Energy harvesting from flow induced vibrations (FIV) in flexible bodies offer opportunities for power generation in biomimicking robotic devices and is an active area of research. The focus of this study is on investigating the underlying physics and qualitatively analysing the energy extraction scenarios in similar structural systems, comprising of a flexible piezoelectric flapper in a low Reynolds number flow regime. A high-fidelity three-way fully coupled fluid-structure-electric energy solver is developed in-house to study the energy harvesting capabilities of such a flapper, its hydrodynamic characteristics and the associated unsteady flow-field. The results indicate that the flapper deformation profiles at the most efficient harvesting regimes resemble the propulsion gaits of natural swimmers. Investigations on the effects of actuation reveal no significant impact on the harvested power at the high yield regime, identified under the passive condition showing biomimetic gait. This study provides mechanics based insights that is expected to be useful for bio-inspired designs of FIV based harvesters. The in-flow condition is then further modified by placing a bluff body at the upstream of the flapper. The flapper oscillation dynamics is seen to be altered based on the frequency of vortex shedding of the bluff body, as well as the spatio-temporal forcing exerted by the rolling vortices onto the flapper. The ongoing study aims to investigate the effect of the gap location and the flexibility of the structure on the FSI dynamics and the associated output power.