Sound generation by a rotor in a thick axisymmetric turbulent boundary layer

The sound generation by a rotor immersed in a thick turbulent boundary layer at the tail end of an axisymmetric body of revolution is investigated using large-eddy simulation and the Ffowcs Williams-Hawkings equation. Two rotor advance ratios corresponding to nominally zero-thrust and thrusting conditions are considered. The computed sound pressure spectra exhibit reasonable agreement with the experimental measurements at Virginia Tech (Hickling et al., AIAA-2019-2571) over a wide range of frequencies. The spectra show broadband noise with haystacking peaks caused by multiple blades cutting through the same turbulence structure. Two-point and space-time velocity correlations are examined to elucidate the evolution of the turbulence structures in the adverse-pressure-gradient boundary layer and their interaction with rotor blades. The strength of blade sectional unsteady-loading dipoles increases with the local chord length, turbulence intensity and rotational speed, and is largest near the location of maximum chord length. Significant blade-to-blade coherence is found and related to the boundary-layer structures and haystacking acoustic peaks.