Open-rotor noise prediction with a RANS informed analytical method

Analytical models are informed by a steady-state RANS calculation with mixing plane in order to predict turbomachinery noise with a short computing time; the method is tested on the example of a contra-rotating open-rotor propulsion system at take-off condition; the results are compared to these of an unsteady RANS calculation with the pressure far field given by the Ffowcs Williams–Hawkings solution for a porous integration surface. Regarding the rotor-alone tones, the sound power level agrees within 0.5 dB between the two methods; for the interaction tones, the discrepancy is around 2 dB reaching up to 10 dB locally on the directivity. So far the sound-generating mechanisms considered by the acoustical models are the steady loading and thickness tonal components and the unsteady periodic loading of the aft rotor due to the interaction with the front-rotor wakes. Inherently, the prediction of interaction noise with the RANS-informed analytical approach lacks accuracy for two main reasons. First, the determination of the unsteady lift relies on a gust response function, the prediction of which ist not commonplace for real loaded blades; the analytical solution for a flat plate is applied in the present study. Second, the wake velocity profiles used as input for the gust response function are mixed out at the interface between the two rotors typically located in the middle of the rotor interval; ideally the values at the aft-rotor leading edge should be used. An evaluation of these two problems shall be undertaken before using the RANS-informed analytical method for aeroacoustic design optimisation.