Numerical Investigation of Rotor Aeroacoustics in Non-Axial Inflow Conditions using the Lattice Boltzmann Method

This work presents high-fidelity numerical simulations of a two-bladed rotor operating under non-axial inflow conditions, targeting the identification of broadband noise mechanisms relevant to eVTOL rotor operation. A Mejzlik 12 by 6 rotor is simulated using the PowerFLOW Lattice Boltzmann Method solver, with far-field acoustics predicted using the FW-H acoustic analogy. Three inflow tilt angle conditions are considered, being edgewise, 12 degrees into the flow, and 30 degrees into the flow, replicating experimental test conditions for acoustic validation. The simulations reproduce the main experimental spectral trends, including the mid-frequency broadband hump observed in the edgewise case and the flatter broadband response at 30 degrees. The 12 degree case shows similar broadband features but with greater under-prediction of the measured levels, indicating sensitivity to the precise wake trajectory and interaction strength. Full 360 degree simulated directivity shows that increasing inflow angle progressively reduces radiated noise, with the largest reductions occurring in the broadband frequency range. This reduction is linked to changes in the position of the preceding blade wake and tip vortex relative to the following blade. Blade-level analysis shows that, in the edgewise case, the tip vortex interacts with the suction side and produces elevated broadband surface pressure fluctuations. At 12 degrees, the interaction is displaced to the pressure side and weakens, while at 30 degrees direct tip-vortex impingement is largely avoided. These results show that wake trajectory is a key driver of broadband rotor noise under non-axial inflow.