Areas with porous ground, such as those covered in plants, can significantly decrease the noise generated by drones and air taxis, helping to lessen disturbances in urban areas as the concept of Urban Air Mobility (UAM) expands.
A study released today in Scientific Reports reveals, for the first time, how treatments that utilize porous surfaces can help reduce noise and enhance propeller efficiency.
Dr. Hasan Kamliya Jawahar, the lead researcher from the University of Bristol’s aeroacoustic team led by Professor Mahdi Azarpeyvand, demonstrated that applying porous ground techniques can lower noise levels by as much as 30 dB in low to mid frequencies and improve thrust and power efficiency compared to regular solid ground. This indicates that covering building roofs, landing areas, and vertiports with porous materials like grass or moss can decrease noise during drone landings.
Dr. Kamliya Jawahar, part of Bristol’s Faculty of Science and Engineering, stated: “It was already understood that the type of ground influences how propellers perform and the noise they generate, especially during take-offs and landings.”
“Although noise challenges are well acknowledged, tailored solutions for urban settings are scarce.”
“I was inspired by natural porous materials, particularly vegetation, which is recognized for its sound-absorbing qualities. This prompted an exploration of engineered porous surfaces as a potential way to lessen sound and enhance aerodynamic performance.”
The research team performed tests in a specially designed anechoic chamber utilizing a pusher propeller positioned over a ground surface. They alternated between solid and porous ground treatments with different levels of porosity and thickness. Acoustic data was captured using microphones in both close and distant placements, while a six-axis load cell assessed aerodynamic forces. By comparing various setups, they calculated the impact of porous surfaces on noise and performance under ground-effect scenarios.
Dr. Kamliya Jawahar explained: “Vegetation serves as a natural porous medium; its intricate structure and material features, like foliage density and moisture levels, enhance its sound-absorbing abilities.
“While it has been implemented in strategies for reducing environmental noise, such as roadside noise barriers and urban greenery, this is the first instance it is being studied for future Urban Air Mobility applications.”
The noise reduction offered by porous ground treatments comes from their capacity to alter and manage airflow dynamics near the surface. When a propeller is in close proximity to a porous area, the material absorbs some energy from the airflow, which diminishes the speed of the tangential wall jet—a rapid outflow of air near the ground—thus reducing the aerodynamic interactions that result in noise.
Moreover, the porous design captures sections of the impacting airflow, lessening its reflection back towards the propeller. This prevents the reintroduction of disturbed air into the propeller, which is a major contributor to tonal and broadband noise. The reduction of reflected turbulence and a stabilized hydrodynamic pressure field further decrease both tonal and broadband noise emissions, leading to quieter operations, especially in ground-effect situations.
These insights can be utilized in UAM operations by allowing for the design of quieter and more efficient vehicles. They also aid in creating noise-reducing surfaces for vertiports, enhancing community acceptance and adherence to urban noise standards.
“Our findings suggest that innovative porous landing surfaces can significantly diminish noise from drones and air taxis, paving the way for quieter and more sustainable urban aerial transportation,” Dr. Kamliya Jawahar concluded.
