Lattice-Boltzmann calculations of rotor aeroacoustics in transitional boundary layer regime

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The goal of this paper is to demonstrate the predictive capabilities of the Lattice-Boltzmann/Very-Large-Eddy-Simulation CFD solver SIMULIA PowerFLOW® when a transitional flow past a small drone rotor blade is simulated, without using any numerical or physical turbulence triggering system. A recently developed variant of PowerFLOW VLES model is validated against measurements performed at Delft University of Technology. A 2-bladed propeller of 0.3m diameter is considered, which is operated at 4000 and 5000 RPM and two different advance ratios, 0.0 and 0.6, for each angular velocity. In such conditions, corresponding to a Reynolds number based on the chord at 75% of the tip radius ranging between 7⋅104 and 9⋅104, the presence of a laminar separation bubble was experimentally observed and related to the occurrence of a high-frequency broadband noise hump in the far-field noise spectra. The numerical results are in a good agreement with the loading, noise and PIV experimental measurements, demonstrating the capability of the new PowerFLOW VLES model to predict boundary layer flow transitional phenomena such as laminar separation, reattachment and transition to turbulence, as well as the corresponding broadband trailing-edge noise radiation.