Suppression of vertical flow separation over steep slopes in open channels by horizontal flow contraction

Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist...

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Bibliographic Details
Published inJournal of fluid mechanics Vol. 885
Main Authors Broekema, Y. B., Labeur, R. J., Uijttewaal, W. S. J.
Format Journal Article
LanguageEnglish
Published Cambridge Cambridge University Press 25.02.2020
Subjects
Boundary layers
Confining
Contraction
Divergence
Experiments
Flow control
Flow separation
Flow velocity
Free surfaces
Hydraulics
Local flow
Nonuniformity
Open channels
Pressure distribution
Pressure gradients
Separation
Shear stress
Slope
Slopes
Topography
Transition points
Upstream
Vertical flow
Vertical mixing
Vertical separation
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Summary:Flow separation and its control have been the subject of intensive research for decades. Flow separation occurs when the boundary layer loses contact with the associated confining wall, which is usually caused by a pressure gradient acting against the local flow direction. Numerous strategies exist to control flow separation, and in this study we demonstrate experimentally that vertical flow separation over steep slopes in shallow free-surface flows may be suppressed by contracting the flow horizontally upstream of the slope. We found that, unexpectedly, introducing lateral non-uniformity in the upstream flow field could suppress vertical flow separation for steep slopes up to 1 in 2. This study reveals the possibility of two different flow states over steep slopes; (i) a vertically attached flow combined with horizontal convergence, and (ii) a vertically detached flow combined with horizontal divergence. A detailed analysis of the dynamics of the two different flow states is presented. Although a predictive relation determining the transition point between the two flow states was not found in the current study, the observed phenomena were shown to be strongly related to the magnitude of the lateral gradient at the upstream edge of the slope. The results demonstrate a significant influence of the vertical flow state – separated or attached – on the shear stress at the confining boundaries of the flow.
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ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2019.972