  The flow-induced flutter has a significant role in aircraft stability,
renewable energy extraction, animal locomotion, among many other applications.
While being a ubiquitous phenomenon, the control of the flutter response has
been primarily limited to simplified systems and, often, with the help of
linear inviscid flow theories. In this paper, we numerically investigate how
the plunging response of a foil can be regulated using an active flap to
improve structural safety or enhance the energy extraction efficiency of the
foil with a tightly coupled fluid-structure interaction algorithm. A broad
range of foil and flap settings was tested and their flow dynamics have been
investigated. A novel multi-scale modal analysis technique suitable for
fluid-structure interaction systems successfully isolates the active
flap-induced and the flow-induced modes. It is observed that the competition
between these two modes dictates the plunging response of the foil. The active
flap can modulate the leading edge vortex shedding with larger flapping
amplitude and regulate the foil heaving motion. The ratio of the competing
modal energy is proposed to evaluate the control efficacy of the morphing
surface, and the onset of the lock-in is associated with the ratio approaching
unity. It is shown that the morphing flap is a good candidate for active flow
control.
