The effect of a chordwise flexible aft-tail of a rigid heaving aerofoil on the dynamical transitions of the trailing-wake is studied here. The two-way coupled fluid–solid dynamics is simulated using an in-house fluid–structure interaction (FSI) platform, comprising a discrete forcing immersed boundary method based incompressible Navier–Stokes solver, weakly coupled with a finite difference method based structural solver. The FSI dynamics is studied in comparison to the corresponding rigid tail configuration. For the latter, mild jet-switching due to quasi-periodic movement of the wake vortices gives way to vigorous jet-switching as the dynamics transitions to a state of intermittency, where the quasi-periodic behaviour gets interspersed with chaotic windows. Introduction of a moderately flexible tail regularises this intermittent dynamics, eliminating jet-switching. The wake exhibits a deflected reverse Kármán pattern with fluctuating angles, governed by quasi-periodicity. With a highly flexible tail (very low rigidity), the wake shows almost a symmetric reverse Kármán street as periodicity is restored. Flexibility of the aft-tail is next controlled by changing its length, and flow is regularised and periodicity retained for moderate rigidity for increased length. Different dynamical states are established through robust nonlinear dynamical tools. The underlying flow-field behaviour, instrumental in suppressing the jet-switching phenomenon, is identified through a detailed investigation of the near-field vortex interactions dictated by the dynamics. A suite of measures has also been derived from the unsteady flow field to quantify the interactions of the key near-field vortices with a view to understanding the mechanism of switching and its subsequent suppression through flexibility.

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The present study is focused on examining the flow-field dynamics of a flapping foil with a flexible aft tail as compared to a rigid configuration where tail flexibility is infinite. The flow around the oscillating body is governed by the incompressible Navier-Stokes equations. An in-house Fluid-Structure Interaction solver has been developed following a discrete forcing type Immersed Boundary Method coupled with an inextensible filament structural model. The flapping amplitude is considered as a bifurcation parameter, and as the bifurcation parameter is increased, the periodic wake transitions into the chaotic patterns. The periodic to chaotic transition happens through an intermittency route. However, the elliptic foil with flexible aft tail exhibits chaotic onsets much later compared to the foil with a rigid tail. Time series analysis techniques, such as frequency spectra and recurrence plots, have been used to establish the intermittency and the chaotic dynamics conclusively.

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