Coherent Structures at the Serrated Trailing-Edge of a NACA 0012

Abstract

In the past, different modifications of trailing-edges have been investigated with the aim to reduce noise emissions. One promising solution, which has been shown to reduce the intensity of trailing-edge noise, is the application of so-called sawtooth serrations with different proportions and sizes. Current research aims at applying new tools of flow field interrogation, first of all tomographic PIV, to the problem of trailing-edge noise and at creating links between the phenomenological description of the flow field in terms of coherent structures and noise spectra in order to improve understanding of the noise generating mechanisms. In the framework of this thesis, sawtooth serrations applied to a NACA 0012 airfoil have been investigated and compared to the straight trailing-edge of the unmodified NACA 0012 baseline model. Acoustic far-field, surface pressure and a series of PIV measurements have been performed for this purpose. In a first step, acoustic far-field measurements on a large selection of serrated models have been performed. On one hand, tonal narrowband noise associated to the shedding of vortices from the blunt parts of the trailing-edge poses a large penalty for the reduction in overall sound pressure level. On the other hand, the acoustic measurements have indicated a small reduction in noise level originating from the serrated trailing-edge for frequencies in the broadband spectrum exceeding the tonal range, which is in agreement with Howe’s theory of trailingedge noise. Overall, a decrease in total sound pressure level could not be observed. In a second step, PIV measurements of the boundary layer close to the trailing-edge have been performed in order to identify a subset of coherent structures, which can be linked to the production of noise and in particular to the production of tonal noise. Large coherent structures appearing close to the trailing-edge or in between the serrations have been visualized, dominate in terms of vorticity level and show the shape of horseshoe vortices. Most of these structures show a large spanwise coherence on the order of the distance between two serrations and reappear at the frequency of the tonal noise. This suggests a link between this category of coherent structures and the tonal noise observed in the acoustic spectra.Aerospace Engineerin