Empirical prediction of flap tip noise.

Abstract

In this paper, DLR’s empirical prediction model for flap tip noise is presented and discussed in details. The prediction scheme is based on a comprehensive acoustic and aerodynamic database acquired in the Acoustic Wind Tunnel Braunschweig. It was verified, through successful scaling of the measured noise spectra, that the cross-flow velocity at the flap tip is an important parameter characterizing the flow mechanism(s) responsible for the noise production. This finding led to the definition of a universal flap tip noise spectral shape in terms of a linear least-squares fit of the corresponding measurement data. Using a similar approach, a model for the flap tip noise directivity was formulated. The prediction model was compared against full-scale fly-over measurement data (B747-400 and A319) and an acceptable agreement of the overall predictions was found. A slight underprediction of the noise levels at high frequencies suggests that additional airframe noise sources might be needed in the complete aircraft noise prediction scheme to get a better agreement between measured and predicted noise levels. It is also found that, for large flap deflection angles, flap tip noise dominates the high frequency part of the predicted complete aircraft high-lift noise spectra. Knowledge of the flap tip noise peak frequency and high-frequency decay are therefore sufficient to account for this noise source in the total aircraft noise prediction. Finally, the limitations of the prediction scheme are discussed and research needs are identified.