The burgeoning Urban Air Mobility (UAM) sector has the potential to noisily disturb urban communities during take-off and landing, but porous surfaces have the potential to reduce this whilst optimising propellor performance.
The team’s findings are detailed in Nature Scientific Reports. Lead author Dr Hasan Kamliya Jawahar from Bristol University’s aeroacoustic group was able to demonstrate that porous ground treatments can reduce noise by up to 30dB in low-mid frequencies and enhance thrust and power coefficients compared to solid ground surfaces. This suggests that treating roofs of buildings, landing pads and vertiports with porous surfaces like grass or mosses will reduce noise when a drone is landing.
In a statement, Dr Kamliya Jawahar from Bristol’s Faculty of Science and Engineering said: “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 near-field and far-field locations captured acoustic data, while a six-axis load cell measured aerodynamic forces. By comparing results across configurations, they calculated 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.”
According to the University, 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.
The porous structure also traps portions of the impinging flow, reducing its reflection back towards the propeller. This minimises the re-ingestion of disturbed airflows into the propeller, which the team said is a significant source of tonal and broadband noise. The reduction in reflected turbulence and the stabilised hydrodynamic pressure field help decrease 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, with the added benefit of fostering greater community acceptance and compliance with urban noise regulations.
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