Impact of solid and hollow bluff bodies on the performance and dynamics of flag-based energy harvester

This study investigates the energy harvesting potential of the wake of different bluff bodies using a piezoelectric membrane. Both hollow and solid C-shaped cylinders with similar dimensions in normal and inverted orientations are placed upstream of the membrane as a kinetic source of energy. A series of water channel tests are performed by varying stream-wise gap (x*) 0.5 <= x* <= 4.0 between the cylinder and membrane and flow velocity (V) 0.12 <= V <= 0.26 m/s. The instability region is expanded remarkably by using different cross-sections. Different flapping modes are observed, including biased, intermittent, and continuous. It is shown that the time-mean wake fluctuates with changes in the cross-section of the bluff body and has a substantial influence on the energy harvesting efficiency and dynamical behaviors of the membrane in terms of peak-to-peak oscillation amplitude and flapping frequency. A threshold flow velocity of 0.2 m/s for continuous energy harvesting is defined for all bluff bodies. The highest electrical energy is obtained at a flow velocity of 0.26 m/s and 2.5 diameters downstream from the hollow C-shaped bluff body when placed in an inverted orientation. A comparison of all the configurations of the bluff bodies indicates that the hollow C-shaped bluff body in inverted orientation produces the strongest wake, which results in a 36 % increase in flapping amplitude and a 35 % improvement in harvested power for the piezo-membrane based energy harvester compared to the baseline case (a solid C-shape).