Bottom outlet conduits are the most important side constructions in dams which are used to control of reservoir water volume in back of dam, regulation of downstream water right and drain of the reservoir sediment. The Flushing operations to drain the reservoir sediments by high-speed flow in bottom outlet conduits of dams can create cavitation damages in these structures. Experiences showed that the flow aeration can decrease the risk of cavitation damages effectively. The investigations of flow characteristics by numerical methods along with physical hydraulic models can be effective in choosing the best choice for aeration and also decrease extra cost and time of physical modeling. In this research the cavitation occurrence in the bottom outlet of Sefidroud Dam was evaluated with different gate openings using FLUENT software in 3D mode. A method for aeration of this structure concluding aerator ramps on floor and side walls of conduit was selected and then the results of numerical and physical model were compared.