شبیه‌سازی عددی جریان بر روی سرریزهای کنگره‌ای مثلثی و قوسی

Weirs are among the structures used in water transfer channels, hydraulic structures, irrigation and drainage channels, and they are used to regulate the water level and control the flow. Weirs are flow diversion tools that are widely used in irrigation and drainage systems and urban sewage systems. Increasing the capacity of each weirs in a certain width is one of the important issues. These changes cannot be implemented in all structures because limiting factors such as topographical conditions and construction materials may not allow. The use of weirs with non-linear crests is one of the ways to increase the capacity of each weir. During the construction of dam structures, due to the high cost, they are looking for a solution to optimize costs, and one of the best ways to build a dam is to use a weir with a non-linear crest. One of the advantages of non-linear weir is to increase the flow rate capacity. Simulation using various software including Flow3d to investigate the flow pattern has become very popular in recent years. The most important reason for using simulation software, in addition to cost reduction, is the possibility of creating different forms of labyrinth weirs in the software. Meanwhile, in the laboratory, it is not possible to investigate the labyrinth weirs of different shapes and dimensions.

In this research, the hydraulic model was tested inside a glass flume with a length of 10 meters, a height of 80 cm and a width of 80 cm. This flume has a pump with a maximum flow rate of 90 lit/s. The flow rate is measured using an ultrasonic flow meter. To measure the water level, several rail gauges points that can move along the flume are used. The labyrinth weir is triangular with 4 mm thick and height of 15 cm. The length of the model is 126 cm and the L/W ratio is 1.58. At first, the laboratory model was compared with the simulated model with the same magnification ratio. For this purpose, 9 discharges with a head ratio of 0.2 to 0.7 were investigated in the laboratory. The purpose was to check the accuracy of the model used in the simulation compared to the laboratory model. Then, three triangular and three arched labyrinth weirs models were simulated with 2, 3 and 4 magnifications. K-Ɛ RNG turbulence model was used to solve the turbulence flow. This model has an additional term compared to other turbulence equations, which will give us a more accurate solution for solving and discretizing the turbulence equations. In the current research, the flow depth boundary conditions were used in the flow entry, the exit flow boundary conditions were used in the exit sections, the wall boundary conditions were used in the walls and the floor, and the symmetry boundary conditions were used in the water surface.

In order to validate the model, comparison of numerical and laboratory of discharge coefficient (Cd) was used. The results showed that RMSE is equal to 3.9% and MSE is equal to 0.16%. Therefore, the error is acceptable and the model will be used in the simulation of triangular and arched labyrinth weirs. The results showed that the discharge coefficient increased at first and then decreased. At first, gradually increasing the flow over the weirs, the amount of air trapped under the nappe decreases. This has caused a decrease in the amount of momentum introduced by the rotating air under the nappe, as well as a decrease in the negative pressure in that area, and as a result, it has resulted in an increase in the discharge coefficient. Investigations showed that with the increase of the water head ratio (HT⁄P) and magnification ratio (L/W), the nappe interference and local submergence increased and caused a decrease in the discharge coefficient. Thus, in the triangular and arched labyrinth weir, the discharge coefficient decreases up to 38% and 39% with the increase of the water head ratio from 0.1 to 1 and up to 29% and 37% with the increase of the magnification ratio from 2 to 4. On the other hand, arched labyrinth weirs have a lower discharge coefficient of up to 14% at a same magnification ratio than triangular labyrinth weirs. The reason for this problem is the smaller angle of the arched labyrinth weir with the channel wall compared to the triangular labyrinth weir, which causes the nappe interference to hit the channel wall and also cause more local submergence in the arched weir. Also, the impact angle of the flow jets at the top of the arched weir is higher than that of the triangular weir, which will cause more disturbance of the flow and decrease the flow coefficient.