Periodic Reporting for period 1 - ANACLETO (Noise and drag reduction by riblets.)
Reporting period: 2020-07-01 to 2022-06-30
The project aims at mitigating the noise produced by an airfoil by applying surface treatments at the trailing-edge, therefore through a passive technique of flow control. These surface treatments are protrusions oriented along the flow, and they are respectively called 'finlets' and 'riblets' depending on whether they are of the order of magnitude of the outer scales or of the viscous scales of the turbulent boundary layer. Finlets as well as riblets can be of different sizes and shapes. In recent research it was found that finlets are capable of mitigating the noise emitted, but they also produce a drag increase of the airfoil. Several research articles showed that riblets can lead to a drag decrease, even though their effects have never been examined in relation to noise. The goal of the present project is to assess the effects of finlets and riblets of different sizes and geometries in terms of drag and noise emission.
The problem that is addressed is of great societal relevance, as trailing-edge noise is among the most significant contributors to the total acoustic emission of a landing aircraft, and it is the main source of noise from a wind turbine. Therefore, two fast-growing sectors that are strategic for the European Union would directly benefit from this research, which are air transportation and clean energy.
- The air traffic has known a massive development over the last decades, and this trend is expected to continue. The acoustic emission from air vehicles represents a strong limiting factor to their further expansion, as noise has been reported to cause health damages to civilians living nearby airports, particularly in relation with prolonged discomfort, stress, and insomnia.
- An important limitation to the on-shore installations of wind turbines is their acoustic emission. On this respect, the regulations on the noise pollution from the on-shore wind farms are becoming growingly more stringent. Mitigating the trailing-edge noise, and, thus, reducing the whole acoustic emission of each wind turbine is expected to enlarge the surface available for their installation and to increase their installation density.
The ultimate objective of this project is to gain noise reduction without paying any drag increase penalty.
The problem that is addressed is of great societal relevance, as trailing-edge noise is among the most significant contributors to the total acoustic emission of a landing aircraft, and it is the main source of noise from a wind turbine. Therefore, two fast-growing sectors that are strategic for the European Union would directly benefit from this research, which are air transportation and clean energy.
- The air traffic has known a massive development over the last decades, and this trend is expected to continue. The acoustic emission from air vehicles represents a strong limiting factor to their further expansion, as noise has been reported to cause health damages to civilians living nearby airports, particularly in relation with prolonged discomfort, stress, and insomnia.
- An important limitation to the on-shore installations of wind turbines is their acoustic emission. On this respect, the regulations on the noise pollution from the on-shore wind farms are becoming growingly more stringent. Mitigating the trailing-edge noise, and, thus, reducing the whole acoustic emission of each wind turbine is expected to enlarge the surface available for their installation and to increase their installation density.
The ultimate objective of this project is to gain noise reduction without paying any drag increase penalty.
Trailing-edge inserts carrying finlets and riblets of different geometries and sizes were connected to an instrumented model of a NACA63018, and tested. Specifically, with regard to the finlets, two different geometries were investigated, which were finlet fences and finlet rails. These inserts were obtained from 3d-printing, which enabled to assess in a parametric analysis the performances of over 20 different inserts. An array of microphones was used for the rigorous measurement of the far-field noise, and, therefore, for a quantification of the noise attenuation resulting from the surface treatment. A wake rake apparatus was also set up for the accurate estimate of the coefficient of drag. The inserts with riblets generated a tonal source of noise, while no important reduction of the drag coefficient was obtained. The application of the inserts with riblets was therefore considered of relatively low interest. On the other hand, the inserts with finlets produced a noise mitigation of up to 6db, particularly at specific frequency bands depending on finlets geometry and size. As expected, this came at the penalty of increasing the coefficient of drag. From finlet fences, a lower noise reduction than from finlet rails was obtained, although they also caused a relatively lower increase of the drag coefficient. Overall, no noise reduction can be obtained without having a certain deterioration of the aerodynamic performances too, in particular an increase of the drag coefficient. However, reducing the lateral spacing between finlets has a stronger beneficial effect on the noise mitigation than increasing their height, whereas increasing the finlet height leads to a heavier increase of the drag coefficient than reducing the transversal spacing. These results give important guidelines for the design and the implementation of the finlets in engineering applications.
