Flow characteristics within different configurations of submerged flexible vegetation

Abstract

Summary The effects of three configurations (aligned, staggered, and columnar) of submerged flexible vegetation on flow structure are investigated experimentally in the laboratory. Time-averaged flow velocity and turbulence behavior are evaluated at different positions in each configuration by using a 3D acoustic Doppler velocimeter (ADV). According to the hydrodynamic regimes in experimental results, the vertical distribution of streamwise velocity can be separated into three layers—the upper non-vegetated layer, middle vegetation layer, and lower sheath layer. This three-layer model, which is associated with different logarithmic equations, can be applied to describe the vertical distribution of streamwise velocity. The local maximum velocity within vegetation occurs at the sheath section of a plant clump (0.10–0.15 vegetation height ( H v )) where the frontal width is minimal. Turbulent intensities in the streamwise ( u rms ) and spanwise ( v rms ) directions peak at the sheath section and at the approximate top of the canopy (0.9 – 1.2 H v ). The maximum Reynolds stresses exist at roughly 0.9 – 1.2 H v , which may be migrated vertically as the frontal width of a plant clump is increased. This high frontal width also increases streamwise velocity above vegetation, leading to increase variations in Reynolds stresses around the canopy top. On the vertical turbulent velocity scale ( w rms ), the vortices above a still canopy rotate faster than those above a waving canopy. Therefore, the experimental results demonstrate that the flow field can vary significantly at the sheath section and at the top of a plant clump due to altered flow pass. These analytical findings will likely prove useful when designing ecological habitats and preventing riverbed erosion.