林带断面形状及布置形式对防护林带防风效应影响的大涡模拟

Effects of cross-sectional shapes and arrangement patterns on windbreak effects of shelterbelts by large eddy simulation

探究不同结构特征的防护林带的风速流场及空气动力学特征,对合理配置防护林带、减轻风沙灾害具有重要意义。本研究将树冠假设为多孔介质,以总高度(H)为7 m、冠层高度为5.8 m、冠层底部宽度为2 m的日本“築地松”林带为例,考虑5种林带断面形状(矩形、迎风直角三角形、背风直角三角形、等腰三角形和抛物线形)和4种布置形式(单条林带、L型林网、U型林网和矩型林网),采用大涡模拟(LES)湍流模型进行计算流体力学(CFD)数值模拟,揭示不同结构特征的防护林带的平均风速流场及湍流结构,研究林带断面形状和布置形式对防护林带防护距离及防护面积等防风效应指标的影响。结果表明:数值模拟与野外实测得到的不同高度处的平均风速和湍动能的平均相对误差较小,分别为5.5%和12%,多孔介质冠层模型可成功再现林带背风区的平均风速和湍动能。矩形断面林带冠层体积最大,对气流的阻碍作用最显著,在靠近林带附近的背风区中上部(z≥0.5H,z表示高度)区域,平均风速和湍动能显著降低,防护范围最大;抛物线形断面林带的防护范围其次;断面形状呈迎风直角、背风直角和等腰三角形的林带再次。在林带下游水平距离(x)≥10H的区域,不同断面形状林带的平均风速和湍动能趋于相同。对比单条林带、L型、U型和矩型林网的流场结构特征发现,与来流风速垂直方向的林带越多,林带冠层后的风速衰减越明显,气流需要更长的距离方可恢复至来流风速,而与来流风速方向平行的防护林带的防风效应极为有限,因而,U型和矩型林网比单条林带和L型林网的防风效应更显著。研究结果可为防护林带的结构配置与优化布局提供参考。

Investigating the wind speed flow field and aerodynamic characteristics of shelterbelts with different structural features is of significant importance for the rational arrangement of shelterbelts and the mitigation of wind-blown sand disasters.Considering five cross-sectional shapes of shelterbelts(rectangle,windward right-angle triangle,leeward right-angle triangle,isosceles triangle,and parabolic)and four layout forms(single shelterbelt,L-shaped network,U-shaped network,and rectangular network),we conducted computational fluid dynamics(CFD)simulations using the large eddy simulation(LES)turbulence model to understand mean wind speed flow field and turbulence structure of shelterbelts with different structural features,and investigated the effects of shelterbelt cross-sectional shapes and layout forms on windbreak indicators,such as protection distance and area.We considered tree canopies as porous media and conducted simulation with the‘Tsujimatsu'shelterbelt in Japan with a total height(H)of 7 m,canopy height of 5.8 m,and a canopy base width of 2 m.The results showed that the average relative errors of mean wind speed and turbulent kinetic energy at different heights obtained by numerical simulations and field measurement were small,being 5.5% and 12%,respectively,indicating that the porous medium canopy model successfully reproduced the mean wind speed and turbulent kinetic energy in the leeward area of the shelterbelt.The rectangular cross-section shelterbelt,with the largest canopy volume,significantly obstructed airflow.The mean wind speed and turbulent kinetic energy showed a notable reduction in the leeward area near the shelterbelt,especially in the upper region(z≥0.5H,where z denoted the height),showing the largest protection range.The parabolic cross-section shelterbelt ranked second in terms of protection range,followed by shelterbelts with windward right-angle,leeward right-angle,and isosceles triangular cross-sections.In the downstream area where horizontal distance x≥10H,the mean wind speed and turbulent kinetic energy of shelterbelts with different cross-sectional shapes tended to be the same.Comparing the flow field structures of single shelterbelts and L-shaped,U-shaped,and rectangular networks,it revealed that the more shelterbelts oriented perpendicular to the incoming wind speed,the more pronounced the wind speed attenuation behind the canopy,a longer distance would be required for airflow to recover to the incoming wind speed.In contrast,the wind protection effect of shelterbelts paralleled to the wind direction was extremely limited,making the U-shaped and rectangular networks more effective in wind protection than single shelterbelts and L-shaped networks.The findings would provide references for the structural configuration and optimal layout of shelterbelt systems.