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Mechanism of secondary currents in open channel flows

Profile image of Shu-Qing YangShu-Qing Yang

2012, Journal of Geophysical Research

https://doi.org/10.1029/2012JF002510
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Abstract

This paper describes the conditions for initiation and maintenance of secondary currents in open channel flows. By analyzing the Reynolds equation in the wall-normal and wall-tangent directions, this study reveals that, like other types of vortices, the secondary currents are originated in the near-boundary region, and the magnitude (or strength) of secondary flow is proportional to the lateral gradient of near-wall velocity. The near-wall secondary flow always moves from the region with lower velocity (or lower boundary shear stress) to the location with higher velocity (or higher boundary shear stress). Subsequently, the near-boundary secondary flow creeps into the main flow and drives circulation within a region enclosed by lines of zero total shear stress, leading to anisotropy of turbulence in the main flow region. This paper also discusses typical secondary currents in open channel flows and presents the relationship between sediment transport and secondary currents. The formation of sand ridges widely observed on the Earth surface is explained in the light of the proposed relationship.

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Secondary Current and Classification of River Channels
patricia Wanjiru

In this research, the secondary current theory is used in investigating the role of phase shift angle between the secondary current and the channel axis displacement in stability analysis of a river channel. To achieve this, a small-perturbation stability analysis is developed for investigation of the role of the secondary current accompanying channel curvature in the initiation and early development of meanders in open channels. The secondary currents are generating in planes perpendicular to the primary direction of motion. The secondary currents form a helical motion in which the water in the upper part of the river is driven outward, whereas the water near the bottom is driven inward in a bend. Force-momentum equations for longitudinal and transverse direction in open channel bends were utilized. Assuming that the transverse force contributed by the bed is negligible, the pressure force associated with the transverse surface inclination is balanced by the centripetal force. Existing equations of the transverse velocity profile were analyzed. Since the magnitude of the vertical velocity is negligible compared to the transverse velocity in secondary currents, this study concentrates on the transverse velocity which is the radial component of the secondary current. This formulation leads to a linear differential equation which is solved for its orthogonal components which give the rates of meander growth and downstream migration. It is shown that instability increases with decrease in phase shift angle. Transition from straight to meandering and then from meandering to braiding occurs when phase shift angle is reduced.

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Related topics

  • Geology
  • Channel Flow

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    Diana De Padova

    2021

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    Formulae of Sediment Transport in Unsteady Flows (Part 2)
    Shu-Qing Yang

    Sediment Transport - Recent Advances [Working Title], 2020

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    Influence of Coriolis Force Upon Bottom Boundary Layers in a Large‐Scale Gravity Current Experiment: Implications for Evolution of Sinuous Deep‐Water Channel Systems
    joel Sommeria

    Journal Of Geophysical Research: Oceans, 2020

    Oceanic density currents in many deep-water channels are strongly influenced by the Coriolis force. The dynamics of the bottom boundary layer in large geostrophic flows and low Rossby number turbidity currents are very important for determining the erosion and deposition of sediment in channelized contourite currents and many large-scale turbidity currents. However, these bottom boundary layers are notoriously difficult to resolve with oceanic field measurements or in previous small-scale rotating laboratory experiments. We present results from a large, 13-m diameter, rotating laboratory platform that is able to achieve both stratified and highly turbulent flows in regimes where the rotation is sufficiently rapid that the Coriolis force can potentially dominate. By resolving the dynamics of the turbulent bottom boundary in straight and sinuous channel sections, we find that the Coriolis force can overcome centrifugal force to switch the direction of near-bed flows in channel bends. This occurs for positive Rossby numbers less than +0.8, defined as Ro R = <U>/Rf, where <U> is the depth and time-averaged velocity, R is the radius of channel curvature, and f is the Coriolis parameter. Density and velocity fields decoupled in channel bends, with the densest fluid of the gravity current being deflected to the outer bend of the channel by the centrifugal force, while the location of velocity maximum shifted with the Coriolis force, leading to asymmetries between left-and right-turning bends. These observations of Coriolis effects on gravity currents are synthesized into a model of how sedimentary structures might evolve in sinuous turbidity current channels at various latitudes. Plain Language Summary Many of the largest currents in the oceans depths are dense, gravity-driven, flows that pour down deep-water channels. These large gravity currents include dense overflows, as well as sediment-laden turbidity currents and contourite flows. The dynamics of these gravity currents can be strongly affected by the Coriolis force. This study examines the effect of the Coriolis force on the flow structure of oceanic gravity currents flowing through straight and sinuous channels. A set of 22 experiments were carried out on the world's largest rotating experimental facility, the LEGI Coriolis platform in Grenoble, France. Our detailed measurements of velocity and internal density structure imply that at higher latitudes Coriolis force dominates and changes the direction of the flow near the bed leading to differences between flows going around a left-turning versus a right-turning bend. Our observations are relevant to the large deep-water channels formed on the ocean floor by successive turbidity currents. Near the Equator these channels tend to be noticeably sinuous; however, recent studies have shown that this sinuosity decreases with latitude. One possibility is that latitudinal variations in the Coriolis force may influence the evolution of these channels through changing near-bed patterns of erosion and deposition.

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