On the submerged low-Cauchy-number canopy dynamics under unidirectional flows

Abstract

The unsteady dynamics and induced turbulence associated with flow development of a fully-submerged canopy with relatively low flexibility were explored experimentally under small Cauchy numbers Ca≲9 using particle image velocimetry (PIV) and particle tracking velocimetry (PTV). Inspection of the flow around a rigid canopy supported the identification of significant modulation of the flow dynamics due to minor blade reconfiguration and oscillations within the first rows. The canopies consisted of relatively short arrays of rectangular blades of height h=37.5mm placed in a staggered layout and extending Δx/h=5 in the streamwise direction. The results show a downstream shift in the onset of the internal boundary layer. Canopy top friction velocity was lower than its rigid counterpart, even where blades undergo negligible deflection, evidencing the impact of the leading structure dynamics on flow adjustment. A transition from vortex-induced blade motions to dominance by natural blade frequency occurs with distance from the canopy edge. A frequency synchronization occurs several rows into the canopy, resulting in peak displacement intensity of the blades. The relationship between flow-induced and blade natural frequencies is described through a developed transfer function, enabling the determination of the characteristic vortex frequency. Natural and flow-induced frequencies dominate the flow-structure interaction near the leading edge region, resulting in increased small-scale turbulence.