Abstract
Limited by large unit discharge above the overflow weir and deep tail water inside the stilling basin, the incoming flow inside stilling basin is seriously short of enough energy dissipation and outgoing flow still carries much energy with large velocity, bound to result in secondary hydraulic jump outside stilling basin and scour downstream river bed. Based on the RNG k-ε turbulence model and the VOF method, this paper comparatively studies flow field between the conventional flat gate pier program and the incompletely flaring gate pier program to reveal energy dissipation mechanism of incomplete flaring gate pier. Results show that incompletely flaring gate pier can greatly promote the longitudinally stretched water jet to laterally diffuse and collide in the upstream region of stilling basin due to velocity gradients between adjacent inflow from each chamber through shrinking partial overflow flow chamber weir chamber, which would lead to large scale vertical axis vortex from the bottom to the surface and enhance mutual shear turbulence dissipation. This would significantly increase energy dissipation inside stilling basin to reduce outgoing velocity and totally solve the common hydraulic problems in large unit discharge and deep tail water projects.
Highlights
The seasonal rainfall requires the low head hydraulic structures to urgently response and increase discharge volume, necessarily raise up tail water depth inside stilling basin and prone to scour downstream river bed due to insufficient energy dissipation and large outgoing velocity outside stilling basin[1,2].Recently, chute sill blocks or baffle blocks are installed above the stilling basin slab to enhance turbulent dissipation and improve flow field
Boundary conditions are set as following: sidewalls x-common no slip, non-porous wall; top z-pressure boundary with gauge pressure equal to zero,bottom z- no slip wall; left y- local stagnation pressure based on upstream total head over overflow weir crest with a hydrostatic pressure distribution, right y- local static pressure based on downstream tailwater with a hydraulic pressure distribution. 50m upstream and 400m downstream of overflow weir are used as boundaries of left and right Y, respectively
Comparing above discharge volume in two crucial conditions, discharge capacity of incompletely flaring gate pier (FGP) program is only 2.0%、1.7% lower than that of the flat gate pier program respectively, indicating such arrangement of incompletely FGP is of no influence to discharge capacity for overflow weir
Summary
The seasonal rainfall requires the low head hydraulic structures to urgently response and increase discharge volume, necessarily raise up tail water depth inside stilling basin and prone to scour downstream river bed due to insufficient energy dissipation and large outgoing velocity outside stilling basin[1,2].Recently, chute sill blocks or baffle blocks are installed above the stilling basin slab to enhance turbulent dissipation and improve flow field. Combined with so many low head hydraulic projects, literature 2 pointed out that flaring gate pier (FGP) would remarkably promote turbulent energy dissipation and shorten stilling basin length. Through shrinking flow section in overflow weir chamber, longitudinal high speed water jet with 3 exposure faces would take shape to continually entrain air in the process of falling down in air and violently impinge on downstream tail water inside stilling basin, which would significantly aggravate turbulent energy dissipation and collision. In order to guarantee discharge capacity for overflow weir and reserve enough room for the running down water jet, the ration of downstream water head inside stilling basin to upstream total water head should meet some requirements, which almost confine FGP in application within high water head dams. Combined with RNG k-ε turbulent model and VOF method, this paper would detailedly present the flow pattern, flow field and velocity distribution inside stilling basin
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