and then jumps to 25% of the chord at u18.5 deg. This corresponds to the arrival of the reversed- ow region (SLV) in its upstream movement along the airfoil surface. The results at x=cD 0:52 and 0.84 (Figs. 2c and 2d) show a similar trend, differing only in the timing of the events. In each case, the initial viscous layer, which is already quite thick, further thickens as the incidence increases to u18.0 deg. At this point it is no longer possible to deŽ ne a vortical layer thickness because the boundary of the vortical layer has moved outside the visualization Ž eld. In each case, during the downward pitching phase of the oscillation, the  ow recovers, completing the hysteresis loop, even though only a part of the thumb curve lies within the Ž eld of vision.