3D Flow Measurements Over Heterogeneous Canopies: Effects of a Local Protrusion

The effects of local, protruding height heterogeneities within an otherwise homogeneous model canopy were experimentally studied in an open, recirculating water channel. The homogeneous canopy model, consisting of staggered rows of square prism elements, spanned the entire width of the channel and was shallowly submerged at H/h = 3, where H is the water free-surface height and h is the canopy height. The height heterogeneity was investigated for a protrusion of h_p/h = 1 and 1.5, where h_p is the protrusion height. The flow above the canopy and in the wake of the heterogeneity was studied with a volumetric three-dimensional particle tracking velocimetry system composed of an array of four high-speed cameras, a high-powered double-pulsed laser, and reflective hollow glass sphere tracer particles. The preliminary results indicate that the flow exhibits an acceleration and deflection in the region above the heterogeneity and a deceleration in the wake, resulting in marked modifications to the spatial distribution and magnitude of the time-averaged velocity. Further, the predominant location of the large spanwise Kelvin-Helmholtz vortical structures is notably altered by the introduction of the height heterogeneity, driving the dominant vortices away from the canopy-flow interface and into the free-stream.