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

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 Δ g / 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 F r ? [0.08, 0.2] and bulk Reynolds numbers R e ? [0.8, 2] × 10 4. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was Δ x / d ≈ 5 for the mean flow and Δ x / 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.