Flow aeration downstream of normal gates in bottom outlet conduits has been found to be an effective and cheap way to eliminate cavitations damages. Studies on aerators have been focused on the question of how much air is entrained into the flow for different geometry of aerators and flow conditions. Designers of the aerators use empirical equations to calculate the quantity of air required by the aerators. The area of the aerator is determined using empirical relationships assuming the air velocity not exceeded a certain values. This paper is based on the results of hydraulic model studies of new bottom outlets recently constructed and examined at Water Research Center of Iran. The results consist of Jareh Karkheh Kosar Dasht-e-Abbas and Eilam bottom outlet dams in Iran. The model studies showed two different mechanism of flow aeration. These mechanisms depend on the geometry of conduits downstream of the gates with respect to the upstream geometry. It is hopped that this information provides a better understanding of the process of aeration for the designers of such structures.