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Research Papers: Flows in Complex Systems

Turbulence Characteristics of Vegetated Channel With Downward Seepage

[+] Author and Article Information
Thokchom Bebina Devi

Department of Civil Engineering,
Indian Institute of Technology Guwahati,
Guwahati 781039, India
e-mail: thokchom@iitg.ernet.in

Anurag Sharma

Department of Civil Engineering,
Indian Institute of Technology Guwahati,
Guwahati 781039, India
e-mail: anurag.sharma@iitg.ernet.in

Bimlesh Kumar

Associate Professor
Department of Civil Engineering,
Indian Institute of Technology Guwahati,
Guwahati 781039, India
e-mail: bimk@iitg.ernet.in

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 31, 2016; final manuscript received May 28, 2016; published online August 3, 2016. Assoc. Editor: Zhongquan Charlie Zheng.

J. Fluids Eng 138(12), 121102 (Aug 03, 2016) (12 pages) Paper No: FE-16-1068; doi: 10.1115/1.4033814 History: Received January 31, 2016; Revised May 28, 2016

Experimental studies were carried out for investigating changes in flow characteristics with the presence of flexible vegetation in a channel. The study focuses on the effect of introducing downward seepage on velocity profiles, Reynolds shear stress (RSS), and different turbulent length scales in a vegetative channel. The presence of vegetation provides drag and reduces the flow velocity. The turbulence generation mainly comes from the oscillations occurring near the top of the vegetation as is evident from the achievement of maximum Reynolds stress near the top of the vegetation. Application of downward seepage results in a higher velocity zone in the lower vegetation zone and a higher Reynolds stress. Quadrant analysis shows that sweep and ejection contribute most to Reynolds stress. The dominance of sweep event over ejection event is more with the application of downward seepage which means more bed transport. Different turbulent length and time scales increase with increase in downward seepage percentage due to reduction in energy dissipation. The increase in the length scale and time scale with downward seepage infers that higher level of turbulence prevail near the bed with an increased eddy size resulting in higher Reynolds stresses with downward seepage. The universal probability distribution functions (PDFs) of velocity fluctuations, RSS, and conditional RSS of vegetative channel follow Gram Charlier (GC) series based on exponential distribution except that a slight departure of inward and outward interactions of conditional RSS is observed which may be due to weaker events.

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Figures

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Fig. 1

Experimental flume setup: (a) plan view and (b) side view showing vegetation arrangement

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Fig. 2

Velocity power spectra showing the fit of Kolmogorov −5/3 scaling law

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Fig. 3

Plot showing (a) velocity and (b) Reynolds stress profiles for no-seepage, 10% seepage, and 15% seepage

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Fig. 4

Stress fraction contribution from each quadrant for plane bed, plane bed with vegetation, 10% seepage, and 15% seepage

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Fig. 10

Comparisons of computed Pτ̂(τ̂) with experimental data for all cases

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Fig. 8

Integral (a) length scale and (b) time scale

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Fig. 7

Taylor (a) length scale and (b) time scale

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Fig. 6

Kolmogorov (a) length scale and (b) time scale

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Fig. 5

(a) Velocity power spectra suu (kw) and (b) estimation of turbulent dissipation rate ε

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Fig. 9

Comparisons of computed Pû(û) and Pŵ(ŵ) with experimental data for all cases

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Fig. 11

Comparisons of computed Pτ̂(τ̂) with experimental data at z = 0.03H for all cases

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Fig. 12

Comparisons of computed Pi(τ̂) with experimental data at z = 0.53H for all cases

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