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Recent research published in Scientific Reports reveals innovative approaches to reduce noise levels and enhance propeller efficiency through the use of porous ground treatments.
Dr. Hasan Kamliya Jawahar, the lead researcher from the University of Bristol’s aeroacoustic research team, overseen by Professor Mahdi Azarpeyvand, has demonstrated that these porous surfaces can diminish noise levels by as much as 30 dB within low to mid-frequency ranges, while also boosting thrust and power coefficients relative to conventional solid ground surfaces. This discovery points to the potential for implementing porous materials, such as grass or moss, on rooftops, landing zones, and vertiports, to lower noise levels associated with drone landings.
Dr. Kamliya Jawahar, part of the Faculty of Science and Engineering, remarked on the known effects of ground surfaces on propeller efficiency and noise, particularly in critical phases of flight such as take-off and landing. He emphasized that addressing noise pollution in urban environments has consistently posed challenges, often with limited solutions available.
He further elaborated: “My research drew inspiration from the inherent sound-absorbing properties of natural materials, particularly plants, which led us to investigate engineered porous surfaces as viable solutions for noise reduction and improved aerodynamics.”
Through a series of experiments conducted in an anechoic chamber, the research team utilized a pusher propeller positioned above a surface that alternated between solid and porous materials of varying porosity and thickness. Acoustic measurements were taken using microphones in both near-field and far-field setups, alongside a six-axis load cell to gauge aerodynamic forces. By contrasting the results of these different configurations, the researchers were able to assess the impact of porous surfaces on noise and performance amid ground-effect conditions.
Dr. Kamliya Jawahar stated: “Vegetation acts as a natural porous medium, where attributes such as the complexity of structure, density of foliage, and moisture content play crucial roles in its ability to absorb sound.”
He noted that while vegetation has been effectively utilized for environmental noise mitigation in scenarios such as roadside noise barriers and urban greenery, this is the inaugural study examining its application in the context of Urban Air Mobility (UAM).
The key to the noise reduction achieved with porous ground treatments lies in their capacity to alter and manage airflow dynamics near the ground. When a propeller operates in proximity to these porous surfaces, the materials absorb some of the flow’s energy, thereby reducing the velocity of the tangential wall jet—a swift outflow of air along the ground. This reduction lessens the aerodynamic interactions that typically contribute to noise generation.
Moreover, the porous structure captures portions of the airflow hitting the surface, limiting the reflection of sound waves back towards the propeller. This leads to a decreased likelihood of re-ingesting disturbed airflows—known contributors to both tonal and broadband noise. The combined effect of reduced turbulent reflections and stabilized hydrodynamic pressure creates a quieter operational environment, especially significant under ground effect conditions.
The implications of these findings are significant for UAM operations, allowing for the design of quieter and more efficient aircraft. Additionally, they pave the way for the creation of noise-reducing surfaces at vertiports, enhancing community acceptance and adherence to urban noise regulations.
“Our research highlights how innovative porous landing surfaces can significantly lessen noise associated with drones and air taxis, thus contributing to the vision of quieter and more sustainable urban aviation,” concluded Dr. Kamliya Jawahar.
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