Effects of Daisy-Shape Inlet on Free Flow over Shaft Spillways

Spillways and bottom outlets are the common hydraulic structures in dam engineering to convey excess water from the reservoir toward the downstream river. Economy and operation are the major factors affecting the type of spillway and its’ crest shape to avoid unfavorable hydraulic phenomena. An appropriate spillway crest results in increasing the discharge coefficient and distributing a uniform pressure inside the conveying conduits. One of the most common types of spillways in dam engineering is shaft spillways with morning glory inlet. Generation of swirling flow at the entrance of the shaft spillways and entraining air into the conduits is an unfavorable phenomenon. Air-entrainment into the system leads to different depressing effects on system operation including reduction of the discharge coefficient and can cause severe damages. To control the swirling flow phenomenon one way is the application of anti-vortex devices. Applying certain shape of inlets e.g. installing a Daisy (Marguerite)-shape inlet over the shaft entrance is another alternation to avoid the swirling flow effects thereby to increase the discharge coefficient. Marguerite-shape inlet has been used in different existing dam projects. Marguerite inlet is a unique inlet compared to other shapes of spillway crests for a constant head. This is in part due to spatially varied flow inside the marguerite inlet which makes it capable of passing greater discharge. Although different types of dam spillways have been the subject of different investigations, there is a lack of study on these types of spillways.
In this study, the effects of Daisy (Marguerite)-shape inlet on free-flow through shaft spillways have been investigated based on model experimentation. Dimensional analysis has been used to determine the effective dimensionless parameters. Experiments were conducted in a cylindrical model of 2 m diameter with two shafts of 10 and 12.5 cm diameters on the tank bottom. The tests have been performed based on a wide range of geometric and hydraulic parameters to study the effects of each dimensionless parameter on flow hydraulics. Finally, applying SPSS software and nonlinear regression analyses empirical correlations were obtained for estimating the discharge coefficient and the threshold depth of orifice flow over Daisy-shape inlets. To validate these correlations, the normalized root-mean-square error (NRMSE), the weighted quadratic deviation (WQD) and the coefficient of determination R2 were applied. Contrary to R2, both NRMSE and WQD must be small to have the best correlations.
Spillways and bottom outlets are the common hydraulic structures in dam engineering to convey excess water from the reservoir toward the downstream river. Economy and operation are the major factors affecting the type of spillway and its’ crest shape to avoid unfavorable hydraulic phenomena. An appropriate spillway crest results in increasing the discharge coefficient and distributing a uniform pressure inside the conveying conduits. One of the most common types of spillways in dam engineering is shaft spillways with morning glory inlet. Generation of swirling flow at the entrance of the shaft spillways and entraining air into the conduits is an unfavorable phenomenon. Air-entrainment into the system leads to different depressing effects on system operation including reduction of the discharge coefficient and can cause severe damages.