The Effect of Applying Suppressed and Non-Suppressed Sill on Discharge Coefficient in Different Positions of the Sill Relative to the Sluice Gate

Background and Objectives
The gate is one of the hydraulic structures in which water passes through it. The most common types of these structures are sluice gates that move up and down in a vertical plane to adjust the opening. Estimating the discharge coefficient and consequently determining the flow rate under the gate is one of the basic and important issues in hydraulic engineering. Daneshfaraz et al. (2016) numerically investigated the effect of sluice gate edge shapes on flow characteristics. Their results showed that the flow contraction coefficient for sharp edges and round-edge gates decreases when the ratio of gate opening to upstream specific energy is less than 0.4 and increases for ratios greater than 0.4. The results of the research (Alhamid 1999) showed that the discharge coefficient increased in the sill mode compared to the no-sill mode. Salmasi and Abraham (2020) conducted an experimental study of sluice gate discharge coefficient with polygonal and non-polygonal sills. In the present study, general formula for calculating the flow rate under the sluice gate for the suppressed sill state has been developed for the non-suppressed sill and presented for the first time. The discharge coefficient was investigated in no-sill state at different openings and with the suppressed and non-suppressed sills in two openings in different positions relative to the sluice gate.
Methodology
In this study, a laboratory flume with a rectangular cross-section of 5 meters long, 0.3 meters wide and 0.5 meters high with walls and floors made of transparent Plexiglas has been used for experiments. The experiments were performed in two states without sill at different gate openings and with sill at two openings. In this research, experiments using polyethylene sills with a thickness of 5 cm and a height of 3 cm in different widths of 2.5, 5, 7.5, 10, 15, 20, 25 and 30 cm was done at different positions relative to sluice gate. In total, 377 experiments were performed in the flow rate range of 150 to 850 L min^-1
Findings
In without sill state, discharge coefficient is inversely related to the gate opening. Applying the sill below and tangentially to the gate leads to an increase in discharge coefficient. Also, with increasing the ratio of the upstream flow depth to the sill width, the discharge coefficient has an increasing trend. By comparing the discharge coefficient in different situations, the discharge coefficient in the upstream tangential position is higher than the sill in the below position of the sluice gate. The reason is related to the position of the sill. For the downward tangent model compared to the sill below the gate, the discharge coefficient is higher, and compared to the upward tangent model is lower. In the upstream tangential model, the amount of water depth upstream of the gate is lower than the below and downward model, and the highest amount is related to the sill state below the gate. At the same opening in the without sill and suppressed sill state, the maximum discharge coefficient is related to the sill state. In the stage-discharge diagram at constant discharge, the water head upstream of the sluice gate is lower in all suppressed sill positions than the no-sill state. In this study, equations were presented for predicting discharge coefficient in with and without sill state.
Conclusion
The results showed that in no-sill state and in different gate openings, the discharge coefficient is inversely related to the gate opening. In the present study, the general equation of discharge calculation was developed for the non-suppressed sill and the calculations were performed based on the new relation presented for the case with the non-suppressed sill. This equation can be used for symmetric and asymmetric sills. Comparison of the results obtained for the discharge coefficient, with sill, and without sill condition indicates the better performance of the existence of a sill in all positions in terms of increasing the discharge coefficient. In addition, the comparison of the results of discharge coefficients between the suppressed sill and no-sill state in the opening of 1 and 2 cm indicates an increase of the discharge coefficient in the suppressed sill state. In this condition, the discharge coefficient of the gate opening of 1 cm is higher than 2 cm. However, the discharge coefficient is higher in both opening modes than in the without sill mode.