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Labyrinth weir is one of the approaches to increase the discharge capacity. An arced ‎configuration improves the orientation of the labyrinth weir cycles to the approach flow and ‎increases the weir crest length for a given width. In this study, the effects of the entrance flow ‎conditions on the hydraulic‏ ‏performance of the arced ‎labyrinth weirs is studied experimentally.‎‏ ‏The effects of the angle between the entrance channel walls (Θ′) on the discharge coefficient ‎and the efficiency are investigated for different values of the ‎headwater ratio (Ho/P), the ‎downstream sidewall angle (α), and the weir arc ‎angle (Θ).‎ Experiments were conducted in a recirculating flume‏ ‏which is 10 m long, 2 m wide, and 0.9 m ‎deep at Tarbiat Modares University. To simulate the reservoir conditions, a specific setup was ‎added to the flume, known as the reservoir simulator. The flume was launched from its two ends ‎by two pipelines. The inflow passes from underneath of the reservoir simulator and enters into it ‎through a semi-circular opening in its horizontal‏ ‏wall. After moving‏ ‏over the horizontal‏ ‏wall, the ‎flow comes up through the gap between the vertical wall. Finally, it flows on the platform and ‎moves towards the downstream channel. All the plates (including the platform and the simulator ‎walls) have a semicircular plan-view with a porosity equal to zero. The‏ ‏weirs were mounted on ‎the platform at the entrance of the downstream channel. Totally 132 experiments were ‎conducted to investigate the effects of the mentioned parameters on hydraulic performance of ‎arced labyrinth weirs.‎ Due to the nappe interference, the local submergence forms in the downstream of the ‎labyrinth ‎weirs. The size‎ of local submergence regions increase by increasing the ‎headwater ratio ‎and the arc angle. However, vice versa trend occurs with the downstream sidewall angle. In ‎addition, for low values of the arc angle, the lateral flow from the side cycles to their adjacent ‎cycles produces the surface turbulences. The results ‎indicate that the discharge coefficient ‎decreases by increasing the ‎headwater ratio and the downstream sidewall angle. For low values ‎of the ‎headwater ratio, the discharge coefficient increases when the arc angle increases. ‎However, a decreasing trend is observed in high head conditions. By increasing the ‎arc angle ‎and decreasing the downstream sidewall angle, the efficiency of a labyrinth weir can be ‎increased. However, the efficiency gains diminish by increasing the ‎headwater ratio.‎ The efficiency of a labyrinth weir can slightly be increased by projecting of the cycles into a ‎reservoir for low values of Ho/P, α, and Θ. However, in the wide range of the research domain, ‎the efficiency decreases‏ when ‏the angle between the entrance channel walls increases. ‎According to the results of this research, the efficiency of a labyrinth weir can be increased up ‎to 20% by channelizing abutments in high head conditions. However, the effect of Θ′ is ‎insignificant for higher values of Θ. In addition, as α decreases, the benefits and the losses of ‎decreasing Θ′ become more ‎severe at higher and lower values of Ho/P, respectively.‎

Keywords: Discharge‎, Entrance channel walls, Discharge coefficient, Efficiency, Arced labyrinth weir

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