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Flow Characteristics of Undular Hydraulic Jumps. Comparison with Near-Critical Flows

By Hubert Chanson


In open channels, the transition from supercritical to subcritical flows is called a hydraulic jump. For low upstream Froude numbers, free-surface undulations develop downstream of the jump. Such a hydraulic jump is called an undular jump. An earlier report (CHANSON 1993) described over 75 experiments on undular hydraulic jumps, performed in a rectangular channel in which the upstream flows were fully developed turbulent shear flows (both in terms of bottom and sidewall boundary layers). Visual and photographic observations indicated five types of undular jumps. One of the main flow characteristics was the presence of lateral shock waves for Froude numbers larger than 1.2. The present document describes a second series of more than 20 new experiments. The experiments were performed in the same flume previously used. The basic features of all the experiments are : a rectangular channel cross-section, and the upstream flows were uniform turbulent flows with fully-developed bottom and sidewall boundary layers. The experimental data include free-surface profiles, velocity distributions, pressure distributions and local head profiles upstream and downstream of undular jumps. The measurements were taken along the jump as well as across the flume width. The results show a major three-dimensional flow redistribution immediately upstream of the first wave crest. Velocity and pressure distributions were recorded at very close intervals in that region. And they provide some understanding of the flow redistribution mechanisms. The analysis of all the data (first and second series of experiments) shows that the energy dissipation occurs by the propagation of a train of stationary waves, lateral shock waves and a breaking wave (roller) mechanism at the first wave crest. The disappearance of the undular jump occurs for Froude numbers ranging from 1.5 to 2.9 and it is a function of the aspect ratio dc/W. Further the flow field properties (pressure, velocity, free-surface height) are functions not only of the upstream Froude number but also of the inflow conditions, the aspect ratio and the sidewall roughness. The flow characteristics of undular hydraulic jumps are compared also with near-critical flows : undular surges, free-surface undulations above broad-crested weirs, undulations downstream of backward-facing steps. After an introduction, the reader will find a bibliographic review on undular hydraulic jumps. Then the experimental apparatus is described. The experimental results are presented in two parts : the free-surface characteristics and the flow field characteristics. Later an analogy with near-critical flows is developed. At the beginning of the report, a table of contents, a list of symbols and a glossary can be found. At the end of the report, the appendices include : 1- a list of constants and fluid properties (App. A), 2- a table of unit conversions (App. B), 3- the experimental data of free-surface flow parameters (App. C, D, E) and 4- the data of pressure, velocity and total head distributions (App. F). A summary of the experiments is presented in appendix G. The experimental data of DARCY and BAZIN (1865), published in French, are given in appendix H

Topics: undular flows, undular hydraulic jumps, undular positive surges and bores, experiments, physical modelling, shock waves, crosswaves, standing waves, free-surface flows, transcritical flows, near-critical flows, open channel flows, 240502 Fluid Physics, 260502 Surfacewater Hydrology, 290000 Engineering and Technology, 290800 Civil Engineering, 290802 Water and Sanitary Engineering, 291100 Environmental Engineering, 291199 Environmental Engineering not elsewhere classified, 291200 Maritime Engineering, 291299 Maritime Engineering not elsewhere classified, 300100 Soil and Water Sciences, 300105 Applied Hydrology (Drainage, Flooding, Irrigation, Quality, etc.)
Year: 1995
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