Investigations on Direct and Hybrid Sound Predictions

As a result of dissipative and dispersive properties of numerical methods the accuracy of direct noise prediction degrades with increasing distance from the source. Hybrid approaches are hence applied to predict the acoustic farfield, which rely on an integration of disturbance quantities extracted from the nearfield flow solutions. In order to elaborate the influence of the integration limits on sound prediction, different approaches of the Ffowcs Williams and Hawkings method are systematically applied to an advanced counter rotating propfan configuration within the present study. The solutions of various permeable and impermeable integration surfaces are analysed with respect to nearfield sound radiation and compared against direct sound predictions from the compressible Reynolds-averaged Navier-Stokes solutions which likewise serve as input for the extrapolation routine. Due to the flexibility of the routine, source terms and zones can be selectively excluded from the surface integration, allowing a systematic identification of the origin of dissimilar sound prediction. Subsequent farfield analyses are used to conclude on the propagation and persistency of differences identified in the nearfield predictions.