عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The main part of available water resources is consuming in agriculture. So, irrigation networks play an important role in the optimal use of these resources. Gates are one of the most common water structures in distribution canals, which transport and deliver water to consumers, to control the flow rate or water level. Determining the relationships governor on these structures can improve the performance of open canals and therefore prevent water loss. In the present study, an experimental investigation has been conducted on elliptical Lopac gates. This gate is a combination of a Lopac gate and elliptical sharped edge weir. The reason for the using of such combination is to achieve a simple and hydraulically constructed gate that has a higher performance than commonly used models. The elliptical Lopac gate is applied as a new structure for water level adjustment. Some advantages of this structure could be mentioned as: overflow behavior and better possibility for water level control, simultaneous passage of sediments and floating bodies, low power requirements, and ease of automation of the structure. These advantages have recently attracted the attention of water managers to apply the structure in irrigation canals. These overflow structures act as weir in terms of hydraulic behavior. The Lopac gates are similar to saloon doors, hinged at vertical walls of the canal, which regulate the upstream water level for different discharges by adjusting their opening angle. So far little studies have been done on these types of gates. In this study, by using of energy relationship and laboratory results, the characteristics of the flow and the equation of discharge and the gate discharge coefficient in free flow conditions were investigated and the new type of this gate, -called elliptical Lopac gate- were mentioned.
The experiments of this study were conducted in Hydraulic Laboratory of the Faculty of Water Engineering, Shahid Chamran University of Ahwaz. After examining all the methods of testing and different types of flumes and channels, a rectangular flume with glass wall and metal floor with the length, width and height of 10.5, 0.8 and 0.6 meter, respectively, was selected for tests. In order to obtain the opening angles, a pair of PVC picket were cut at the desired angle and then applied. Also, for testing in two free and submerged modes, a fiat sliding gate at the downstream of the channel was installed for this purpose. In free flow conditions, five different discharges (20, 30, 40, 50 and 60 l/s) were tested for 5 different opening angles (20, 22.5, 30, 37.5 and 45 degrees).The flow discharge was measured by an ultrasonic flow meter with the accuracy of ± 1 l/s. In each experiment, for a given discharge and the angle of opening, the depth of water was taken on the upstream of the elliptical Lopac gate, at the 7 sections in the length and 5 points in width. After dimensional analysis of the effective quantities on the discharge coefficient of elliptical Lopac gate, the effective dimensionless parameters were obtained. These quantities include: Froude number, Reynolds number, Flume width to the upstream water depth ratio B/y, and Opening ratio b/B. The Reynolds number expresses the effect of viscosity forces. This factor plays a role only for large gate opening angles, where the upstream water depth decreases. According to the tests in the range of turbulent flow, the impact of the viscosity force was negligible. In the following, due to the free flow condition, a theoretical relationship was determined for discharge by the energy equation. Subsequently by using a regression on the results of the experiments, acceptable propositions were estimated for the discharge equation and the discharge coefficient.
The discharge coefficient in free flow conditions and for both types of rectangular Lopac gate and elliptical Lopac gate was a function of the gate opening ratio and its value increases with increasing gate opening. The observations showed that the most effective dimensionless parameter on the discharge coefficient was the opening ratio. The two parameters of the Froud number and the ratio of flume width to the upstream depth were both the function of the opening ratio. This relative on provided similar accuracy as the main equation with Root Mean Square Error of 0.00185, Mean Absolute Percentage Error of 3.143%, and Relative Error of ±10%.