Low-speed flutter of artificial stalk-leaf and its application in wind energy harvesting

Developing wind energy harvester (WEH) by mimicking the leaf flutter may provide an innovative way for increasing the power efficiency and decreasing the cut-in speed. The low-speed flutter mechanism of the stalk-leaf system is investigated through introducing a frictionless hinge into the stalk-leaf finite element model. The aeroelastic system is established by the usage of doublet-lattice aerodynamics and the spline interpolation between structural motion and flow downwash. The critical flutter speed and frequency are analyzed via V-g method. The evolution of damping and frequency with wind speed which various from static air to Beaufort level 5 are simulated. The influence of inclined angle of the stalk on the flutter characteristics is studied. The stainless-steel artificial stalk-leaf systems with inclined angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90° are fabricated and tested in wind tunnel. The wind energy harvesting performance is also measured by attaching macro-fiber composite patches on root of the stalk. It is found that the 30° stalk-leaf WEH has the lowest critical flutter speed, while the energy harvesting output (voltage and power) increases slowly with the increase of the wind speed. On the contrary, although the 90° stalk-leaf (vertical stalk) WEH has the steepest velocity-voltage and velocity-power curve, it also has the highest cut-in speed. In the preliminary tests, the 30° stalk-leaf WEH outputs steady power density of 47.46 μW cm⁻³ with stable oscillating frequency of 6.6 Hz at 11 m s⁻¹ wind, while the 90° stalk-leaf WEH outputs power density of 92.88 μW cm⁻³ with oscillating frequency of 7.2 Hz at the same wind speed. The stalk-leaf design presents a possible way to balance the performance between the high efficiency and the low cut-in speed for the WEHs.