Flow and Turbulence Statistics of a Two-Height Canopy Model in a Wind Tunnel

Modelling the flow and mass transport in canopies requires knowledge of how roughness heterogeneity impacts them. We present wind-tunnel experiments that enable quantifying the (time-space) average flow statistics of a two-height model canopy. We use two-dimensional particle image velocimetry measurements of the flow in multiple planes inside the canopy layer (CL), the roughness sublayer, and above it. The canopy comprises thin metal plates of two heights, h and h/2, which form a specific heterogeneity that can be viewed as two horizontal layers, “top” and “bottom”. The overall frontal index (λ= 0.5) resides in a less-studied dynamical zone between “dense” (> 0.5) and “sparse” (0.1 < λ< 0.5). The effects of the canopy’s heterogeneity are highlighted by comparing the flow statistics, with published results from dense and sparse uniform canopies made of similar element shapes. The plane mixing layer analogy was reflected in the first-, second-, and third-order moments of the velocity fluctuations, as well as in the quadrant analysis of the Reynolds shear stress, with few reservations: (i) the shear length scale at canopy height, which quantifies the inverse of the mean shear, is relatively smaller than would have been expected from the momentum penetration depth h- d; (ii) An additional inflection point, obeying the inviscid Fjørtoft’s criterion, is observed in the average streamwise velocity at the bottom-layer height (h/2), where a local increase in sweep/ejection events of the Reynolds shear stress are prominent; (iii) unlike typical dense canopies, the estimated contribution from wake to the production of TKE near the top CL is smaller relative to the contributions from shear; (iv) the dispersive fluxes of the shear stress and kinetic energy differ from that of uniform canopies that are either denser or sparser. These observations provide new insights into the possible impact that vertical roughness heterogeneity has on the flow statistics in plant and urban canopies.