Bifurcating flows of plunging airfoils at high Strouhal numbers


Cleaver, D. J., Wang, Z. and Gursul, I., 2012. Bifurcating flows of plunging airfoils at high Strouhal numbers. Journal of Fluid Mechanics, 708, pp. 349-376.

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    Force and particle image velocimetry measurements were conducted on a NACA 0012 aerofoil undergoing small-amplitude high-frequency plunging oscillation at low Reynolds numbers and angles of attack in the range 0–$2{0}^{\ensuremath{\circ} } $. For angles of attack less than or equal to the stall angle, at high Strouhal numbers, significant bifurcations are observed in the time-averaged lift coefficient resulting in two lift-coefficient branches. The upper branch is associated with an upwards deflected jet, and the lower branch is associated with a downwards deflected jet. These branches are stable and highly repeatable, and are achieved by increasing or decreasing the frequency in the experiments. Increasing frequency refers to starting from stationary and increasing the frequency very slowly (while waiting for the flow to reach an asymptotic state after each change in frequency); decreasing frequency refers to impulsively starting at the maximum frequency and decreasing the frequency very slowly. For the latter case, angle of attack, starting position and initial acceleration rate are also parameters in determining which branch is selected. The bifurcation behaviour is closely related to the properties of the trailing-edge vortices. The bifurcation was therefore not observed for very small plunge amplitudes or frequencies due to insufficient trailing-edge vortex strength, nor at larger angles of attack due to greater asymmetry in the strength of the trailing-edge vortices, which creates a preference for a downward deflected jet. Vortex strength and asymmetry parameters are derived from the circulation measurements. It is shown that the most appropriate strength parameter in determining the onset of deflected jets is the circulation normalized by the plunge velocity.


    Item Type Articles
    CreatorsCleaver, D. J., Wang, Z. and Gursul, I.
    DepartmentsFaculty of Engineering & Design > Mechanical Engineering
    Research CentresAerospace Engineering Research Centre
    Publisher StatementJFM_Final_-_600dpi.pdf: This paper has been accepted for publication and will appear in a revised form, subsequent to peer review and/or editorial input by Cambridge University Press, in Journal of Fluid Mechanics published by Cambridge University Press, © Cambridge University Press.
    ID Code30278


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