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Characterization of the Nonaerated Flow Region in a Stepped Spillway by PIV

[+] Author and Article Information
António Amador

Hydraulic and Environmental Department., Technologic School of Barreiro, IPS, R. Stinville No. 14 Quimiparque, Barreiro 2830-144, Portugalantonio.amador@estbarreiro.ips.pt

Martí Sánchez-Juny

Hydraulic, Maritime and Environmental Department, Polythecnic University of Catalonia (UPC), C. Jordi Girona Campus Nord 1/3 Ed. D1, Barcelona 08034, Spainmarti.sanchez@upc.edu

Josep Dolz

Hydraulic, Maritime and Environmental Department, Polythecnic University of Catalonia (UPC), C. Jordi Girona Campus Nord 1/3 Ed. D1, Barcelona 08034, Spainj.dolz@upc.edu

J. Fluids Eng 128(6), 1266-1273 (Apr 07, 2006) (8 pages) doi:10.1115/1.2354529 History: Received February 09, 2005; Revised April 07, 2006

The development of the roller-compacted concrete (RCC) as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewed interest in stepped spillways. Previous research has focused on studying the air-water flow down the stepped chute with the objective of obtaining better design guidelines. The nonaerated flow region enlarges as the flow rate increases, and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present, study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the von Kármán integral momentum equation. The shear strain, vorticity, and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also, the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudobottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudobottom. Finally, the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Developing flow region in stepped spillway with q=0.11m2∕s, h=0.05m, α=51.3deg. (b) Image captured by CCD camera for steps E34 (X∕ks=13.04) and E33 at (X∕ks=15.09).

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Figure 2

Contour map of mean absolute velocity (∣U∣)

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Figure 3

Evolution of the δ(◇), δ*(◻), θ(엯)

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Figure 4

Flow resistance coefficient (cf) along the stepped spillway

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Figure 5

Mean velocity profiles (U∕U0) normal to the pseudo-bottom, along the step cavities (Lcav) analyzed

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Figure 6

Mean velocity profiles (U∕U0) and variation of yα along the step cavities analyzed. Symbols as in Fig. 5.

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Figure 7

Contour map of mean shear strain (εxy)

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Figure 8

Contour map of mean spanwise vorticity (ωz)

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Figure 9

Contour map of swirling strength

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Figure 10

Contour map of turbulent kinetic energy (k)

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Figure 11

Turbulence intensity (Tu) profiles normal to the pseudobottom, along the step cavities analyzed. Symbols as in Fig. 5.

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Figure 12

Definition sketch of the u′v′ plane (after (23))

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Figure 13

Contour map of the dominant quadrant in terms of occurrence probability for H=0

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Figure 14

Contour map of the dominant quadrant in terms of occurrence probability for H=2

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