The propulsive ability of aquatic swimmers to move through viscous fluid has been attracting attention as a thorough understanding of the movement strategies of fishes, paves the way towards development of futuristic aquatic bio-robots. Besides the rich mechanics and flow dynamics involved in fish swimming, the large body deformation of aquatic swimmers has the potential to be used for energy harvesting by using smart materials for man-made similar aquatic devices. The ability of fishes to extract energy from flow disturbances for its locomotion, enables less demand on its internal energy. It is this characteristic feature of fishes that gives the idea of the potential of harvesting energy from flows, using flexible flapping systems. There have been extensive experimental, theoretical and numerical studies on the mechanics of locomotion, hydrodynamics of the surrounding flow , fish physiology as well as neurology . However, the detection of favourable flow regimes and ideal structural deformation for the design of robust and practically feasible energy harvesters, need systematic investigation. This work aims to study the dynamics of flow interaction of fluid with a highly deformable fish like flapper and to develop a fish-mimetic model for harvesting energy from the flow by developing a numerical solver, that has three way coupling between solid , fluid and energy equations.