Jet flipping and scouring below a sluice gate

• Main Author: Lim, Bi Shi.
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Environmental engineering
• Online Access: http://hdl.handle.net/10356/53856
• Institution: Nanyang Technological University

The sediment bed downstream of hydraulic structures is often susceptible to an intense local bed degradation or scour. In the present study, the scour characteristics and its associated jet flipping phenomenon caused by a two-dimensional submerged wall jet issuing from a sluice opening are investigated. Experiments were conducted under various tailwater depths, with and without an apron. A total of three flow patterns, namely bed jet, surface jet and jet flipping, are observed when an apron is applied. On the contrary, no jet flipping exists when no apron is applied. The flow pattern depends on the tailwater depth, and it is observed that a critical range of tailwater depths exists at which jet flipping would occur and beyond which only bed jet or surface jet would exist. An interesting phenomenon is observed at very shallow submergence where the tailwater depth is almost equal to the jet thickness, and would be discussed in Section 4.1. Moreover, the flow patterns have a significant effect on the transport mechanism of bed sediments. The presence or absence of a solid apron also greatly affects the correlation between maximum scour depth and tailwater depth. The intermittent shifting of jet direction, form bed to water surface or vice versa, during the scouring process is known as the jet flipping phenomenon. Visual observations of the jet flipping phenomenon are discussed in Section 4.2 using centerline scour profiles and dye injection tests. In Section 4.3, the possible triggering mechanism behind this phenomenon is proposed with relate to the Coanda effect. It is believed that the sudden flick in jet direction or the abrupt change in jet attachment to the bed or water surface is due to the preference of the jet to attach to the nearest boundary; described as the effect of ‘relative nearness’ in this report. Moreover, the occurrence and frequency of jet flipping are collectively determined by parameters such as tailwater depth, jet velocity and apron length. Velocity distributions in the scour region are presented in Section 4.4 to understand the flow fields associated with the jet flipping phenomenon. Various flow fields diagrams are used to illustrate the flow directions, velocity magnitudes and turbulent kinetic energies during the digging and filling phases. These diagrams aid the visualisation of water flow fields that cannot be seen through naked eyes and enhance the understanding of jet flipping phenomenon.