  The separated flow response to single and multiple burst mode actuation over
a 2-D airfoil at $12^o$ angle of attack was studied experimentally. For the
single-burst actuation case, surface pressure signals were correlated with the
flowfield observations of the roll-up and convection of a large-scale vortex
structure that follows the actuator burst input. A spatially localized region
of high pressure occurs below and slightly upstream of a "kink" that forms in
the shear layer, which is responsible for the lift reversal that occurs within
$2.0t^+$ after the burst signal was triggered. Proper orthogonal decomposition
of the single-burst flow field shows that the time-varying coefficients of the
first two modes correlate with the negative of the lift coefficient and
pitching moment coefficient. The dynamic mode decomposition (DMD) of the
single-burst flow field data identified the modes related to the kinetic energy
growth of the disturbance. The mode with the largest growth rate had a Strouhal
number close to that associated with the separation bubble dynamics. However,
when multiple bursts are used to control the separation, the interactions
between the bursts were observed which depend on the time intervals between the
bursts. The convolution integral and DMD were performed on the multi-burst flow
field datasets. The results indicate that the nonlinear burst-burst interaction
only affects the reverse flow strength within the separation bubble, which is
related to the main trend of the lift. On the other hand, the linear
burst-burst interaction contributes to the high-frequency lift variation
associated with the bursts.
