Destabilising effects of plant flexibility in air and aquatic vegetation canopy flows

With an analytical model coupling a mixing layer flow with an oscillating vegetation canopy through a drag force, we show that for both the cases of wind over a crop field and a water stream over aquatic plants, the dynamics of the plants is responsible for increasing the growth rate of the coupled instability in the lock-in range through two mechanisms. Because the flow and the vegetation canopy move in phase and thus minimise their interactions, the drag dissipation is decreased. The correlation between the two components of the perturbation flow velocity in the mixing layer is increased and makes the perturbation flow more efficient at extracting energy from the mean flow.