The findings, published today in Scientific Reports, show, for the first time, how porous ground treatments can mitigate noise and optimise propellor performance.
Lead author Dr Hasan Kamliya Jawahar from the University of Bristol's aeroacoustic group managed by Professor Mahdi Azarpeyvand was able to demonstrate that porous ground treatments, can significantly reduce noise by up to 30 dB in low-mid frequencies and enhance thrust and power coefficients compared to solid ground surfaces. This suggests that treating roofs of building, landing pads and vertiports with porous surfaces like grass or mosses will reduce noise when drone is landing.
Dr Kamliya Jawahar based in Bristol's Faculty of Science and Engineering explained: "It was known that ground effects influence propeller performance and noise, particularly during take-off and landing.
"While noise issues are well-documented, solutions tailored to urban environments are limited.
"I drew inspiration from natural porous materials, such as vegetation, known for their noise-damping properties. This led to exploring engineered porous surfaces as a potential solution to reduce noise and improve aerodynamics."
The team conducted experiments in an anechoic chamber using a pusher propeller mounted above a ground plane. The ground was alternated between solid and porous treatments with varying porosity and thickness. Microphones placed in both near-field and far-field locations captured acoustic data, while a six-axis load cell measured aerodynamic forces. By comparing results across configurations, they were able to calculate how porous surfaces influence noise and performance under ground-effect conditions.
Dr Kamliya Jawahar said: "Vegetation is known to function as a natural porous medium, where its structural complexity and material properties such as foliage density and moisture content contribute to its noise absorption capabilities.
"It has been widely used in environmental noise reduction strategies such as roadside barriers and urban green spaces but this is the first time it is being investigated for futuristic Urban Air Mobility."
The noise reduction effect of porous ground treatments stems from their ability to modify and manage the flow dynamics near the ground. When a propeller operates close to a porous surface, the porous material absorbs some of the energy from the flow impingement reducing the velocity of the tangential wall jet—a high-speed outwash of air along the ground—thereby mitigating the aerodynamic interactions that contribute to noise.
Additionally, the porous structure traps portions of the impinging flow, reducing its reflection back towards the propeller. This minimizes the re-ingestion of disturbed airflows into the propeller, which are a significant source of tonal and broadband noise. The reduction in reflected turbulence and the stabilized hydrodynamic pressure field help decrease both tonal and broadband noise emissions, resulting in quieter operations. These effects are particularly pronounced in ground effect conditions.
These findings can be applied to UAM operations by enabling quieter and more efficient vehicle designs. They also support the development of noise-reducing vertiport surfaces, fostering greater community acceptance and compliance with urban noise regulations.
"Our research demonstrates that innovative porous landing surfaces can drastically reduce noise from drones and air taxis, paving the way for quieter and more sustainable urban skies," added Dr Kamliya Jawahar.
Paper:
'Porous ground treatments for propeller noise reduction in ground effect' by Hasan Kamliya Jawahar, Liam Hanson, Md. Zishan Akhter and Mahdi Azarpeyvand in Scientific Reports.
