Abstract
The vegetation density λ affects turbulent flow type in the submerged vegetated river. This laboratory study investigates different types of vegetated turbulent flow, especially the flow at 0.04 < λ < 0.1 and λ = 1.44 by setting the experimental λ within a large range. Vertical distributions of turbulent statistics (velocity, shear stress and skewness coefficients), turbulence kinetic generation rate and turbulence spectra in different λ conditions have been presented and compared. Results indicate that for flow at 0.04 < λ < 0.1, the profiles of turbulent statistics manifest characteristics that are similar to those of both the bed-shear flow and the free-shear flow, and the turbulence spectral curves are characterized with some slight humps within the low-frequency range. For λ = 1.44, the turbulent statistics above the vegetation top demonstrate the characteristics of boundary-shear flow. The spectral curves fluctuate intensely within the low-frequency range, and the spectra of low-frequency eddies above vegetation top are significantly larger than the values below. The change of turbulent flow type induced by an increase of λ would increase the maximum value of turbulence kinetic generation rate GS and change the point where GS is vertically maximum upwards to the vegetation top.
Highlights
Aquatic vegetation is ubiquitous in rivers, which affects the sediment transport, scalar dispersion, channel evolution and aquatic biodiversity by interfering with the flow field [1,2,3,4]
In an open channel with submerged vegetation, the turbulent flow type in the longitudinalvertical (x–z) plane varies with an increase of vegetation density, from the bed-shear flow to the free-shear flow and the secondary boundary-shear flow [5,6]
It is sufficiently accurate to represent overall flow characteristic by an average of velocities measured at Locations 1# & 2#; For a modest value of 0.04 < λ < 0.1, characteristics of profiles for turbulent statistics are similar to both the bed-shear flow one and the free-shear flow one
Summary
Aquatic vegetation is ubiquitous in rivers, which affects the sediment transport, scalar dispersion, channel evolution and aquatic biodiversity by interfering with the flow field [1,2,3,4]. In an open channel with submerged vegetation, the turbulent flow type in the longitudinalvertical (x–z) plane varies with an increase of vegetation density, from the bed-shear flow to the free-shear flow and the secondary boundary-shear flow [5,6]. The bed-shear flow happens because of bed drag near the flume bottom, with vegetation contributing to the bed roughness and increasing the bed friction [5]. For the medium dense vegetation, flow discontinuity caused by canopy drag would occur at the vegetation top, generating the free-shear flow in certain vertical region. Yan et al [7]
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