Causes of noise feedback loops in vertical landings identified

A research team from the FAMU-FSU College of Engineering at Florida State University and the Florida Center for Advanced Aero-Propulsion (FCAAP) have jointly discovered the exact causes behind the noise feedback loops that occur during landings of Short Takeoff and Vertical Landing (STOVL) aircraft, such as the military aircraft F-35B Lightning II. The exhaust plumes of these aircraft generate resonance waves of more than 140 dB upon hitting the ground, which can lead to structural problems with the aircraft and health impairments for ground personnel.

The resonance waves that can occur during the landing of a STOVL aircraft pose various dangers. Firstly, the acoustic energy and the resonances caused by them can impair the structural integrity of the aircraft. Additionally, dangerous low-pressure zones can form, pulling the aircraft towards the ground. Secondly, the resonance waves, which are over 140 dB loud, can cause permanent hearing damage and internal injuries to ground personnel – even if they are wearing hearing protection and safety clothing.

The scientists conducted investigations to understand the exact causes in the study “Role of convecting disturbances and acoustic standing waves in supersonic impinging jet,” which was published in the Journal of Fluid Mechanics and online by Cambridge University Press. For this purpose, researchers from FAMU-FSU and FCAAP conducted extensive experiments with a Mach 1.5 supersonic jet, simulating the conditions during vertical landing in the FCAAP's STOVL facility using modern flow diagnostics.

Identifying causes for noise feedback loops

The researchers used high-speed cameras for Schlieren photography to visualize and analyze flow disturbances and sound waves in real-time. Additionally, the scientists recorded acoustic data with highly sensitive microphones. They observed the flow disturbances and the reflected sound waves that occurred at different distances between the engine outlet, the aircraft fuselage, and a simulated landing surface.

The scientists found that when the highest noise level was reached, the flow disturbances and the returning sound waves synchronized into a repeating sequence. The pitch of the noise is mainly determined by acoustic standing waves. These occur stationary in the area between the aircraft fuselage and the ground. The scientists' observations revealed that the pitch is not primarily dependent on the propagation speed of the disturbance. This finding offers a new perspective for understanding resonance feedback. Slower disturbances tend to be larger and consequently cause higher noise levels.

This is a surprising finding for the researchers. It means that acoustic standing waves significantly determine the pitch, while the size and speed of the disturbances determine the loudness of the generated noise. If the propagation speed of the disturbance has only a minor influence on the pitch, information about acoustic standing waves is sufficient to predict the pitch of the noise.

With this knowledge, developers can more easily predict noise frequencies when designing aircraft and planning landing sites. The scientists involved in the study assume that this will help reduce or interrupt the noise feedback mechanism, thereby better protecting the aircraft structure and ground personnel from acoustic influences. The study provides indications, for example, on the redesign of STOVL aircraft nozzles, landing platforms, and operational procedures.

(olb)