Canopy Shear Layer Perturbation by Localized Height Heterogeneity

The effects of a local heterogeneity on the turbulent flow over an otherwise homogenous canopy were investigated. Specifically, the effects of a local increase in canopy height were studied for a canopy resembling aquatic vegetation; this local height increase is analogous to invasive species of seagrass. The considered canopy is categorized as a dense, such that tightly packed elements affect the flow on a canopy scale, instead of an element scale only. Canopy flow is separated into slow-moving flow through the canopy and fast-moving flow above the canopy, resulting in a shear layer at the canopy-flow interface. To better understand the effects of a local increase in canopy height, particle image velocimetry flow fields for various disturbance heights (h_d/h = 1.25, 1.5, 2) were analyzed; here, h denotes the homogenous canopy height. The effects of the local heterogeneity are investigated for two submergence depths, H/h = 2 and H/h = 3, with respective Reynolds numbers of 12 000 and 19 000. The local heterogeneity restricts the flow area, increasing vertical flows before and after the disturbance as well as elevating free stream velocity at a rate proportional to the disturbance size. The disturbance size is also proportional to the turbulence in the disturbance wake; this wake disperses downstream, distributing turbulence and decreasing the size of coherent vortical structures until the shear layer reattaches to the homogenous canopy and begins recovery. Homogenous canopy flow was not recovered four canopy heights downstream of the disturbance and submergence cases analyzed, and future research could investigate the rate of canopy boundary layer recovery as a function of disturbance size, submergence, and Reynolds number.