Laboratory Investigation of the Performance of Triangular and Semi-circular Homologous Labyrinth Weirs

Labyrinth weirs reduce the flood height compared to linear weirs. Labyrinth weirs have a longer length than linear weirs and therefore the flood will pass over it with a lower height. But different forms of Labyrinth weirs have different efficiency. Considering the many effective parameters in the discharge coefficient of Labyrinth weirs, in this research, semi-circular and triangular homologous labyrinth weirs are investigated. Therefore, only changes in the angle of the weir head and the angle with the channel wall will be compared and investigated.

Experiments were performed in a channel with a width of 0.8 meters and a flow rate of up to 70 liters per second. 10 weir models with a thickness of 4 mm were made, five of which are triangular weirs and 5 of which are semi-circular weirs. The height of the weirs is 15 cm and the ratio of the length of the weirs to the width of the channel (L/W) is equal to 1.26. Each triangular labyrinth weir was compared with its homologous semi-circular weir. This means that the single-cycle semi-circular Labyrinth weir with the upward apex will be compared with the triangular single-cycle Labyrinth weir with the upward apex. Other Labyrinth weirs will also be compared with their corresponding triangular Labyrinth weirs. This causes the angle of the head of the weir as well as the angle of the weir with the wall of the downstream channel to be investigated and other parameters effective in the discharge coefficient are constant in this investigation. Finally, according to the measured data, a relation for the discharge coefficient will be presented.

The comparison of the triangular labyrinth weir with the identical semi-circular weir showed that the discharge coefficient of the triangular weirs is generally higher. The maximum reduction of the discharge coefficient of the semi-circular labyrinth weir was 9%, which is for the single-cycle labyrinth weir with the with downward apex. With the increase of cycles and the placement of the weirs apex towards the upstream, the distance between the discharge coefficient of the identical semi-circular and triangular labyrinth weirs decreases and their efficiency becomes almost the same. The smaller angle of the semi-circular labyrinth weir with downward apex with channel wall causes more interference of Nappe with the channel wall and cause more local submergence and reduce its efficiency. Also, the angle of impact of the flow jets at the apex of the semi-circular labyrinth weir with downward apex is greater than that of the triangular labyrinth weir, which causes more turbulence and as a result reduces the efficiency more at the apex of the semi-circular labyrinth weir. By placing the apex of the labyrinth weir towards the upstream, the impact of the flow jets at the apex is eliminated and the angle of the weir with the wall of the downstream channel increases, which ultimately increases the efficiency of the weir. The increase in efficiency in a semi-circular weir with the apex towards the upstream is more than in a similar triangular weir, and as a result, the discharge coefficient of the two weirs is close to each other. For triangular labyrinth weir, the discharge coefficient is equal to -0.146Ln(HT/P)+0.4959 with a regression of 0.94 and for semicircular labyrinth weir, the discharge coefficient is equal to -0.158Ln(HT /P)+ 0.4731 with a regression of 0.9, which indicates that these relationships fit the data well.

The results showed that the labyrinth weirs with the upward apex have a better performance than the labyrinth weirs with the downward apex. Also, semi-circular labyrinth weirs had lower efficiency than triangular labyrinth weirs. Of course, with the placement of the apex of the labyrinth weirs upstream, the performance of the semi-circular labyrinth weirs increases and they approach the triangular labyrinth weirs. Therefore, in labyrinth weirs with upward apex, which have better performance, the use of each of semi-circular and triangular labyrinth weirs will not make much difference.