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

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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.

Original languageEnglish
Title of host publicationAIAA Scitech 2019 Forum
Subtitle of host publication7-11 January 2019, San Diego, California, USA
PublisherAmerican Institute of Aeronautics and Astronautics Inc. (AIAA)
Number of pages15
ISBN (Electronic)978-1-62410-578-4
Publication statusPublished - 2019
EventAIAA Scitech Forum, 2019 - San Diego, United States
Duration: 7 Jan 201911 Jan 2019


ConferenceAIAA Scitech Forum, 2019
Country/TerritoryUnited States
CitySan Diego
Internet address

Bibliographical note

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.


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