape of the jet as it leaves the engine.

The idea is to reduce the noise by changing the pattern of the turbulence, although much of this design work remains a guessing game.

By using Argonne’s supercomputer, dubbed Mira, Nichols and his team are applying computational fluid dynamics to remove some of that guesswork.

They start by conducting high-fidelity large eddy simulations that accurately capture the physics of the turbulence that is making the noise.

From those simulations they extract reduced-order, or more concise, models that explain what part of the turbulence actually makes the sound. In addition to improving scientific understanding of jet noise, these reduced-order models also provide a fast, yet accurate, means for engineers to evaluate new designs.

The use of Mira is necessary as the simulations are extremely complex, containing millions of data points within.

The jet turbulence model requires the use of an unstructured mesh - a non-uniform 3-D grid - to represent the dynamics involved. In this case, one simulation could have 500 million grid points. This is multiplied by five to account for pressure, density and three components of velocity to describe the flow at every grid point. Ultimately this equates to billions of degrees of freedom, or the number of variables Mira uses to simulate jet noise.

This simulation could help engineers more precisely direct the modelling of jet engine nozzle geometries by determining, for instance, the ideal number and length of chevrons.

“What distinguishes Joe’s work from those of the other computational fluid dynamics projects at ALCF is that it involves the development of a method that could mature into becoming a design tool for aero-acoustics,” said ALCF’s Balakrishnan.

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