Effect of bed roughness on the mixing layers in a 90° asymmetrical confluence

In this paper, the mixing layer between the two incoming flows in a 90 degree, asymmetrical open channel confluence is investigated. Specific attention is given towards looking into the effect of bed roughness on the flow patterns in the confluence. This analysis is performed in a Serret-Frenet type axis system, in order to come up with a more convenient way of representing the measurement data. The effects on the velocity distribution, velocity difference over the mixing layer, width and location of the mixing layer are studied, and found to be influenced

Influence of cross-sectional shape on flow patterns in an open-channel confluence

The state-of-the-art hydrodynamic knowledge on schematized open-channel confluences is largely based on laboratory experiments in channels with a rectangular cross-section. Since in practice man-made canals often have a nonrectangular cross-section, the question can be raised to which extent the knowledge on open-channel confluences still holds for confluences of that type of canals. Therefore, this paper examines the influence of three cross-sectional shapes on the flow patterns in an asymmetric 90° open-channel confluence with fixed concordant bed and subcritical flows throughout. Large-eddy simulations are performed for a rectangular, a chamfered rectangular and a trapezoidal section. The simulations for the non-rectangular sections show that the location of both the mixing layer between the incoming flows and the shear layer between the separation zone and the contracted section are altered in comparison to the rectangular section case. Also the evolution of the separation zone is found to be different. It is demonstrated that the distribution of incoming momentum and the shape of the confluence corners are primary causes of the observed deviations

Influence of bed elevation discordance on flow patterns and head losses in an open-channel confluence

Confluences play a major role in the dynamics of networks of natural and man-made open channels, and field measurements on river confluences reveal that discordance in bed elevation is common. Studies of schematized confluences with a step at the interface between the tributary and the main channel bed reveal that bed elevation discordance is an important additional control for the confluence hydrodynamics. This study aimed to improve understanding of the influence of bed elevation discordance on the flow patterns and head losses in a right-angled confluence of an open channel with rectangular cross-sections. A large eddy simulation (LES)-based numerical model was set up and validated with experiments by others. Four configurations with different bed discordance ratios were investigated. The results confirm that, with increasing bed elevation discordance, the tributary streamlines at the confluence interface deviate less from the geometrical confluence angle, the extent of the recirculation zone (RZ) gets smaller, the ratio of the water depth upstream to that downstream of the confluence decreases, and the water level depression reduces. The bed elevation discordance also leads to the development of a large-scale structure in the lee of the step. Despite the appearance of the large-scale structure, the reduced extent of the RZ and associated changes in flow deflection/contraction reduce total head losses experienced by the main channel with an increase of the bed discordance ratio. It turns out that bed elevation discordance converts the lateral momentum from the tributary to streamwise momentum in the main channel more efficiently. (C) 2019 Hohai University. Production and hosting by Elsevier B.V

Influence of hydraulic resistance on flow features in an open channel confluence

A numerical model based on the 3D shallow water equations is set up for a 90° angle open channel confluence. The model is first calibrated and validated using experimental data by (Shumate, 1998). Then a series of numerical simulations is carried out, systematically increasing the friction coefficient, in order to investigate the impact of hydraulic resistance on the flow features in an open channel confluence. The properties of the separation zone (width and length) are found to be substantially altered by the hydraulic resistance. The hydrodynamic processes are analysed zooming in onto lateral momentum fluxes

On analytical formulae for navigation lock filling-emptying and overtravel

In culvert-based navigation lock filling-emptying systems, inertia effects have a significant influence on the filling-emptying time and cause a (damped) oscillation of the water surface in the lock chamber around its equalization level, referred to as the overtravel phenomenon. In this paper, the derivation of analytical formulae for the lock filling-emptying time and overtravel peak of systems consisting of a number of identical culverts is revisited. In comparison to earlier publications, the underlying assumptions are made explicit and the importance of accounting for the surface area ratio of lock chamber to upper reservoir in case of filling (or lower reservoir in case of emptying) is pointed out. Additionally, it is shown how the applicability of the analytical formulae can be extended to lock filling-emptying systems with more complex lay-outs by using an "equivalent culvert" approach. The validity of the analytical formulae is thoroughly assessed, first by comparing to an accurate numerical solution of the governing non-linear second-order differential equations, and second, by means of experiments in a physical model

Experimental study of the surface oscillations induced by a shallow flow past a lateral cavity

The present work presents an experimental study in which resonant surface oscillations inside a lateral cavity are reconstructed, often denoted as se- iching, which are excited by a shallow main stream flowing past the horizontal basin. Firstly, the flow configurations that trigger transverse and/or longitu- dinal seiching are studied using pressure measurements in the corners of the cavity, which shows that a transitional Froude number exists, approximately 0.63, at which the dominant seiching mode changes from transverse to longi- tudinal seiching. For both types of resonant conditions, the surface shape is analyzed in detail using a three-dimensional particle tracking velocimetry (3D- PTV) setup. Based upon floating seeding particles, the 3D surface is recon- structed with a superior spatial resolution compared to traditional measurement techniques, which confirms the multimodal aspect of the surface oscillations