Cellular Secondary Currents in Straight Conduit

Nezu, Iehisa; Nakagawa, Hiroji

doi:10.1061/(asce)0733-9429(1984)110:2(173)

Cited by 124 publications

(91 citation statements)

References 8 publications

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“…The observation of secondary current over the entire width in wide and shallow natural rivers [12,13,23,25,33] is attributed to a positive feedback between flow and sediment transport, which leads to a non-uniform bed roughness. This widespread theory is in line with laboratory experiments with longitudinal roughness strips [19,22,23,28,34,35] that result in secondary currents over the entire width in wide channels. Ikeda [11] was the first to argue that the basic mechanism causing the formation of secondary currents over the entire width is the interaction between flow and sediment transport.…”

Section: Introductionsupporting

confidence: 81%

Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight open-channels

Blanckaert

Duarte

Schleiss

2010

Advances in Water Resources

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Boundary shear stress and flow variability due to its interaction with main flow and secondary currents were investigated under conditions that extend previous research on trapezoidal channels. Secondary currents that scale with the flow depth were found over the entire width in all experiments. These findings contradict the widespread perception that secondary currents die out at a distance of 2.5 times the flow depth from the bank, a perception which is largely based on experiments with smooth boundaries. The reported results indicate that a stable pattern of secondary currents over the entire channel width can only be sustained over a fixed horizontal bed if the bed's roughness is sufficient to provide the required transverse oscillations in the turbulent shear stresses. Contrary to laboratory flumes, alluvial river bed always provide sufficient roughness. The required external forcing of this hydrodynamic instability mechanism is provided by the turbulence-generated near-bank secondary currents. The pattern of near-bank secondary currents depends on the inclination and the roughness of the bank. In all configurations, secondary currents result in a reduction of the bed shear stress in the vicinity of the bank and a heterogeneous bank shear stress that reaches a maximum close to the toe of the bank. Moreover, these currents cause transverse variability of 10-15% for the streamwise velocities and 0.2u * 2 -0.3u * 2 for the bed shear stress. These variations are insufficient to provide the flow variability required in river restoration projects, but nevertheless must be accounted for in the design of stable channels.

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Section: Introductionsupporting

confidence: 81%

Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight open-channels

Blanckaert

Duarte

Schleiss

2010

Advances in Water Resources

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“…Relation between the near-wall strength of secondary flow and the near-wall Reynolds shear stress and its distribution. ments by Nezu and Nakagawa (1984) obviously confirm that the secondary currents are always associated with or occur at the place where the near-bed regional velocity, qu/qz = 0. It can be predicted that no secondary currents would be induced if the ridge was placed on the upper flow region, instead of the bed.…”

Section: ½31mentioning

confidence: 73%

Mechanism for initiating secondary currents in channel flows

Yang

2009

Can. J. Civ. Eng.

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This study investigates the underlying mechanisms that initiate secondary flow in developing turbulent flow along a corner. This is done by theoretical examination of the total shear stress, which is the time-averaged product of instantaneous streamwise velocity U and the velocity Vn normal to the interface. The study shows that lines of zero total shear stress exist in the flow region, which delineate the region of secondary flow. Therefore, the flow region is dividable and eight vortices occur in a duct flow. The theoretical and experimental results show that the division line, separating the neighboring secondary currents in a corner, is not always identical to the bisector of the corner, but deviates from the corner bisector if the aspect ratio is b/h = 1. By simplifying Reynolds equation in the near-bed region, we find that theoretically a lateral variation of streamwise velocity initiates the wall-tangent flow that drives the vortex in the region bounded by zero total shear stress. A simplified method for estimating the vortex center, near-bed secondary velocity, and shape of secondary currents has been proposed, and a good agreement between the measured and predicted features is achieved.Key words: dip-phenomenon, division line, Reynolds shear stress, secondary currents of Prandtl's second kind, turbulent energy, velocity distribution.Résumé : La présente étude examine les mécanismes sous-jacents qui initient un écoulement secondaire dans un écoule-ment turbulent en développement le long d'un coin. Cela est accompli en examinant de manière théorique la contrainte totale en cisaillement, laquelle est le produit pondéré dans le temps de la composante longitudinale instantanée de la vitesse U et la vitesse Vn perpendiculaire à l'interface. L'étude montre que, dans la région de l'écoulement, il existe des lignes de zéro contrainte totale de cisaillement qui délimitent la région d'écoulement secondaire. C'est pourquoi la région d'écou-lement peut être divisée et que huit vortex surviennent dans un écoulement en canalisation. Les résultats théoriques et expérimentaux montrent que la ligne de division, séparant les courants secondaires avoisinants dans un coin, n'est pas toujours identique à la bissectrice du coin, mais qu'elle dévie de la bissectrice du coin si le rapport de forme b/h = 1. En simplifiant l'équation de Reynolds dans la région près du lit, les résultats théoriques montrent qu'une variation latérale de la composante longitudinale de la vitesse initie l'écoulement tangent au mur qui génère le vortex dans la région limitée par la contrainte totale de cisaillement zéro. Une méthode simplifiée pour estimer le centre du vortex, la vitesse secondaire près du lit et la forme des courants secondaires a été proposée et une bonne corrélation a été obtenue entre les paramètres mesurés et prédits.

