ACADEMIA

If you’ve ever lived near the airport, or sat in the back row of a 747, you know that jets are noisy. Today’s aircraft have many sources of noise, but among them, the clatter of the exhaust system is a prime contributor. The higher the speed of the jet, the louder the noise it generates.

But with the help of supercomputers, the roar overhead will soon be softened.

“Jet noise has been a subject of intensive research since early 1950s,” said Ali Uzun, research associate at Florida State University. “One way to minimize jet noise is to modify the turbulent mixing process using special control devices, such as chevrons. Since noise is a by-product of the turbulent mixing of jet exhaust with ambient air, one can attempt to reduce the noise by modifying the mixing process.”

In aerodynamics, a chevron refers to a triangle-shaped protrusion at the end of the nozzle. A crown of four to six chevrons set around the end of the nozzle [see figure, left] has been shown to reduce turbulence, and consequently noise, as compared to the usual circular nozzle. Chevron-shaped nozzles are one of the most promising jet noise reduction devices, but it remains unclear why chevrons reduce noise, and how their designs can be optimized to minimize output.

Uzun uses the Rangersupercomputer at the Texas Advanced Computing Center (TACC) as a 21st century virtual wind-tunnel, simulating the turbulence and noise generated by virtual exhaust passing though a virtual engine.

A picture depicting a two-dimensional cut through the jet flow. The picture visualizes the turbulence in the jet flow and the resulting noise radiation away from the jet. The dark solid line represents the control surface for the Ffowcs Williams - Hawkings (FWH) method, which is a special technique that is used for calculating noise propagation to distances far away from the jet flow.

Working with a grant from National Aeronautics and Space Administration (NASA), his research is answering fundamental questions about turbulence and noise, including how complex physical phenomena generate sound waves in a jet exhaust flow, and how noise suppression devices, such as chevrons, modify the way exhaust mixes with air to reduce noise levels.

For decades, experimental testing was the only way to understand the underlying physics of turbulence. But such tests are expensive, time-consuming, and only capable of studying a few physical prototypes at a time. Increasingly, computational simulations are taking a leading role in the study of turbulent interactions.

“Properly validated computational simulations can provide a lot more useful information about the problem of interest than physical experiments,” Uzun said. “Also, computational simulations can be used to study many different designs without actually building the physical models. Once the computer simulations point out the most promising designs, experiments can be performed only on those few models of interest to confirm that they are indeed working as intended. This saves time and money.”

However, as Uzun suggests, for simulations to be useful for prediction and design, researchers needed to prove that they could replicate real results with virtual models. Which is what Uzun and his colleagues set out to do.

To determine how a given design would react to high-speed jet exhaust, Uzun first created a computer model of the chevron-shaped exhaust nozzle. This was then integrated into a parallel simulation code that calculated the turbulence of the air as exhaust was forced through the nozzle.

“Dr. Uzun’s computations of chevron nozzles are pushing the state-of-the-art in computational fluid dynamics (CFD) as applied to turbulent aerodynamics,” said Nicholas J. Georgiadis from NASA Glenn Research Center, technical manager for the project. “While most other efforts using large eddy simulations for jet computations have typically used on the order of one to 10 million grid points, Dr. Uzun’s computations have used up to 400 million grid points, and as a result are capturing a broader spectrum of the turbulent flow than has been done previously. Such computations require the use of a massively parallel computer platform to handle the size of the computer model under investigation.”

A comparison of experimental (left) and simulated (right) data shows a close match between results.

Uzun' ‘first of their kind’ calculations were made by possible by the generous computer time allocations on [the National Science Foundation’s] TeraGrid resources. The group relied on HPC systems at the National Center for Supercomputing Applications (NCSA) and the Louisiana Optical Network Initiative (LONI), as well as at TACC, to compute their high-resolution nozzle simulations. In 2008, the project used more than eight million computing hours, and in 2009, it will use up to 15 million computing hours, making it one of the most computationally-intensive science projects on the TeraGrid.

Performing numerical simulations of test cases for which there are experimental measurements available, Uzun’s group matched the physical results with a high degree of accuracy [see image, left, for side-by-side comparisons]. According to Uzun, the results prove that computer simulations now have the ability to closely match experimental data, while providing far more detailed information about physical processes. “This means that we have the capability to produce reliable predictions that can be used with confidence in jet noise research,” he said.

 

Ali Uzun, research associate at Florida State University

But Uzun’s test cases are only the first step of a long design optimization process. The arc of the research extends from validating their computational methods, to identifying the key physical factors responsible for noise generation, to designing a new engine that can significantly minimize exhaust noise.

The group’s turbulence research has important ramifications for both military and civilian communities. Hearing loss is one of the most pervasive and expensive problems the military faces. Much of the hearing damage is caused by prolonged exposure to jet noise, which could be alleviated with quieter engines (and better hearing protection devices, as discussed in an April TACC feature).

But a more powerful impetus may come from the public where restrictions on noise pollution near airports have been strengthening worldwide. The U.S. aviation industry is a significant contributor to the nation's economy, boasting annual sales in excess of $36 billion and providing nearly one million jobs. New noise reduction legislation has inspired manufacturers to produce quieter engines for more powerful planes on short order — a task with which NASA, TACC and the NSF TeraGrid are happy to help.

“One of NASA's primary goals is to conduct and support scientific research that will help the U.S. aviation industry maintain its global competitiveness,” Uzun said. “Quieter engines will create more jobs in the U.S. and help the economy.”

Recent papers by Uzun:

American Institute of Aeronautics and Astronautics (AIAA) 2009-3194 paper, "High-Fidelity Numerical Simulation of a Chevron Nozzle Jet Flow," presented at the AIAA Aeroacoustics Conference in Miami, May 2009.

AIAA 2007-3596 paper, "Noise Generation in the Near-Nozzle Region of a Chevron Nozzle Jet Flow," presented at the AIAA Aeroacoustics Conference in Rome, Italy, May 2007. (Accepted for publication in the AIAA Journal)

Ali Uzun's research is funded by NASA.

Aaron Dubrow
Texas Advanced Computing Center
Science and Technology Writer