Investigation of blunt trailing edge noise from an asymmetric airfoil under different aerodynamic loads

This work deals with the noise from the trailing edge of a two-dimensional airfoil segment in airflow. Of interest is the mechanism of flow-induced noise generation at the trailing edge with special consideration of its bluntness.
Unlike in many well-known studies, a typical asymmetric airfoil is chosen for the segment, here the DU93W210 airfoil. This is frequently used for wind turbines, for example. By shortening the chord length, the bluntness of the trailing edge is increased in several steps. Furthermore, the inflow velocity and the angle of attack are varied. An enlargement of the angle of attack corresponds to an increase of the aerodynamic load on the airfoil. The study is limited to the case of a fully turbulent boundary layer from leading to trailing edge on both, the suction and pressure side. This is achieved by operation at relatively large values of the chord-based REYNOLDS number (up to 1.2x106) in combination with a carefully chosen boundary layer tripping. The results of the study are primarily obtained using the numerical Lattice- BOLTZMANN method as a computational aeroacoustic method, but are successfully validated randomly by measurements in an aeroacoustic wind tunnel.
The observed flow-induced 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 is increased, the tone disappears and only broadband noise is present. The frequency of the tone decreases with the bluntness and increases with the REYNOLDS number. The criterion of BLAKE for the existence of a tone at blunt trailing edges turns out to be necessary but not sufficient. Already an analysis of the time-averaged velocity field shows that with increase of the bluntness a second vortex gradually develops in the near wake. The vortices alternately detach from the trailing edge and a vortex street is generated. Using methods of modal analysis, the flow field could be decomposed into coherent structures. One of these coherent structures, which is spatially and temporally resolved in detail, represents the vortex street described above. At high bluntness and comparatively low aerodynamic loading of the airfoil segment, the shed vortices are located close to the trailing edge. As the aerodynamic loading of the airfoil increases, the asymmetry of the flow between the suction- and pressure-side boundary layers becomes larger. As a result, even with comparatively blunt trailing edges, the vortices form further downstream from the trailing edge, which explains why the typical "blunt trailing edge" signature is absent.