On the Effect of the Tip Clearance on the Aerodynamic and Aeroacoustics of a Diffuser-Augmented Wind Turbine

A computational study of two Diffuser-Augmented Wind Turbines (DAWTs) is carried out to investigate both the hydrodynamic flow field and the far-field noise. The two configurations differ for the tip-clearance ratio, defined as the ratio between the tip clearance and the rotor radius. The DonQi® wind turbine, a three blades ducted rotor, is adopted as baseline configuration because of the availability of reference data. It has a tip-clearance ratio of 2.5%. The second configuration is obtained from the first one by elongating the rotor radius such to force the interaction between the turbulent boundary layer, developing over the suction side of the diffuser, and the tip of the blades, thus resulting in a tip-clearance ratio of 0.7%. The rotor with the longer blades shows a reduction of the thrust coefficient because of the lower lift generated by the diffuser that results in a lower axial velocity at the rotor plane. It is shown that this is caused by the smaller tip gap that forces the break down of the rotor tip vortex in smaller turbulent structures immediately after the rotor plane and that induces earlier flow separation along the suction side of the diffuser. The tip-clearance ratio has also a strong effect on the far-field noise. For angles between 60◦ and 120◦, where 0◦ corresponds to the axial upstream direction, the blade tonal noise, at frequency equal to the blade passing frequency and higher harmonics, is the dominant source. For other angular directions, noise increase is found for the smaller tip-clearance ratio case associated to an additional noise source, that becomes dominant, linked to an increase of the energy content of velocity fluctuations in the gap region. This noise source, that can be modeled as a monopole source located in the gap, causes an increase of broadband noise at frequencies higher than the third blade passing frequency and a tonal peak at a frequency equal to 4.5 times the blade passing frequency and higher harmonics.