Three-dimensional model construction and wind simulation of different tree species in farmland shelter

Abstract

<p>Protective forests are the ecological barriers of oases in arid sand areas and can effectively prevent and control wind and sand hazards. The structural characteristics of individual trees, as the basic unit of protective forests, are the key factors affecting the protective benefits. With the typical leafless tree species of Ulan Buh Desert oasis, i.e., Populus alba var. pyramidalis, Populus nigra var. thevestina, and Populus popularis, as the research objects, and by using the ground-based LiDAR and through computational fluid dynamics (CFD), we fully explored the structural characteristics of individual trees and their surrounding aerodynamic characteristics on the basis of real 3D models. We further established the relationship between structural parameters of individual trees and wind field index. The results showed that combining AdQSM and MeshLab to build tree models had high accuracy. The wind field around the individual trees could be roughly divided into six regions, including the attenuation zone of the windward side of the plant, the acceleration zone at the top of the plant, the eddy zone, the calm zone, the transition zone, and the recovery zone of leeward side of the plant. The pressure field around individual trees showed a gradual change of high pressure on the windward side to low pressure on the leeward side. Horizontally, in the range of 20% to 50% reduction in relative wind speed, the effective protection distances were 0.21H-1.51H, 0.20H-0.91H, and 0.25H-1.64H (H was the corresponding tree height) for P. alba var. pyramidalis, P. nigra var. thevestina, and P. popularis, corresponding to effective protection areas of 18-294, 15-227, and 18-261 m2, respectively. The maximum wind speed decay rate in the vertical direction was at 0.3H height for P. alba var. pyramidalis and P. popularis, and was reflected at 0.5H height for P. nigra var. thevestina. The correlation and stepwise regression analysis of the single tree structure parameters with the wind field indicators clearly indicated that optical porosity and volume porosity dominated the protection effect. Among the wind field factors, the best regression models related to the porous coefficient were screened for three factors, including diameter at breast height, tree surface area, and optical porosity. The regression variables screened for effective protection distance and effective protection area differed among the classes.</p>

防护林是干旱沙区绿洲的生态屏障，能够有效防治风沙灾害。 单木作为构成防护林的基础单元，其结构特征是影响防护效益的关键因素。 本研究以乌兰布和沙漠绿洲无叶期典型树种新疆杨、箭杆杨、小美旱杨为研究对象，借助地面激光雷达，通过计算机数值模拟，在建立单木真实三维模型的基础上，充分探究单木结构及其周围空气动力学特征，并建立单木结构参数与风场指标间的关系。 结果表明： 结合AdQSM与MeshLab构建树木模型的方法精度高。 单木周围风场大致分为６个区域，包括植株迎风面的衰减区、植株顶端的加速区、植株背风面的涡旋区、平静区、过渡区、恢复区。 单木周围压力场呈现迎风面压力大、背风面压力小的梯度变化。 水平方向上，在相对风速降低20%-50%的范围内，新疆杨、箭杆杨、小美旱杨的有效防护距离分别为0.21H～1.51H、0.20H～0.91H、0.25H～1.64H(H为对应树高)，对应的有效防护面积分别为18～294、15～227、18～261m2 。垂直方向上，新疆杨和小美旱杨在0.3H高度处的风速衰减率最大，箭杆杨则体现在0.5H高度。 综合单木结构参数与风场指标相关性和逐步回归分析，明确透光疏透度和体积孔隙度对防护效应的影响占主导地位。 风场指标中，基于胸径、树木表面积、透光疏透度建立的透风系数回归模型最优；各等级有效防护距离和有效防护面积中，筛选的回归变量不尽相同。