Laboratory tests show promise for reducing jet noise in commercial and military aviation.
Aerospace engineers at the University of Cincinnati and the Naval Research Laboratory have come up with a new nozzle design for F-18 fighter jets that they hope will dampen the deafening roar of the engines without compromising performance.
Distinguished Professor Ephraim Gutmark, a distinguished researcher from Ohio, and his students at UC’s College of Engineering and Applied Sciences designed and tested the new nozzles on 1 / 28th scale jet engines in his aeroacoustics laboratory.
The interior of the nozzles features triangular fins like rows of shark teeth which significantly reduced jet engine noise in UC lab tests. The project is a collaboration between UC, the US Naval Research Laboratory and the Naval Air Station Patuxent River. This fall, NAVAIR will test the designs and performance of the UC on the F-18 Super Hornets, the tactical fighter aircraft used by the United States Marines and Navy.
“They are simple accessories that modify the behavior of the outflow of the engine with minimal effect on its performance,” said Gutmark.
UC lab tests have shown that the new nozzle can reduce engine noise by 5 to 8 decibels. It may not seem like much. But unlike linear scales like a ruler where an inch is always an inch, decibels are measured on a logarithmic scale in which 20 decibels is ten times louder.
âIt’s very important,â Gutmark said. âAs a general rule, engine manufacturers are happy to get even a half-decibel improvement, because the decibels represent a logarithmic scale. “
Preliminary results from UC’s laboratory show promise for reducing jet noise in commercial and military aviation. UC and the Navy have filed for a joint patent.
âIf the design works with this engine, it can be applied to any other engine with very minor modifications,â Gutmark said.
The project was funded under the Strategic Environmental Research and Development Program by the US Department of Defense Environmental Research Program.
Hearing loss and tinnitus are the leading causes of military disability claims, affecting more than 2.6 million former military personnel, according to figures from the US Department of Veterans Affairs. The VA spends over $ 1 billion per year on hearing loss cases, which represents about 15% of new disability claims filed with the VA each year.
Jet noise in particular represents a serious health risk in military and commercial aviation. According to the Naval Research Advisory Committee, Navy personnel on flight decks are exposed to noise of more than 150 decibels.
âOn aircraft carriers, the crew working with the pilots on the flight deck must be very close to the aircraft when it takes off. Due to the short track of the aircraft carrier, they have to run the engine with afterburner, so it’s very noisy, âsaid Gutmark.
The jets are so loud that noise and vibration can affect even the aircraft itself – a phenomenon called sound load, Gutmark said.
“By suppressing the noise, you are helping the crew but also the longevity of the aircraft itself,” added Gutmark.
âIt’s a pleasure to know that you work on fighter jets. There is a cool factor.
– Mohammad Saleem, aerospace engineering student at UC
UC has been working on the project with the Naval Research Lab and NAVAIR for two years. The Navy provides computer analysis and aircraft information to supplement the design and experimentation work of the UC.
Gutmark’s diverse research has included engine combustion and propulsion technology, acoustics, and even biomedical research. He taught the principles of aerospace engineering to pilots in the US Navy’s Strike Fighter Tactics Instructor program, the school that inspired the movie “Top Gun”.
âIt was exciting teaching them because they were really interested,â said Gutmark. âThey really wanted to know more about what happens when the plane does this or that. And getting feedback from someone who understands what the plane does practically when it is in the air has been a special opportunity. “
Students at Gutmark’s lab wear industrial grade hearing protection when working with jet engines in an underground lab in Rhodes Hall. The motors are mounted on the floor inside an anechoic chamber – a room designed to completely absorb the reflections of acoustic waves. Students can turn on jet engines remotely outside the chamber and use a suite of sensors to measure and analyze the noise of the exhaust plumes.
âIn our lab, we have four different measurement techniques, including signal processing with acoustic and optical measurements and laser equipment,â said Aatresh Karnam, doctoral student at UC. âAs in any field of engineering, you have to adapt several techniques to know what data to capture, how to process it and how to interpret it. It is a difficult challenge.
An array of sensitive microphones surrounds the jet in the chamber. It is perhaps not surprising that the noise is different depending on the relative position of each relative to the jet plume.
“Each of these noise components propagates in different directions,” said Professor Gutmark. “The microphones are distributed in an arc around the jet so that we can detect noise that is going downstream or upstream or to the sides.”
UC engineers test their new nozzle designs on cold, heated jets with exhaust gases that burn up to 1,100 degrees. Fahrenheit. The jet is a scale model of the F-18 F404 engine manufactured by General Electric Aviation, headquartered in Cincinnati.
âThe afterburning of the exhaust gases from a real jet will get even hotter,â Karnam said.
UC doctoral student Mohammad Saleem said planes are quieter today than they were just 20 years ago. Commercial and military aviation have a strong public and financial interest in reducing noise. Even incremental improvements can have a profound impact, he said.
âThese noise reduction technologies are extremely beneficial to the communities living around air bases and personnel working on aircraft carriers,â Saleem said.
Saleem said he had always been interested in aerospace engineering. But he likes the challenge presented by a project as complicated as the noise of jet engines.
âI chose UC because it is a solid school in the field of propulsion systems,â he said.
But he admits that his friends and family are in awe when he talks about working with the F-18 Super Hornets.
âIt’s a pleasure to know that you work on fighter jets. There is a cool factor, âSaleem said.
âEngineering is about solving problems. The joy is in finding a system that works. I think it is satisfactory.