Effects of a conical enlarging shaft on hydraulic performances of siphon-shaft spillways

Siphon-shaft spillways can discharge large amounts of water down to the crest level in a narrow reservoir thanks to the siphonic pressures. However, cavitational pressures in the siphon-shaft limit the operating head of these spillways. An enlargement at the shaft mouth can reduce the vacuum pressures and velocities within the siphonic flows, thereby removing the risk of cavitation occurring. In this study, four different enlarging shaft profiles with various coning angles of 0 degrees, 10 degrees, 15 degrees, and 20 degrees were applied to a siphon-shaft spillway model to eliminate cavitation pressures in the shaft. These models are analyzed in three dimensions by computational fluid dynamics based on the RANS turbulence model coupled with the Volume of Fluid (VOF) method to simulate fluid motion. The numerical uncertainties of the simulation were calibrated with some experimental results and techniques in the literature. The results showed that the enlargement in the shaft mouth with a conical profile significantly decreased the vacuum pressures and velocities in the siphon-shaft. Thus, the use of conical profiles considerably reduced the cavitation numbers along the shaft surface and increased the discharge performance by about 11%.