  The experiment of separated flow response to a single-burst actuation over a
2-D NACA-0009 airfoil at $12^o$ angle of attack was conducted. The mechanism of
the lift and pitching moment reversal following the single-burst actuation was
studied. A spatially localized region of high pressure caused by a
vortices-induced downwash is responsible for the lift and pitching moment
reversal. Proper orthogonal decomposition (POD) of the flowfield shows that
mode 2 shares a similar structure that produces the downwash and is responsible
for the lift and pitching moment reversal. On the other hand, POD mode 1, which
represents the direction and strength of the reverse flow on the suction side
of the airfoil is responsible for the lift enhancement.
