Simulation of two-phase turbulent flow of pivot weirs with different crest shapes

Pivot weirs are one of the most important structures to control and regulate the water level. Three-Pivot elevator weirs can be installed as one or more gates in a row in the waterways. Each of them has an independent hoist system to change the weir angle relative to the bed. The hydraulic conditions of this type of weirs (especially in multi-gates and different angles) are not studied. Therefore, the hydraulic conditions of these weirs were investigated.

In this study, flow modeling was performed to analyze the weir discharge coefficient and select the appropriate turbulence model using Ansys CFX software. The model was evaluated using Wahlin and Replogle experimental data this for should be omitted for different angles, and discharges. Also, RNG K-e, K-v, standard k-e, and SST turbulent models were compared. By determining the turbulence model, the optimal shape of the crest was studied in 3 types: Sharp, circular (upstream and downstream of crest edge in round shape), and semicircular (upstream of crest edge in a round shape).

Comparison of the model output results for different turbulence models showed that the standard k-e turbulence model is generally more consistent with laboratory readings so that for low angles the relative error calculated was between 1.4 to 3.1% less than the other models. The should be added error between should be omitted was calculated to be less than 4.4%, which showed a very good agreement between the model output and laboratory results. 

The results of calculating the discharge coefficient in elevator weirs showed that the discharge coefficient for weirs with an angle of 70 degrees and the semicircular crest is 0.7 to 7.9 percent higher than the weirs with circular and sharp-crested weirs, respectively. Similarly, the increase of discharge coefficient for weirs with an angle of 27.8 degrees was obtained between 0.4 to 3.2 percent. Therefore, weirs with semicircular crest edges have the highest discharge coefficient.