Experimental Study of Hydraulic Properties of Flow over Vertical Shaft Spillway with Different Shape of Inlet

Abstract
Background and Objectives
Spillway as a hydraulic structure is used to controlled release of water from a dam or levee downstream. One of the types of spillway used in dams is vertical shaft spillway or the same as morning glory spillway (MGS). Shaft spillways are used where there is not enough space for conventional spillways in the dam reservoirs,. The morning glory spillway (MGS) is formed by a crest, a conical transition, a vertical shaft, a bend, and an outlet tunnel. Swirling flow is a fundamental problem encountered in hydraulic engineering, resulting in air-entraining vortex at pipe intakes and shaft spillways. Forming at hydraulic intakes, these vortices reduce intake efficiency and can lead to unstable operation and vibration in downstream hydraulic machinery. One of the methods to increase the flow rate and reduce the occurrence of vortex currents in the spillway is to modify the inlet shape of the spillways. In recent years, spillways with daisy, piano key and zigzag inlets have been proposed known as spiral spillways. A number of researchers have studied vertical shafts with different inlets and defined their own characteristics. A review of the results of previous research shows that the common goal in the study of vertical shaft spillways is to pass the flood flows with the highest possible discharge and the lowest water level and reducing the destructive effects of vortex currents. In the previous researches, issues such as the effect of the inlet type of vertical shaft spillway and geometric parameters of the structure on the discharge coefficient and conditions of vortex occurrence and hydraulic of vertical shaft spillway flows have been studied. In this research new shapes of vertical shaft inlet in form of Bow-tied and Spindle spillways were introduced and discharge coefficient of these spillways was investigated using a series of experiments and the results compared with common shapes of vertical shaft inlet in form of the Daisy, Morning Glory, Square, Hexagonal and Octagonal spillways.
Methodology
In this research, the effect of different input shapes on the flow hydraulics of vertical shaft spillway was studied. For this propose, models of Bow tie spillway with middle and without arches, Spindle, Daisy, Hexagonal, Octagonal and Square with the same total length were used and discharge coefficient of the models were calculated and the results were compared with a Morning Glory spillway. The experiments of this research were performed in a metal cube tank with 1.5 m length, 1 m height and 1 m width with transparent Plexiglas side walls in the hydraulic laboratory of Department of Water Engineering at Zanjan University. In this research, by preparing eight models of the studied spillways with a total length of 60 cm and considering the hydraulic conditions (H/D), 63 experiments were run to investigate the effect of the geometric shape of the spillway on the discharge coefficient.
Findings
The results showed that the Bow-tie without middle arch model cause to uniform the flow regime and has a significant effect on reducing water head and increase the discharge through the shaft, so that Bow-tie without arch model has a higher discharge coefficient than other models. It has better hydraulic performance (10.4%) compared to Morning Glory spillway and also has higher economic benefits (30.88%). Comparison of the Spindle, Daisy, Square, Hexagonal and Octagonal models with the Morning Glory spillway shows that in low and high discharge, the Daisy and square models have better performance, respectively (3.45 and 5.1%, respectively). Also, the Bow-tie with middle arch model did not have a significant effect on the discharge coefficient and its hydraulic performance decreased in compared to the Morning Glory spillway (-2.3%), which could be due to low internal surface and interference of downstream flows.
Conclusion
Comparison of the hydraulic performance of Bow-tie and Spindle models with the other shapes shows that the model of Bow-tie without arch has the highest hydraulic performance and a better economic advantage. In the construction of reservoir dams, especially earthen dams, the major part of the executive cost is related to the implementation of concrete structures of emergency spillway. In this situation, by choosing Bow-tied spillway with optimal geometry, a suitable economic savings can be achieved along with improving the hydraulic conditions of the flow.