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Influence of end plate placement on the reduction of airfoil tip vortex formation noise

The tonal noise generated at the tip is a major contribution to the aerodynamic noise generated by three-dimensional airfoils. One way to reduce this noise is the use of end plates, which was investigated in the present study for three different circular end plate geometries applied to a cambered NACA 4412 airfoil with aspect ratio 2 and forced boundary layer transition. Microphone array measurements were performed in a small aeroacoustic wind tunnel for chord based Reynolds numbers between 75,000 and 200,000 and geometric angles of attack between 0° and 20°. The acoustic far-field noise spectra obtained for the baseline configuration show a broadband hump centered at a chord based Strouhal number of 13 that is associated with the noise generation at the tip and scales with the third power of the flow velocity. The application of end plates reduces this peak and is most effective for end plates which bound the flow on the suction side of the airfoil. Hot-wire measurements taken for one configuration show that the end plates diffuse the turbulence intensity in the relevant frequency range and reduce the fluid transfer along the pressure gradient at the tip. The suction side end plate additionally prevents the interaction of turbulent structures with the trailing edge and is therefore more effective in reducing noise. Surface flow visualizations for this configuration reveal a separation line extending along the full span up to the tip while the surface flow of the baseline and pressure configuration is strongly affected by the flow swept around the tip. Therefore, the placement of the end plate on the suction side is more effective in reducing tip noise.