Experimental Study of Energy Harvesting Flag under the Influence of Wakes

Abstract

In this dissertation, an energy harvesting system based on wake flow energy is proposed for microelectromechanical devices that require few watts of energy for their operation. A potential source of small-scale renewable energy that draws a lot of attention is to harvest energy linking up with the piezo-flag’s vibration which is caused by fluid force on it. The stability of the structure is depending upon this applied force and at a critical velocity, it starts to oscillate. To be benefited from this phenomenon a piezoelectric flag is used just like a cantilever beam and placed in the wake of a bluff body. The piezoelectric flag is excited by the shedded vortices in the wake region where a continuous energy transfer process, from fluid to structure, occurs. This oscillation of the piezo flag produces strain energy in the piezo-material that can be transformed into electrical energy and stored in a battery or directly supply power to small sensors using an electrical circuit. The flag in the wake of the cylinder was strongly influenced by the vortices shed from the upstream cylinder under the vortex-vortex and vortex-body interactions. Geometric and flow parameters are optimized for a flexible flag subjected to flapping. For investigating the effect of critical parameters and interactions between flexible bodies and vortices, the current study examined the flexible flags in the viscous flow. Extensive experimentation is performed to see the impact of key parameters along with the bluff body’s shape change on the energy harvesting from piezoelectric flags. Several cross-sections of the bluff body covering hollow, solid, and range of cut angles (α= 0° – 180°) are studied. Other parameters including the geometry of the bluff body, L/D (ratio of flag’s length to cylinder diameter), streamwise gap (Gx=S/D, the ratio of the distance between cylinder and flag to the diameter), flow velocity U and bending rigidity γ are optimized and it is found that harvested energy is sensitive to these parameters. Particle image velocimetry (PIV) and videography techniques are used for the bluff body wake and dynamical behavior of piezo-flag. However, PIV is done for selective cases but videography is performed for every case. PIV results show that the variation in the time-mean wake for different bluff bodies has a significant impact on the energy harvesting system’s performance. Two different flag configurations are studied to improve the energy harvesting: a single flag behind different bluff bodies, and two tandem formations behind a hollow inverted C-shape cylinder. Variation in synchronization of wake flow with the flag is found with the variation in stated parameters which describes the poorly coupled to an optimally coupled motion state, where the piezo-flag oscillates at the undisturbed wake’s natural frequency. Furthermore, unidirectional and bidirectional bending is found which explained the variations in the harvested energy. The aim of the wind and hydrodynamic testing is to maximize the strain/mechanical energy by coupling the unsteady wake flow with the vibration of the piezo-flag.