| Author: | Arif, Muhammad Irsalan |
| Title: | Airfoil tonal noise reduction by means of localized flow-induced panel vibration |
| Advisors: | Leung, Chi Kin Randolph (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Acoustical engineering Aerofoils -- Noise Noise control Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxix, 286 pages : color illustrations |
| Language: | English |
| Abstract: | In this research a novel passive control method for airfoil tonal noise using localized flow-induced vibration is proposed and numerically explored with a short elastic panel flush mounted on the suction surface of a NACA 0012 airfoil at a low Reynolds number of 5 × 10⁴ and Mach number of 0.4. The numerical model is governed by two-dimensional compressible Navier-Stokes equations together with equation of state and solved by direct aeroacoustic simulation (DAS) solver based on the conservation element and solution element (CE/SE) method. The key idea is to absorb the energy of natural instabilities arising in the laminar boundary layer by locally self-sustained flow-induced vibration of the short panel which results in reduced flow instabilities for scattering at airfoil trailing edge and weakened aeroacoustic-feedback loop responsible for tonal noise radiation without any adverse effect on airfoil aerodynamics. In the first part of research, a complete methodology of elastic panel design based on its material, structural properties, location etc is developed and the noise reduction potential of the designed airfoil configurations with panel is evaluated using a reduced order model, namely perturbation evolution method (PEM). The developed PEM technique allows much quicker panel design iterations with inputs of reasonable approximation and only requires 10% of the computing time required for a corresponding full DAS. The effects of panel resonance and non-resonance condition under the fluid loading are also evaluated by PEM which reveal that a resonant panel located just ahead of the sharp growth of natural boundary layer instability within the airfoil separation bubble provides the maximum noise reduction. Secondly, high-fidelity DAS calculations are carried out for the optimum elastic panel airfoil configuration to uncover the mechanism of tonal noise reduction using localized flow-induced vibration in a quantitative manner. The analysis of numerical results reveals that a resonating elastic panel just at the onset of sharp growth of boundary layer instability provides an overall tonal noise reduction up to 3 dB. Such significant noise reduction is achieved without any sacrifice in the original aerodynamic characteristics of the airfoil. In the latter part of this research, the designed approach for airfoil tonal noise reduction is further enhanced by introducing structural coupling of elastic panels over the airfoil. The results of comprehensive aerodynamic and acoustic analyses, using high fidelity direct aeroacoustic simulation, of airfoil-panel configurations, show that an average and maximum noise reduction up to 7.6 dB and 7.9 dB can be achieved respectively, without any adverse effect on overall airfoil aerodynamics when strong coupled structural resonance between the panels prevails. This noise reduction is higher than twice of that from the configuration with a single panel which firmly illustrates the synergy of coupled flow-induced structural resonance of the panels prevailing in noise reduction. Finally, an airfoil with multi-panel configuration is proposed which could provide tonal noise reduction for a range of angles of attack. A detailed design concept is presented based on the rigid airfoil characteristics at different angles of attack. Different extent of noise reduction by designed airfoil configuration is observed for the range of angle of attack making it a promising approach for modifying the acoustics of existing aerodynamic or wing profiles operating at variable loading conditions. |
| Rights: | All rights reserved |
| Access: | open access |
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