Effect of Outlet Submergence in Bottom Outlet on Aeration between Emergency and Service Gates at high Dams

Using bottom outlets for drain and regulation of water level in reservior has significant importance in performance mahagment of dam. Amount of service and emergency gates opening have important role in pressure, velocity and discharge coefficient variations and finally dam performance. In the present research, flow aeration between emergency and service gates was studied with different hydraulic conditions in Karron 4 and GhizGhalasi dams with free and submerged downstream condtirions. Aeration coefficient was measured experimentally. Numerical simulations were also performed to analyze the pressure fluctuation in stilling basin after the gate using Flow3D software. Results showed that outlet submergence has reductive effect about 50% on flow aeration rate. On the other words, the submergence condition at outlet, will lead to increase cavitation occurrence probability, and therfore larger air vent will be needed. Also it was senn that aeration decreased pressure flaccuation on the stilling basin that it leads to eliminate cavitation occurance.