Wake dynamics of actively pitching and passively heaving tandem flappers

The study investigates the wake dynamics of two actively pitching and passively heaving elliptic foils in a tandem configuration in the low Reynolds number regime. The numerical simulations are carried out using an inhouse fluid-structure interaction solver based on the Cut-Finite Element Method (CutFEM) on hierarchical b-spline grids. In this study, both the foils are free to heave in the cross-flow direction as they are elastically mounted along the plunge degree-of-freedom and undergo a prescribed sinusoidal pitch oscillation about the foil centre. A bifurcation study for the passive heaving response is carried out with the increasing oncoming flow velocity, keeping solid-to-fluid mass ratio, natural frequency of the elastic support, structural damping, pitching amplitude and frequency constant. The flow-induced vibration characteristics are compared between two significantly different foil geometries with aspect ratios of 1.5 and 6. The effect of the gap flow on manifesting different wake patterns is studied for three different intermediate gaps between the two foils. Furthermore, the influence of the intermediate gap flow on the dynamical wake transition is analysed, and the synchronisation behaviour in the collective dynamics of these two foils is revealed. The underlying flow physics is unravelled by identifying various wake modes and underlying vortex-structure interactions.