Compound-channel-driven shear layer impinging upon streamwise-oriented plates of different solidities

Abstract

The impingement of a compound open-channel flow upon vertical and streamwise-oriented plates of different solidities located at the edge between main channel and floodplain are investigated experimentally. The plates model finite-length thin hedges along the river bank. The incident flow is characterised by a turbulent shear layer dominated by Kelvin–Helmholtz structures of length $L_s$. The solidity of the plates is varied from 17% (i.e. 83% porosity) to 100% (solid plate) in order to represent different vegetation densities, while the plate length is fixed to $0.56L_s$. The velocity field at the free-surface is measured by means of Large-Scale Particle Image Velocimetry (LS-PIV) with a field of view of length $3L_s$ and covering the channel width. While the solid plate is shown to induce a significant decrease in the incoming turbulent shear stress, the porous plates instead lead to an overshoot in the turbulent shear stress. All plates induce a reduction of the lateral turbulence intensity, while the longitudinal turbulence intensity is never reduced and even strongly overshoots the incident value for the porous plates. The individual Kelvin–Helmholtz structures are altered when passing the plates but are not destroyed. The alteration of the structures increases with the plates’ solidity, but the vortex cores as well as their spatial periodicity are always maintained, and the structures reform to their original state within a relatively short distance downstream of the plates of about $1.5L_s$. While for the solid plate the vorticity within the vortex cores decreases by the passage of the plate, in the case of the porous plates the vorticity increases. Vorticity shed from the holes of the porous plates is surmised to be the cause of this increase. The ensemble-averaged Kelvin–Helmholtz structure downstream of each plate reveals that for the least solid plates, the upstream ejection associated with the vortex core is strongly strengthened, which accounts for the observed overshoots in the turbulent stresses. When the plate’s solidity is higher than 67% however, the Kelvin–Helmholtz vortex downstream of the plate has shrunk and weakened, but is accompanied by secondary vortices, generated by the interaction with the plates.