On the effect of the tip-clearance ratio on the aeroacoustics of a diffuser-augmented wind turbine

Lattice-Boltzmann Very-Large-Eddy Simulations of two Diffuser-Augmented Wind Turbines are carried out to investigate the effect of the tip-clearance (TC) ratio on both the flow field and the far-field noise. The DonQi® wind turbine, a three blades ducted rotor with nominal TC of 2:5%, is chosen as reference test case. The second configuration has TC equal to 0:7%. The latter shows flow separation on the diffuser suction side causing lower velocity at the rotor plane and a reduction of 5% of the thrust coefficient. Flow separation is associated with the break down of the tip vortex immediately after the rotor plane. The TC has an effect on the far-field noise. For angles between 60+ and 120+, where 0+ corresponds to the axial upstream direction, the blade tonal noise is the dominant source. For other angular directions, noise increase is found for the smaller TC and it associated to an additional noise source located into the gap that can be modeled as a monopole source. It causes an increase of broadband noise at frequencies higher than the third blade passing frequency and tonal peaks at frequencies equal to 4.5 times the blade passing frequency and higher harmonics