Surface layer response to heterogeneous tree1canopy distributions: roughness regime regulates secondary flow polarity

LES was used to model atmospheric surface layer flow over spanwise-heterogeneous vegetative canopies of elements with height h. Simulations used to study the flow response in the outer later and within the canopy due to spanwise and streamwise heterogeneity in the leaf area density. Synthetic trees were modeled using the well-established canopy drag model, which accounts for forcing due to the presence of trees. Previous experimental and simulation studies have shown the presence of Prandtl’s secondary flows of second kind due to spanwise heterogeneity in the aerodynamic roughness length. However, the role of streamwise gap on the flow dynamics has not been pursued. We took advantage of previous knowledge of the criterion for the presence of the secondary flows (δ-scale distance between the parallel streamwise aligned canopy rows) and varied the streamwise gap to quantify flow response. Results indicated that for δ1/h < 5 (d-type roughness), low momentum pathways (LMP) are present above the canopy row while the high-momentum pathways (HMP) are situated in the valley between the parallel rows. In contrast, for δ1/h > 5 (k-type roughness), the seccondary flow polarity reverses.