Energy Dissipation of Supercritical Flow with Use of Different Geometric Shapes of Sills

Background an objectives
In the past, many studies have been carried out on the hydraulics of sliding valves. In addition to flow control and measurement, this structure is used for energy consumption downstream. This structure plays an important role in controlling and regulating the speed downstream of the structure. The supercritical flow passing through sliding valves has attracted the attention of researchers due to its high energy dissipation. The high velocity of the flow in the downstream of the hydraulic structures is one of the important issues of water engineering and usually causes damages if there is no control in the downstream. The creation of hydraulic jump is associated with the transformation of supercritical to subcritical flow, and as a result, energy consumption occurs. This phenomenon can be seen downstream of structures such as dams, rapids and valves. So far, successive methods have been used to reduce the kinetic energy of the flow passing through the sliding valves, which are discussed below. Among the first researches in this field, the studies of Rajaratnam (1967), Rajaratnam (1968) and Alhamid, (1994) can be mentioned. Daneshfaraz et al. (2022a) investigated the hysteretic behavior of the supercritical flow which occurs with two different flow behaviors under the same hydraulic conditions. The results showed in the primary flow, the amount of these depths indicates the subcritical regime, and in the secondary flow, with the formation of the hysteresis phenomenon in some flow rates, it indicates the supercritical regime. The hysteretic phenomenon has a different effect on the amount of energy consumption depending on the type of flow.
Methodology
The experiments were performed in a laboratory flume 5 m long, 0.30 m wide, and 0.45 m deep. The laboratory channel has a floor and walls made of Plexiglass and is equipped with a point depth gauge with an accuracy of ±1 mm. A 1 cm thick sluice gate is installed one meter away from the beginning of the flume. In all experiments, the gate opening was considered constant and equal to 5 cm. The discharge applied in the present study ranged from 700 to 900 l/min. The effect of the threshold with semi-cylindrical, cylindrical, pyramidal, and rectangular cube geometric shapes and with different widths of 5, 7, 10, 15, and 20 cm was investigated experimentally. In the present paper, the sill was installed in the different position, including under, the tangential state downstream and upstream of sliding valve.
Findings
The energy dissipation of the sluice gate was investigated in the non-sill mode and with increasing discharge. Laboratory studies showed that the increase in flow rate caused an increase in the flow speed and consequently the descent number of the supercritical region. As a result, the depth of flow in section A has decreased and it has caused energy consumption downstream of the sliding valve. Energy consumption by installing sill in different positions showed that all four geometries, including semi-cylindrical, cylindrical, pyramidal and rectangular cubic sill, had the maximum energy loss in the position under the valve. Results showed that for undergate sill, the maximum of energy dissipation is related to pyramidal, semi- cylindrical, cylindrical and rectangular cubic sills, respectively. The increase in the jetting streamlines due to the application of the pyramid sill is evident in the tangential position downstream of the valve. The results showed that placing circular sills, including semi-cylindrical and cylindrical, in the downstream of the sliding valve, cause the uniformity of the flow lines. Therefore, the depth of the flow in the initial section of the hydraulic jump is reduced compared to the polygon sills.
Conclusion
The results showed that:
1. The amount of energy loss increases with the increase in sill width.
2. Due to having the slope of the downstream side in the same direction as the flow, the pyramid threshold increases the speed of the flow and therefore causes a decrease in the initial depth of the flow.
3. Circular sills including semi-cylindrical, cylindrical and rectangular cube were included, respectively, with the greatest initial depth
4. By changing the position of the threshold to the tangent position downstream of the sliding valve, the highest amount of energy loss was assigned to semi-cylindrical, cylindrical, rectangular cube and pyramidal thresholds, respectively.