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“…Vaughan (2013) found an increase of 1.5 W from the paddlewheel for the addition of a single DWVG, and in reality an array of DWVGs would be required to achieve su cient mixing. Additionally, it was observed by Godfrey (2012) Citerone (2016) used PVC half-pipe ridges on the bed of a shallow, straight flume to generate cellular, secondary currents after the fashion of Nezu and Nakagawa (1984). It was found that the gas transfer velocity did increase 9-15% with the addition of the PVC ridges.…”

Section: Open Rwts and Existing Studiesmentioning

confidence: 98%

“…Conversely, in the straight sections of the reactor it is expected that secondary currents of Prandtl's second kind will form, especially if enhanced with modifications along the bed. In the straight regions, assuming the V 6 term is negligible for a boundary layer flow, the streamwise vorticity equation reduces to: Nezu and Nakagawa (1984) found in their studies of straight conduit that the V 4 and V 5…”

Section: Streamwise Vorticity Equationmentioning

confidence: 99%

“…This was further confirmed by the report of sand ribbons in the longitudinal direction after flooding subsided (Culbertson, 1967;Karcz, 1973). It was suggested by Kinoshita that these sediment "boils" were caused by counter-rotating secondary vortices with a diameter equal to the flow depth (see Figure 1.5; Nezu & Nakagawa, 1984). However, the mechanism for initiation of these currents was unknown.…”

Section: Experimental and Field Studiesmentioning

confidence: 99%

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Identifying the dominant physical processes for mixing in full-scale raceway tanks

2018

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Biodiesels from microalgae are a promising alternative to fossil fuels with a number of unique benefits over other alternative fuel sources. However, to date, their viability in the fuels market remains infeasible due in part to a number of ine ciencies in the cultivation process. This has led to a growing interest in the hydrodynamics of open raceway tank (RWT) reactors, and cultivating conditions that are favorable to algae growth. In particular, two impediments require attention: the lack of vertical mixing in the straight portions of the reactor, and the need for e cient introduction of atmospheric carbon into the growth medium. It is proposed that the promotion of cellular, secondary currents by introduction of longitudinal ridges along the bed of the reactor can both promote vertical mixing and enhance gas transfer across the free surface of the growth medium. Experiments are performed in a full-scale open RWT at the University of Illinois, Urbana-Champaign, Ecohydraulics and Ecomorphodynamics Laboratory (EEL), in which the bed of the RWT is modified with longitudinal ridges of two di↵erent sizes, triangular and semi-circular cross-sections, and variable spacing.The 3-dimensional velocity components are monitored at significant locations in the reactor by means of acoustic doppler velocimetry (ADV) and surface particle image velocimetry (sPIV), and the gas transfer across the free-surface is measured by re-aeration curves of dissolved oxygen (DO). It is found that the cellular, secondary cells can be visualized in the velocity data after the introduction of longitudinal ridges. However, the impacts at the free surface are limited and consequently so is the gas transfer. A range of flow structures are observed that impact the relative significance of these cellular currents, including: vortex shedding from bend vanes, secondary currents of Prandtl's first kind induced by the ii bends, paddlewheel pulsation, and boundary layer development. These structures are found to overwhelm the presence of the cellular currents, and indicate that flows in the straight portions of the RWT reactor are a complex composite of boundary layer flow, secondary currents, resonance structures, energy input conditions, and shear flows. If cellular, secondary currents are to be used to promote the ideal algae growth conditions, further developments on this method will be required to overwhelm or work cooperatively with the dominant bend-induced dynamics.iii To Jarah.

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Cellular Secondary Currents in Straight Conduit

Cited by 124 publications

References 8 publications

Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight open-channels

Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight open-channels

Mechanism for initiating secondary currents in channel flows

Identifying the dominant physical processes for mixing in full-scale raceway tanks

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