Empirical Rotor Broadband Noise Prediction Using CFD Boundary Layer Parameter Extraction

In this study, for small unmanned aerial vehicle aeroacoustic predictions, an in-house acoustic code was extended for broadband noise using the empirical Brooks–Pope–Marcolini (BPM) method. The acoustic code was coupled with a comprehensive analysis tool (FLIGHTLAB) or Reynolds-averaged Navier–Stokes (RANS) method to predict the noise from a DJI 9443 CF rotor. The effects of two input parameters of the BPM method on the broadband noise were investigated, including the effective angle of attack and boundary layer parameters. The empirical formula for the boundary layer parameters was replaced with predictions using two-dimensional RANS, allowing arbitrary airfoils other than NACA0012. The boundary layer parameters were also predicted using three-dimensional RANS to capture a three-dimensional flow effect, which can be dominant in modern propellers operating at high rotational speeds. As a result, the three-dimensional effect on the boundary layer was confirmed to conflict with the BPM method, which was developed based on a two-dimensional chordwise flow database. Finally, mid- and high-frequency noise spectra were predicted from the three-dimensional hybrid RANS/LES simulation to be combined with the BPM results, thus improving the noise spectra predictions at midrange frequencies.