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Published November 2016 | metadata_only
Journal Article

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil


Time-resolved velocity fields around a pitching and surging NACA 0018 airfoil were analyzed to investigate the influence of three independent timescales associated with the unsteady flowfield. The first of these timescales, the period of the pitch/surge motion, is directly linked to the development of dynamic stall. A simplified model of the flow using only a time constant mode and the first two harmonics of the pitch surge frequency has been shown to accurately model the flow. Full stall and leading-edge flow separation, however, were found to take place before the maximum angle of attack, indicating that a different timescale was associated with leading-edge vortex formation. This second, leading-edge vortex, timescale was found to depend on the airfoil convection time and compare well with the universal vortex formation time. Finally, instantaneous non-phase-averaged measurements were investigated to identify behavior not directly coupled to the airfoil motion. From this analysis, a third timescale associated with quasi-periodic Strouhal vortex shedding was found before flow separation. The interplay between these three timescales is discussed in detail, particularly as they relate to the periodic velocity and angle-of-attack change apparent to the blades of a vertical axis wind turbine.

Additional Information

© 2016 by the American Institute of Aeronautics and Astronautics, Inc. Received 1 October 2015; revision received 26 March 2016; accepted for publication 29 April 2016; published online 18 July 2016. This work was supported by the Gordon and Betty Moore Foundation through grant GBMF#2645 to the California Institute of Technology. The authors thank Morteza Gharib for the use of the free surface water channel facility, Hsieh-Chen Tsai and Tim Colonius for discussion on the Coriolis effect, and John Dabiri for his insight from vertical axis wind turbines field research.

Additional details

August 20, 2023
August 20, 2023