Abstract
Due to the constricting and localization of flow by orifices, these hydraulic structures are commonly used in dams and water impoundment reservoirs to flush deposited sediments. They are also one of the most important flow measurement structures. In this study, the flow characteristics at upstream of a large circular orifice located at the wall of a water reservoir, in the case that the sediments were accumulated behind it, was investigated. The experiments were performed at the hydraulic laboratory of Shahid Bahonar university of Kerman, in a model of rectangular reservoir. The experiments were performed in two parts. First, the flow behavior during the scour development at upstream of the orifice and the extent of sediment erosion due to the orifice flow was investigated. Next, a semi-confined structure was located at the upstream of orifice to increase the sediment removal efficiency. The effect of the geometrical characteristics of structure on the dimensions of scour hole was determined. It was observed that the interactions of water, sediment and structure generated strong vortices upstream of the orifice, which caused the lifting and sweeping away of a large amount of sediment from the reservoir and let to formation of a semi-cone hole upstream of the orifice.
1. Introduction
Orifices are hydraulic structures that are used for different purposes. In dams, they are used to flush deposited sediments from the reservoir. They are also one of the main flow measurement structures. Streeter et al. [24] defined the orifice as an opening through which the fluid flows. Generally, the orifice flow condition is established when the head above the center of orifice become 2–2.2 times of the orifice diameter [10]. If the head is less than 5 times the diameter of the orifice, the orifice is categorized as a large orifice (Goyal, 2015).
4. Conclusion
In this research the flow characteristics and sediment scour at the upstream of a large orifice, when the sediments are accumulated up to the invert of orifice, were investigated. Next, a semi-confined piles group structure was used to strengthen the power of orifice flow with the purpose of improving the sediment removal efficiency. The effect of geometrical properties of proposed structure on the dimensions of flushing cone was investigated. The results showed that by using the proposed structure, both the relative depth and the relative radius of flushing cone increased. In this way, piles diameter had a significant effect on the dimensions of scoured region. It was observed that by increasing the piles spacing, the maximum depth of flushing cone was firstly increased and then decreased. Furthermore, it was found that as the distance of piles from the orifice increased, a larger amount of sediment was scoured from the flushing cone. Then, the time development of flushing cone was followed with and without the structure. It was observed that in the presence of proposed structure, the rate of temporal development of flushing cone increased significantly. The observations indicated that, the amount of flushed sediments increased by up to 250% compared to the reference test.
