Aerodynamic Effects Of Farmland Windbreak System

The characteristics of the surface boundary layer on farmland windbreak system were analyzed and the regional wind protective model of sparse trees was evolved. This model and that of belt-type windbreak system (Zhu Tingyao, 1992,1993) construct the theoretical frame of regional wind protective evaluation of farmland windbreak system. Based on wind tunnel simulation experiment and field experiment, aerodynamic models of sparse trees and windbreak system were developed from physical law and hypothesis. The following conclusions seem justified under near-neutral measurements:1. Isotach leeward isolated tree was nearly ellipse in horizontal and vertical plane. Drag of isolated tree model can be measured and calculated by pendulum method and momentum balance law. Drag coefficient of isolated tree predicted by C_T=0.5A(1- α²). This prediction agrees well with data of wind tunnel experiment.2. Theoretical models of parameters u., Z₀ and d in windbreak system surface were developed using dimensional analysis and physical hypothesis. The predictions agree well with data of wind tunnel experiment. Maximum value of u.,Z₀ appears when the two parameters vary versus windbreak or sparse trees density.3. Theoretical model of regional wind protective evaluation of sparse trees was developed based on energy balance law. The model gives prediction which agree well with data of wind tunnel experiment.4. Regional wind protective effect of farmland windbreak system in Zhangwu county were calculated by geostrophic deviation method. The average effect is 14%.5. Drag and momentum distribution models are developed as indexes of self-wind-protective effect ( wind resisting ability ) of windbreak. The model show that self-wind-protective effect is closely related to the structure of windbreak. At same dynamic porosity, uniformly porous windbreak has better self-wind-protective effect than that of ventilated windbreak, windbreak which has the triangle section form (lower at front and higher at back, or higher in middle and lower at two verges) has the better effect than that of rectangle section form. Changes of section form from rectangle to...