Erik Schneehagen has successfully defended his PhD thesis: "The aeroacoustic noise reduction effect of end plates for finite airfoils".
The expansion of wind energy and growing air traffic demand for low noise airfoils to reduce nuisance of neighboring residents of onshore wind farms or airports. The aeroacoustic self noise associated with wind turbine blades or landing flaps consists of multiple sound sources and source regions that depend on the airfoil’s operating conditions. At high angles of attack, the cross flow from the pressure to the suction side creates a vortex at the free end of the airfoil which generates sound due to the turbulent interaction with the airfoil surface. This tip vortex formation noise can be a relevant contribution to the overall self noise spectrum for mid to high frequencies.
The application of winglet type end plates is a known method to reduce the vortex generation but research has been focused on the aerodynamic impact rather than the aeroacoustic effect. This thesis aims to investigate the potential noise reduction effect of such end plates that prevent the flow around the free end. For this purpose experimental and numerical studies on a symmetric NACA 0012 and cambered NACA 4412 are conducted.
Microphone array measurements in aeroacoustic wind tunnels are performed to quantify the noise reduction effect on three different subsets of end plates. Aerodynamic measurements, hot-wire measurements in the tip wake as well as surface flow visualizations are employed to understand the effect of the end plates on the flow field. Additionally, for two selected cases a numerical hybrid approach of LES and Curle’s analogy is used to assess the information of the flow field and surface pressures in more detail.
The results suggest that the end plates greatly reduce the transfer of fluid from the pressure to the suction side and, subsequently, diffuse the turbulence intensity at the tip. High pressure fluctuations at the tip trailing edge are reduced which leads to substantial noise reduction in the tip region compared to base configuration. Airfoils with the end plates applied achieve higher maximum lift and the spanwise dependency of the flow is reduced so that the flow field rather resembles a two dimensional configuration. This working principle is qualitative similar for all considered end plates and the quantitative differences are discussed in the thesis. The application of end plates on the suction side is most beneficial for the underlying considerations. Overall, when the contribution of the tip is relevant, end plates effectively reduce the aeroacoustic noise and can potentially help to achieve quieter airfoils.