• aeroacoustics
• aerodynamics
• computational fluid dynamics
• diffuser
• noise
• sound pressure level
• tip vortex
• wind energy
• wind turbines

The purpose of the study is to analyze the effect of the tip clearance on the aerodynamic and acoustic performance of a Ducted Wind Turbine. Because of the availability of reference data, the commercial DonQi Wind Turbine is used as reference geometry. Two tip clearance distances (TC) equal to 4% and 1% of the radius are considered. They are respectively double and half of the nominal DonQi wind turbine tip clearance, which correspond to the size of the turbulent boundary layer thickness over the diffuser at the rotor plane.

The hydrodynamic flow field is computed using the Lattice-Boltzmann solver Exa-PowerFLOW, while the acoustic field is obtained with the FW-H analogy.

Results show that the thrust coefficient is weakly affected by the tip clearance size. On the other hand, the acoustic response is strongly sensitive to the tip clearance size. The TC 4% geometry is quieter than the TC 1% one with maximum Overall Sound Pressure Level (OASPL) reduction up to 4 dB. This is linked to the interaction taking place between the blade tip vortex and the turbulent boundary layer developing over the diffuser.

Furthermore the fluid dynamics characterizing the two tip clearance cases is different. The tip clearance 4% shows a tip vortex shed by the blade which is maintaining its coherence longer. The tip clearance 1% instead, presents an instable tip vortex which is quickly breaking up due to its interaction with the diffuser boundary layer.