The impact of non-frozen turbulence on the noise prediction for serrated trailing edges

Abstract

Serrations are widely used to reduce the turbulent boundary layer trailing edge noise. However, pronounced discrepancies are still observed between model predictions and experiments, which may be due to the frozen turbulence assumption. This paper aims to relax the frozen turbulence assumption and develop a prediction model that accounts for the intrinsic non-frozen nature of turbulent boundary layers. To achieve this, we simulate a fully developed flat plate turbulent boundary layer using the Large Eddy Simulation (LES) method, with the inlet boundary condition generated by the Digital Filter Method (DFM). The one-point frequency spectra and space-time correlations are studied. It is shown that the coherence function decays in the streamwise direction, which is hypothesized to be unity in the frozen turbulence assumption. By assuming an exponential decay, the prediction model for serrated trailing edges is modified and then simplified. A correction coefficient is introduced by properly approximating the response function. The comparison between the modified and original models indicates that the modified model predicts lower noise reductions. Furthermore, it is shown that the streamwise correlation length is an important parameter that needs to be determined accurately in future studies. Overall, this study sheds light on the limitations of the frozen turbulence assumption and provides a more accurate prediction model for the effectiveness of serrated trailing edges in reducing noise.