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Title: Jet-flipping phenomenon in local scour below a sluice gate
Authors: Xie, Chen
Keywords: DRNTU::Engineering::Environmental engineering::Water supply
DRNTU::Engineering::Civil engineering::Water resources
Issue Date: 2014
Source: Xie, C. (2014). Jet-flipping phenomenon in local scour below a sluice gate. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: This study shows that a jet-flipping phenomenon may occur during the local scour process caused by 2-D submerged wall jets over an apron downstream of a sluice gate. The jet-flipping consists of a digging and a filling phase. The former scours the bed and the latter re-fills the scour hole. The process is cyclical and there is no equilibrium scour condition. A novel webcam method is developed to record the scouring process continuously for a long period. At the end of digging, a dynamic equilibrium condition is reached with a stabilized scour profile which may last up to 10 days before the next filling phase occurs. Hence, if the run time is too short, the jet-flipping may not be observed. The scour dimensions were found to change cyclically in accordance with the digging and filling phases, and multiple ridges may form. At the end of the digging and filling phases over many cycles, the scour profiles were found to be similar. The results showed that the scour depth during digging can be 60% more than the filling phase. A comparison of the maximum scour depth from past and present data shows that the scour is more severe with jet-flipping compared to the no jet flip conditions. The velocity distributions during digging and filling phases were measured in two fixed-bed models at the threshold of jet-flipping. The flow pattern shows that for the digging phase, the jet dives and impinges on the bed after leaving the apron. The velocities are high near the bed which contributes to the severe scouring action. For the filling phase, the jet approaches the water surface immediately after leaving the apron. The high velocities are found near the apron and the path to the water surface and the latter causes the sediments on the ridge to slide down and back-fill the scour hole. The results show that jet-flipping could be triggered by the Coanda effect due to the jet’s preference to attach to a relatively nearer boundary. The effects of jet-flipping on the scour depth and the frequencies of jet-flipping were studied systematically. It was found that the maximum scour depth for both digging and filling phases increase as the tailwater depth becomes deeper, decrease as the apron length becomes longer, and increase as the densimetric Froude number becomes larger. These results suggest that no jet-flipping will be observed if the tailwater is sufficiently deep or if the apron is too long. The results also show that the presence of an apron below the gate is necessary for jet-flipping to occur. The results for the jet-flipping frequencies in terms of the Strouhal number show that they increase with the increase in densimetric Froude number and apron length but decrease as the tailwater increases. Based on the present data, prediction equations are proposed for the maximum scour depth at the end of digging and filling phases and for the frequencies of jet-flipping. A threshold diagram to predict the occurrence of jet-flipping is also presented and could be applied in design to avoid the undesirable phenomenon in local scour below a sluice gate.
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Appears in Collections:CEE Theses

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