There exists for flows past certain airfoils, near the onset of stall a naturally occuring unsteady flow oscillation that is very low in frequency, with the Strouhal number typically on the order of .02. Here the Strouhal number is defined as S_r = f c sin a/Uh where the f is the oscillation requency, c is the airfoil chord, a is the angle of attack, and Uh is the freestream speed. This low-frequency flow oscillation was studied in detail at low Reynolds numbers for an LRN(1)-1007 airfoil by Zaman et al.,¹ who concluded that the flow oscillation involved quasiperiodic switching between stalled and unstalled states. Evidence gathered from oil-flow and laser-sheet-flow visualization led Bragg et al.² to suggest that the unsteady stall was related to the growth and bursting of a laminar seperation bubble. The oil-flow visualization also showed that the flowfield leading up to stall was two dimensional.³

Similar low-frequency unsteady stalling phenomena have been observed on several airfoils over a large range of Reynolds number.^4-6 Although much research has been carried out, the details of the unsteady flowdfield and the dynamics of the laminar seperation bubble remain unclear. In an effort to resolve these questions, conditionally averaged laser Doppler velocimeter (LDV) measurements were performed for the upper surface flowfield of the LRN(1)-1007 airfoil. The results of the present measurements quantified the changing character of the flowfield as a cycle of a leading-edge bubble formation, growth, merger with a growing trailing-edge seperation, and resulting airfoil stall, which completes one period of the oscillation. The purpose of this Note is to present these results.

¹Zaman, K. B. M. Q., McKenzie, D. J., and Rumsey, C. L., "A Natural Low-Frequency Oscillation over Airfoils near Stalling Conditions." Journal of Fluid Mechanics, Vol. 202, 1989, pp.403-442.

²Bragg, M. B., Heinrich, D. C., Balow, F. A., and Zaman, K. B. M. Q., "Flow Oscillation over an Airfoil near Stall," AIAA Journal, Vol. 34, No. 1, 1996, pp. 199-201.

³Heinrich, D. C., "An Experimentall Study of a Low-Reynolds Number Airfoil near Stall," M.S. Thesis, Dept. of Aeronautical and Astronautical Engineering, Univ. of Illinois, Urbana, IL, 1994.

⁴Farren, W. S., "The Reaction on a Wing Whose Angle of Incidence is Changing Rapidly—Wind-Tunnel Experiments with a Short Period Recording Balance," Aeronautical Research Council, ARC R&M 1648, His Majesty’s Stationary Office, London, Jan. 1935.

⁵Bragg, M. B., Khodadoust, A., and Spring, S. A., "Measyrements in a Leading-Edge Seperation Bubble Due to a Simulated Airfoil Ice Accretion," AIAA Journal, Vol. 30, No. 6, 1992, pp. 1462-1467.

⁶Mabey, D. G., "Review of Normal Force Fluctuations on Aerofoils with Seperated Flow," Progress in Aerospace Sciences, Vol. 29, No. 1, 1992, pp. 43-80.