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Analysis of turbulent flow structures in the straight rectangular open channel with floating vegetated islands

Environmental Science and Pollution Research volume 27, pages 26856–26867 (2020)Cite this article

Abstract

Floating vegetated islands (FVIs) are extensively implemented in various river ecology restoration projects, given their capability of decontaminating pollutants. The fluid dynamical behaviors of turbulence through FVIs are studied in the flume by using the SonTek Acoustic Doppler Velocimetry. Through conventional spectral and quadrant analyses, flow characteristics, such as energy content and turbulent momentum exchange, are investigated as the flow encountered a series of root canopies. A shear layer with corresponding coherent vortex structures at the bottom of root canopies occurred, which is generated by Kelvin–Helmholtz instabilities. These instabilities are usually derived from velocity differences between root canopy and gap region. Shear- and stem-scale vortices are identified by using spectral analysis. The power spectral density function on measured vertical velocity fluctuations in the flow direction near the bottom of root canopies from the leading edge of FVIs is computed. Given the flow developing downstream, a series of the spectral curves has gradually showed one dominant dimensionless frequency at 0.046. The sweep and ejection events have contributed prominently to the Reynolds stress in whole vertical direction. Momentum flux carried by sweeps outweighs its counterpart carried by ejections inside root canopies. However, the situation is different outside root canopies. The sweep–ejection contributions are brief but crucial to the total turbulent momentum exchange, which is in good agreement with considerable studies on turbulent flow through canopies.

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Abbreviations

a :

root canopy density (m−1)

n :

number of cylinders per bed area (m−2)

d :

cylinder diameter (m)

s :

vegetation spacing (m)

S 0 :

channel bed slope (vertical:horizontal) (-)

Q :

flow rate (m3/s)

b :

width of floating vegetated islands (m)

B :

flume width (m)

h r :

height of root canopy (m)

h g :

height of gap region (m)

H :

flow depth (m)

H p :

Penetration depth (m)

L veg :

length of floating vegetated islands (m)

ν :

Kinematic viscosity (m2/s)

g :

Gravitational acceleration (m/s2)

x, y, z :

streamwise, lateral and vertical directions (-)

U, V, W :

time-averaged velocity components in x, y, z directions (m/s)

u, v, w :

instantaneous velocity components in x, y, z directions (m/s)

u΄, v΄, w΄ :

fluctuating velocities in x, y, z directions (m/s)

u ∗b :

bulk friction velocity (m/s)

U m :

cross-sectional average streamwise velocity of the flow (m/s)

U a :

characteristic streamwise velocity in Eq. (3) (m/s)

U d1 :

stable time-averaged streamwise velocity in high-stream velocity layer (m/s)

U d2 :

stable time-averaged streamwise velocity in low-stream velocity layer (m/s)

〈U〉:

arithmetic average of Ud1 and Ud2 (m/s)

ΔU :

velocity difference between Ud1 and Ud2 (m/s)

Re :

bulk Reynolds number (-)

Fr :

bulk Froude number (-)

St :

Strouhal number (-)

R H :

hydraulic radius (m)

T KE :

turbulent kinetic energy (m2/s2)

θ :

momentum thickness (m)

δ :

mixing layer thickness (m)

\( -\overline{u^{\prime }w^{\prime }} \) :

Reynolds stress with respect to vertical plane (m2/s2)

S ww :

power spectral density (cm2/s)

H 0 :

threshold value (-)

\( {{S_i}_{,}}_{H_0} \) :

contribution of various flow types to Reynolds stress (cm2/s2)

\( {{C_i}_{,}}_{H_0}(t) \) :

averaging condition in Eq. (4)

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Funding

This work was supported by the National Natural Science Foundation of China (grant numbers 11872285 and 11672213).

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Affiliations

  1. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, 430072, Hubei, China

    Xuecheng Fu, Feifei Wang, Mengyang Liu & Wenxin Huai

Authors
  1. Xuecheng Fu
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  2. Feifei Wang
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  3. Mengyang Liu
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  4. Wenxin Huai
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Correspondence to Wenxin Huai.

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Highlights

Turbulence characteristics, including momentum thickness, velocity, and the Reynolds stress distribution, were studied.

Power spectral density analysis was used to explore vortex structures.

Quadrant analysis was adopted to elucidate momentum transporting properties of vortices.

Responsible editor: Philippe Garrigues

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Fu, X., Wang, F., Liu, M. et al. Analysis of turbulent flow structures in the straight rectangular open channel with floating vegetated islands. Environ Sci Pollut Res 27, 26856–26867 (2020). https://doi.org/10.1007/s11356-020-09087-3

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  • DOI: https://doi.org/10.1007/s11356-020-09087-3

Keywords

  • Floating vegetated islands
  • Coherent vortex structures
  • Quadrant analysis
  • Spectral analysis