  Effects of chord-wise flexibility as an instrument to control chaotic
transitions in the wake of a flexible flapping foil have been studied here
using an immersed boundary method-based in-house fluid-structure-interaction
solver. The ability of the flapping foil at an optimum level of flexibility to
inhibit chaotic transition, otherwise encountered in a similar but rigid
configuration, has been highlighted. The rigid foil manifests chaotic
transition through a quasi-periodic-intermittency route at high dynamic plunge
velocities; whereas, increasing the level of flexibility gradually regularises
the aperiodic behaviour through a variety of interesting wake patterns. If
flexibility is increased beyond an optimum level, aperiodicity sets in again
and robust chaos is restored at very high flexibility levels. The mechanisms of
triggering the order-to-chaos transition are different between the rigid and
the high flexibility cases. Along the route to order and back to chaos, the
flexible foil exhibits different flow-field behaviours, including far-wake
switching, primary \& secondary vortex streets, bifurcated wakes and
interactive vortices between the bifurcated wakes. The underlying interaction
mechanisms of the flow-field vortices responsible for the associated dynamical
signatures of the wake have been closely tracked. This study further examines
the optimum propulsive performance range of the flexible flapper and
investigates its connection with the periodicity/regularity of the system.