After this parametric analysis was completed, numerical simulations as well as experiments of particle image velocimetry were performed on the inserts carrying the most promising finlets geometries. The goal of this second part of the project was to gain a deeper understanding into the physical mechanisms for noise mitigation. It was found that finlets attenuate the turbulent energy in proximity to the trailing-edge, and move the energetic structures of the turbulent boundary layer away from the wall. This attenuates the wall pressure fluctuations which ultimately mitigates the scattering phenomena that lead to noise generation. An additional mechanism that contributes to the mitigation of noise is the reduction of the transversal size of the turbulence structures.
After this parametric analysis was completed, numerical simulations as well as experiments of particle image velocimetry were performed on the inserts carrying the most promising finlets geometries. The goal of this second part of the project was to gain a deeper understanding into the physical mechanisms for noise mitigation. It was found that finlets attenuate the turbulent energy in proximity to the trailing-edge, and move the energetic structures of the turbulent boundary layer away from the wall. This attenuates the wall pressure fluctuations which ultimately mitigates the scattering phenomena that lead to noise generation. An additional mechanism that contributes to the mitigation of noise is the reduction of the transversal size of the turbulence structures.
In the literature, several recent studies, both numerical and experimental, involved finlet fences, while only one publication examined the aeroacoustic behaviour of finlets of a geometry alternative to fences, the so-called finlet rails. The results on the aeroacoustic performances of the finlet rails from the aforementioned sole work evidenced that this geometry can lead to higher levels of noise attenuation compared with the finlet fences. However, only a limited number of configurations was tested so far, and absolutely no quantifications were reported in relation to how the surface treatment affects the drag coefficient. Another aspect that was left unexplored is the mechanism by which finlet rails produced a noise mitigation, and how the flow field around the airfoil with inserts is modified in consequence of bespoke treatment. These aspects were for first time addressed in this project.
The research results on finlet fences presented some remarkable elements of novelty and breakthrough findings too. A wider range of configurations, in terms of finlets height and transversal spacing, was in fact tested compared to past analyses. And for each configuration, both the far-field noise spectrum and the drag coefficient of the airfoil were measured as a function of of the angle of attack and the Reynolds number. It was found that the transversal spacing has a stronger impact on the noise mitigation than the height, while the opposite is true for the drag coefficient, with the height being dominant. This is a very important finding, as it suggests that an optimum exists in the design of the finlet fences, which maximizes the aeroacoustic performances and minimizes the drag penalty.
Overall, the potential socio-economic impact of this research lies in the attenuation of the acoustic emissions of aircrafts and of on-shore wind turbines, while minimizing the inevitable penalties in terms of drag, and thus the reduced efficiency associated with these treatments. Civil aviation and clean energy are sectors that have experienced a considerable increase in the past decade, and that are pivotal to the wealth of the European Union in the years ahead. Reducing their impact on civilians is the main long-term achievement that this project is envisaged to produce, with the ultimate goal to make air transportation sustainable, and to accelerate the transition from fossil fuels to renewable sources of energy, such as wind energy is.
The research results on finlet fences presented some remarkable elements of novelty and breakthrough findings too. A wider range of configurations, in terms of finlets height and transversal spacing, was in fact tested compared to past analyses. And for each configuration, both the far-field noise spectrum and the drag coefficient of the airfoil were measured as a function of of the angle of attack and the Reynolds number. It was found that the transversal spacing has a stronger impact on the noise mitigation than the height, while the opposite is true for the drag coefficient, with the height being dominant. This is a very important finding, as it suggests that an optimum exists in the design of the finlet fences, which maximizes the aeroacoustic performances and minimizes the drag penalty.
Overall, the potential socio-economic impact of this research lies in the attenuation of the acoustic emissions of aircrafts and of on-shore wind turbines, while minimizing the inevitable penalties in terms of drag, and thus the reduced efficiency associated with these treatments. Civil aviation and clean energy are sectors that have experienced a considerable increase in the past decade, and that are pivotal to the wealth of the European Union in the years ahead. Reducing their impact on civilians is the main long-term achievement that this project is envisaged to produce, with the ultimate goal to make air transportation sustainable, and to accelerate the transition from fossil fuels to renewable sources of energy, such as wind energy is.