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Research Papers: Flows in Complex Systems

Drag and Turbulent Characteristics of Mobile Bed Channel With Mixed Vegetation Densities Under 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

Rishabh Daga, Sumit Kumar Mahto

Department of Civil Engineering,
Indian Institute of Technology Guwahati,
Guwahati 781039, India

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 June 27, 2015; final manuscript received January 19, 2016; published online April 22, 2016. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 138(7), 071104 (Apr 22, 2016) (13 pages) Paper No: FE-15-1431; doi: 10.1115/1.4032753 History: Received June 27, 2015; Revised January 19, 2016

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.

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Figures

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Fig. 1

Schematic diagram showing (a) experimental flume setup and (b) pattern of placing vegetation

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Fig. 2

Velocity profiles of different vegetation pattern for no-seepage, 10% seepage, and 15% seepage cases (dashed lines show the top of the vegetation): (a) 5 mm upstream–10 mm downstream and (b) 10 mm upstream–5 mm downstream

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Fig. 3

Reynolds stress profiles of different vegetation patterns for no-seepage, 10% seepage, and 15% seepage cases (dashed lines show the top of the vegetation): (a) 5 mm upstream–10 mm downstream and (b) 10 mm upstream–5 mm downstream

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Fig. 4

Turbulent Intensities in streamwise direction, σu (+, ◊, ○) and vertical direction, σw (+,,•) for no-seepage, 10% seepage, and 15% seepage cases: (a) 5 mm upstream–10 mm downstream and (b) 10 mm upstream–5 mm downstream

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Fig. 5

Profiles showing third-order moments of 5 mm diameter upstream–10 mm diameter downstream for no-seepage, 10% seepage, and 15% seepage: (a) upstream, (b) center, and (c) downstream

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Fig. 6

Profiles showing third-order moments of 10 mm diameter upstream–5 mm diameter downstream for no-seepage, 10% seepage, and 15% seepage: (a) upstream, (b) center, and (c) downstream

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Fig. 7

Profiles showing fractional stress contribution to Reynolds stress of 5 mm diameter upstream and 10 mm diameter downstream: (a) upstream, (b) center, and (c) downstream

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Fig. 8

Profiles showing fractional stress contribution to Reynolds stress of 10 mm diameter upstream and 5 mm diameter downstream: (a) upstream, (b) center, and (c) downstream

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Fig. 9

Drag coefficient of different vegetation pattern (a)–(d) and average CD (e) for no-seepage, 10% seepage and 15% seepage

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