WIND TUNNEL INVESTIGATION OF THE AERODYNAMIC CHARACTERISTICS OF PURPLE WISTERIA COMPOUND LEAVES

The capability of reconfiguration of tree leaves is of significance in the designs of solar panel, aerofoil, bionic antenna, and wind power generation tree. The leaf was clamped at the base end of the rachis and vertically suspended in the center of wind tunnel test section, and tested with its front and back surface facing on-coming stream respectively at step-by-step increasing wind speed from 0 to 25 m/s. Results show that the changing process of the leaf can be divided into three stages: earlier steady, intermediate transition, and later steady, and critical wind speeds are observed. In earlier stage, the downstream bending curvature of the rachis increases rapidly with wind speed, and multi-layer wing steady and multi-layer multi-shape steady states exist. In intermediate stage, large amplitude low frequency rachis vibration, and small amplitude high frequency lobules vibration are observed. In later steady stage, two-layer structure or single streamlined body of conic, or wedge or U-shape cross section can be found. As wind speed increasing, the number of lobule layers and the width of the compound leaf b decrease, until the single streamlined body formed. As Re increasing, the drag coefficient of the leaf decreases rapidly at first, then slowly approaching to a constant. The absolute value of the negative Vogel component \vert \alpha \vert decreases with the increase of lobule number of the leaf. \vert \alpha \vert of the leaf with its back surface is larger than that with its front surface facing wind, but they tend to converge with the increase of lobule number. Rachis vibration occurs if the frequency of vortex shedding from the leaf is close to the natural frequency of the rachis. The second critical wind speed V_2/\sqrtE/\rho, at which the rachis vibration begins, is shown to be the function of b/l and d/l, where E, \rho , d, and l are respectively elastic module, mass density, diameter and length of the rachis, b is width of the deformed leaf, and a figure about this function is drawn using experimental data.