THE DEFORMATION AND VIBRATION OF TULIP LEAVES IN WIND

The study of aerodynamic and solid-fluid coupling characteristics of tree leaves is of significance in tree protection, new power generation technology and solar panel design. Vogel first observed that a tree leaf could reconfigure itself at high winds to avoid damage. Vogel's leaf was freely supported at its petiole end, which is quite different from the natural way of petiole-branch connection. In our study, the leaf was clamped at the end of its petiole. The lamina was vertically hanging, with its front or back surface facing wind. Two types of lamina steady status, i.e, wing steady and conic steady, three types of lamina vibration, i.e, low frequency sway, 1st and 2nd high frequency vibration, and 5 critical wind speeds were observed in the range of wind speed 0~27 m/s. The probability of existance of every status and the expected value of each critical wind speed were obtained by statistics of the results of more than 70 leaves. The phenomenon of vortex shedding from a deformed leaf was found by flow visualization. Wind tunnel balance measurement revealed that the leaf drag coefficient decreased with the increase of lamina Re, and finally reached 0.1. A cantilevered beam model was introduced, and the measured aerodynamic force on the lamina was used to simulate the static bending curve of a petiole. Results showed that, the downstream bending increased rapidly with the increase of wind speed from 0 to 5 m/s, but it slowed down from 5 m/s to higher ones.