Experimental electro-hydrodynamic investigation of flag-based energy harvesting in the wake of inverted C-shape cylinder

Highlights: • An inverted C-shape cylinder is used to enhance energy efficiency of piezo-flag systems. • The effect of cut angles of the inverted C-shape cylinder are studied experimentally. • The inverted C-shape cylinder increases the instability region as compared to circular cylinder. • 120⁰ inverted C-shape cylinder yields an exceptional increase in energy efficiency of the system. Series of water tunnel experiments are performed to study the improvement in energy harvesting by vortex-induced vibrations. Inverted C-shape cylinders with different cut angles are placed in the uniform fluid flow and electrical energy is harvested using the undulating behavior of the piezo-flag in the downstream vortices. Experimental results demonstrate different flapping modes like poorly and optimal coupling with the wake flow. It is also showed that the streamwise gap and flow speed have a significant impact on the amplitude and flapping frequency, which results in the variation of the energy output of piezo-flag. The results indicate that the highest gain in output power is 66% for an inverted C-shape cylinder with a 120° cut angle compared to a circular cylinder performance. For each cylinder and flow velocity, it is also observed that there exists a critical streamwise gap for which vortex shedding does not produce any energy using piezo-flag due to poor coupling with wake flow. The results show that there is no significant difference in the performance of circular and 60° inverted C-shape cylinders regarding energy harvesting. However, changing the shape and cut angle leads to a remarkable increase in the flapping amplitude and its growth rate along with the dominant frequency. Particle Image Velocimetry (PIV) experimentation also endorses the results as wake dynamics is in good agreement with the energy efficiency improvement. Therefore, a 120° cut angle configuration holds a vivid preeminence over a circular cylinder as the kinetic source of a fluid energy harvester. The present study contributes effectively to harvesting energy from impinging vortices by tuning the streamwise gap, flow velocity, and cut angle of the inverted C-shape cylinder.