Impact of gaps on the flow statistics in an emergent rigid canopy

Abstract

High-resolution large eddy simulations and complementary laboratory experiments using particle image velocimetry were performed to provide a detailed quantitative assessment of flow response to gaps in cylinder arrays. The base canopy consists of a dense array of emergent rigid cylinders placed in a regular staggered pattern. The gaps varied in length from Δg/d=4 to 24, in intervals of 4d, where d is the diameter of the cylinders. The analysis was performed under subcritical conditions with Froude numbers Fr∈[0.08,0.2] and bulk Reynolds numbers Re∈[0.8, 2]×104. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was Δx/d≈5 for the mean flow and Δx/d≈3 for the second-order statistics. Dimensionless time-averaged streamwise velocity within the gap exhibited minor variability with gap spacing; however, in-plane turbulent kinetic energy, k, showed a consistent decay rate when normalized with that at x/d≥1 from the beginning of the gap. The emergent canopy acts as a passive turbulence generator for the gap flow for practical purposes. The streamwise dependence of k follows an exponential trend within 1≤x/d≲2.5 and transitions to a power-law at x/d≥4. The substantially lower maximum values of k within the gap compared to k within the canopy evidence a limitation of gap measurements representative of canopy flow statistics. We present a base framework for estimating representative in-canopy statistics from measurements in the gap.