This study investigates energy harvesting from vortex induced vibrations of a flexible cantilevered flapper placed in the wake of a rigid circular cylinder. The effect of the gap between the cylinder and the flapper on the energy harvested is investigated through wind tunnel experiments and numerical simulations. As the flow speed is varied, a transition in the flapper dynamics is observed, which in turn affects the power extracted by the harvester. Numerical investigations reveal that the flapper dynamics is different depending on whether the vortices are shed ahead or behind the flapper. This study concludes that the choice of the gap influences the energy harvesting potential of such harvesters. © 2017 Author(s).