Experimental Study of the Discharge Coefficient in Side Weirs with the Piano Key and the Rectangular Labyrinth Crests

Document Type : Research Article


1 Department of Civil Engineering, Faculty of Engineering, Bu-Ali Sina University, Hamedan, I.R.IRAN

2 Assistant Professor, Department of Civil Engineering, Bu-Ali Sina University, Hamadan, Iran

3 Assistant Professor, Department of Civil Engineering, Faculty of Engineering, Bu-Ali Sina University, Hamedan, I.R.IRAN


Introduction: The weirs or spillways are the oldest and the most important hydraulic structures. They have several applications such as evacuation of excess water flow due to floods, control of water level in the reservoir, flow distraction, reducing river erosion and measurement the flow discharge. The side weir is one of different type of spillway which used to control the flow level, diversion and flood damage prevention in dams and hydraulic structures. Also, the side weirs can be divided to the linear and nonlinear crest. Nonlinear weirs come in a variety of forms, such as Labyrinth weir and Piano Key weir. They are used to increase the length of the crest and their discharge capacity where there is length limitation for constructing the weir. Due to importance of the discharge coefficients in the side weirs with the Piano Key and the Labyrinth crest shapes, in the present work a vast range of experimental studies were performed on those types of weirs with different height and geometries. The results of experiments are then used to compare the Key Piano side weir with the Labyrinth one.
Methodology: The experiments were carried out 10 m flume at the Bu-Ali Sina University, Civil Engineering Department. A simple rectangular cross section was selected with almost 10 m long, 0.60 m wide and 0.60 m height. The Rectangular Labyrinth and Piano Key weirs experimental models are made using 5mm Plexiglas material in 3 cycles, and 4 heights of 5 cm, 10 cm, 15 cm and 20 cm. The side weirs models had 57 cm length and fixed in the wall opening near the flume end. Since in this research the flow condition is the spatial varied flow, the De Marchi relationship and dimensional analysis have been used to investigate the discharge coefficients in the Piano Key weirs and the Rectangular Labyrinth weirs.
Results and Discussion: The study shows that in general, in the Rectangular Labyrinth weir, by increasing weir height the weir’s capacity and consequence the discharge coefficient increases. For example in Rectangular Labyrinth weir with 20cm height the discharge coefficient is almost 34 %, 7.3 % and 14.1 % bigger than that for weir with height of 5 cm, 10 cm and 15 cm respectively. Also, a comparison between the 5, 10 and 15cm weir, with the 20 cm height weir, reviled that the weir efficiency has increased by 35 %, 7.8 % and 14.5 %, respectively. Meanwhile, in the Rectangular Labyrinth weir with heights of 5 cm, 10 cm, 15 cm and 20 cm by increasing Ht / P from 0.95, 0.66, 0.46 and 0.32, respectively, the weir efficiency decreases significantly and its performance get close to linear weir. Also comparing to the Piano key weirs with 5 cm, 10 cm and 20 cm heights, in the weir with 15 cm height, the averaged discharge coefficient increased by 9.3 %, 5.5 % and 9.2 %, respectively. The results of experiments on the Piano key weir shows that by choosing 15 cm as the weir height, the average weir efficiency increases by 9.5 %, 3.5 % and 9.4 % respectively ( to compare with 5 cm, 10 cm and 20 cm weir ). Also according to the experimental results on the Piano key weir with 5 cm, 10 cm, 15 cm and 20 cm height, by increasing the Ht / P ratio from 0.88, 0.6, 0.44 and 0.35 values, the weir performance also get close to linear weir and the weir efficiency reduced considerably. In rectangular Labyrinth weir and Piano key weir, the interference of the flow shedding blades causes a weir at the end of the outlet keys, which is the beginning of a significant decrease in the weir efficiency; and as the interference of these shedding blades increases, the weirs flow gradually deviates from its original function and acts as a linear weir.
Conclusion: For the weir with specific value of Ht / P ratio, the smallest weir has the highest discharge coefficient and the lowest discharge capacity. Previous study on the Labyrinth and Piano key weirs indicated that when the weir axis is perpendicular to the flow direction, the efficiency of the Piano key weir is much more than that for the Rectangular Labyrinth weir. However, for side weirs where the weir axis is parallel to the flow direction the Rectangular Labyrinth weir shows better efficiency and performance to compare with the Piano key weir. The Type A Piano key side weir performs better than the Type C Piano key side weir.


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