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Serrations are commonly employed to mitigate the turbulent boundary layertrailing-edge noise. However, significant discrepancies persist between modelpredictions and experimental observations. In this paper, we show that thisresults from the frozen turbulence assumption. A fully-developed turbulentboundary layer over a flat plate is first simulated using the large eddysimulation (LES) method, with the turbulence at the inlet generated using thedigital filter method (DFM). The space-time correlations and spectralcharacteristics of wall pressure fluctuations are examined. The simulationresults demonstrate that the coherence function decays in the streamwisedirection, deviating from the constant value of unity assumed in the frozenturbulence assumption. By considering an exponential decay function, we relaxthe frozen turbulence assumption and develop a prediction model that accountsfor the intrinsic non-frozen nature of turbulent boundary layers. To facilitatea direct comparison with frozen models, a correction coefficient is introducedto account for the influence of non-frozen turbulence. The comparison betweenthe new and original models demonstrates that the new model predicts lowernoise reductions, aligning more closely with the experimental observations. Thephysical mechanism underlying the overprediction of the noise model assumingfrozen turbulence is discussed. The overprediction is due to the decoherence ofthe phase variation along the serrated trailing edge. Consequently, the ratioof the serration amplitude to the streamwise frequency-dependent correlationlength is identified as a crucial parameter in determining the correctprediction of far-field noise.

The impact of non-frozen turbulence on the modelling of the noise from serrated trailing edges