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Spatial evolution of coherent motions in finite-length vegetation patch flow

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Abstract

A number of experimental studies on submerged canopy flows have focused on fully-developed flow and turbulent characteristics. In many natural rivers, however, aquatic vegetation occurs in patches of finite length. In such vegetated flows, the shear layer is not formed at the upstream edge of the vegetation patch and coherent motions develop downstream. Therefore, more work is neededz to reveal the development process for large-scale coherent structures within vegetation patches. For this work, we considered the effect of a limited length vegetation patch. Turbulence measurements were intensively conducted in open-channel flows with submerged vegetation using Particle Image Velocimetry (PIV). To examine the transition from boundary-layer flow upstream of the vegetation patch to a mixing-layer-type flow within the patch, velocity profiles were measured at 33 positions in a longitudinal direction. A phenomenological model for the development process in the vegetation flow was developed. The model decomposed the entire flow region into four zones. The four zones are the following: (i) the smooth bed zone, (ii) the diverging flow zone, (iii) the developing zone and (iv) the fully-developed zone. The PIV data also confirmed the efficiency of the mixing-layer analogy and provided insight into the spatial evolution of coherent motions.

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Abbreviations

\(a\) :

Vegetation density

\(b\) :

Vegetation-element width

\(C_{D }\) :

Canopy drag coefficient

Fr:

\(U_{m}/({gH})^{1/2}\), Froude number

\(H\) :

Flow depth

\(h\) :

Vegetation height

\(h_{p}\) :

Penetration depth of the Reynolds stress

\(h_{1}\) :

Lower limit position of mixing-layer

\(h_{2}\) :

Upper limit position of mixing-layer

\(L_{v},B_{v}\) :

Neighboring vegetation spacings in streamwise and spanwise directions, respectively

\(L_{DF}\) :

Length scale of diverging flow region

\(L_{DV}\) :

Length scale of developing region

Re:

\({HU}_{m}/{\nu }\), Reynolds number

\(t_h \) :

vegetation element thickness

\(U,\, V,\, W \) :

double-averaged streamwise, vertical and spanwise velocity

\(U_{c}\) :

Convection velocity

\(U_{h}\) :

Time-averaged velocity at the vegetation edge

\(U_{m}\) :

Bulk mean velocity

\(U_{*}\) :

Friction velocity

\(U_{1}\) :

The constant velocity of the low speed zone

\(U_{2}\) :

The constant velocity of the high speed zone

Vo :

Vegetation zone volume

\(x,\, y,\, z\) :

Streamwise, vertical and spanwise coordinates

\(x_{DF}\) :

Downstream end of the diverging flow zone

\(x_{DV}\) :

Downstream end of the developing zone

\(x_{SM}\) :

Downstream end of the smooth bed zone

\(\lambda _f \) :

Dimensionless vegetation density

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Authors and Affiliations

  1. Department of Civil Engineering, Kyoto University, Kyoto, 615-8540, Japan

    Taka-aki Okamoto & Iehisa Nezu

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  1. Taka-aki Okamoto
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  2. Iehisa Nezu
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Correspondence to Taka-aki Okamoto.

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Okamoto, Ta., Nezu, I. Spatial evolution of coherent motions in finite-length vegetation patch flow. Environ Fluid Mech 13, 417–434 (2013). https://doi.org/10.1007/s10652-013-9275-6

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  • DOI: https://doi.org/10.1007/s10652-013-9275-6

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