Mach 1.5 tests reveal noise feedback loops from supersonic jets

Researchers from the FAMU-FSU College of Engineering and the Florida Center for Advanced Aero-Propulsion, or FCAAP, are helping to solve a safety challenge in military aviation: the extreme noise generated by supersonic jets during takeoff and landing. The research, published in the Journal of Fluid Mechanics, demonstrates a new model for understanding how supersonic jets of air collide with the ground or other structures to create a resonant feedback loop that produces extreme noise that can reach dangerous volume levels.

The team examined jets like those found in a type of aircraft known as Short Takeoff and Vertical Landing jets, or STOVL. The ability to operate without a traditional runway gives these aircraft, such as the F-35B Lightning II, critical tactical advantages.

But as they descend toward the ground, their exhaust plumes interact with landing surfaces and generate intense noise, often exceeding 140 decibels, posing serious dangers to both aircraft structure and nearby personnel.

"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 the Department of Mechanical and Aerospace Engineering and former founding director of the Institute for Strategic Partnerships, Innovation, Research, and Education, or InSPIRE, and founding director of FCAAP.

"The intense noise produced by jet engines can cause structural damage to the aircraft and damage the hearing of personnel on the ground. 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. In fact, we have already had some success in developing techniques that can reduce jet noise."

Why it matters

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. This establishes a repeating, back-and-forth interaction and creates resonance, an example of a feedback loop, causing loud and repeating noise. For aircraft, these resonant vibrations accelerate structural fatigue and can generate hazardous low-pressure zones that can pull the aircraft toward the ground.

For crewmembers on the ground, sustained exposure to sound levels over 140 decibels can cause permanent hearing damage, even when wearing protective equipment. At peak intensities, extreme acoustic pressure can even cause organ damage.

A new approach to modeling jet resonance

The research team tested a supersonic, Mach 1.5 jet—1.5 times the speed of sound—and adjusted nozzle pressure and the jet's distance from the ground to simulate takeoff/landing and make a range of measurements.

To see the airflow, they used a high-speed camera and a specialized visualization technique called schlieren imaging that allowed them to "see" the jet flow—including its large-scale disturbances and the sound waves generated in real time. At the same time, a highly sensitive microphone also 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. By matching images to a specific point in the cycle, 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—of the noise was primarily governed by acoustic standing waves, which appear stationary in space between the body of the plane and the ground. The findings reveal that the pitch is not primarily governed by disturbance velocity, thereby offering another perspective on the existing understanding of the resonance feedback. They also found that slower disturbances tend to be larger, consequently creating louder noise.

"That was surprising," said postdoctoral researcher Myungjun Song, the study's lead author. "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 offered the research team an insight. Because the disturbance speed has little effect on pitch, information about acoustic standing waves would be enough to predict the noise pitch.

The new model enables engineers to predict noise frequencies more easily during aircraft and landing pad design, a critical step toward protecting both aircraft structures and personnel from acoustic trauma.

World-class research facilities drive discovery

The experiments were conducted at FCAAP's specialized research facilities, designed for advanced high-speed aerodynamic studies at the FAMU-FSU College of Engineering.

Researchers used the FCAAP's STOVL facility, which offers cutting-edge flow diagnostic capabilities, and the hot jet facility, which can generate high-temperature, high-speed airflow in an anechoic chamber to allow for highly accurate acoustic measurements under realistic jet conditions.

"While jet propulsion is an important focus of our work, our research is not limited to it," Alvi said. "The university and the college, through FCAAP, operate a polysonic wind tunnel that simulates supersonic flows up to Mach 6—supersonic to hypersonic conditions. We also use our anechoic wind tunnel and subsonic wind tunnels for numerous other aerospace related research projects. Together, these facilities and the expertise of our researchers create a one-of-a-kind ecosystem for conducting leading-edge research in aerospace and aviation."

An associated initiative, InSPIRE is an FSU-led effort to establish a new aerospace and advanced manufacturing hub in Bay County, Florida. The program builds on FCAAP's foundation to develop complementary facilities for larger hypersonic wind tunnels that can handle a wider range of conditions for applied, industry-relevant research.

"In partnership with industry, InSPIRE is also integrating advanced manufacturing capabilities that will allow much more efficient test and evaluation and assist our industry partners to innovate manufacturing processes in a realistic factory-modeled setting," said Alvi, the former director of InSPIRE. "Working with industry partners allows our researchers to use their expertise to solve the pressing and difficult problems that are directly relevant for industry."

Provided by Florida State University