'But what if inside the jet turbulence there is a skeleton of coherent flow structures that we can describe with just 50 degrees of freedom,' suggested Nichols. That equates to billions of degrees of freedom, or the number of variables Mira uses to simulate jet noise. Multiply that by five to account for pressure, density and three components of velocity to describe the flow at every grid point.

In this case, one simulation could have 500 million grid points. Simulating complex geometries like jet turbulence requires the use of an unstructured mesh - a non-uniform 3-D grid - to represent the dynamics involved. 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. They start by conducting high-fidelity large eddy simulations that accurately capture the physics of the turbulence that is making the noise.įrom those simulations they extract reduced-order, or more concise, models that explain what part of the turbulence actually makes the sound. Working with ALCF computational scientist Ramesh Balakrishnan and Argonne's supercomputer Mira, Nichols and his team are applying computational fluid dynamics to remove some of that guesswork. But much of the design work remains a guessing game. The idea is to reduce the noise by changing the pattern of the turbulence. The turbulence produced from this instability becomes the roar of the engine.Īeronautic engineers incorporate chevrons, broken eggshell-shaped patterns, into exhaust nozzle designs to change the shape of the jet as it leaves the engine. As the exhaust stream meets relatively still air, it creates tremendous shear that quickly becomes unstable. And planes are loudest when they move slowly, such as at takeoff or at landing. 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. That is why jet noise presents both a challenging and a beautiful problem for Nichols. 'Noise tells you something about the fundamental nature of turbulence, because noise reveals order that is otherwise hidden in complex, highly nonlinear, chaotic phenomena,' he said. The results may lead to novel engineering designs that reduce noise over commercial flight paths and on aircraft carrier decks. Department of Energy (DOE) Office of Science User Facility within the DOE's Argonne National Laboratory, to create high-fidelity computer simulations to determine how jet turbulence produces noise. Nichols is working with the Argonne Leadership Computing Facility (ALCF), a U.S. They are, said Joe Nichols, among the loudest sources of human-made noise that exist.Īn assistant professor of Aerospace Engineering and Mechanics at the University of Minnesota, Nichols is fascinated by sound and its ability to find order in chaos - and by applying that understanding to the development of new technologies that can reduce noise in aircraft. But try as they might, bands just can't compete with the decibel ranges produced by jet engines.

Jet engine decibel