Elham Mina - PhD. Of Hydraulic Structures Tarbiat Modarres University, Tehran, Iran
Mohammad Reza Kavianpur - Assistant professor ,Civil EngineeringKhaje Nasir Toosi University ,Tehran, Iran
Saleh Neishaburi - professor,Tarbiat Modarres University ,Tehran, Iran

Pressure flushing is a means of discharging sediments from reservoirs. Bottom outlets are among the main lateral structures of dams, which are used to control reservoir water pressure and discharge dam sediments. Due to the high flow rate and pressure drop, problems such as cavitation can affect these structures. The present research objective was to conduct a hydraulic analysis of the effect of the depth-to-width ratio of bottom outlets and increased efficiency of flushing through the escalatio n of eddy currents around the gate. This i nvestigates the discharge characteristics of a bottom outlet with a moving gate by Computational fluid dynamics. This study also includes the effect of the height to wid th ratio of the conduit on aeration and flo w field downstream of the gate. Changes are applied in thre e cases of constant height (depth) and variable width, constant width and variable height, and both vari able height and width to make equal gate sr ea. Results indicated that more aeration while heightening an d widening of the outlet. Also velocity incre ase and pressure decrease is observed while friction head is co nstant. The pressure drop is mainly caused by the increase in gate height by which transverse turbulenc e on water surface declines and decrease and the Vortex number increase. Therefore, the increase in res ervoir gate height positively and significantly affects flushing and Sedimentation .It was possible to reduc e pressure fluctuations and omit negative pressures by placing an air-entraining system after the bottom gate. In addition, the flow of air increase s pressure relatively and decrease the risk of cavitation.

Pressure Flushing, Dam reservoir , Bottom Outlet, Vortex number, turbulence models CFD simulation, VOF model