Delay of Stall With Self-Vibrating Mini-Flag Attached to an Airfoil

Junchen Tan, Zhijin Wang, Ismet Gursul

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Experiments were carried out in a wind tunnel to study the effects of a small flag attached to the surface of an airfoil. It was found that the flag exhibited quasi-periodic oscillations between the airfoil surface and the freestream at post-stall angles of attack of the clean airfoil when it was attached near the leading-edge. Digital image correlation (DIC) measurements of deformation field and particle image velocimetry (PIV) measurements of velocity field indicated that the limit cycle oscillations of compliant latex flags produced leading-edge vortices when the flag tip reached its maximum displacement, thus forming a separation bubble, and promoting flow reattachment to the airfoil surface. As a result, the stall angle could be delayed significantly, and the lift coefficient could be increased remarkably, e.g., up to 73% increase in the mean lift coefficient of the clean airfoil in the post-stall regime was observed in the present investigation. Proper orthogonal decomposition analyses of the flag deformation field showed that the dominant mode of flag oscillations occurred mostly in the first beam mode, whereas the higher modes in spanwise direction revealed the three-dimensionality of the flag deformation field. Flags reinforced with plastic shims, which have larger bending rigidity, however, can improve the two-dimensionality of the flag displacement fields, thus improving the lift enhancement and stall-delay even further. The oscillation frequencies of the flags suggest coupling with wake instabilities and wake resonance at the post-stall angles of attack. This is a passive flow control method that relies on unsteady effects and requires no external power.
Original languageEnglish
Number of pages19
Publication statusPublished - 29 Dec 2021
EventAIAA Science and Technology Forum and Exposition (SciTech 2022) - San Diego, CA, USA United States
Duration: 3 Jan 20227 Jan 2022


ConferenceAIAA Science and Technology Forum and Exposition (SciTech 2022)
Country/TerritoryUSA United States
CitySan Diego, CA

ASJC Scopus subject areas

  • Aerospace Engineering


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