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

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.