Discharge Coefficient of Elliptical Lopac Gate with Gradual Transition in Submergence Condition

Introduction: The discharge coefficient (Cd) of irrigation gates is one of the important parameters for predicting the flow discharge through these structures. This parameter depends on the geometrical conditions of the gate as well as the upstream and downstream flow conditions. During the past decades attempts have been done to develop relations for different types of gates. Most of such studeis have been carrie out by researchers in the laboratory and therefore various types of relationships have been proposed. In the present study, the Cd value of the elliptical lopac gate with a gradual transition upstream was investigated. Rectangular lopac gate were invented in the 1980s by Peter Langman et al. to measure and manage water level fluctuations in irrigation canals and their successful applications have been reported in several projects [3]. This structure acts as a lopac lid that can adjust the flow of water upstream of the surface by adjusting the flow direction [8]. This structure acts like a gate that can also be adjusted the flow through the upstream surface by installing it in the flow direction [8]. Different equations for the coefficient of discharge of rectangular lopac gate with and without upstream transition have been extracted by different researchers. For the elliptical lopac gate without upstream conversion, Pilbala et al. (2018), Pilbala(2018) and Nesi et al (2018) have reported ectensive laboratory investigation to presented the required expressions for determination of Cd. In a wide channel, for beter performance and saving the cost of operation, it is advise to install narrower gate. Therefore a gradually transition at the upstream of the gate is usually designed and installed to help passing of the flow smoothly. By the knowledge of the authors, the effect of such structures on Cd have not been studied yet and thus the main goal of this paper is to conducted experimental tests to provide required data.
Methodology: In this study first, using the Buckingham theory the effective non-dimensional parameters were extracted (Eq. 4) and the experimental program was carried out accordingly. Experiments were conducted on a flume with a length of 800 cm, a height of 80 cm and width of 60 cm. At the beginning of the flume, a rock filled basket was installed to calm the flow which enters the flume. Four meters away from the basket, The Elliptical lopac gate models made of galvanized iron with a thickness of 2 mm and a width and height of 40 cm and a radius of 5 cm were installed. The gate was connected to the flume wall at a distance of 15 cm from the wall of the flume by a sudden conversion of the PVC sheet. Then, using this type of sheet, gradual transition was made and installed.
Results and discussion: In this study total of 108 experiments including 27 experiments with no presence of gradual transition and 81 experiments with three different models of gradual upstream transition were performed at three different angles (Table 1). For each experiment, the discharge coefficient was calculated from Eq.5 and the trend of discharge coefficient variation was studied by changing the dimensionless parameters obtained in dimensional analysis including upstream gradient angle, relative gate opening rate as well as the submergence. At the end data were analysed and presented in form of graphically or expression. The most finding of the present stduy can be outline as follows:
Conclusion: In general, the results show that the coefficient of discharge increases with the increase in the gate’s opening ratio. From the data analysis it was found that with the relative increase of the gate opening from 0.41 to 0.49, the discharge coefficient increased from 39.4% up to 77.7%. It was also found that increasing the rate of submergence decreases the discharge coefficient. This trend was investigated by decreasing the percentage of submergence from the maximum increase of discharge coefficient and it was observed that with decrease of submergence from 0.9 to 0.7, the discharge coefficient increased between 89.6 to 58.7%. Another parameter that was investigated was the gradual upward angle of the gate upstream. The lowest coefficient of discharge obtained for the case of no transition upstream or sudden transition with angle of 90 degree. The elliptic lopac gate discharge coefficient with the gradual transition of 22.5 degree was 37% higher than that of the sudden transition. SPSS version 25 software was appleid to developed expression for predicting Cd (Eq. 9). This relation was extracted using 80% of obtained expremental data and the remaining 20% of the data was applied to validated the relation. Comparing the predicted values of Cd and the experimental data show high accuracy of this relationship.