A wind tunnel study of the effects of vegetation structural characteristics on the airflow field

Abstract

Implementing vegetation is the preferred measure for controlling soil wind erosion. The physical variations in the wind velocity profile, such as the friction wind velocity (u*), aerodynamic roughness (z0), zero-plane displacement (d) and drag coefficient (Cd), are key to understanding soil wind erosion on vegetation-covered surfaces. However, the lack of systematic and in-depth research in the literature has led to significant controversy and uncertainty. Based on the wind tunnel data of wind velocities at 18 different heights near the ground surface for four plant heights (H = 5, 10, 15, and 20 cm) and densities (ds = 27, 46, 97, and 193 N/m2) and six free stream velocity (u∞ = 6, 8, 10, 12, 14, and 16 m/s) scenarios, the relationships among u*, z0, d, the parameters of u∞ and the vegetation structural characteristics, including the plant height (H), density (ds), and lateral cover (λ), are investigated in this paper. The conclusions are as follows: (1) The near-surface wind velocity profiles of the vegetation-covered surface can be divided into three segments with little variation in the wind velocity with height, a rapid increase in the wind velocity with height, and close to the free stream velocity. Their corresponding heights ranged from the surface to heights of 0.5 – 0.75H, 0.5 – 0.75H to 50 cm, and exceeding the height of 50 cm, respectively. (2) The zero-plane displacement increased with the plant height and density but decreased with the free stream velocity. The relationship between them can be expressed by Eq. (5). (3) The friction wind velocity of the vegetation-covered surface increased with the plant height, density, and free stream velocity. The relationship between them can be expressed by Eq. (6). The drag coefficient increased rapidly at first and then slowly decreased with increasing lateral cover, which satisfied the equation Cd = 0.00155 + 0.0111λ/(λ + 0.124). (4) The aerodynamic roughness increased with the plant height and density but decreased with the free stream velocity. The aerodynamic roughness of the vegetation-covered surface under different free stream velocities and lateral cover conditions can be expressed by Eq. (8). The study results are a basis for the modeling of the dynamics of wind erosion on vegetation-covered surfaces.