High-speed PIV analysis of trailing edge aeroacoustics

Abstract

Tonal noise generated by airfoils observed at low to moderate Reynolds numbers is related to laminar boundary layer instabilities, which has lead to the term laminar boundary layer instability noise. The particular features of the acoustic spectrum have been discussed and a number of theories have been proposed in literature over the past 50 years. Previous research suggests that the appearance of tonal noise is related to a feedback between the acoustic waves scattered at the trailing edge and the receptive part of the boundary layer [4]. Reported studies have been performed on the basis of hot-wire anemometry, laser doppler velocimetry, phase-locked PIV, acoustic measurements, numerical simulations or theoretical models. In recent years, PIV has become an alternative for the investigation of aeroacoustic sources. In particular the aeroacoustic sources for trailing edge noise have been investigated by Schröder et al. [10] using time-resolved PIV and Shannon and Morris [11] based on phase-locked PIV. Nakano et al. [8] focussed on the subject of laminar boundary layer instability noise on an airfoil and related their occurrence on the pressure side to noise emissions based on a correlation based technique. In the present study, planar high-speed PIV is performed simultaneously with acoustic far-field measurements. This combination allows to associate features of the acoustic emissions to events in the source field near the trailing edge for a better understanding of the tonal noise generation on an airfoil. In the past a wide range of Reynolds numbers has been investigated for different airfoil models, most notably the NACA0012 which is also selected for the present study. In particular, it is found that a periodic amplitude modulation of rapidly growing instabilities on the pressure side of the airfoil is responsible for the occurrence of multiple tones for the present configuration