Although open-channel junctions are common in many hydraulic structures, no comprehensive data set has been compiled that describes the 3D flow field within the junction itself. This physical model study examined a 90°, sharp-edged, open-channel junction for channels of equal width. Depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region. The average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location. In addition, a 2D mapping of the water surface was performed on a 76.2 mm square grid throughout the channel junction. This paper presents the details of the experimental procedure and the general flow characteristics observed. The full data set generated during this experimental work is available for downloading on the Internet. Using a small portion of the data recorded, an evaluation of several previously proposed theori...

Experiments on flow at a 90° open-channel junction

Channel confluence is one of the important features of each river system and some hydraulic structures. The features that can dominantly control flow characteristics in a confluence are confluence ...

Three-dimensional numerical study of flow structure in channel confluences

Despite the ratio of incoming discharges being recognized as a key parameter in open-channel confluence hydrodynamics, little is known about the flow patterns when the tributary provides more than 90% of the total discharge This paper offers a systematic study of flow features when the tributary becomes increasingly dominant in a 90° confluence with a fixed concordant bed Large-eddy simulations are used to investigate the three-dimensional complex flow patterns for three different discharge ratios It is found that the tributary flow impinges on the opposing bank when the tributary flow becomes sufficiently dominant, causing a recirculating eddy in the upstream channel of the confluence, which induces significant changes in the incoming velocity distribution Moreover, it results in stronger helicoidal cells in the downstream channel, along with zones of upwelling flow In turn, the changed flow patterns also influence the mixing layer and the flow recovery Finally, intermittent events of stronger upwelling flow are discerned Improved understanding of flow patterns at confluences where the tributary is dominant is applicable to both engineering and earth sciences

https://www.mdpi.com/2073-4441/7/9/4724/pdf

Flow Patterns in an Open Channel Confluence with Increasingly Dominant Tributary Inflow

If the hydraulics of a flow is to be understood, then at least a few parameters must be known accurately. When the depth and the unit discharge are measured correctly, many other parameters can be calculated, e.g. the mean velocity of the flow, the friction, the Reynolds Number, the Froude Number, and, if the study is to be connected with erosion measurements, the shear stress. A method is therefore developed for the direct measurement of the depth of disturbed water films, by weighing the entire run; the depth of the flow is thus equal to the weight of the water contained in it, one gram being the equivalent of one cm3, divided by the area of the run. Corrections to the measurement are, however, necessary because the water accelerates along the run to a steady state velocity, because the weight drops due to an acceleration in the gravity field, because the weight of the spattered drops has to be known if simulated rainfall is applied, and because the impact of the drops causes a weight. The accuracy of the method is 10 μ if the flow conditions are not to be studied on too steep slopes and if the discharge used is not too great. The main results are plotted in Figure 6 which combines the formulas for laminar, transitional and turbulent flow that are obtained. A comparison with available literature suggests mat sheet flow can either be laminar or purely turbulent, but that mixed flows prevail on low slopes, under 0.5 per cent combined with greater water depths. Since the flow equations on steep slopes obey physical laws, sin S is to be used instead of S, which stands for tan S.

The experiments have shown that the effect of the rainfall impact on the Darcy-Weisbach f does not exceed 20 per cent in the case of laminar flow on gentle slopes. This influence diminishes when the discharge increases, or when the Reynolds Number increases, as well as when the slope angle increases.

The hydraulics of sheet flow on a smooth surface and the effect of simulated rainfall

The hydraulic flow features associated with a hydraulic jump over positive and negative steps are investigated. Attention is paid to the extreme positions of the phenomena, which allow the design o...

Hydraulic jumps at positive and negative steps

The main flow features of subcritical junction flows are explored, based on an extended hydraulic model study. The flow conditions chosen resulted in flows that are governed by the Froude similarity law. So-called simple junctions with junction angles of 30°, 60°, and 90° were tested. The main emphasis was made in determining the characteristics of the lateral flow and the flow contraction in the tailwater branch. Further, expressions for the momentum correction coefficients, the lateral wall pressure force, and the ratio of flow depths in the lateral and upstream branches were provided. A rational approach for the momentum contribution of the lateral branch is presented and applied for the prediction of the backwater effect across a simple junction. The complex flow pattern is further documented with selected photographs such that a rather complete description of junction flow is now available.

Subcritical Junction Flow

Experimental investigations have been carried out to study the supercritical flow over a square section sill for all flow situations, except the jump condition. The supercritical flow has been categorized into two distinct types depending upon the ratio of the sill height to upstream depth and the Froude number. This classification also corresponds to the splashing and nonsplashing conditions. The stagnation, wake, and reattachment pressures have been investigated. The flow conditions have been explored when the downstream face pressures and the bed pressures become negative. An almost constant drag coefficient value, obtained from the present experiments, substantiated the drag characteristics of bluff bodies in open channels.

Supercritical Flow Over Sills

A laboratory investigation has been conducted to study the conditions mainly responsible for the stabilization and prediction of the hydraulic jump in a rectangular, horizontal-bed channel with a square-shaped bottom sill at the incipient jump condition (i.e., at the verge of formation of the jump, however, without it actually forming). A theoretical analysis based upon the momentum and continuity principles has been carried out. A formula for the design of the sills, when the ratio of the sill height to the upstream depth is smaller than 1.25, is suggested. A relationship between the drag experienced by the sill and the upstream Froude number is also given. The theoretical formulation has been checked by laboratory experimentation.

Karam S. Karki

Papers

Subcritical Junction Flow

Supercritical Flow Over Sills

Supercritical Flow over Sills at Incipient Jump Conditions

Discussion of “Subcritical Open-Channel Junction Flow” by Chung-Chieh Hsu, Wen-Jung Lee, and Cheng-Hsi Chang