Investigation Of The Aerodynamic Characteristics Of Purple Wisteria Pinnately Compound Leaves

In order to study the aerodynamic characteristics of pinnately compound leaves of wisteria,the experiments in this paper were tested in the test section of cyclic wind tunnel which was able to provide 0.5?40m/s speed.The wind tunnel test section is 2m long,0.6m wide and 0.6m tall,surrounded by transparent glass.Set up a 500 frame/s high-speed camera on the side and bottom of the test section for shooting and recording.A six component balance force measurement system was set up in order to measure the aerodynamic forces of the pinnate compound leaves of wisteria.The force measurement system was mainly composed of the force measurement balance,signal amplifier,data acquisition card and data acquisition software.The aerodynamic characteristics of compound blades are studied by analyzing their state change,aerodynamic force,vibration attitude and critical wind speed.Firstly,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 20m/s.Results show that the changing process of the leaf can be divided into 3 stages:earlier steady,intermediate transition,and later steady,and 5 critical wind speed are observed.In earlier stage,the downstream bending curvature of the rachis changes rapidly,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 |?| decreases with the increase of number of lobules.|?| 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 number of lobules.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/(?),at which the rachis vibration begins,is shown to be the equation model of b/l and d/l where E,?,d,and l are respectively elastic module,mass density,diameter and length of the rachis,b is width of the deformed leaf.Finally,a comparative study was made on the shape,resistance coefficient,axial inclination angle and critical wind speed through the method of cutting out the apical lobule.The compound leaves appear to be stable in different ways after the parietal lobe has been removed.Furthermore,the difference of rachis inclined angle can reach 30°,and the critical wind speed will be changed their speed and number.