Numerical study of discharge coefficient of trapezoidal labyrinth weir by changing the angle using Flow3D model

Among the regulator structures used in water projects, weirs have remarkable applications like measuring flow, maintaining water level, controlling sediment transport, and managing variable water inflows. Weirs can be classified on the weir axis direction as a normal, side, or a labyrinth weir. labyrinth weirs are hydraulic structures used to regulate water levels and control flow in reservoirs of dams, canals and rivers.The discharge coefficient in the weirs is directly proportional to the crest length of weir. If the width of the channel or reservoir where the weir made is limited, one of the ways to increase the capacity is to increase the crest length of weir by zigzagging the weir in the plan. Due to their complex geometry labyrinth weirs are expensive to build. Therefore, in laboratory studies, high cost is one of the reasons that has made it difficult to investigate this type of weir comprehensively. Numerical methods are a very good option for hydraulic analysis of labyrinth weirs hydraulic. The history of the construction of labyrinth weirs goes back to before the year 1920 (Darvas, 1971). labyrinth weirs were first investigated by Gentellini )1940(. Vahab nezhad (2017) studied the changes in discharge coefficient for the three angles of 15, 18 and 23 degrees. Zamiri, et al. (2016) showed that increasing the thickness of the labyrinth weir wall increases the depth and velocity of the flow and ultimately reduces the discharge coefficient. The results showed that the discharge coefficient increases with increasing the weir angle, because as the weir angle increases, the weir length decreases and the discharge coefficient increase.In this study, the weir performance of a trapezoidal labyrinth with different angles (α) was investigated. For this purpose, labyrinth weir modeling was performed with three angles of 15, 23 and 30 degrees for two crest length of 5 and 7 cm and three heights of 10, 14 and 18 cm. The geometry of the weirs was created in the Inventor software and simulation was performed in Flow3D software. In order to validate the simulation and adjust the software parameters, valid laboratory data were used and the results of the model were in good agreement with the results of the laboratory data. For this purpose, the discharge coefficient of numerical models (Cd (CFD)) and laboratory (Cd (EXP)) for different values of water to overflow ratio (h / p) were compared and the error value was calculated. The highest error rate is 5.88 and the lowest is zero. In fact, with increasing the ratio (h/p) the error rate has increased, but this increase is also acceptable and the results show that Flow3D software has a very good ability to simulate labyrinth weir. Flow field study was performed with different turbulence models in Flow3D software using laboratory data. The RNG model was used in all simulations. To mesh the model, the channel was divided into three parts: initial, middle and end. The dimensions of the cube of the initial and final parts are 1 cm and the middle part is 5 mm.Analysis of the results showed that increasing the weir angle in a fixed crest length, increases the discharge coefficient (cd). The inverse relationship between the relative hydraulic load (h/p) and the discharge coefficient was also determined. Increasing the crest length of the weir by 2 cm, the trend of increasing the discharge coefficient due to increasing the weir angle, decreased by 9%. Also, increasing the angle 15 degrees, the trend of increasing the discharge coefficient due to increasing the crest length, decreased by 5%. This means that at higher angles, the effect of increasing the crest length on the discharge coefficient decreased and during shorter crest length, the effect of increasing the angle on discharge coefficient increased. Due to the direct relationship between the discharge coefficient and the average flow velocity, simultaneous increase crest length and weir angle increased the average flow velocity by 4 times at the beginning of the channel floor impact. In cases where the purpose of the study is to increase the discharge coefficient by increasing the angle and length of the weir crest, it should be noted that the effect of increasing the angle and length of the weir crest is more noticeable at low altitudes, So that with increasing the height of the weir this amount decreases.