Effect of the Granularity of Heterogeneous Forest Cover on the Drag Coefficient

Abstract

We investigated the effect of the granularity of heterogeneous forest cover on the momentum absorption by conducting a large-eddy simulation of the flow over a surface covered by square-shaped forest/non-forest patches ranging in size from 64 to 512 m. The modification of the drag coefficient by heterogeneity is analyzed using integral identity equations, which decompose the drag coefficient into contributions from the momentum fluxes, pressure, and forest form drag. From a macroscopic viewpoint, the drag coefficient significantly differs in each case, and its tendency is not monotonic with the granularity. The identity equation shows that the effect of the vertical momentum flux becomes persistent on the non-forest area with an increase in patch size, with the spatial development altering the drag contribution, assigning increasing importance to the forest area. The drag coefficient defined from a local viewpoint shows a strong overshoot at the front edge of the forest patch, with a sudden decrease away from the front edge. A distance of almost 10 forest heights is required to arrive at a nearly homogeneous condition. However, spatial development is still obvious in the mean advection and pressure deviation terms with spanwise heterogeneity inducing an overshoot of the drag coefficient by enhancing the secondary flow. As such, the effect of the edge remains in a transitional manner, even if a locally homogeneous region is observed in the downstream and at the centre of patches.