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Kathrin Stahl
Investigation of Blunt Trailing Edge Noise from an Asymmetric Airfoil under Different Aerodynamic Loads
Of interest in this work is the mechanism of flow-induced noise generation at the trailing edge of a two-dimensional airfoil segment with special consideration of its bluntness.
Unlike in many well-known studies, an asymmetric airfoil is chosen. By shortening the chord length, the bluntness of the trailing edge is increased. Furthermore, the velocity and the angle of attack are varied. The study is limited to the case of a fully turbulent boundary layer on both airfoil sides. This is achieved by operation at relatively large values of the chord-based REYNOLDS number in combination with boundary layer tripping. The results of the study are primarily obtained using the numerical Lattice-BOLTZMANN method but successfully validated by measurements in an aeroacoustic wind tunnel.
The observed noise is broadband with a case-dependent pronounced tonal component. As long as the aerodynamic load on the airfoil, i.e. the angle of attack is comparatively low, a dominant tone exists with a sound pressure level that increases with the bluntness of the trailing edge. If the load increases, the tone disappears and only broadband noise is present. The frequency of the tone decreases with bluntness and increases with the Reynolds number. With increasing bluntness a second vortex gradually develops in the near wake. The vortices alternately detach from the trailing edge, a vortex street is generated. At high bluntness and comparatively low aerodynamic loading, the shed vortices are located close to the trailing edge. As the aerodynamic loading of the airfoil increases, the asymmetry of the flow becomes larger. As a result, even with comparatively blunt trailing edges, the vortices form further downstream, which explains why the typical "blunt trailing edge" signature is absent.
Unlike in many well-known studies, an asymmetric airfoil is chosen. By shortening the chord length, the bluntness of the trailing edge is increased. Furthermore, the velocity and the angle of attack are varied. The study is limited to the case of a fully turbulent boundary layer on both airfoil sides. This is achieved by operation at relatively large values of the chord-based REYNOLDS number in combination with boundary layer tripping. The results of the study are primarily obtained using the numerical Lattice-BOLTZMANN method but successfully validated by measurements in an aeroacoustic wind tunnel.
The observed noise is broadband with a case-dependent pronounced tonal component. As long as the aerodynamic load on the airfoil, i.e. the angle of attack is comparatively low, a dominant tone exists with a sound pressure level that increases with the bluntness of the trailing edge. If the load increases, the tone disappears and only broadband noise is present. The frequency of the tone decreases with bluntness and increases with the Reynolds number. With increasing bluntness a second vortex gradually develops in the near wake. The vortices alternately detach from the trailing edge, a vortex street is generated. At high bluntness and comparatively low aerodynamic loading, the shed vortices are located close to the trailing edge. As the aerodynamic loading of the airfoil increases, the asymmetry of the flow becomes larger. As a result, even with comparatively blunt trailing edges, the vortices form further downstream, which explains why the typical "blunt trailing edge" signature is absent.
Schlagwörter: blunt trailing edge noise; vortex shedding noise; airfoil; turbomachinery; wind turbine noise; lattice-Boltzmann method; LBM; wind tunnel experiments; far-field acoustics; spectral proper orthogonal decomposition; SPOD; POD; high Reynolds number flow
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DOI 10.2370/9783844092851
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