Extrapolation of RANS flow data for imrpoved analytical fan tone prediction

The context of this work is the analytical prediction of fan noise based on flow data extracted from a steady-state RANS (Reynolds Averaged Navier-Stokes) simulation. In turbomachinery RANS simulations the flow field is circumferentially averaged at the interface (so-called mixing plane) between two successive blade rows with different rotation speeds. As a consequence no information about the wake and the potential field is exchanged via the mixing plane. Such information is however mandatory for predicting tonal interaction noise in a fan stage. A method to extrapolate the RANS flow solution beyond the mixing plane was derived by the authors in the past. The present study is dedicated to the application and assessment of that method on two realistic test cases: a contra-rotating openrotor at take-off condition, and the Ultra-High Bypass Ratio Fan designed by DLR at approach conditions. The extrapolation method is coupled to the in-house analytical tool PropNoise. The acoustic results are compared to highly resolved unsteady RANS calculations and for the UHBR fan additionally to experimental and empirical results. It is shown that a substantial increase in prediction accuracy is achieved by extrapolating the RANS data beyond the mixing plane. The results also show that the RANS-informed analytical noise prediction approach delivers very realistic estimations of the sound field.