Unsteady Boundary Layer Flow Reversal in a Longitudinally Oscillating Flow.
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OHIO
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Measurements of boundary layer flow reversal made in a wind tunnel on a stationary NACA 0012 airfoil immersed in a longitudinally oscillating subsonic flow are described. A split-film sensor provided flow reversal data in regions above the airfoil surface throughout the period of flow oscillation. At angles of attack below the steady-flow stall angle of attack and for small amplitude oscillations, regions of boundary layer flow reversal near the leading edge were observed to vary nominally in phase with the time-varying external velocity. However, an increase in the oscillation amplitude produced a phase lead and an increased down-stream extent of the flow reversal region. Variations in the oscillation frequency produced only small changes in the shape of the flow reversal region. For angles of attack greater than the steady-flow stall angle, the growth and decay of reversed flow regions were observed to be stronger functions of the velocity oscillation frequency and amplitude. Increases in oscillation amplitude produced larger reversal regions which nominally lagged the phase of the free stream velocity. It is shown that boundary layer flow reversal on a stationary airfoil in an oscillating free stream responds to changes in the flow unsteadiness in a manner similar to the flow reversal observed during the onset of dynamic stall on an airfoil oscillating in pitch.
- Fluid Mechanics