4 results
Flow separation at convex banks in open channels
- K. Blanckaert
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- Journal:
- Journal of Fluid Mechanics / Volume 779 / 25 September 2015
- Published online by Cambridge University Press:
- 17 August 2015, pp. 432-467
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Laboratory experiments in an open channel bend provide insight into the physics of convex bank flow separation occurring in a variety of channel configurations, including confluences and bifurcations. The edge of the zone of flow separation is characterized by a shear layer, enhanced velocity gradients, tke, turbulent shear stresses and reversal of the streamwise vorticity and vertical velocity. The latter result from turbulence-induced secondary flow near the convex bank. When bankline curvature abruptly increases, flow tends to move away from the convex bank along a straight path, as represented by the inertial forces – including the centrifugal force – in the transverse momentum equation written in curvilinear coordinates. Mass accumulation at the opposite bank leads to a transverse tilting of the water surface, and a pressure gradient towards the convex bank that causes the flow to change direction. The pressure gradient force lags spatially behind the inertial forces, which promotes flow separation. Flow separation typically occurs downstream of the location of maximum change in the bankline curvature, because an abrupt increase in bankline curvature also leads to water surface gradients that cause local flow redistribution towards the convex bank that opposes flow separation. The zone of convex bank flow separation is shaped by the secondary flow induced by streamline curvature and turbulence. The latter is conditioned by the production rate of tke, which crucially depends on the accurate description of the Reynolds stresses. Hydrodynamic, geometric and sedimentologic control parameters of convex bank flow separation are identified and discussed.
Large-eddy simulation of a mildly curved open-channel flow
- W. VAN BALEN, W. S. J. UIJTTEWAAL, K. BLANCKAERT
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- Journal:
- Journal of Fluid Mechanics / Volume 630 / 10 July 2009
- Published online by Cambridge University Press:
- 10 July 2009, pp. 413-442
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After validation with experimental data, large-eddy simulation (LES) is used to study in detail the open-channel flow through a curved flume. Based on the LES results, the present paper addresses four issues. Firstly, features of the complex bicellular pattern of the secondary flow, occurring in curved open-channel flows, and its origin are investigated. Secondly, the turbulence characteristics of the flow are studied in detail, incorporating the anisotropy of the turbulence stresses, as well as the distribution of the kinetic energy and the turbulent kinetic energy. Moreover, the implications of the pattern of the production of turbulent kinetic energy is discussed within this context. Thirdly, the distribution of the wall shear stresses at the bottom and sidewalls is computed. Fourthly, the effects of changes in the subgrid-scale model and the boundary conditions are investigated. It turns out that the counter-rotating secondary flow cell near the outer bank is a result of the complex interaction between the spatial distribution of turbulence stresses and centrifugal effects. Moreover, it is found that this outer bank cell forms a region of a local increase of turbulent kinetic energy and of its production. Furthermore, it is shown that the bed shear stresses are amplified in the bend. The distribution of the wall shear stresses is deformed throughout the bend due to curvature. Finally, it is shown that changes in the subgrid-scale model, as well as changes in the boundary conditions, have no strong effect on the results.
Turbulence structure in sharp open-channel bends
- K. BLANCKAERT, H. J. DE VRIEND
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- Journal:
- Journal of Fluid Mechanics / Volume 536 / 10 August 2005
- Published online by Cambridge University Press:
- 26 July 2005, pp. 27-48
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In spite of its practical relevance, little is known about the turbulence characteristics in sharp open-channel bends, which may largely be attributed to a lack of accurate experimental data. This paper reports an experimental investigation of the turbulence structure in one cross-section of an open-channel bend. The flow pattern in this section is characterized by a bicellular pattern of cross-stream circulation (secondary flow) and, in the outer part, a strongly reduced turbulence activity as compared with straight uniform shear flow. The turbulence structure differs fundamentally from that in straight uniform shear flow. The velocity fluctuations are atypically coherent over the channel width, whence the measured signal is decomposed into slow width-coherent fluctuations and a fast background signal. The width-coherent fluctuations reflect a bulk spatio-temporal oscillation of the pattern of circulation cells whereas the background signal represents developed turbulence. A spectral analysis shows that the width-coherent fluctuations have the characteristics of a wavelike motion, i.e. they contribute significantly to the turbulent normal stresses but only weakly to the shear stress, whereas the background turbulence is characterized by efficient shear stress generation. The reduced turbulence activity and the tendency of the secondary-flow pattern to oscillate are both effects of the streamline curvature. Similar observations on reduced turbulence activity and the tendency to wavelike motion have been reported in literature for flows in curved wind tunnels and density-stratified flows. Our experimental results indicate that these phenomena are potentially important in curved open-channel flows, where they affect the mixing and transport capacity of the flow.
Secondary flow in sharp open-channel bends
- K. BLANCKAERT, H. J. De VRIEND
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- Journal:
- Journal of Fluid Mechanics / Volume 498 / 10 January 2004
- Published online by Cambridge University Press:
- 27 January 2004, pp. 353-380
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Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully three-dimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.