Author: Wu, Di
Title: A numerical study of airfoil tonal noise and its reduction using fluid-structure interaction
Degree: Ph.D.
Year: 2019
Subject: Hong Kong Polytechnic University -- Dissertations
Noise control
Aerofoils -- Noise
Aeroacoustics
Pages: xxiv, 190 pages : color illustrations
Language: English
Abstract: The sound generated by airfoil trailing edge widely exists in industrial applications, such as the blade of the wind turbine, cooling fan and the propeller of aircraft. However, this sound generation is only a by-product of airfoil's aerodynamic performance, which can be regarded as noise. In the present study, an investigation of fow inducing airfoil tonal noise and its reduction using fluid-structure interaction is implemented in this thesis. The unsteady flow field and aeroacoustic field around a NACA0012 airfoil with inflow condition of a 5° angle of attack, 0.4 Mach number and 5 × 10 4 Reynolds number based on airfoil chord are simulated by the direct aeroacoustic simulation of solving two-dimensional compressible N-S equations and the ideal gas equation of state. The Conservation Element and Solution Element method is selected as the numerical solver because it is a second order accuracy method and broad application in solving kinds of physical problems, such as the shock wave capture, acoustic wave feedback. A validation process is carried out to verify the grid settings and calculation code with previous researchers works. In order to give an in-depth understanding of tonal noise generation, two-dimensional linear stability analysis is applied and coupled with the numerical solver to further estimate the development of infinitesimal perturbation in a laminar boundary layer. Based on the solutions of linear stability analysis, the mechanism of airfoil tonal noise generation is discussed. It is found that an acoustic feedback loop is existed and dominate the noise generation.
With the purpose of suppressing the acoustic feedback loop, the flexible component is introduced to the airfoil. Several cases with different flexible component locations of x =0.4, 0.45 and 0.9, denoted as FP1, FP2 and FP3, are built, respectively. Two-dimensional linear stability analysis is applied again as an effective tool to predict the flow responds. It is found that FP1 with the flexible component location of x =0.4 receives the most significant possibility to lead a noise reduction. This result is entirely different with previous researchers views that applying a geometry modification on airfoil leading or trailing edge to reduce the noise. Further complete numerical simulation on airfoil with flexible component (FP1) verifies the prediction results by linear stability analysis. A detail investigation on flexible component vibration and flow field responds is implemented. The relationship in fluid-structural interaction is also discussed. The calculation results show even without a resonance between the flexible component vibration and hydrodynamic fluctuation on airfoil suction side, a remarkable noise reduction is found. Moreover, the application of flexible component will not give a fatal influence to the airfoil aerodynamic performance. The decreasing of ratio of lift to drag is only 1.298%. Coupled with the noise reduction, the present work shows there is an excellent potential to lead a tonal noise reduction by using flexible component with a negligible loss of aerodynamic performance. The main contributions of the present work can be summarized as follows. - The fluid-structural interaction shows a great potential to reduce the airfoil tonal noise without a noticeable aerodynamic performance loss. - The mechanism of tonal noise generation, known as the feedback loop, is firstly considered in the attempt of noise control. - The linear stability analysis is firstly used to predict the performance of the fluid-structural interaction. - The location of the flexible panel is entirely different with previous researchers' opinions that applying leading or trailing edge modification to reduce the noise.
Access: open access

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