Piezoelectric flag flutter triggered by bluff body wake for energy harvesting at low airspeeds

This study examines the potential of cylindrical bluff bodies of different shapes to enhance the energy harvesting performance of a piezoelectric flag. A thin triangular flag with a macro-fiber composites patch is placed at various positions downstream of the bluff body to identify favorable regimes, focusing on reduced flutter thresholds and enhanced output power. The flag under streamlined flow undergoes subcritical bifurcation, with the upper flutter onset point near 7.5 m/s. The maximum output power of ≈ 26 μ W is achieved across an external load resistance of 0.22 MΩ. When the flag is placed behind a bluff body, its bifurcation behavior transitions to supercritical, and the flutter onset approaches the lower threshold (3.9 m/s) observed under streamlined flow. Among the various bluff body shapes, the 3 cm diameter full cylinder and the 180 cut U-shaped cylinder demonstrate superior performance. Closely spaced configurations yield higher output power, while farther spaced ones significantly expand the usable flow speed regime for energy harvesting. A maximum of approximately 27% amplification in output power is achieved using a full cylinder of 3 cm diameter placed closely in front of the flag at a distance of 0.5 times the cylinder diameter. These findings are useful for energy harvesting at low airspeeds (4.0–8.0 m/s), commonly encountered in urban wind environments.