The experimental study of the effect of splitter on the flow discharge Piano key weir

Document Type : Research Article


1 msc water and hydraulic engineering at tarbiat modares University

2 Tarbiat Modarres

3 Ph.D. Candidate, Water and Hydraulic Structures Engineering, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran


Introduction: Introduction: The newest type of nonlinear labyrinth weirs are piano key weirs. The initial study on this weir indicated that it increases the discharge significantly and has a simple and economical structure. In the past years, different researches are done to check effective factors on flow discharge and optimization of this weir. But to a limited extent is referenced about ventilation mechanism and aeration of this weir at outlet and the only solution that has been presented for aeration at outlet is gallery aeration at downstream. Another problem that is discussed in the case of piano key weirs and generally over flow of weirs is Nappe oscillation after the cross of weirs crest. Past research considered the use of splitter to be effective in reducing the nappe oscillation in linear weir. By getting ideas from this method (use of splitter), three piers with different geometries (circular, square, and rectangular) has been used in piano key weir to reduce nappe oscillation.
Methodology: All experiments of this research were performed in a rectangular channel with a width of 75 cm, metal floor, unbreakable glass walls and a height of 80 cm in the hydraulic laboratory of the Department of Water Engineering and Hydraulic Structures, Tarbiat Modares University, Tehran The water flow from the underground tank is entered to the flow calming tank by two pumps with a maximum discharge of 85 liters each per second, and it reaches to weirs after passing through the calming plates, and falls into the underground tank by passing over weir at the end of the flume. This cycle continues during various tests under different hydraulic conditions.The discharge was measured by ultrasonic flowmeter with an accuracy of 0.01 liters per second, after the pumping and before entering to the calming tank. All experiments were performed under free flow conditions. Upstream flow deep was measured by a point gauge with an accuracy of ±1 mm. This depth gauge is moved by rails on the wall of the channel and the water depth is measured at desired points.The piano key weir used in this research is of three types of A piano key weir with different rectangular, triangular and trapezoidal designs in the plan. splitter with three circular, square and rectangular cross-section geometries were installed on the weir crown and in the downstream corners of the weir keys.
Results and discussion: The results showed that splitter, in addition to separating the flow after the splitter and creating a space for the connection of free surface air with the lower part of the outflow from the weir, also reduces the nappe oscillation intensity of passing through it. The use of splitter does not have a negative effect on the flow discharge in the rectangular and trapezoidal piano key weir and only in the triangular piano key weir, it has reduced the discharge by 3%.The performance of square and rectangular splitter was similar on water discharge and separation. The effect of flow separation in a rectangular splitter is evaluated better than on a square base. Also, the effect of these two splitter on the current separation is more appropriate than a circular splitter. Regarding the comparison of the discharge coefficient of these three types of weir, it was observed that at H_t/P<0.2, the discharge coefficient in the rectangular and trapezoidal piano key weir is higher than the triangular piano key weir, which is due to more flow suction (due to vacuum created below the weir inlet key) in this head for these two weirs. By increasing H_t/P up to 0.4, the difference between the discharge coefficient of the rectangular and trapezoidal piano key weir relative to the triangle gradually decreases to the point that after H_t/P>0.4, the discharge coefficient of the triangular piano key weir relative to the trapezoidal and rectangular piano key weir has increased, which ventilation performance of this weir type is one of the main reasons for that.
Conclusion: Regarding the increasing use of piano key weir due to its advantages and the need for aeration in the downstream to improve its performance, this study aims to provide an economic solution (using splitter instead of aeration gallery) to improve ventilation performance in the downstream of the piano key weir. Therefor it has studied the effect of splitter with different circular, square and rectangular sections on the flow discharge of this weir type with three rectangular, triangular and trapezoidal designs in the plan. In general, the results obtained in this study can be summarized as follows: In the total head less than 0.08 m, the rectangular piano key weir in constant discharge, about 5 and 15%, has a smaller head than the trapezoidal and triangular piano key weir. But at a total head greater than 0.08 m, a trapezoidal piano key weir at a constant discharge, has a smaller head than a rectangular piano key weir about 5% and on average about 8% than a triangular piano key weir. Splitter show the best flow separation performance in the case of H_t/P<0.6, but in the case of H_t/P>0.6 this performance is affected by high water flow, so that with increasing discharge, the correlation the bottom of flow is reduced with open air and it is not possible to ensure relative to the complete correlation of the open air with the downstream and the outlet of the piano key weir. In terms of the splitter geometric shape, square and rectangular splitter showed similar performance in flow discharge and separation. But the separation of the current at the rectangular splitter is evaluated better than the square splitter. Also, the geometry of these two splitters is more suitable for flow separation than circular geometry.


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