CFD simulation of the wind field over a terrain with sand fences: Critical spacing for the wind shear velocity

Sand fences are often erected to reduce wind speed, prevent aeolian soil focus our investigation on the behaviour of the near-surface wind shear velocity, which is defined here using a simple model based on wind speed values at height 20% of the fence height. Our simulations reveal novel insights about the three-dimensional structure of the turbulent wind flow between fences that are important for developing a theoretical description of this flow. We find that the area of soil associated with values of near-surface wind shear velocity that are below the minimal threshold for sand transport has two regimes, depending on the spacing Lx between the fences. When Lx is smaller than a critical value Lxc, the wake zones associated with each fence are inter-connected (regime A), while these wake zones appear separated from each other (regime B) when Lx exceeds this critical value of spacing. The system undergoes a second order phase transition at Lx=Lxc, with the cross-wind width of the protected zone scaling with 1-Lx/Lxcβ in regime A, with β≈0.32. Moreover, we also perform a comparative investigation of arrays of constant and multiple fence heights on the basis of the two-dimensional shear velocity field obtained from our simulations. Our findings have implication for a better understanding of aeolian transport in the presence of sand fences, as well as to develop optimization strategies for measures to protect soils from wind erosion.