An air-core vortex is a common dynamic fluid phenomenon that usually generates in various types, shapes, geometries, and sizes of the tanks that store liquids such as water, fuel, or hydraulics. An air-core may form when a rotated fluid column in a tank drains the fluid through an outlet. The blocking portion of the drain port caused by residual that forms due to this phenomenon would decrease the flow rate, the efficiency of fluid output during drainage, the lifetime of the tank, and produce cavitation of the drain port. In this research, a comprehensive study was carried out to suppress air-core vortex formation by applying two types of simple bluff body geometries as a passive air-control mechanism. The two types of bluff body geometries, such as cone and prism, are used to investigate the effects in a cylindrical tank on fluid drainage time, residual left, and water velocity at outlet port during draining. The Sketcher Workbench in CATIA v5 is used to model a tank and the flow is simulated using ANSYS CFX. Comparisons have been made between the cone and prism geometry in minimizing air-core vortex generation at the outlet port of the cylindrical tank. Two different sizes have been tested, which are 20 and 25 mm with three different positions of air-core suppression mechanism at 3, 5, and 8 mm from the outlet port of the cylindrical tank. The result shows that the prism 25d that is located at 8 mm from the tank's outlet port has been determined as the most optimal configuration mechanism due to high efficiency in suppressing air-core vortex formation compared to other mechanisms.