A team of Florida researchers is working to solve a long-standing military aviation problem -- reducing the extreme noise from supersonic jets during takeoff and landing. The researchers are engineers from Florida A & M University and Florida State University College of Engineering and the Florida Center for Advanced Aero-Propulsion, or FCAAP.
“Only a tiny fraction of the jet’s energy is transformed into sound, but this small fraction has a major impact,” said Farrukh S. Alvi, professor in FAMU’s Department of Mechanical and Aerospace Engineering, in a statement on the research obtained by Military.com. “The intense noise produced by jet engines can cause structural damage to the aircraft and damage the hearing of personnel on the ground.”
The research team includes Professor Alvi, postdoctoral researcher Myung Jun Song, and doctoral student Allie Gagne. Their research, published in the Journal of Fluid Mechanics, seeks to understand how supersonic jets of air collide with the ground or other structures to produce extreme noise that can reach dangerous and even damaging volumes.
“We are trying to understand the physics behind these supersonic jets and the noise they produce so that we can develop tools that can reduce their impacts,” said Professor Alvi. “We have already had some success in developing techniques that can reduce jet noise.”
During the course of their research, engineers examined short takeoff and vertical landing jets, or STOVL, which are widely used by the military for their ability to operate without a traditional runway. The F-35B Lightning II is one of the aircraft heavily used to give the military critical tactical advantages.
But as these aircraft descend for a landing, the researchers say their exhaust plumes interact with the ground or other solid surfaces and generate intense noise, often above 140 decibels.
“When the high-speed air coming from jet engines mixes with the ambient air, it creates large-scale disturbances that hit the ground, producing strong sound waves that propagate back toward the jet engine,” reads a summary of the research, which has been reviewed by Military.com. “This establishes a repeating, back‑and‑forth interaction and creates resonance, causing loud and repeating noise.”
Jet Noise Can Cause Serious Body Damage
For crewmembers on the ground, long-term exposure to sound levels over 140 decibels can cause permanent hearing damage, even while wearing ear protection. Exposure to higher decibels for a prolonged period of time, could even cause organ damage.
To mitigate the noise and potential danger, the research team tested a supersonic, Mach 1.5 jet at 1.5 times the speed of sound. They adjusted the nozzle pressure from the jet’s engine as well as the jet’s distance from the ground to simulate take-off and landing.
To see the airflow, they used a high‑speed camera and a specialized visualization technique to ‘see’ the jet flow in real time. Additionally, a highly sensitive microphone recorded the sound produced by the jet.
“When the jet is loud, the jet flow and the sound waves repeat at a regular rhythm, which is a characteristic of a resonant cycle,” said the research summary. “The researchers developed a clear picture of the airflow and measured how fast large-scale disturbances in air moved and how sound waves traveled back toward the nozzle.”
The researchers found that for many cases, the pitch, how the human brain perceives the frequency of sound waves, was primarily made by acoustic standing waves and not disturbance velocity. They also found that slower disturbances tend to be larger, creating louder noise.
“That was surprising,” said postdoctoral researcher Myungjun Song, the study’s lead author, in the statement obtained by Military.com. “We found that these acoustic standing waves are much more important in determining the pitch, while the size and speed of the disturbances decide the level or ‘loudness’ of the noise produced.”
The discovery helped engineers to predict noise frequencies more easily during aircraft and landing pad design, a critical step toward protecting both aircraft structures and personnel from the extreme noise.
The project was a collaborative effort. Funding was provided by the Office of Naval Research, with additional support from the National Science Foundation, the Air Force Office of Scientific Research, the Florida Center for Advanced Aero-Propulsion, or FCAAP, and the Don Fuqua Eminent Scholar Fund.
